diff --git a/.github/actions/setup-ocaml/action.yml b/.github/actions/setup-ocaml/action.yml
index fc8c7886107..18026b37146 100644
--- a/.github/actions/setup-ocaml/action.yml
+++ b/.github/actions/setup-ocaml/action.yml
@@ -82,7 +82,8 @@ runs:
       run: |
         dependency_flags_key="${DEPENDENCY_FLAGS//--/}"
         dependency_flags_key="${dependency_flags_key// /_}"
-        key="$CACHE_PREFIX-$OS-3.8.0-$COMPILER-$dependency_flags_key-${{ hashFiles('*.opam') }}"
+        dependency_hash="${{ hashFiles('*.opam', '.github/opam-repository/repo', '.github/opam-repository/packages/**') }}"
+        key="$CACHE_PREFIX-$OS-3.8.0-$COMPILER-$dependency_flags_key-$dependency_hash"
         echo "value=${key//,/-}" >> "$GITHUB_OUTPUT"
         echo "setup-version=3.8.0" >> "$GITHUB_OUTPUT"
 
@@ -104,6 +105,11 @@ runs:
       uses: ocaml/setup-ocaml@v3.8.0
       with:
         ocaml-compiler: ${{ inputs.compiler }}
+        # Use the HTTP repository to prevent intermittent CI setup failures:
+        # the Git-backed default can race with detached Git maintenance while
+        # OPAM scans it (https://github.com/ocaml/opam/issues/7031).
+        opam-repositories: |
+          default: https://opam.ocaml.org
         opam-pin: false
         cache: false
 
@@ -125,12 +131,21 @@ runs:
       env:
         DEPENDENCY_FLAGS: ${{ inputs.dependency-flags }}
       run: |
+        opam repository add rescript-overrides "$GITHUB_WORKSPACE/.github/opam-repository"
         read -ra dependency_flags <<< "$DEPENDENCY_FLAGS"
+        if [[ "$RUNNER_OS" == "Windows" ]]; then
+          export AR=x86_64-w64-mingw32-ar
+          export RANLIB=x86_64-w64-mingw32-ranlib
+          export STRIP=x86_64-w64-mingw32-strip
+          export NM=x86_64-w64-mingw32-nm
+          export DLLTOOL=x86_64-w64-mingw32-dlltool
+          export OBJDUMP=x86_64-w64-mingw32-objdump
+        fi
         opam install . "${dependency_flags[@]}"
 
     - name: Cache OPAM environment
       # Caches created by pull_request runs are scoped to that PR's merge ref
-      # and cannot seed other PRs or branches. Only pushes create shared caches.
+      # and cannot seed other PRs or branches, so only push runs save them.
       if: steps.cache.outputs.cache-hit != 'true' && github.event_name == 'push'
       uses: actions/cache/save@v6
       with:
diff --git a/.github/opam-repository/README.md b/.github/opam-repository/README.md
new file mode 100644
index 00000000000..35e088bb9a0
--- /dev/null
+++ b/.github/opam-repository/README.md
@@ -0,0 +1,14 @@
+# ReScript opam overrides
+
+This repository contains narrowly scoped dependency fixes needed by CI before
+they are available from the main opam repository.
+
+The Luv 0.5.14 override preserves the ARM64 musl compiler's
+`-mno-outline-atomics` requirement while building vendored libuv. Without it,
+the static executable fails to link because libuv refers to glibc's internal
+`__getauxval` symbol. The patch should be contributed upstream and this
+override removed once a fixed Luv release is available.
+
+Luv 0.5.14 also vendors libuv 1.48.0. Evaluating and contributing an update to
+a current libuv 1.x release is a separate upstream follow-up; changing the
+vendored library is intentionally outside the OCaml rewatch port.
diff --git a/.github/opam-repository/packages/luv/luv.0.5.14/files/arm64-musl-outline-atomics.patch b/.github/opam-repository/packages/luv/luv.0.5.14/files/arm64-musl-outline-atomics.patch
new file mode 100644
index 00000000000..674f1b37c4c
--- /dev/null
+++ b/.github/opam-repository/packages/luv/luv.0.5.14/files/arm64-musl-outline-atomics.patch
@@ -0,0 +1,20 @@
+--- a/src/c/dune
++++ b/src/c/dune
+@@ -82,8 +82,14 @@
+  (action (progn
+   (bash "cp -r vendor/configure/* vendor/libuv/")
+   (chdir vendor/libuv (progn
+    (bash
+-    "sh configure --host `ocamlc -config | awk '/^host:/ {print $NF}'` \
+-      'CC=%{cc}' CFLAGS=-DNDEBUG --silent --enable-silent-rules")
++    "architecture=`ocamlc -config-var architecture`
++     c_compiler=`ocamlc -config-var c_compiler`
++     extra_cflags=
++     case \"$architecture:$c_compiler\" in
++       arm64:*musl*) extra_cflags=-mno-outline-atomics ;;
++     esac
++     sh configure --host `ocamlc -config | awk '/^host:/ {print $NF}'` \
++       'CC=%{cc}' CFLAGS=\"-DNDEBUG $extra_cflags\" --silent --enable-silent-rules")
+    (ignore-outputs (bash
+     "$([ '%{os_type}' = Unix ] && echo %{make} || echo make) V=0 -j 4 \
+       -o aclocal.m4 -o Makefile.in -o configure \
diff --git a/.github/opam-repository/packages/luv/luv.0.5.14/opam b/.github/opam-repository/packages/luv/luv.0.5.14/opam
new file mode 100644
index 00000000000..e64c76a6dd5
--- /dev/null
+++ b/.github/opam-repository/packages/luv/luv.0.5.14/opam
@@ -0,0 +1,42 @@
+opam-version: "2.0"
+name: "luv"
+version: "0.5.14"
+synopsis: "Binding to libuv: cross-platform asynchronous I/O"
+description: """\
+Luv is a binding to libuv, the cross-platform C library that does
+asynchronous I/O in Node.js and runs its main loop.
+
+Besides asynchronous I/O, libuv also supports multiprocessing and
+multithreading. Multiple event loops can be run in different threads. libuv also
+exposes a lot of other functionality, amounting to a full OS API, and an
+alternative to the standard module Unix."""
+maintainer: "Anton Bachin <antonbachin@yahoo.com>"
+authors: "Anton Bachin <antonbachin@yahoo.com>"
+license: "MIT"
+homepage: "https://github.com/aantron/luv"
+doc: "https://aantron.github.io/luv"
+bug-reports: "https://github.com/aantron/luv/issues"
+depends: [
+  "base-unix" {build}
+  "ctypes" {>= "0.14.0"}
+  "dune" {>= "2.7.0"}
+  "ocaml" {>= "4.03.0"}
+  "alcotest" {with-test & >= "0.8.1"}
+  "base-unix" {with-test}
+  "odoc" {with-doc & = "2.4.0"}
+]
+build: ["dune" "build" "-p" name "-j" jobs]
+patches: ["arm64-musl-outline-atomics.patch"]
+extra-files: [
+  [
+    "arm64-musl-outline-atomics.patch"
+    "sha256=c15c4c4d34c9b545197c5160e19c69a128cc186b65a54e0e9a779c2543528a4f"
+  ]
+]
+dev-repo: "git+https://github.com/aantron/luv.git"
+url {
+  src:
+    "https://github.com/aantron/luv/releases/download/0.5.14/luv-0.5.14.tar.gz"
+  checksum:
+    "sha256=8e01b4a50c8876cdd98d8e245c0687c4dc4d883aed161ad9c5ace1fb1fdaae99"
+}
diff --git a/.github/opam-repository/repo b/.github/opam-repository/repo
new file mode 100644
index 00000000000..013b84db617
--- /dev/null
+++ b/.github/opam-repository/repo
@@ -0,0 +1 @@
+opam-version: "2.0"
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c26588ac4b0..7826ca538db 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -117,7 +117,7 @@ jobs:
         with:
           compiler: ${{ matrix.ocaml_compiler }}
           os: ${{ matrix.os }}
-          cache-prefix: opam-env-v8
+          cache-prefix: opam-env-v9
 
       - name: Compiler build state key
         id: compiler-build-state-key
@@ -212,8 +212,61 @@ jobs:
         run: echo "C:\Program Files\Git\bin" >> $GITHUB_PATH
         shell: bash
 
-      - name: Run rewatch tests
-        run: ./rewatch/tests/suite.sh rewatch/target/release/rescript
+      - name: Run OCaml rewatch parity tests
+        timeout-minutes: 20
+        run: |
+          bash rewatch-ocaml/tests/check_rust_test_coverage.sh --require-complete
+          bash rewatch-ocaml/tests/check_canonical_test_coverage.sh
+          bash rewatch-ocaml/tests/check_config_acceptance.sh \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
+          bash rewatch-ocaml/tests/check_command_validation.sh \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
+        shell: bash
+
+      - name: Run OCaml rewatch interactive output tests
+        # Terminal rendering depends on the host OS, not the CPU architecture.
+        # Use the faster ARM runners for one Linux and one macOS check. Windows
+        # does not provide a reliable native equivalent of the `script` PTY.
+        if: matrix.node-target == 'linux-arm64' || matrix.node-target == 'darwin-arm64'
+        timeout-minutes: 15
+        run: |
+          bash rewatch-ocaml/tests/check_interactive_output.sh \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
+        shell: bash
+
+      - name: Run OCaml rewatch verbose output tests
+        timeout-minutes: 10
+        run: |
+          bash rewatch-ocaml/tests/check_verbose_output.sh \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
+            packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
+        shell: bash
+
+      - name: Run OCaml rewatch unit tests
+        timeout-minutes: 10
+        run: |
+          opam exec -- dune build tests/rewatch_ounit_tests/rewatch_ounit_tests_main.exe
+          # Run the executable directly so a stuck test leaves live progress;
+          # dune otherwise buffers the test output until the action exits.
+          opam exec -- ./_build/default/tests/rewatch_ounit_tests/rewatch_ounit_tests_main.exe -runner sequential
+        shell: bash
+
+      - name: Run OCaml rewatch focused integration tests
+        timeout-minutes: 30
+        run: |
+          sh rewatch-ocaml/tests/run.sh packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
+        shell: bash
+
+      - name: Run OCaml rewatch canonical tests
+        run: ./rewatch/tests/suite.sh packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
+        shell: bash
+
+      - name: Run Rust rewatch reference tests on Windows
+        if: runner.os == 'Windows'
+        run: ./rewatch/tests/suite.sh packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe
         shell: bash
 
       - name: Run syntax benchmarks
@@ -623,6 +676,10 @@ jobs:
         shell: bash
         working-directory: rewatch/testrepo
 
-      - name: Run rewatch integration tests
+      - name: Run installed default OCaml rewatch integration tests
         run: ./rewatch/tests/suite.sh rewatch/testrepo/node_modules/.bin/rescript
         shell: bash
+
+      - name: Run installed Rust rewatch reference integration tests
+        run: ./rewatch/tests/suite.sh rewatch/testrepo/node_modules/.bin/rescript-rust
+        shell: bash
diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index af2f13cbbed..733caa109e0 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -44,7 +44,7 @@ jobs:
         with:
           compiler: ${{ env.OCAML_COMPILER }}
           os: ${{ env.OS }}
-          cache-prefix: opam-coverage-v1
+          cache-prefix: opam-coverage-v2
           dependency-flags: --deps-only --with-test --with-dev-setup
 
       - name: Install OCaml coverage tooling
diff --git a/Makefile b/Makefile
index 1bc5e8cd7d6..f0fda0191d3 100644
--- a/Makefile
+++ b/Makefile
@@ -79,6 +79,8 @@ $(YARN_INSTALL_STAMP): $(YARN_INSTALL_SOURCES)
 
 REWATCH_SOURCES = $(shell find rewatch/src -name '*.rs') rewatch/Cargo.toml rewatch/Cargo.lock rewatch/rust-toolchain.toml
 RESCRIPT_EXE = $(BIN_DIR)/rescript.exe
+RESCRIPT_RUST_EXE := $(BIN_DIR)/rescript-rust.exe
+PACKAGED_RUST_EXE := $(RESCRIPT_RUST_EXE)
 ifdef CI
 	REWATCH_PROFILE := release
 	REWATCH_CARGO_FLAGS := --release
@@ -88,22 +90,22 @@ else
 endif
 REWATCH_TARGET := rewatch/target/$(REWATCH_PROFILE)/rescript$(PLATFORM_EXE_EXT)
 
-rewatch: $(RESCRIPT_EXE)
+rewatch: $(PACKAGED_RUST_EXE)
 
-$(RESCRIPT_EXE): $(REWATCH_TARGET)
+$(PACKAGED_RUST_EXE): $(REWATCH_TARGET)
 	$(call COPY_EXE,$<,$@)
 
 $(REWATCH_TARGET): $(REWATCH_SOURCES)
 	cargo build --manifest-path rewatch/Cargo.toml $(REWATCH_CARGO_FLAGS)
 
 clean-rewatch:
-	cargo clean --manifest-path rewatch/Cargo.toml && rm -rf rewatch/target && rm -f $(RESCRIPT_EXE)
+	cargo clean --manifest-path rewatch/Cargo.toml && rm -rf rewatch/target && rm -f $(PACKAGED_RUST_EXE)
 
 # Compiler
 
-COMPILER_SOURCE_DIRS := compiler tests analysis tools
-COMPILER_SOURCES = $(shell find $(COMPILER_SOURCE_DIRS) -type f \( -name '*.ml' -o -name '*.mli' -o -name '*.dune' -o -name dune -o -name dune-project \))
-COMPILER_BIN_NAMES := bsc rescript-editor-analysis rescript-tools
+COMPILER_SOURCE_DIRS := compiler tests analysis tools rewatch-ocaml
+COMPILER_SOURCES = $(shell find $(COMPILER_SOURCE_DIRS) -type f \( -name '*.ml' -o -name '*.mli' -o -name '*.c' -o -name '*.h' -o -name '*.dune' -o -name dune -o -name dune-project \))
+COMPILER_BIN_NAMES := bsc rescript-editor-analysis rescript-tools rescript
 COMPILER_EXES := $(addsuffix .exe,$(addprefix $(BIN_DIR)/,$(COMPILER_BIN_NAMES)))
 
 compiler: $(COMPILER_EXES)
@@ -279,9 +281,13 @@ COVERAGE_TEST_ENV := BISECT_FILE=$(COVERAGE_BISECT_PREFIX) BISECT_SILENT=YES
 .PHONY: coverage-build
 coverage-build: | $(YARN_INSTALL_STAMP)
 	dune build --instrument-with bisect_ppx
-	@$(foreach bin,$(COMPILER_BIN_NAMES), \
+	@$(foreach bin,$(filter-out rescript,$(COMPILER_BIN_NAMES)), \
 		cp $(DUNE_BIN_DIR)/$(bin)$(PLATFORM_EXE_EXT) $(BIN_DIR)/$(bin).exe && \
 		chmod 755 $(BIN_DIR)/$(bin).exe;)
+ifneq ($(OS),Windows_NT)
+	cp _build/default/rewatch-ocaml/rescript_ocaml.exe $(RESCRIPT_EXE)
+	chmod 755 $(RESCRIPT_EXE)
+endif
 
 .PHONY: coverage-prepare
 coverage-prepare: clean-coverage coverage-build
diff --git a/REWATCH_OCAML.md b/REWATCH_OCAML.md
new file mode 100644
index 00000000000..0c8c8af5b99
--- /dev/null
+++ b/REWATCH_OCAML.md
@@ -0,0 +1,239 @@
+# Goal: Port rewatch from Rust to OCaml
+
+Implement an OCaml version of the ReScript build system currently in `rewatch/`, reproducing its existing behavior. Continue invoking `bsc` as an external process, including parallel subprocess execution.
+
+The OCaml implementation must run multiple `bsc` subprocesses concurrently, preserving Rust rewatch’s dependency-aware parallel scheduling.
+
+Direct in-process compiler integration and compiler-state caching are outside this goal. OCaml 5 domains are permitted if useful for implementing the build tool, but are not required: compilation parallelism comes from running independent `bsc` processes.
+
+Build the OCaml implementation alongside Rust rewatch, preferably in `rewatch-ocaml/`. Preserve the Rust implementation and unrelated worktree changes. Do not switch the production default as part of this task.
+
+## Working approach
+
+Follow `AGENTS.md` and read the relevant area guides. Start with:
+
+- `rewatch/README.md`
+- `rewatch/CompilerConfigurationSpec.md`
+- `rewatch/MonorepoSupport.md`
+- `rewatch/Features.md`
+- `rewatch/src/`, particularly configuration, package discovery, build scheduling, and watching
+- `rewatch/tests/` and `rewatch/testrepo/`
+
+Inventory the commands, options, configuration fields, platform behavior, and tests. Treat the current Rust implementation as the behavioral reference. Record the reference commit so ongoing upstream changes do not silently change the target.
+
+Implement idiomatic OCaml rather than translating Rust structure mechanically. Keep configuration, module graphs, build state, subprocess management, artifact handling, and watching in cohesive modules with explicit ownership. Local mutation is fine; avoid unnecessary process-wide globals.
+
+Choose dependencies pragmatically. A small native C/Rust watcher component or helper process is acceptable if it provides dependable platform support. Compare existing OCaml watcher libraries, libuv bindings, and established implementations before building a backend. Respect licenses and document packaging requirements.
+
+This is an implementation task. Progress autonomously through the milestones, including tests and review. Milestone gates are verification checkpoints, not requests for routine user approval.
+
+## Required compatibility
+
+Cover all current rewatch responsibilities, including:
+
+- Commands, CLI options, configuration validation and precedence.
+- Package dependencies, monorepos, namespaces, source discovery, generated and feature-gated directories.
+- Compiler discovery, environment overrides, PPX and compiler argument construction.
+- Parsing through `bsc`, dependency extraction, cycle detection, and dependency-ordered compilation.
+- Bounded parallel subprocess scheduling.
+- Incremental invalidation, interface changes, and stale artifact cleanup.
+- Diagnostics, exit status, verbosity, and supported tracing behavior.
+- Watch-event handling, configuration changes, error recovery, and shutdown.
+- Supported-platform path, process, and filesystem behavior.
+
+Do not silently ignore unsupported options or configuration. Document temporary gaps precisely.
+
+Exact parser and Unicode semantics of Rust's `regex` crate are not required for
+`--filter`. The OCaml port may expose a documented, visibly rejected subset
+through a maintained native OCaml matcher. Known parser and non-ASCII matching
+differences must remain documented and covered at the compatibility boundary.
+
+## Milestones and gates
+
+### 1. Working one-shot build
+
+Add build integration and a separately named experimental executable. Implement a complete single-package build: configuration → discovery → parsing → dependency graph → compilation → artifacts and diagnostics.
+
+Support multiple modules, `.res`/`.resi` pairs, dependency cycles, compilation failures, and a subsequent successful build.
+
+**Gate:** Selected existing fixtures produce equivalent results under Rust and OCaml rewatch.
+
+### 2. Configuration and workspace parity
+
+Complete configuration handling, commands and options, namespaces, dependency packages, monorepos, generated directories, and feature-gated sources.
+
+Maintain a concise compatibility matrix covering the inventoried behavior.
+
+**Gate:** Relevant existing fixtures pass, and every configuration field and command is accounted for.
+
+### 3. Parallel subprocess scheduling
+
+Run independent `bsc` processes concurrently with bounded parallelism. Schedule work only when prerequisites are satisfied. Handle failed prerequisites, output collection, child-process cleanup, interruption, and shutdown without deadlocks or conflicting writes.
+
+The compiler remains an external executable; this milestone does not require parallel OCaml compiler execution.
+
+**Gate:** Sequential and parallel builds agree, failure paths are tested, and clean-build performance is compared with Rust rewatch.
+
+### 4. Incremental builds
+
+Port dirty-state propagation and artifact ownership. Handle source and interface edits, additions, deletions, renames, dependency changes, configuration changes, and recovery after failed builds.
+
+Temporary over-invalidation is acceptable if documented; under-invalidation is not.
+
+**Gate:** After every step in representative edit sequences, incremental results match a clean reference build.
+
+### 5. Watch mode
+
+Integrate a watcher backend. Handle recursive watching, newly created directories, editor atomic saves, duplicate or reordered events, batching, configuration changes, and changes arriving during a build.
+
+Ensure coherent diagnostics, reliable error recovery, clean shutdown, and bounded resource usage.
+
+**Gate:** Applicable existing watch tests pass reliably. Platform support and unverified platforms are stated explicitly.
+
+### 6. Full compatibility and evaluation
+
+Run the complete applicable rewatch suite. Account for every failure and close implementation gaps without weakening tests. Compare clean builds, unchanged builds, edit latency, watch responsiveness, peak memory, and packaging requirements.
+
+**Gate:** Deliver a working port with a factual compatibility and performance report. Do not replace Rust rewatch or begin in-process compiler integration.
+
+## Testing
+
+Reuse existing fixtures and integration infrastructure. Parameterize the runner or add a thin alternative runner rather than duplicating the fixture tree.
+
+Compare:
+
+- Exit status and diagnostics.
+- Compiler invocation arguments where relevant.
+- Generated JavaScript and compiler artifacts.
+- Created and removed files.
+- Behavior after edit sequences and failed builds.
+
+Compare deterministic output exactly. Where normalization or semantic comparison is necessary, explain why and ensure it does not hide differences.
+
+Add focused unit tests for configuration, graph algorithms, invalidation, argument construction, and event normalization. Use end-to-end tests to establish observable behavior.
+
+Do not use fixed sleeps for asynchronous tests. Wait for explicit observable conditions. Run focused tests during development and broader relevant checks at milestone gates. Serialize tests that mutate shared fixtures.
+
+## Code quality
+
+Produce code that a maintainer can understand and extend:
+
+- Prefer idiomatic OCaml and established repository conventions.
+- Keep interfaces narrow and state ownership clear.
+- Avoid speculative abstractions, trivial wrapper layers, duplicated logic, and oversized catch-all modules.
+- Do not add future compiler-integration machinery.
+- Do not hard-code fixture-specific behavior.
+- Do not swallow errors or substitute success-shaped defaults.
+- Pass subprocess arguments directly rather than constructing interpolated shell commands.
+- Clean up processes, file descriptors, temporary files, and watcher resources on success and failure.
+- Remove dead code, abandoned experiments, stale comments, and placeholders before completing a milestone.
+- Comments should explain invariants and non-obvious decisions rather than restate the code.
+  Start with why the code or invariant is needed, give enough context for a
+  reader who is not a specialist in every relevant OCaml, build-system,
+  compiler, or operating-system detail, and make the explanation stand on its
+  own. Refer to the Rust implementation only when that compatibility
+  relationship is itself the reason for the decision.
+- Do not suppress warnings or weaken tests to make the port pass.
+- Measure before introducing performance-driven complexity.
+- Treat performance parity as a work-equivalence gate, not only a wall-clock
+  ratio. Inventory the Rust implementation's avoidance strategies (including
+  filesystem traversal, metadata calls, parsing, graph construction, artifact
+  checks, subprocess creation, and output capture), and implement applicable
+  missing strategies before accepting the benchmark. Use syscall or equivalent
+  tracing where available to detect superfluous work.
+- Keep proposed optimizations that are not present in Rust in a separate,
+  prioritized backlog. For each proposal, record whether it addresses a measured
+  bottleneck or is still a hypothesis, its expected benefit, complexity and
+  correctness risk, Windows implications, and the benchmark plus equivalence
+  checks required before adoption. Do not mix speculative improvements into the
+  compatibility port merely to improve headline timings.
+
+## Review gates
+
+After every milestone:
+
+1. The implementation agent reviews the complete milestone diff, simplifies unnecessary code, checks parity against Rust, and runs formatting, compilation, and relevant tests.
+2. A separate reviewer with fresh context reviews the code, corresponding Rust behavior, tests, and acceptance criteria.
+3. The implementer addresses findings, explains any disagreement with evidence, and reruns relevant checks. Material fixes receive a focused follow-up review.
+
+Review both correctness and maintainability. Require concrete findings with affected code and consequences; avoid speculative redesigns and style churn.
+
+For subprocess scheduling, incremental invalidation, watch mode, and final evaluation, use two independent reviewers with complementary scopes: behavioral correctness, and design/resource/concurrency concerns.
+
+Do not call a milestone complete while confirmed material findings remain unresolved.
+
+### Final code-quality gate
+
+After behavior, work equivalence, and performance gates pass, perform a distinct
+whole-port maintainability pass before release:
+
+- Split modules whose size or mixed responsibilities obstruct review; keep test
+  code and benchmark tooling separate from production implementation.
+- Review module, file, type, function, field, and test names for clear ownership
+  and consistent terminology. Remove misleading Rust-derived names and unclear
+  abbreviations, while keeping established ReScript concepts recognizable.
+  Review opened modules at the same time: explicitly qualify calls when doing
+  so makes ownership or side effects clearer, especially for generic utility
+  names, but retain an open when qualification would only add repetitive noise.
+- Simplify duplicated control flow and remove dead code, stale compatibility
+  scaffolding, abandoned experiments, and avoidable allocations without
+  regressing measured performance.
+- Audit optional booleans and other encodings with unnamed states. Prefer a
+  normal variant when each state has distinct meaning so invalid combinations
+  are unrepresentable and compiler errors name the missing case.
+- Document the unit, focused, canonical, full-repository, work-equivalence,
+  performance, filesystem-call, and source-size tooling so future changes can
+  reproduce the gates.
+- Require warning-free builds, formatting, and available linters without warning
+  suppressions.
+- Audit every process, pipe descriptor, watcher handle, lock, and staged or
+  temporary output across success, failure, interruption, and partial-launch
+  paths.
+- Audit the platform boundary for hidden Unix assumptions and type-check both
+  selected and unselected implementations. Keep the Windows implementation in
+  the tree, but defer native Windows runtime validation and switching Windows
+  from Rust to a separate follow-up PR.
+- Review dependency maintenance, licenses/notices, static packaging, and the
+  final npm artifact manifest.
+- Remove the temporary `save-pr-cache` setup-OCaml input and its
+  `rewatch-ocaml` branch settings from CI and coverage once the new OPAM cache
+  key is available on the default branch, and in all cases before merge.
+- Review test isolation and reliability, replacing fragile sleeps with observable
+  polling where possible and retaining tests for every intentional Rust
+  divergence or corrected Rust bug.
+- Recheck public diagnostics, exit classes, redirected/interactive output, and
+  CLI discoverability.
+- Record final production/test/tooling line counts and largest modules as review
+  signals, not optimization targets.
+- Publish separate final inventories of (a) compatibility behavior retained even
+  though it appears odd or inconsistent, (b) documented Rust bugs or simple
+  inefficiencies intentionally corrected by the OCaml port, and (c) possible
+  post-parity performance improvements absent from Rust. Include rationale,
+  coverage, and a future cleanup or validation path for every entry.
+
+**Gate:** The whole-port review has no unresolved material correctness,
+resource, portability, maintainability, documentation, or packaging finding,
+and all behavior/performance gates still pass after cleanup.
+
+After that release-quality gate, perform the broad source-comment pass for
+ownership, concurrency, platform, cleanup, and algorithmic invariants that are
+not evident from the code. Do not add comments that merely paraphrase
+statements. Run formatting and a build check after this comment-only pass.
+
+## Models
+
+Use **GPT-5.6 Sol at medium reasoning** for implementation and ordinary independent reviews. Use **GPT-5.6 Terra at medium reasoning** for bounded tasks with clear acceptance criteria.
+
+After each milestone, perform an implementation self-review and one independent review. Fix confirmed findings and rerun relevant tests. Request a second review only when substantial fixes, unresolved concerns, or particularly complex scheduling or invalidation logic justify it. Perform a final whole-port review.
+
+Escalate to Sol high reasoning for a specific difficult issue when medium repeatedly fails to resolve it. Use Astra only with explicit user approval. Do not automatically increase reasoning effort based on milestone number or task size.
+
+Evaluate model suitability after the first working build milestone using behavioral correctness, code clarity, review findings, and rework required. Keep medium as the default if those results are satisfactory.
+
+## Completion and reporting
+
+Maintain one concise progress document containing the reference commit, completed milestones, compatibility gaps, tests, measurements, review outcomes, and next actions. Avoid generating a collection of redundant planning documents.
+
+The goal is complete when the OCaml port reproduces current rewatch behavior, passes the applicable suite, still invokes `bsc` externally, and includes clear build/run/test instructions. Required behavior that remains unsupported means the goal is incomplete; unavailable platform verification must be disclosed.
+
+At milestones, report what works, what was verified, remaining gaps, and material decisions. If execution is interrupted, leave a buildable, tested checkpoint and precise continuation instructions. Resume from that checkpoint rather than treating partial progress as completion.
diff --git a/biome.json b/biome.json
index 94358cad64f..09994b663ab 100644
--- a/biome.json
+++ b/biome.json
@@ -64,6 +64,7 @@
       "!**/tests/tests/**/src",
       "!**/tests/tools_tests/**/src",
       "!**/rewatch",
+      "!**/rewatch-ocaml",
       "!**/lib/es6",
       "!**/lib/js",
       "!**/lib/bs",
diff --git a/cli/common/bins.js b/cli/common/bins.js
index 5800a54f5ea..4d11717a1ad 100644
--- a/cli/common/bins.js
+++ b/cli/common/bins.js
@@ -6,7 +6,13 @@ const minimumNodeVersion = "20.11.0";
  * @typedef {import("@rescript/linux-x64")} BinaryModuleExports
  */
 
-const target = `${process.platform}-${process.arch}`;
+// Windows on ARM runs the published x64 toolchain through the OS emulation
+// layer. Native ARM64 Node must therefore resolve the same package as x64 Node.
+const binaryArch =
+  process.platform === "win32" && process.arch === "arm64"
+    ? "x64"
+    : process.arch;
+const target = `${process.platform}-${binaryArch}`;
 
 const supportedPlatforms = [
   "darwin-arm64",
@@ -43,6 +49,7 @@ export const {
     rescript_editor_analysis_exe,
     rescript_tools_exe,
     rescript_exe,
+    rescript_rust_exe,
   },
 } = mod;
 
diff --git a/cli/common/runBuildSystem.js b/cli/common/runBuildSystem.js
new file mode 100644
index 00000000000..09ce5ae8427
--- /dev/null
+++ b/cli/common/runBuildSystem.js
@@ -0,0 +1,72 @@
+// @ts-check
+
+import * as child_process from "node:child_process";
+import { runtimePath } from "./runtime.js";
+
+/** @type {Record<string, number>} */
+const signalToNumber = { SIGINT: 2, SIGTERM: 15, SIGHUP: 1, SIGQUIT: 3 };
+
+/**
+ * Run a build-system executable with the package runtime and forward terminal
+ * signals so watch mode can clean up before the Node launcher exits.
+ *
+ * @param {string} executable
+ */
+export function runBuildSystem(executable) {
+  const child = child_process.spawn(executable, process.argv.slice(2), {
+    stdio: "inherit",
+    env: { ...process.env, RESCRIPT_RUNTIME: runtimePath },
+  });
+
+  let forwardedSignal = false;
+  /** @param {NodeJS.Signals} signal */
+  const handleSignal = signal => {
+    if (forwardedSignal) return;
+    forwardedSignal = true;
+    // Ctrl+C is delivered to every process attached to the Windows console.
+    // child.kill("SIGINT") uses TerminateProcess there and can kill the build
+    // system before its console handler releases locks and owned descendants.
+    if (process.platform === "win32" && signal === "SIGINT") return;
+    try {
+      if (child.exitCode === null && child.signalCode == null) {
+        child.kill(signal);
+      }
+    } catch {
+      // Signal forwarding is best effort if the child exited concurrently.
+    }
+  };
+
+  process.on("SIGINT", handleSignal);
+  process.on("SIGTERM", handleSignal);
+  process.on("SIGHUP", handleSignal);
+  process.on("SIGQUIT", handleSignal);
+
+  process.on("exit", () => {
+    if (child.exitCode === null && child.signalCode == null) {
+      try {
+        child.kill("SIGTERM");
+      } catch {
+        // The child may already have exited.
+      }
+    }
+  });
+
+  child.on("exit", (code, signal) => {
+    process.removeListener("SIGINT", handleSignal);
+    process.removeListener("SIGTERM", handleSignal);
+    process.removeListener("SIGHUP", handleSignal);
+    process.removeListener("SIGQUIT", handleSignal);
+
+    if (signal) {
+      const number = signalToNumber[signal];
+      process.exit(typeof number === "number" ? 128 + number : 1);
+    } else {
+      process.exit(typeof code === "number" ? code : 0);
+    }
+  });
+
+  child.on("error", error => {
+    console.error(error?.message ?? String(error));
+    process.exit(1);
+  });
+}
diff --git a/cli/rescript-rust.js b/cli/rescript-rust.js
new file mode 100755
index 00000000000..af48395a16f
--- /dev/null
+++ b/cli/rescript-rust.js
@@ -0,0 +1,8 @@
+#!/usr/bin/env node
+
+// @ts-check
+
+import { rescript_rust_exe } from "./common/bins.js";
+import { runBuildSystem } from "./common/runBuildSystem.js";
+
+runBuildSystem(rescript_rust_exe);
diff --git a/cli/rescript.js b/cli/rescript.js
index 236e1847e82..8256b0b76c0 100755
--- a/cli/rescript.js
+++ b/cli/rescript.js
@@ -1,93 +1,6 @@
 #!/usr/bin/env node
 
-// @ts-check
-
-import * as child_process from "node:child_process";
 import { rescript_exe } from "./common/bins.js";
-import { runtimePath } from "./common/runtime.js";
-
-const args = process.argv.slice(2);
-
-// We intentionally use spawn (async) instead of execFileSync (sync) here.
-// Rationale:
-// - execFileSync blocks Node's event loop, so Ctrl+C (SIGINT) causes Node to
-//   exit immediately without giving us a chance to forward the signal to the
-//   child and wait for its cleanup. In watch mode, the Rust watcher prints
-//   "Exiting..." on SIGINT and performs cleanup; with execFileSync that output
-//   may appear after the shell prompt and sometimes requires an extra keypress.
-// - spawn lets us install signal handlers, forward them to the child, and then
-//   exit the parent with the correct status only after the child has exited.
-const child = child_process.spawn(rescript_exe, args, {
-  stdio: "inherit",
-  env: { ...process.env, RESCRIPT_RUNTIME: runtimePath },
-});
-
-// Map POSIX signal names to conventional exit status numbers so we can
-// reproduce the usual 128 + signal behavior when exiting due to a signal.
-/** @type {Record<string, number>} */
-const signalToNumber = { SIGINT: 2, SIGTERM: 15, SIGHUP: 1, SIGQUIT: 3 };
-
-let forwardedSignal = false;
-/**
- * @param {NodeJS.Signals} signal
- */
-const handleSignal = signal => {
-  // Intercept the signal in the parent, forward it to the child, and let the
-  // child perform its own cleanup. This ensures ordered shutdown in watch mode.
-  // Guard against double-forwarding since terminals or OSes can deliver
-  // multiple signals (e.g., repeated Ctrl+C).
-  // Prevent Node from exiting immediately; forward to child first
-  if (forwardedSignal) return;
-  forwardedSignal = true;
-  try {
-    if (child.exitCode === null && child.signalCode == null) {
-      child.kill(signal);
-    }
-  } catch {
-    // best effort
-  }
-};
-
-process.on("SIGINT", handleSignal);
-process.on("SIGTERM", handleSignal);
-process.on("SIGHUP", handleSignal);
-process.on("SIGQUIT", handleSignal);
-
-// Cross-platform note:
-// - On Unix, Ctrl+C sends SIGINT to the process group; we also explicitly
-//   forward it to the child to be robust.
-// - On Windows, Node maps kill('SIGINT'/'SIGTERM') to console control events;
-//   the Rust watcher (via the ctrlc crate) handles these and exits cleanly.
-
-// Ensure no orphaned process if parent exits unexpectedly
-process.on("exit", () => {
-  if (child.exitCode === null && child.signalCode == null) {
-    try {
-      child.kill("SIGTERM");
-    } catch {
-      // ignore
-    }
-  }
-});
-
-child.on("exit", (code, signal) => {
-  process.removeListener("SIGINT", handleSignal);
-  process.removeListener("SIGTERM", handleSignal);
-  process.removeListener("SIGHUP", handleSignal);
-  process.removeListener("SIGQUIT", handleSignal);
-
-  // If the child exited due to a signal, emulate the conventional exit status
-  // (128 + signalNumber). Otherwise, pass through the child's numeric exit code.
-  if (signal) {
-    const n = signalToNumber[signal];
-    process.exit(typeof n === "number" ? 128 + n : 1);
-  } else {
-    process.exit(typeof code === "number" ? code : 0);
-  }
-});
+import { runBuildSystem } from "./common/runBuildSystem.js";
 
-// Surface spawn errors (e.g., executable not found) and exit with failure.
-child.on("error", err => {
-  console.error(err?.message ?? String(err));
-  process.exit(1);
-});
+runBuildSystem(rescript_exe);
diff --git a/compiler/sync/dune b/compiler/sync/dune
index e11fdfacaae..c8f334b877e 100644
--- a/compiler/sync/dune
+++ b/compiler/sync/dune
@@ -7,10 +7,10 @@
 ; cause no timestamp churn downstream.
 ;
 ; One rule per platform; %{system}/%{architecture} come from `ocamlc -config`
-; (note: x64 is "amd64" there). Windows copies without stripping, matching
-; the historical packaging step. The browser profile is excluded because it
-; builds a playground-flavoured compiler that must never overwrite the
-; native binaries.
+; (note: x64 is "amd64" there). Every package receives the OCaml build system
+; as rescript.exe; Cargo separately promotes the Rust reference as
+; rescript-rust.exe. The browser profile is excluded because it builds a
+; playground-flavoured compiler that must never overwrite the native binaries.
 
 (rule
  (enabled_if
@@ -18,11 +18,16 @@
    (<> %{profile} browser)
    (= %{system} macosx)
    (= %{architecture} arm64)))
- (targets bsc.exe rescript-editor-analysis.exe rescript-tools.exe)
+ (targets
+  bsc.exe
+  rescript-editor-analysis.exe
+  rescript-tools.exe
+  rescript.exe)
  (deps
   ../bsc/rescript_compiler_main.exe
   ../../analysis/bin/main.exe
-  ../../tools/bin/main.exe)
+  ../../tools/bin/main.exe
+  ../../rewatch-ocaml/rescript_ocaml.exe)
  (mode
   (promote
    (until-clean)
@@ -31,7 +36,8 @@
   (progn
    (run strip -o bsc.exe ../bsc/rescript_compiler_main.exe)
    (run strip -o rescript-editor-analysis.exe ../../analysis/bin/main.exe)
-   (run strip -o rescript-tools.exe ../../tools/bin/main.exe))))
+   (run strip -o rescript-tools.exe ../../tools/bin/main.exe)
+   (run strip -o rescript.exe ../../rewatch-ocaml/rescript_ocaml.exe))))
 
 (rule
  (enabled_if
@@ -39,11 +45,16 @@
    (<> %{profile} browser)
    (= %{system} macosx)
    (= %{architecture} amd64)))
- (targets bsc.exe rescript-editor-analysis.exe rescript-tools.exe)
+ (targets
+  bsc.exe
+  rescript-editor-analysis.exe
+  rescript-tools.exe
+  rescript.exe)
  (deps
   ../bsc/rescript_compiler_main.exe
   ../../analysis/bin/main.exe
-  ../../tools/bin/main.exe)
+  ../../tools/bin/main.exe
+  ../../rewatch-ocaml/rescript_ocaml.exe)
  (mode
   (promote
    (until-clean)
@@ -52,7 +63,8 @@
   (progn
    (run strip -o bsc.exe ../bsc/rescript_compiler_main.exe)
    (run strip -o rescript-editor-analysis.exe ../../analysis/bin/main.exe)
-   (run strip -o rescript-tools.exe ../../tools/bin/main.exe))))
+   (run strip -o rescript-tools.exe ../../tools/bin/main.exe)
+   (run strip -o rescript.exe ../../rewatch-ocaml/rescript_ocaml.exe))))
 
 (rule
  (enabled_if
@@ -60,11 +72,16 @@
    (<> %{profile} browser)
    (= %{system} linux)
    (= %{architecture} arm64)))
- (targets bsc.exe rescript-editor-analysis.exe rescript-tools.exe)
+ (targets
+  bsc.exe
+  rescript-editor-analysis.exe
+  rescript-tools.exe
+  rescript.exe)
  (deps
   ../bsc/rescript_compiler_main.exe
   ../../analysis/bin/main.exe
-  ../../tools/bin/main.exe)
+  ../../tools/bin/main.exe
+  ../../rewatch-ocaml/rescript_ocaml.exe)
  (mode
   (promote
    (until-clean)
@@ -73,7 +90,8 @@
   (progn
    (run strip -o bsc.exe ../bsc/rescript_compiler_main.exe)
    (run strip -o rescript-editor-analysis.exe ../../analysis/bin/main.exe)
-   (run strip -o rescript-tools.exe ../../tools/bin/main.exe))))
+   (run strip -o rescript-tools.exe ../../tools/bin/main.exe)
+   (run strip -o rescript.exe ../../rewatch-ocaml/rescript_ocaml.exe))))
 
 (rule
  (enabled_if
@@ -81,11 +99,16 @@
    (<> %{profile} browser)
    (= %{system} linux)
    (= %{architecture} amd64)))
- (targets bsc.exe rescript-editor-analysis.exe rescript-tools.exe)
+ (targets
+  bsc.exe
+  rescript-editor-analysis.exe
+  rescript-tools.exe
+  rescript.exe)
  (deps
   ../bsc/rescript_compiler_main.exe
   ../../analysis/bin/main.exe
-  ../../tools/bin/main.exe)
+  ../../tools/bin/main.exe
+  ../../rewatch-ocaml/rescript_ocaml.exe)
  (mode
   (promote
    (until-clean)
@@ -94,7 +117,8 @@
   (progn
    (run strip -o bsc.exe ../bsc/rescript_compiler_main.exe)
    (run strip -o rescript-editor-analysis.exe ../../analysis/bin/main.exe)
-   (run strip -o rescript-tools.exe ../../tools/bin/main.exe))))
+   (run strip -o rescript-tools.exe ../../tools/bin/main.exe)
+   (run strip -o rescript.exe ../../rewatch-ocaml/rescript_ocaml.exe))))
 
 (rule
  (enabled_if
@@ -102,11 +126,16 @@
    (<> %{profile} browser)
    (= %{system} mingw64)
    (= %{architecture} amd64)))
- (targets bsc.exe rescript-editor-analysis.exe rescript-tools.exe)
+ (targets
+  bsc.exe
+  rescript-editor-analysis.exe
+  rescript-tools.exe
+  rescript.exe)
  (deps
   ../bsc/rescript_compiler_main.exe
   ../../analysis/bin/main.exe
-  ../../tools/bin/main.exe)
+  ../../tools/bin/main.exe
+  ../../rewatch-ocaml/rescript_ocaml.exe)
  (mode
   (promote
    (until-clean)
@@ -115,4 +144,5 @@
   (progn
    (copy ../bsc/rescript_compiler_main.exe bsc.exe)
    (copy ../../analysis/bin/main.exe rescript-editor-analysis.exe)
-   (copy ../../tools/bin/main.exe rescript-tools.exe))))
+   (copy ../../tools/bin/main.exe rescript-tools.exe)
+   (copy ../../rewatch-ocaml/rescript_ocaml.exe rescript.exe))))
diff --git a/dune b/dune
index 91a5df6eca9..c8ac51fbe74 100644
--- a/dune
+++ b/dune
@@ -1 +1 @@
-(dirs compiler tests analysis tools)
+(dirs compiler tests analysis tools rewatch-ocaml)
diff --git a/dune-project b/dune-project
index 8e34a29f118..8ab17cd367a 100644
--- a/dune-project
+++ b/dune-project
@@ -32,6 +32,14 @@
    (and :with-test (= 0.29.0)))
   (yojson
    (= 3.0.0))
+  (spawn
+   (= v0.17.0))
+  (cmdliner
+   (= 2.1.1))
+  (luv
+   (= 0.5.14))
+  (re
+   (= 1.14.0))
   (ounit2
    (and :with-test (= 2.2.7)))
   (odoc :with-doc)
diff --git a/package.json b/package.json
index 50c42cffed2..3f676a7e7d2 100644
--- a/package.json
+++ b/package.json
@@ -45,6 +45,7 @@
   "bin": {
     "bsc": "cli/bsc.js",
     "rescript": "cli/rescript.js",
+    "rescript-rust": "cli/rescript-rust.js",
     "rescript-tools": "cli/rescript-tools.js"
   },
   "scripts": {
diff --git a/packages/@rescript/darwin-arm64/RE_LICENSE.md b/packages/@rescript/darwin-arm64/RE_LICENSE.md
new file mode 100644
index 00000000000..f79ddd2b654
--- /dev/null
+++ b/packages/@rescript/darwin-arm64/RE_LICENSE.md
@@ -0,0 +1,521 @@
+This Software is distributed under the terms of the GNU Lesser
+General Public License version 2.1 (included below), or (at your
+option) any later version.
+
+As a special exception to the GNU Library General Public License, you
+may link, statically or dynamically, a "work that uses the Library"
+with a publicly distributed version of the Library to produce an
+executable file containing portions of the Library, and distribute
+that executable file under terms of your choice, without any of the
+additional requirements listed in clause 6 of the GNU Library General
+Public License.  By "a publicly distributed version of the Library",
+we mean either the unmodified Library, or a modified version of the
+Library that is distributed under the conditions defined in clause 3
+of the GNU Library General Public License.  This exception does not
+however invalidate any other reasons why the executable file might be
+covered by the GNU Library General Public License.
+
+----------------------------------------------------------------------
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+		       Version 2.1, February 1999
+
+ Copyright (C) 1991, 1999 Free Software Foundation, Inc.
+     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+[This is the first released version of the Lesser GPL.  It also counts
+ as the successor of the GNU Library Public License, version 2, hence
+ the version number 2.1.]
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+Licenses are intended to guarantee your freedom to share and change
+free software--to make sure the software is free for all its users.
+
+  This license, the Lesser General Public License, applies to some
+specially designated software packages--typically libraries--of the
+Free Software Foundation and other authors who decide to use it.  You
+can use it too, but we suggest you first think carefully about whether
+this license or the ordinary General Public License is the better
+strategy to use in any particular case, based on the explanations below.
+
+  When we speak of free software, we are referring to freedom of use,
+not price.  Our General Public Licenses are designed to make sure that
+you have the freedom to distribute copies of free software (and charge
+for this service if you wish); that you receive source code or can get
+it if you want it; that you can change the software and use pieces of
+it in new free programs; and that you are informed that you can do
+these things.
+
+  To protect your rights, we need to make restrictions that forbid
+distributors to deny you these rights or to ask you to surrender these
+rights.  These restrictions translate to certain responsibilities for
+you if you distribute copies of the library or if you modify it.
+
+  For example, if you distribute copies of the library, whether gratis
+or for a fee, you must give the recipients all the rights that we gave
+you.  You must make sure that they, too, receive or can get the source
+code.  If you link other code with the library, you must provide
+complete object files to the recipients, so that they can relink them
+with the library after making changes to the library and recompiling
+it.  And you must show them these terms so they know their rights.
+
+  We protect your rights with a two-step method: (1) we copyright the
+library, and (2) we offer you this license, which gives you legal
+permission to copy, distribute and/or modify the library.
+
+  To protect each distributor, we want to make it very clear that
+there is no warranty for the free library.  Also, if the library is
+modified by someone else and passed on, the recipients should know
+that what they have is not the original version, so that the original
+author's reputation will not be affected by problems that might be
+introduced by others.
+
+  Finally, software patents pose a constant threat to the existence of
+any free program.  We wish to make sure that a company cannot
+effectively restrict the users of a free program by obtaining a
+restrictive license from a patent holder.  Therefore, we insist that
+any patent license obtained for a version of the library must be
+consistent with the full freedom of use specified in this license.
+
+  Most GNU software, including some libraries, is covered by the
+ordinary GNU General Public License.  This license, the GNU Lesser
+General Public License, applies to certain designated libraries, and
+is quite different from the ordinary General Public License.  We use
+this license for certain libraries in order to permit linking those
+libraries into non-free programs.
+
+  When a program is linked with a library, whether statically or using
+a shared library, the combination of the two is legally speaking a
+combined work, a derivative of the original library.  The ordinary
+General Public License therefore permits such linking only if the
+entire combination fits its criteria of freedom.  The Lesser General
+Public License permits more lax criteria for linking other code with
+the library.
+
+  We call this license the "Lesser" General Public License because it
+does Less to protect the user's freedom than the ordinary General
+Public License.  It also provides other free software developers Less
+of an advantage over competing non-free programs.  These disadvantages
+are the reason we use the ordinary General Public License for many
+libraries.  However, the Lesser license provides advantages in certain
+special circumstances.
+
+  For example, on rare occasions, there may be a special need to
+encourage the widest possible use of a certain library, so that it becomes
+a de-facto standard.  To achieve this, non-free programs must be
+allowed to use the library.  A more frequent case is that a free
+library does the same job as widely used non-free libraries.  In this
+case, there is little to gain by limiting the free library to free
+software only, so we use the Lesser General Public License.
+
+  In other cases, permission to use a particular library in non-free
+programs enables a greater number of people to use a large body of
+free software.  For example, permission to use the GNU C Library in
+non-free programs enables many more people to use the whole GNU
+operating system, as well as its variant, the GNU/Linux operating
+system.
+
+  Although the Lesser General Public License is Less protective of the
+users' freedom, it does ensure that the user of a program that is
+linked with the Library has the freedom and the wherewithal to run
+that program using a modified version of the Library.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.  Pay close attention to the difference between a
+"work based on the library" and a "work that uses the library".  The
+former contains code derived from the library, whereas the latter must
+be combined with the library in order to run.
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License Agreement applies to any software library or other
+program which contains a notice placed by the copyright holder or
+other authorized party saying it may be distributed under the terms of
+this Lesser General Public License (also called "this License").
+Each licensee is addressed as "you".
+
+  A "library" means a collection of software functions and/or data
+prepared so as to be conveniently linked with application programs
+(which use some of those functions and data) to form executables.
+
+  The "Library", below, refers to any such software library or work
+which has been distributed under these terms.  A "work based on the
+Library" means either the Library or any derivative work under
+copyright law: that is to say, a work containing the Library or a
+portion of it, either verbatim or with modifications and/or translated
+straightforwardly into another language.  (Hereinafter, translation is
+included without limitation in the term "modification".)
+
+  "Source code" for a work means the preferred form of the work for
+making modifications to it.  For a library, complete source code means
+all the source code for all modules it contains, plus any associated
+interface definition files, plus the scripts used to control compilation
+and installation of the library.
+
+  Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running a program using the Library is not restricted, and output from
+such a program is covered only if its contents constitute a work based
+on the Library (independent of the use of the Library in a tool for
+writing it).  Whether that is true depends on what the Library does
+and what the program that uses the Library does.
+
+  1. You may copy and distribute verbatim copies of the Library's
+complete source code as you receive it, in any medium, provided that
+you conspicuously and appropriately publish on each copy an
+appropriate copyright notice and disclaimer of warranty; keep intact
+all the notices that refer to this License and to the absence of any
+warranty; and distribute a copy of this License along with the
+Library.
+
+  You may charge a fee for the physical act of transferring a copy,
+and you may at your option offer warranty protection in exchange for a
+fee.
+
+  2. You may modify your copy or copies of the Library or any portion
+of it, thus forming a work based on the Library, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) The modified work must itself be a software library.
+
+    b) You must cause the files modified to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    c) You must cause the whole of the work to be licensed at no
+    charge to all third parties under the terms of this License.
+
+    d) If a facility in the modified Library refers to a function or a
+    table of data to be supplied by an application program that uses
+    the facility, other than as an argument passed when the facility
+    is invoked, then you must make a good faith effort to ensure that,
+    in the event an application does not supply such function or
+    table, the facility still operates, and performs whatever part of
+    its purpose remains meaningful.
+
+    (For example, a function in a library to compute square roots has
+    a purpose that is entirely well-defined independent of the
+    application.  Therefore, Subsection 2d requires that any
+    application-supplied function or table used by this function must
+    be optional: if the application does not supply it, the square
+    root function must still compute square roots.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Library,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Library, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote
+it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Library.
+
+In addition, mere aggregation of another work not based on the Library
+with the Library (or with a work based on the Library) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may opt to apply the terms of the ordinary GNU General Public
+License instead of this License to a given copy of the Library.  To do
+this, you must alter all the notices that refer to this License, so
+that they refer to the ordinary GNU General Public License, version 2,
+instead of to this License.  (If a newer version than version 2 of the
+ordinary GNU General Public License has appeared, then you can specify
+that version instead if you wish.)  Do not make any other change in
+these notices.
+
+  Once this change is made in a given copy, it is irreversible for
+that copy, so the ordinary GNU General Public License applies to all
+subsequent copies and derivative works made from that copy.
+
+  This option is useful when you wish to copy part of the code of
+the Library into a program that is not a library.
+
+  4. You may copy and distribute the Library (or a portion or
+derivative of it, under Section 2) in object code or executable form
+under the terms of Sections 1 and 2 above provided that you accompany
+it with the complete corresponding machine-readable source code, which
+must be distributed under the terms of Sections 1 and 2 above on a
+medium customarily used for software interchange.
+
+  If distribution of object code is made by offering access to copy
+from a designated place, then offering equivalent access to copy the
+source code from the same place satisfies the requirement to
+distribute the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  5. A program that contains no derivative of any portion of the
+Library, but is designed to work with the Library by being compiled or
+linked with it, is called a "work that uses the Library".  Such a
+work, in isolation, is not a derivative work of the Library, and
+therefore falls outside the scope of this License.
+
+  However, linking a "work that uses the Library" with the Library
+creates an executable that is a derivative of the Library (because it
+contains portions of the Library), rather than a "work that uses the
+library".  The executable is therefore covered by this License.
+Section 6 states terms for distribution of such executables.
+
+  When a "work that uses the Library" uses material from a header file
+that is part of the Library, the object code for the work may be a
+derivative work of the Library even though the source code is not.
+Whether this is true is especially significant if the work can be
+linked without the Library, or if the work is itself a library.  The
+threshold for this to be true is not precisely defined by law.
+
+  If such an object file uses only numerical parameters, data
+structure layouts and accessors, and small macros and small inline
+functions (ten lines or less in length), then the use of the object
+file is unrestricted, regardless of whether it is legally a derivative
+work.  (Executables containing this object code plus portions of the
+Library will still fall under Section 6.)
+
+  Otherwise, if the work is a derivative of the Library, you may
+distribute the object code for the work under the terms of Section 6.
+Any executables containing that work also fall under Section 6,
+whether or not they are linked directly with the Library itself.
+
+  6. As an exception to the Sections above, you may also combine or
+link a "work that uses the Library" with the Library to produce a
+work containing portions of the Library, and distribute that work
+under terms of your choice, provided that the terms permit
+modification of the work for the customer's own use and reverse
+engineering for debugging such modifications.
+
+  You must give prominent notice with each copy of the work that the
+Library is used in it and that the Library and its use are covered by
+this License.  You must supply a copy of this License.  If the work
+during execution displays copyright notices, you must include the
+copyright notice for the Library among them, as well as a reference
+directing the user to the copy of this License.  Also, you must do one
+of these things:
+
+    a) Accompany the work with the complete corresponding
+    machine-readable source code for the Library including whatever
+    changes were used in the work (which must be distributed under
+    Sections 1 and 2 above); and, if the work is an executable linked
+    with the Library, with the complete machine-readable "work that
+    uses the Library", as object code and/or source code, so that the
+    user can modify the Library and then relink to produce a modified
+    executable containing the modified Library.  (It is understood
+    that the user who changes the contents of definitions files in the
+    Library will not necessarily be able to recompile the application
+    to use the modified definitions.)
+
+    b) Use a suitable shared library mechanism for linking with the
+    Library.  A suitable mechanism is one that (1) uses at run time a
+    copy of the library already present on the user's computer system,
+    rather than copying library functions into the executable, and (2)
+    will operate properly with a modified version of the library, if
+    the user installs one, as long as the modified version is
+    interface-compatible with the version that the work was made with.
+
+    c) Accompany the work with a written offer, valid for at
+    least three years, to give the same user the materials
+    specified in Subsection 6a, above, for a charge no more
+    than the cost of performing this distribution.
+
+    d) If distribution of the work is made by offering access to copy
+    from a designated place, offer equivalent access to copy the above
+    specified materials from the same place.
+
+    e) Verify that the user has already received a copy of these
+    materials or that you have already sent this user a copy.
+
+  For an executable, the required form of the "work that uses the
+Library" must include any data and utility programs needed for
+reproducing the executable from it.  However, as a special exception,
+the materials to be distributed need not include anything that is
+normally distributed (in either source or binary form) with the major
+components (compiler, kernel, and so on) of the operating system on
+which the executable runs, unless that component itself accompanies
+the executable.
+
+  It may happen that this requirement contradicts the license
+restrictions of other proprietary libraries that do not normally
+accompany the operating system.  Such a contradiction means you cannot
+use both them and the Library together in an executable that you
+distribute.
+
+  7. You may place library facilities that are a work based on the
+Library side-by-side in a single library together with other library
+facilities not covered by this License, and distribute such a combined
+library, provided that the separate distribution of the work based on
+the Library and of the other library facilities is otherwise
+permitted, and provided that you do these two things:
+
+    a) Accompany the combined library with a copy of the same work
+    based on the Library, uncombined with any other library
+    facilities.  This must be distributed under the terms of the
+    Sections above.
+
+    b) Give prominent notice with the combined library of the fact
+    that part of it is a work based on the Library, and explaining
+    where to find the accompanying uncombined form of the same work.
+
+  8. You may not copy, modify, sublicense, link with, or distribute
+the Library except as expressly provided under this License.  Any
+attempt otherwise to copy, modify, sublicense, link with, or
+distribute the Library is void, and will automatically terminate your
+rights under this License.  However, parties who have received copies,
+or rights, from you under this License will not have their licenses
+terminated so long as such parties remain in full compliance.
+
+  9. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Library or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Library (or any work based on the
+Library), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Library or works based on it.
+
+  10. Each time you redistribute the Library (or any work based on the
+Library), the recipient automatically receives a license from the
+original licensor to copy, distribute, link with or modify the Library
+subject to these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties with
+this License.
+
+  11. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Library at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Library by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Library.
+
+If any portion of this section is held invalid or unenforceable under any
+particular circumstance, the balance of the section is intended to apply,
+and the section as a whole is intended to apply in other circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  12. If the distribution and/or use of the Library is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Library under this License may add
+an explicit geographical distribution limitation excluding those countries,
+so that distribution is permitted only in or among countries not thus
+excluded.  In such case, this License incorporates the limitation as if
+written in the body of this License.
+
+  13. The Free Software Foundation may publish revised and/or new
+versions of the Lesser General Public License from time to time.
+Such new versions will be similar in spirit to the present version,
+but may differ in detail to address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Library
+specifies a version number of this License which applies to it and
+"any later version", you have the option of following the terms and
+conditions either of that version or of any later version published by
+the Free Software Foundation.  If the Library does not specify a
+license version number, you may choose any version ever published by
+the Free Software Foundation.
+
+  14. If you wish to incorporate parts of the Library into other free
+programs whose distribution conditions are incompatible with these,
+write to the author to ask for permission.  For software which is
+copyrighted by the Free Software Foundation, write to the Free
+Software Foundation; we sometimes make exceptions for this.  Our
+decision will be guided by the two goals of preserving the free status
+of all derivatives of our free software and of promoting the sharing
+and reuse of software generally.
+
+			    NO WARRANTY
+
+  15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO
+WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
+EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR
+OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY
+KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE
+LIBRARY IS WITH YOU.  SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME
+THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
+WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY
+AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU
+FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
+CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
+LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
+RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
+FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
+SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+DAMAGES.
+
+		     END OF TERMS AND CONDITIONS
+
+           How to Apply These Terms to Your New Libraries
+
+  If you develop a new library, and you want it to be of the greatest
+possible use to the public, we recommend making it free software that
+everyone can redistribute and change.  You can do so by permitting
+redistribution under these terms (or, alternatively, under the terms of the
+ordinary General Public License).
+
+  To apply these terms, attach the following notices to the library.  It is
+safest to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least the
+"copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the library's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Lesser General Public
+    License as published by the Free Software Foundation; either
+    version 2 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+Also add information on how to contact you by electronic and paper mail.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the library, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the
+  library `Frob' (a library for tweaking knobs) written by James Random Hacker.
+
+  <signature of Ty Coon>, 1 April 1990
+  Ty Coon, President of Vice
+
+That's all there is to it!
diff --git a/packages/@rescript/darwin-arm64/THIRD_PARTY_NOTICES_REWATCH.md b/packages/@rescript/darwin-arm64/THIRD_PARTY_NOTICES_REWATCH.md
new file mode 100644
index 00000000000..9a2c787e47d
--- /dev/null
+++ b/packages/@rescript/darwin-arm64/THIRD_PARTY_NOTICES_REWATCH.md
@@ -0,0 +1,85 @@
+# Third-party notices for the OCaml ReScript build system
+
+The `rescript.exe` OCaml build-system binary includes the following
+third-party software. These notices supplement the package's declared license.
+
+## Cmdliner
+
+Copyright (c) 2011 The cmdliner programmers
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+## Yojson
+
+Copyright (c) 2010-2012, Martin Jambon
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+- Neither the name of the copyright holder nor the names of its contributors
+  may be used to endorse or promote products derived from this software
+  without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+## Re
+
+Copyright (C) 2001 Jerome Vouillon
+
+Re is distributed under the GNU Lesser General Public License version 2.1 or,
+at your option, any later version, with the OCaml linking exception. The full
+license and exception are included in `RE_LICENSE.md`.
+
+## Spawn, Luv, libuv, ctypes, and integers
+
+- Spawn: Copyright (c) 2016-2018 Jane Street Group, LLC
+- Luv: Copyright (c) 2018-2023 Anton Bachin
+- libuv: Copyright (c) 2015-present libuv project contributors
+- ctypes: Copyright (c) 2013 Jeremy Yallop
+- integers: Copyright (c) 2013-2016 Jeremy Yallop
+
+Each project listed in this section is distributed under the MIT License:
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/packages/@rescript/darwin-arm64/bin.d.ts b/packages/@rescript/darwin-arm64/bin.d.ts
index f6fa8daaca5..d6e39948452 100644
--- a/packages/@rescript/darwin-arm64/bin.d.ts
+++ b/packages/@rescript/darwin-arm64/bin.d.ts
@@ -7,4 +7,5 @@ export type BinaryPaths = {
   rescript_tools_exe: string;
   rescript_editor_analysis_exe: string;
   rescript_exe: string;
+  rescript_rust_exe: string;
 };
diff --git a/packages/@rescript/darwin-arm64/bin.js b/packages/@rescript/darwin-arm64/bin.js
index aff7c9c9d93..290e8d7a241 100644
--- a/packages/@rescript/darwin-arm64/bin.js
+++ b/packages/@rescript/darwin-arm64/bin.js
@@ -12,4 +12,5 @@ export const binPaths = {
     "rescript-editor-analysis.exe",
   ),
   rescript_exe: path.join(binDir, "rescript.exe"),
+  rescript_rust_exe: path.join(binDir, "rescript-rust.exe"),
 };
diff --git a/packages/@rescript/darwin-arm64/package.json b/packages/@rescript/darwin-arm64/package.json
index e5c0bf036fb..eb1c693319a 100644
--- a/packages/@rescript/darwin-arm64/package.json
+++ b/packages/@rescript/darwin-arm64/package.json
@@ -35,7 +35,8 @@
       "./bin/bsc.exe",
       "./bin/rescript-editor-analysis.exe",
       "./bin/rescript-tools.exe",
-      "./bin/rescript.exe"
+      "./bin/rescript.exe",
+      "./bin/rescript-rust.exe"
     ]
   },
   "engines": {
@@ -50,6 +51,8 @@
   "files": [
     "bin.js",
     "bin.d.ts",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "RE_LICENSE.md",
     "bin/"
   ],
   "exports": "./bin.js",
diff --git a/packages/@rescript/darwin-x64/RE_LICENSE.md b/packages/@rescript/darwin-x64/RE_LICENSE.md
new file mode 100644
index 00000000000..f79ddd2b654
--- /dev/null
+++ b/packages/@rescript/darwin-x64/RE_LICENSE.md
@@ -0,0 +1,521 @@
+This Software is distributed under the terms of the GNU Lesser
+General Public License version 2.1 (included below), or (at your
+option) any later version.
+
+As a special exception to the GNU Library General Public License, you
+may link, statically or dynamically, a "work that uses the Library"
+with a publicly distributed version of the Library to produce an
+executable file containing portions of the Library, and distribute
+that executable file under terms of your choice, without any of the
+additional requirements listed in clause 6 of the GNU Library General
+Public License.  By "a publicly distributed version of the Library",
+we mean either the unmodified Library, or a modified version of the
+Library that is distributed under the conditions defined in clause 3
+of the GNU Library General Public License.  This exception does not
+however invalidate any other reasons why the executable file might be
+covered by the GNU Library General Public License.
+
+----------------------------------------------------------------------
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+		       Version 2.1, February 1999
+
+ Copyright (C) 1991, 1999 Free Software Foundation, Inc.
+     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+[This is the first released version of the Lesser GPL.  It also counts
+ as the successor of the GNU Library Public License, version 2, hence
+ the version number 2.1.]
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+Licenses are intended to guarantee your freedom to share and change
+free software--to make sure the software is free for all its users.
+
+  This license, the Lesser General Public License, applies to some
+specially designated software packages--typically libraries--of the
+Free Software Foundation and other authors who decide to use it.  You
+can use it too, but we suggest you first think carefully about whether
+this license or the ordinary General Public License is the better
+strategy to use in any particular case, based on the explanations below.
+
+  When we speak of free software, we are referring to freedom of use,
+not price.  Our General Public Licenses are designed to make sure that
+you have the freedom to distribute copies of free software (and charge
+for this service if you wish); that you receive source code or can get
+it if you want it; that you can change the software and use pieces of
+it in new free programs; and that you are informed that you can do
+these things.
+
+  To protect your rights, we need to make restrictions that forbid
+distributors to deny you these rights or to ask you to surrender these
+rights.  These restrictions translate to certain responsibilities for
+you if you distribute copies of the library or if you modify it.
+
+  For example, if you distribute copies of the library, whether gratis
+or for a fee, you must give the recipients all the rights that we gave
+you.  You must make sure that they, too, receive or can get the source
+code.  If you link other code with the library, you must provide
+complete object files to the recipients, so that they can relink them
+with the library after making changes to the library and recompiling
+it.  And you must show them these terms so they know their rights.
+
+  We protect your rights with a two-step method: (1) we copyright the
+library, and (2) we offer you this license, which gives you legal
+permission to copy, distribute and/or modify the library.
+
+  To protect each distributor, we want to make it very clear that
+there is no warranty for the free library.  Also, if the library is
+modified by someone else and passed on, the recipients should know
+that what they have is not the original version, so that the original
+author's reputation will not be affected by problems that might be
+introduced by others.
+
+  Finally, software patents pose a constant threat to the existence of
+any free program.  We wish to make sure that a company cannot
+effectively restrict the users of a free program by obtaining a
+restrictive license from a patent holder.  Therefore, we insist that
+any patent license obtained for a version of the library must be
+consistent with the full freedom of use specified in this license.
+
+  Most GNU software, including some libraries, is covered by the
+ordinary GNU General Public License.  This license, the GNU Lesser
+General Public License, applies to certain designated libraries, and
+is quite different from the ordinary General Public License.  We use
+this license for certain libraries in order to permit linking those
+libraries into non-free programs.
+
+  When a program is linked with a library, whether statically or using
+a shared library, the combination of the two is legally speaking a
+combined work, a derivative of the original library.  The ordinary
+General Public License therefore permits such linking only if the
+entire combination fits its criteria of freedom.  The Lesser General
+Public License permits more lax criteria for linking other code with
+the library.
+
+  We call this license the "Lesser" General Public License because it
+does Less to protect the user's freedom than the ordinary General
+Public License.  It also provides other free software developers Less
+of an advantage over competing non-free programs.  These disadvantages
+are the reason we use the ordinary General Public License for many
+libraries.  However, the Lesser license provides advantages in certain
+special circumstances.
+
+  For example, on rare occasions, there may be a special need to
+encourage the widest possible use of a certain library, so that it becomes
+a de-facto standard.  To achieve this, non-free programs must be
+allowed to use the library.  A more frequent case is that a free
+library does the same job as widely used non-free libraries.  In this
+case, there is little to gain by limiting the free library to free
+software only, so we use the Lesser General Public License.
+
+  In other cases, permission to use a particular library in non-free
+programs enables a greater number of people to use a large body of
+free software.  For example, permission to use the GNU C Library in
+non-free programs enables many more people to use the whole GNU
+operating system, as well as its variant, the GNU/Linux operating
+system.
+
+  Although the Lesser General Public License is Less protective of the
+users' freedom, it does ensure that the user of a program that is
+linked with the Library has the freedom and the wherewithal to run
+that program using a modified version of the Library.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.  Pay close attention to the difference between a
+"work based on the library" and a "work that uses the library".  The
+former contains code derived from the library, whereas the latter must
+be combined with the library in order to run.
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License Agreement applies to any software library or other
+program which contains a notice placed by the copyright holder or
+other authorized party saying it may be distributed under the terms of
+this Lesser General Public License (also called "this License").
+Each licensee is addressed as "you".
+
+  A "library" means a collection of software functions and/or data
+prepared so as to be conveniently linked with application programs
+(which use some of those functions and data) to form executables.
+
+  The "Library", below, refers to any such software library or work
+which has been distributed under these terms.  A "work based on the
+Library" means either the Library or any derivative work under
+copyright law: that is to say, a work containing the Library or a
+portion of it, either verbatim or with modifications and/or translated
+straightforwardly into another language.  (Hereinafter, translation is
+included without limitation in the term "modification".)
+
+  "Source code" for a work means the preferred form of the work for
+making modifications to it.  For a library, complete source code means
+all the source code for all modules it contains, plus any associated
+interface definition files, plus the scripts used to control compilation
+and installation of the library.
+
+  Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running a program using the Library is not restricted, and output from
+such a program is covered only if its contents constitute a work based
+on the Library (independent of the use of the Library in a tool for
+writing it).  Whether that is true depends on what the Library does
+and what the program that uses the Library does.
+
+  1. You may copy and distribute verbatim copies of the Library's
+complete source code as you receive it, in any medium, provided that
+you conspicuously and appropriately publish on each copy an
+appropriate copyright notice and disclaimer of warranty; keep intact
+all the notices that refer to this License and to the absence of any
+warranty; and distribute a copy of this License along with the
+Library.
+
+  You may charge a fee for the physical act of transferring a copy,
+and you may at your option offer warranty protection in exchange for a
+fee.
+
+  2. You may modify your copy or copies of the Library or any portion
+of it, thus forming a work based on the Library, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) The modified work must itself be a software library.
+
+    b) You must cause the files modified to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    c) You must cause the whole of the work to be licensed at no
+    charge to all third parties under the terms of this License.
+
+    d) If a facility in the modified Library refers to a function or a
+    table of data to be supplied by an application program that uses
+    the facility, other than as an argument passed when the facility
+    is invoked, then you must make a good faith effort to ensure that,
+    in the event an application does not supply such function or
+    table, the facility still operates, and performs whatever part of
+    its purpose remains meaningful.
+
+    (For example, a function in a library to compute square roots has
+    a purpose that is entirely well-defined independent of the
+    application.  Therefore, Subsection 2d requires that any
+    application-supplied function or table used by this function must
+    be optional: if the application does not supply it, the square
+    root function must still compute square roots.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Library,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Library, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote
+it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Library.
+
+In addition, mere aggregation of another work not based on the Library
+with the Library (or with a work based on the Library) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may opt to apply the terms of the ordinary GNU General Public
+License instead of this License to a given copy of the Library.  To do
+this, you must alter all the notices that refer to this License, so
+that they refer to the ordinary GNU General Public License, version 2,
+instead of to this License.  (If a newer version than version 2 of the
+ordinary GNU General Public License has appeared, then you can specify
+that version instead if you wish.)  Do not make any other change in
+these notices.
+
+  Once this change is made in a given copy, it is irreversible for
+that copy, so the ordinary GNU General Public License applies to all
+subsequent copies and derivative works made from that copy.
+
+  This option is useful when you wish to copy part of the code of
+the Library into a program that is not a library.
+
+  4. You may copy and distribute the Library (or a portion or
+derivative of it, under Section 2) in object code or executable form
+under the terms of Sections 1 and 2 above provided that you accompany
+it with the complete corresponding machine-readable source code, which
+must be distributed under the terms of Sections 1 and 2 above on a
+medium customarily used for software interchange.
+
+  If distribution of object code is made by offering access to copy
+from a designated place, then offering equivalent access to copy the
+source code from the same place satisfies the requirement to
+distribute the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  5. A program that contains no derivative of any portion of the
+Library, but is designed to work with the Library by being compiled or
+linked with it, is called a "work that uses the Library".  Such a
+work, in isolation, is not a derivative work of the Library, and
+therefore falls outside the scope of this License.
+
+  However, linking a "work that uses the Library" with the Library
+creates an executable that is a derivative of the Library (because it
+contains portions of the Library), rather than a "work that uses the
+library".  The executable is therefore covered by this License.
+Section 6 states terms for distribution of such executables.
+
+  When a "work that uses the Library" uses material from a header file
+that is part of the Library, the object code for the work may be a
+derivative work of the Library even though the source code is not.
+Whether this is true is especially significant if the work can be
+linked without the Library, or if the work is itself a library.  The
+threshold for this to be true is not precisely defined by law.
+
+  If such an object file uses only numerical parameters, data
+structure layouts and accessors, and small macros and small inline
+functions (ten lines or less in length), then the use of the object
+file is unrestricted, regardless of whether it is legally a derivative
+work.  (Executables containing this object code plus portions of the
+Library will still fall under Section 6.)
+
+  Otherwise, if the work is a derivative of the Library, you may
+distribute the object code for the work under the terms of Section 6.
+Any executables containing that work also fall under Section 6,
+whether or not they are linked directly with the Library itself.
+
+  6. As an exception to the Sections above, you may also combine or
+link a "work that uses the Library" with the Library to produce a
+work containing portions of the Library, and distribute that work
+under terms of your choice, provided that the terms permit
+modification of the work for the customer's own use and reverse
+engineering for debugging such modifications.
+
+  You must give prominent notice with each copy of the work that the
+Library is used in it and that the Library and its use are covered by
+this License.  You must supply a copy of this License.  If the work
+during execution displays copyright notices, you must include the
+copyright notice for the Library among them, as well as a reference
+directing the user to the copy of this License.  Also, you must do one
+of these things:
+
+    a) Accompany the work with the complete corresponding
+    machine-readable source code for the Library including whatever
+    changes were used in the work (which must be distributed under
+    Sections 1 and 2 above); and, if the work is an executable linked
+    with the Library, with the complete machine-readable "work that
+    uses the Library", as object code and/or source code, so that the
+    user can modify the Library and then relink to produce a modified
+    executable containing the modified Library.  (It is understood
+    that the user who changes the contents of definitions files in the
+    Library will not necessarily be able to recompile the application
+    to use the modified definitions.)
+
+    b) Use a suitable shared library mechanism for linking with the
+    Library.  A suitable mechanism is one that (1) uses at run time a
+    copy of the library already present on the user's computer system,
+    rather than copying library functions into the executable, and (2)
+    will operate properly with a modified version of the library, if
+    the user installs one, as long as the modified version is
+    interface-compatible with the version that the work was made with.
+
+    c) Accompany the work with a written offer, valid for at
+    least three years, to give the same user the materials
+    specified in Subsection 6a, above, for a charge no more
+    than the cost of performing this distribution.
+
+    d) If distribution of the work is made by offering access to copy
+    from a designated place, offer equivalent access to copy the above
+    specified materials from the same place.
+
+    e) Verify that the user has already received a copy of these
+    materials or that you have already sent this user a copy.
+
+  For an executable, the required form of the "work that uses the
+Library" must include any data and utility programs needed for
+reproducing the executable from it.  However, as a special exception,
+the materials to be distributed need not include anything that is
+normally distributed (in either source or binary form) with the major
+components (compiler, kernel, and so on) of the operating system on
+which the executable runs, unless that component itself accompanies
+the executable.
+
+  It may happen that this requirement contradicts the license
+restrictions of other proprietary libraries that do not normally
+accompany the operating system.  Such a contradiction means you cannot
+use both them and the Library together in an executable that you
+distribute.
+
+  7. You may place library facilities that are a work based on the
+Library side-by-side in a single library together with other library
+facilities not covered by this License, and distribute such a combined
+library, provided that the separate distribution of the work based on
+the Library and of the other library facilities is otherwise
+permitted, and provided that you do these two things:
+
+    a) Accompany the combined library with a copy of the same work
+    based on the Library, uncombined with any other library
+    facilities.  This must be distributed under the terms of the
+    Sections above.
+
+    b) Give prominent notice with the combined library of the fact
+    that part of it is a work based on the Library, and explaining
+    where to find the accompanying uncombined form of the same work.
+
+  8. You may not copy, modify, sublicense, link with, or distribute
+the Library except as expressly provided under this License.  Any
+attempt otherwise to copy, modify, sublicense, link with, or
+distribute the Library is void, and will automatically terminate your
+rights under this License.  However, parties who have received copies,
+or rights, from you under this License will not have their licenses
+terminated so long as such parties remain in full compliance.
+
+  9. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Library or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Library (or any work based on the
+Library), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Library or works based on it.
+
+  10. Each time you redistribute the Library (or any work based on the
+Library), the recipient automatically receives a license from the
+original licensor to copy, distribute, link with or modify the Library
+subject to these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties with
+this License.
+
+  11. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Library at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Library by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Library.
+
+If any portion of this section is held invalid or unenforceable under any
+particular circumstance, the balance of the section is intended to apply,
+and the section as a whole is intended to apply in other circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  12. If the distribution and/or use of the Library is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Library under this License may add
+an explicit geographical distribution limitation excluding those countries,
+so that distribution is permitted only in or among countries not thus
+excluded.  In such case, this License incorporates the limitation as if
+written in the body of this License.
+
+  13. The Free Software Foundation may publish revised and/or new
+versions of the Lesser General Public License from time to time.
+Such new versions will be similar in spirit to the present version,
+but may differ in detail to address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Library
+specifies a version number of this License which applies to it and
+"any later version", you have the option of following the terms and
+conditions either of that version or of any later version published by
+the Free Software Foundation.  If the Library does not specify a
+license version number, you may choose any version ever published by
+the Free Software Foundation.
+
+  14. If you wish to incorporate parts of the Library into other free
+programs whose distribution conditions are incompatible with these,
+write to the author to ask for permission.  For software which is
+copyrighted by the Free Software Foundation, write to the Free
+Software Foundation; we sometimes make exceptions for this.  Our
+decision will be guided by the two goals of preserving the free status
+of all derivatives of our free software and of promoting the sharing
+and reuse of software generally.
+
+			    NO WARRANTY
+
+  15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO
+WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
+EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR
+OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY
+KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE
+LIBRARY IS WITH YOU.  SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME
+THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
+WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY
+AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU
+FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
+CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
+LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
+RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
+FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
+SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+DAMAGES.
+
+		     END OF TERMS AND CONDITIONS
+
+           How to Apply These Terms to Your New Libraries
+
+  If you develop a new library, and you want it to be of the greatest
+possible use to the public, we recommend making it free software that
+everyone can redistribute and change.  You can do so by permitting
+redistribution under these terms (or, alternatively, under the terms of the
+ordinary General Public License).
+
+  To apply these terms, attach the following notices to the library.  It is
+safest to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least the
+"copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the library's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Lesser General Public
+    License as published by the Free Software Foundation; either
+    version 2 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+Also add information on how to contact you by electronic and paper mail.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the library, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the
+  library `Frob' (a library for tweaking knobs) written by James Random Hacker.
+
+  <signature of Ty Coon>, 1 April 1990
+  Ty Coon, President of Vice
+
+That's all there is to it!
diff --git a/packages/@rescript/darwin-x64/THIRD_PARTY_NOTICES_REWATCH.md b/packages/@rescript/darwin-x64/THIRD_PARTY_NOTICES_REWATCH.md
new file mode 100644
index 00000000000..9a2c787e47d
--- /dev/null
+++ b/packages/@rescript/darwin-x64/THIRD_PARTY_NOTICES_REWATCH.md
@@ -0,0 +1,85 @@
+# Third-party notices for the OCaml ReScript build system
+
+The `rescript.exe` OCaml build-system binary includes the following
+third-party software. These notices supplement the package's declared license.
+
+## Cmdliner
+
+Copyright (c) 2011 The cmdliner programmers
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+## Yojson
+
+Copyright (c) 2010-2012, Martin Jambon
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+- Neither the name of the copyright holder nor the names of its contributors
+  may be used to endorse or promote products derived from this software
+  without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+## Re
+
+Copyright (C) 2001 Jerome Vouillon
+
+Re is distributed under the GNU Lesser General Public License version 2.1 or,
+at your option, any later version, with the OCaml linking exception. The full
+license and exception are included in `RE_LICENSE.md`.
+
+## Spawn, Luv, libuv, ctypes, and integers
+
+- Spawn: Copyright (c) 2016-2018 Jane Street Group, LLC
+- Luv: Copyright (c) 2018-2023 Anton Bachin
+- libuv: Copyright (c) 2015-present libuv project contributors
+- ctypes: Copyright (c) 2013 Jeremy Yallop
+- integers: Copyright (c) 2013-2016 Jeremy Yallop
+
+Each project listed in this section is distributed under the MIT License:
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/packages/@rescript/darwin-x64/bin.d.ts b/packages/@rescript/darwin-x64/bin.d.ts
index f6fa8daaca5..d6e39948452 100644
--- a/packages/@rescript/darwin-x64/bin.d.ts
+++ b/packages/@rescript/darwin-x64/bin.d.ts
@@ -7,4 +7,5 @@ export type BinaryPaths = {
   rescript_tools_exe: string;
   rescript_editor_analysis_exe: string;
   rescript_exe: string;
+  rescript_rust_exe: string;
 };
diff --git a/packages/@rescript/darwin-x64/bin.js b/packages/@rescript/darwin-x64/bin.js
index aff7c9c9d93..290e8d7a241 100644
--- a/packages/@rescript/darwin-x64/bin.js
+++ b/packages/@rescript/darwin-x64/bin.js
@@ -12,4 +12,5 @@ export const binPaths = {
     "rescript-editor-analysis.exe",
   ),
   rescript_exe: path.join(binDir, "rescript.exe"),
+  rescript_rust_exe: path.join(binDir, "rescript-rust.exe"),
 };
diff --git a/packages/@rescript/darwin-x64/package.json b/packages/@rescript/darwin-x64/package.json
index 3d7e7e7e4f7..6d03cca3d13 100644
--- a/packages/@rescript/darwin-x64/package.json
+++ b/packages/@rescript/darwin-x64/package.json
@@ -35,7 +35,8 @@
       "./bin/bsc.exe",
       "./bin/rescript-editor-analysis.exe",
       "./bin/rescript-tools.exe",
-      "./bin/rescript.exe"
+      "./bin/rescript.exe",
+      "./bin/rescript-rust.exe"
     ]
   },
   "engines": {
@@ -50,6 +51,8 @@
   "files": [
     "bin.js",
     "bin.d.ts",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "RE_LICENSE.md",
     "bin/"
   ],
   "exports": "./bin.js",
diff --git a/packages/@rescript/linux-arm64/RE_LICENSE.md b/packages/@rescript/linux-arm64/RE_LICENSE.md
new file mode 100644
index 00000000000..f79ddd2b654
--- /dev/null
+++ b/packages/@rescript/linux-arm64/RE_LICENSE.md
@@ -0,0 +1,521 @@
+This Software is distributed under the terms of the GNU Lesser
+General Public License version 2.1 (included below), or (at your
+option) any later version.
+
+As a special exception to the GNU Library General Public License, you
+may link, statically or dynamically, a "work that uses the Library"
+with a publicly distributed version of the Library to produce an
+executable file containing portions of the Library, and distribute
+that executable file under terms of your choice, without any of the
+additional requirements listed in clause 6 of the GNU Library General
+Public License.  By "a publicly distributed version of the Library",
+we mean either the unmodified Library, or a modified version of the
+Library that is distributed under the conditions defined in clause 3
+of the GNU Library General Public License.  This exception does not
+however invalidate any other reasons why the executable file might be
+covered by the GNU Library General Public License.
+
+----------------------------------------------------------------------
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+		       Version 2.1, February 1999
+
+ Copyright (C) 1991, 1999 Free Software Foundation, Inc.
+     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+[This is the first released version of the Lesser GPL.  It also counts
+ as the successor of the GNU Library Public License, version 2, hence
+ the version number 2.1.]
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+Licenses are intended to guarantee your freedom to share and change
+free software--to make sure the software is free for all its users.
+
+  This license, the Lesser General Public License, applies to some
+specially designated software packages--typically libraries--of the
+Free Software Foundation and other authors who decide to use it.  You
+can use it too, but we suggest you first think carefully about whether
+this license or the ordinary General Public License is the better
+strategy to use in any particular case, based on the explanations below.
+
+  When we speak of free software, we are referring to freedom of use,
+not price.  Our General Public Licenses are designed to make sure that
+you have the freedom to distribute copies of free software (and charge
+for this service if you wish); that you receive source code or can get
+it if you want it; that you can change the software and use pieces of
+it in new free programs; and that you are informed that you can do
+these things.
+
+  To protect your rights, we need to make restrictions that forbid
+distributors to deny you these rights or to ask you to surrender these
+rights.  These restrictions translate to certain responsibilities for
+you if you distribute copies of the library or if you modify it.
+
+  For example, if you distribute copies of the library, whether gratis
+or for a fee, you must give the recipients all the rights that we gave
+you.  You must make sure that they, too, receive or can get the source
+code.  If you link other code with the library, you must provide
+complete object files to the recipients, so that they can relink them
+with the library after making changes to the library and recompiling
+it.  And you must show them these terms so they know their rights.
+
+  We protect your rights with a two-step method: (1) we copyright the
+library, and (2) we offer you this license, which gives you legal
+permission to copy, distribute and/or modify the library.
+
+  To protect each distributor, we want to make it very clear that
+there is no warranty for the free library.  Also, if the library is
+modified by someone else and passed on, the recipients should know
+that what they have is not the original version, so that the original
+author's reputation will not be affected by problems that might be
+introduced by others.
+
+  Finally, software patents pose a constant threat to the existence of
+any free program.  We wish to make sure that a company cannot
+effectively restrict the users of a free program by obtaining a
+restrictive license from a patent holder.  Therefore, we insist that
+any patent license obtained for a version of the library must be
+consistent with the full freedom of use specified in this license.
+
+  Most GNU software, including some libraries, is covered by the
+ordinary GNU General Public License.  This license, the GNU Lesser
+General Public License, applies to certain designated libraries, and
+is quite different from the ordinary General Public License.  We use
+this license for certain libraries in order to permit linking those
+libraries into non-free programs.
+
+  When a program is linked with a library, whether statically or using
+a shared library, the combination of the two is legally speaking a
+combined work, a derivative of the original library.  The ordinary
+General Public License therefore permits such linking only if the
+entire combination fits its criteria of freedom.  The Lesser General
+Public License permits more lax criteria for linking other code with
+the library.
+
+  We call this license the "Lesser" General Public License because it
+does Less to protect the user's freedom than the ordinary General
+Public License.  It also provides other free software developers Less
+of an advantage over competing non-free programs.  These disadvantages
+are the reason we use the ordinary General Public License for many
+libraries.  However, the Lesser license provides advantages in certain
+special circumstances.
+
+  For example, on rare occasions, there may be a special need to
+encourage the widest possible use of a certain library, so that it becomes
+a de-facto standard.  To achieve this, non-free programs must be
+allowed to use the library.  A more frequent case is that a free
+library does the same job as widely used non-free libraries.  In this
+case, there is little to gain by limiting the free library to free
+software only, so we use the Lesser General Public License.
+
+  In other cases, permission to use a particular library in non-free
+programs enables a greater number of people to use a large body of
+free software.  For example, permission to use the GNU C Library in
+non-free programs enables many more people to use the whole GNU
+operating system, as well as its variant, the GNU/Linux operating
+system.
+
+  Although the Lesser General Public License is Less protective of the
+users' freedom, it does ensure that the user of a program that is
+linked with the Library has the freedom and the wherewithal to run
+that program using a modified version of the Library.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.  Pay close attention to the difference between a
+"work based on the library" and a "work that uses the library".  The
+former contains code derived from the library, whereas the latter must
+be combined with the library in order to run.
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License Agreement applies to any software library or other
+program which contains a notice placed by the copyright holder or
+other authorized party saying it may be distributed under the terms of
+this Lesser General Public License (also called "this License").
+Each licensee is addressed as "you".
+
+  A "library" means a collection of software functions and/or data
+prepared so as to be conveniently linked with application programs
+(which use some of those functions and data) to form executables.
+
+  The "Library", below, refers to any such software library or work
+which has been distributed under these terms.  A "work based on the
+Library" means either the Library or any derivative work under
+copyright law: that is to say, a work containing the Library or a
+portion of it, either verbatim or with modifications and/or translated
+straightforwardly into another language.  (Hereinafter, translation is
+included without limitation in the term "modification".)
+
+  "Source code" for a work means the preferred form of the work for
+making modifications to it.  For a library, complete source code means
+all the source code for all modules it contains, plus any associated
+interface definition files, plus the scripts used to control compilation
+and installation of the library.
+
+  Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running a program using the Library is not restricted, and output from
+such a program is covered only if its contents constitute a work based
+on the Library (independent of the use of the Library in a tool for
+writing it).  Whether that is true depends on what the Library does
+and what the program that uses the Library does.
+
+  1. You may copy and distribute verbatim copies of the Library's
+complete source code as you receive it, in any medium, provided that
+you conspicuously and appropriately publish on each copy an
+appropriate copyright notice and disclaimer of warranty; keep intact
+all the notices that refer to this License and to the absence of any
+warranty; and distribute a copy of this License along with the
+Library.
+
+  You may charge a fee for the physical act of transferring a copy,
+and you may at your option offer warranty protection in exchange for a
+fee.
+
+  2. You may modify your copy or copies of the Library or any portion
+of it, thus forming a work based on the Library, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) The modified work must itself be a software library.
+
+    b) You must cause the files modified to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    c) You must cause the whole of the work to be licensed at no
+    charge to all third parties under the terms of this License.
+
+    d) If a facility in the modified Library refers to a function or a
+    table of data to be supplied by an application program that uses
+    the facility, other than as an argument passed when the facility
+    is invoked, then you must make a good faith effort to ensure that,
+    in the event an application does not supply such function or
+    table, the facility still operates, and performs whatever part of
+    its purpose remains meaningful.
+
+    (For example, a function in a library to compute square roots has
+    a purpose that is entirely well-defined independent of the
+    application.  Therefore, Subsection 2d requires that any
+    application-supplied function or table used by this function must
+    be optional: if the application does not supply it, the square
+    root function must still compute square roots.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Library,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Library, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote
+it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Library.
+
+In addition, mere aggregation of another work not based on the Library
+with the Library (or with a work based on the Library) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may opt to apply the terms of the ordinary GNU General Public
+License instead of this License to a given copy of the Library.  To do
+this, you must alter all the notices that refer to this License, so
+that they refer to the ordinary GNU General Public License, version 2,
+instead of to this License.  (If a newer version than version 2 of the
+ordinary GNU General Public License has appeared, then you can specify
+that version instead if you wish.)  Do not make any other change in
+these notices.
+
+  Once this change is made in a given copy, it is irreversible for
+that copy, so the ordinary GNU General Public License applies to all
+subsequent copies and derivative works made from that copy.
+
+  This option is useful when you wish to copy part of the code of
+the Library into a program that is not a library.
+
+  4. You may copy and distribute the Library (or a portion or
+derivative of it, under Section 2) in object code or executable form
+under the terms of Sections 1 and 2 above provided that you accompany
+it with the complete corresponding machine-readable source code, which
+must be distributed under the terms of Sections 1 and 2 above on a
+medium customarily used for software interchange.
+
+  If distribution of object code is made by offering access to copy
+from a designated place, then offering equivalent access to copy the
+source code from the same place satisfies the requirement to
+distribute the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  5. A program that contains no derivative of any portion of the
+Library, but is designed to work with the Library by being compiled or
+linked with it, is called a "work that uses the Library".  Such a
+work, in isolation, is not a derivative work of the Library, and
+therefore falls outside the scope of this License.
+
+  However, linking a "work that uses the Library" with the Library
+creates an executable that is a derivative of the Library (because it
+contains portions of the Library), rather than a "work that uses the
+library".  The executable is therefore covered by this License.
+Section 6 states terms for distribution of such executables.
+
+  When a "work that uses the Library" uses material from a header file
+that is part of the Library, the object code for the work may be a
+derivative work of the Library even though the source code is not.
+Whether this is true is especially significant if the work can be
+linked without the Library, or if the work is itself a library.  The
+threshold for this to be true is not precisely defined by law.
+
+  If such an object file uses only numerical parameters, data
+structure layouts and accessors, and small macros and small inline
+functions (ten lines or less in length), then the use of the object
+file is unrestricted, regardless of whether it is legally a derivative
+work.  (Executables containing this object code plus portions of the
+Library will still fall under Section 6.)
+
+  Otherwise, if the work is a derivative of the Library, you may
+distribute the object code for the work under the terms of Section 6.
+Any executables containing that work also fall under Section 6,
+whether or not they are linked directly with the Library itself.
+
+  6. As an exception to the Sections above, you may also combine or
+link a "work that uses the Library" with the Library to produce a
+work containing portions of the Library, and distribute that work
+under terms of your choice, provided that the terms permit
+modification of the work for the customer's own use and reverse
+engineering for debugging such modifications.
+
+  You must give prominent notice with each copy of the work that the
+Library is used in it and that the Library and its use are covered by
+this License.  You must supply a copy of this License.  If the work
+during execution displays copyright notices, you must include the
+copyright notice for the Library among them, as well as a reference
+directing the user to the copy of this License.  Also, you must do one
+of these things:
+
+    a) Accompany the work with the complete corresponding
+    machine-readable source code for the Library including whatever
+    changes were used in the work (which must be distributed under
+    Sections 1 and 2 above); and, if the work is an executable linked
+    with the Library, with the complete machine-readable "work that
+    uses the Library", as object code and/or source code, so that the
+    user can modify the Library and then relink to produce a modified
+    executable containing the modified Library.  (It is understood
+    that the user who changes the contents of definitions files in the
+    Library will not necessarily be able to recompile the application
+    to use the modified definitions.)
+
+    b) Use a suitable shared library mechanism for linking with the
+    Library.  A suitable mechanism is one that (1) uses at run time a
+    copy of the library already present on the user's computer system,
+    rather than copying library functions into the executable, and (2)
+    will operate properly with a modified version of the library, if
+    the user installs one, as long as the modified version is
+    interface-compatible with the version that the work was made with.
+
+    c) Accompany the work with a written offer, valid for at
+    least three years, to give the same user the materials
+    specified in Subsection 6a, above, for a charge no more
+    than the cost of performing this distribution.
+
+    d) If distribution of the work is made by offering access to copy
+    from a designated place, offer equivalent access to copy the above
+    specified materials from the same place.
+
+    e) Verify that the user has already received a copy of these
+    materials or that you have already sent this user a copy.
+
+  For an executable, the required form of the "work that uses the
+Library" must include any data and utility programs needed for
+reproducing the executable from it.  However, as a special exception,
+the materials to be distributed need not include anything that is
+normally distributed (in either source or binary form) with the major
+components (compiler, kernel, and so on) of the operating system on
+which the executable runs, unless that component itself accompanies
+the executable.
+
+  It may happen that this requirement contradicts the license
+restrictions of other proprietary libraries that do not normally
+accompany the operating system.  Such a contradiction means you cannot
+use both them and the Library together in an executable that you
+distribute.
+
+  7. You may place library facilities that are a work based on the
+Library side-by-side in a single library together with other library
+facilities not covered by this License, and distribute such a combined
+library, provided that the separate distribution of the work based on
+the Library and of the other library facilities is otherwise
+permitted, and provided that you do these two things:
+
+    a) Accompany the combined library with a copy of the same work
+    based on the Library, uncombined with any other library
+    facilities.  This must be distributed under the terms of the
+    Sections above.
+
+    b) Give prominent notice with the combined library of the fact
+    that part of it is a work based on the Library, and explaining
+    where to find the accompanying uncombined form of the same work.
+
+  8. You may not copy, modify, sublicense, link with, or distribute
+the Library except as expressly provided under this License.  Any
+attempt otherwise to copy, modify, sublicense, link with, or
+distribute the Library is void, and will automatically terminate your
+rights under this License.  However, parties who have received copies,
+or rights, from you under this License will not have their licenses
+terminated so long as such parties remain in full compliance.
+
+  9. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Library or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Library (or any work based on the
+Library), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Library or works based on it.
+
+  10. Each time you redistribute the Library (or any work based on the
+Library), the recipient automatically receives a license from the
+original licensor to copy, distribute, link with or modify the Library
+subject to these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties with
+this License.
+
+  11. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Library at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Library by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Library.
+
+If any portion of this section is held invalid or unenforceable under any
+particular circumstance, the balance of the section is intended to apply,
+and the section as a whole is intended to apply in other circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  12. If the distribution and/or use of the Library is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Library under this License may add
+an explicit geographical distribution limitation excluding those countries,
+so that distribution is permitted only in or among countries not thus
+excluded.  In such case, this License incorporates the limitation as if
+written in the body of this License.
+
+  13. The Free Software Foundation may publish revised and/or new
+versions of the Lesser General Public License from time to time.
+Such new versions will be similar in spirit to the present version,
+but may differ in detail to address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Library
+specifies a version number of this License which applies to it and
+"any later version", you have the option of following the terms and
+conditions either of that version or of any later version published by
+the Free Software Foundation.  If the Library does not specify a
+license version number, you may choose any version ever published by
+the Free Software Foundation.
+
+  14. If you wish to incorporate parts of the Library into other free
+programs whose distribution conditions are incompatible with these,
+write to the author to ask for permission.  For software which is
+copyrighted by the Free Software Foundation, write to the Free
+Software Foundation; we sometimes make exceptions for this.  Our
+decision will be guided by the two goals of preserving the free status
+of all derivatives of our free software and of promoting the sharing
+and reuse of software generally.
+
+			    NO WARRANTY
+
+  15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO
+WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
+EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR
+OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY
+KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE
+LIBRARY IS WITH YOU.  SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME
+THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
+WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY
+AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU
+FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
+CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
+LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
+RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
+FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
+SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+DAMAGES.
+
+		     END OF TERMS AND CONDITIONS
+
+           How to Apply These Terms to Your New Libraries
+
+  If you develop a new library, and you want it to be of the greatest
+possible use to the public, we recommend making it free software that
+everyone can redistribute and change.  You can do so by permitting
+redistribution under these terms (or, alternatively, under the terms of the
+ordinary General Public License).
+
+  To apply these terms, attach the following notices to the library.  It is
+safest to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least the
+"copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the library's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Lesser General Public
+    License as published by the Free Software Foundation; either
+    version 2 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+Also add information on how to contact you by electronic and paper mail.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the library, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the
+  library `Frob' (a library for tweaking knobs) written by James Random Hacker.
+
+  <signature of Ty Coon>, 1 April 1990
+  Ty Coon, President of Vice
+
+That's all there is to it!
diff --git a/packages/@rescript/linux-arm64/THIRD_PARTY_NOTICES_REWATCH.md b/packages/@rescript/linux-arm64/THIRD_PARTY_NOTICES_REWATCH.md
new file mode 100644
index 00000000000..9a2c787e47d
--- /dev/null
+++ b/packages/@rescript/linux-arm64/THIRD_PARTY_NOTICES_REWATCH.md
@@ -0,0 +1,85 @@
+# Third-party notices for the OCaml ReScript build system
+
+The `rescript.exe` OCaml build-system binary includes the following
+third-party software. These notices supplement the package's declared license.
+
+## Cmdliner
+
+Copyright (c) 2011 The cmdliner programmers
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+## Yojson
+
+Copyright (c) 2010-2012, Martin Jambon
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+- Neither the name of the copyright holder nor the names of its contributors
+  may be used to endorse or promote products derived from this software
+  without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+## Re
+
+Copyright (C) 2001 Jerome Vouillon
+
+Re is distributed under the GNU Lesser General Public License version 2.1 or,
+at your option, any later version, with the OCaml linking exception. The full
+license and exception are included in `RE_LICENSE.md`.
+
+## Spawn, Luv, libuv, ctypes, and integers
+
+- Spawn: Copyright (c) 2016-2018 Jane Street Group, LLC
+- Luv: Copyright (c) 2018-2023 Anton Bachin
+- libuv: Copyright (c) 2015-present libuv project contributors
+- ctypes: Copyright (c) 2013 Jeremy Yallop
+- integers: Copyright (c) 2013-2016 Jeremy Yallop
+
+Each project listed in this section is distributed under the MIT License:
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/packages/@rescript/linux-arm64/bin.d.ts b/packages/@rescript/linux-arm64/bin.d.ts
index f6fa8daaca5..d6e39948452 100644
--- a/packages/@rescript/linux-arm64/bin.d.ts
+++ b/packages/@rescript/linux-arm64/bin.d.ts
@@ -7,4 +7,5 @@ export type BinaryPaths = {
   rescript_tools_exe: string;
   rescript_editor_analysis_exe: string;
   rescript_exe: string;
+  rescript_rust_exe: string;
 };
diff --git a/packages/@rescript/linux-arm64/bin.js b/packages/@rescript/linux-arm64/bin.js
index aff7c9c9d93..290e8d7a241 100644
--- a/packages/@rescript/linux-arm64/bin.js
+++ b/packages/@rescript/linux-arm64/bin.js
@@ -12,4 +12,5 @@ export const binPaths = {
     "rescript-editor-analysis.exe",
   ),
   rescript_exe: path.join(binDir, "rescript.exe"),
+  rescript_rust_exe: path.join(binDir, "rescript-rust.exe"),
 };
diff --git a/packages/@rescript/linux-arm64/package.json b/packages/@rescript/linux-arm64/package.json
index 710ce52c049..415d22d9d93 100644
--- a/packages/@rescript/linux-arm64/package.json
+++ b/packages/@rescript/linux-arm64/package.json
@@ -35,7 +35,8 @@
       "./bin/bsc.exe",
       "./bin/rescript-editor-analysis.exe",
       "./bin/rescript-tools.exe",
-      "./bin/rescript.exe"
+      "./bin/rescript.exe",
+      "./bin/rescript-rust.exe"
     ]
   },
   "engines": {
@@ -50,6 +51,8 @@
   "files": [
     "bin.js",
     "bin.d.ts",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "RE_LICENSE.md",
     "bin/"
   ],
   "exports": "./bin.js",
diff --git a/packages/@rescript/linux-x64/RE_LICENSE.md b/packages/@rescript/linux-x64/RE_LICENSE.md
new file mode 100644
index 00000000000..f79ddd2b654
--- /dev/null
+++ b/packages/@rescript/linux-x64/RE_LICENSE.md
@@ -0,0 +1,521 @@
+This Software is distributed under the terms of the GNU Lesser
+General Public License version 2.1 (included below), or (at your
+option) any later version.
+
+As a special exception to the GNU Library General Public License, you
+may link, statically or dynamically, a "work that uses the Library"
+with a publicly distributed version of the Library to produce an
+executable file containing portions of the Library, and distribute
+that executable file under terms of your choice, without any of the
+additional requirements listed in clause 6 of the GNU Library General
+Public License.  By "a publicly distributed version of the Library",
+we mean either the unmodified Library, or a modified version of the
+Library that is distributed under the conditions defined in clause 3
+of the GNU Library General Public License.  This exception does not
+however invalidate any other reasons why the executable file might be
+covered by the GNU Library General Public License.
+
+----------------------------------------------------------------------
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+		       Version 2.1, February 1999
+
+ Copyright (C) 1991, 1999 Free Software Foundation, Inc.
+     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+[This is the first released version of the Lesser GPL.  It also counts
+ as the successor of the GNU Library Public License, version 2, hence
+ the version number 2.1.]
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+Licenses are intended to guarantee your freedom to share and change
+free software--to make sure the software is free for all its users.
+
+  This license, the Lesser General Public License, applies to some
+specially designated software packages--typically libraries--of the
+Free Software Foundation and other authors who decide to use it.  You
+can use it too, but we suggest you first think carefully about whether
+this license or the ordinary General Public License is the better
+strategy to use in any particular case, based on the explanations below.
+
+  When we speak of free software, we are referring to freedom of use,
+not price.  Our General Public Licenses are designed to make sure that
+you have the freedom to distribute copies of free software (and charge
+for this service if you wish); that you receive source code or can get
+it if you want it; that you can change the software and use pieces of
+it in new free programs; and that you are informed that you can do
+these things.
+
+  To protect your rights, we need to make restrictions that forbid
+distributors to deny you these rights or to ask you to surrender these
+rights.  These restrictions translate to certain responsibilities for
+you if you distribute copies of the library or if you modify it.
+
+  For example, if you distribute copies of the library, whether gratis
+or for a fee, you must give the recipients all the rights that we gave
+you.  You must make sure that they, too, receive or can get the source
+code.  If you link other code with the library, you must provide
+complete object files to the recipients, so that they can relink them
+with the library after making changes to the library and recompiling
+it.  And you must show them these terms so they know their rights.
+
+  We protect your rights with a two-step method: (1) we copyright the
+library, and (2) we offer you this license, which gives you legal
+permission to copy, distribute and/or modify the library.
+
+  To protect each distributor, we want to make it very clear that
+there is no warranty for the free library.  Also, if the library is
+modified by someone else and passed on, the recipients should know
+that what they have is not the original version, so that the original
+author's reputation will not be affected by problems that might be
+introduced by others.
+
+  Finally, software patents pose a constant threat to the existence of
+any free program.  We wish to make sure that a company cannot
+effectively restrict the users of a free program by obtaining a
+restrictive license from a patent holder.  Therefore, we insist that
+any patent license obtained for a version of the library must be
+consistent with the full freedom of use specified in this license.
+
+  Most GNU software, including some libraries, is covered by the
+ordinary GNU General Public License.  This license, the GNU Lesser
+General Public License, applies to certain designated libraries, and
+is quite different from the ordinary General Public License.  We use
+this license for certain libraries in order to permit linking those
+libraries into non-free programs.
+
+  When a program is linked with a library, whether statically or using
+a shared library, the combination of the two is legally speaking a
+combined work, a derivative of the original library.  The ordinary
+General Public License therefore permits such linking only if the
+entire combination fits its criteria of freedom.  The Lesser General
+Public License permits more lax criteria for linking other code with
+the library.
+
+  We call this license the "Lesser" General Public License because it
+does Less to protect the user's freedom than the ordinary General
+Public License.  It also provides other free software developers Less
+of an advantage over competing non-free programs.  These disadvantages
+are the reason we use the ordinary General Public License for many
+libraries.  However, the Lesser license provides advantages in certain
+special circumstances.
+
+  For example, on rare occasions, there may be a special need to
+encourage the widest possible use of a certain library, so that it becomes
+a de-facto standard.  To achieve this, non-free programs must be
+allowed to use the library.  A more frequent case is that a free
+library does the same job as widely used non-free libraries.  In this
+case, there is little to gain by limiting the free library to free
+software only, so we use the Lesser General Public License.
+
+  In other cases, permission to use a particular library in non-free
+programs enables a greater number of people to use a large body of
+free software.  For example, permission to use the GNU C Library in
+non-free programs enables many more people to use the whole GNU
+operating system, as well as its variant, the GNU/Linux operating
+system.
+
+  Although the Lesser General Public License is Less protective of the
+users' freedom, it does ensure that the user of a program that is
+linked with the Library has the freedom and the wherewithal to run
+that program using a modified version of the Library.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.  Pay close attention to the difference between a
+"work based on the library" and a "work that uses the library".  The
+former contains code derived from the library, whereas the latter must
+be combined with the library in order to run.
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License Agreement applies to any software library or other
+program which contains a notice placed by the copyright holder or
+other authorized party saying it may be distributed under the terms of
+this Lesser General Public License (also called "this License").
+Each licensee is addressed as "you".
+
+  A "library" means a collection of software functions and/or data
+prepared so as to be conveniently linked with application programs
+(which use some of those functions and data) to form executables.
+
+  The "Library", below, refers to any such software library or work
+which has been distributed under these terms.  A "work based on the
+Library" means either the Library or any derivative work under
+copyright law: that is to say, a work containing the Library or a
+portion of it, either verbatim or with modifications and/or translated
+straightforwardly into another language.  (Hereinafter, translation is
+included without limitation in the term "modification".)
+
+  "Source code" for a work means the preferred form of the work for
+making modifications to it.  For a library, complete source code means
+all the source code for all modules it contains, plus any associated
+interface definition files, plus the scripts used to control compilation
+and installation of the library.
+
+  Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running a program using the Library is not restricted, and output from
+such a program is covered only if its contents constitute a work based
+on the Library (independent of the use of the Library in a tool for
+writing it).  Whether that is true depends on what the Library does
+and what the program that uses the Library does.
+
+  1. You may copy and distribute verbatim copies of the Library's
+complete source code as you receive it, in any medium, provided that
+you conspicuously and appropriately publish on each copy an
+appropriate copyright notice and disclaimer of warranty; keep intact
+all the notices that refer to this License and to the absence of any
+warranty; and distribute a copy of this License along with the
+Library.
+
+  You may charge a fee for the physical act of transferring a copy,
+and you may at your option offer warranty protection in exchange for a
+fee.
+
+  2. You may modify your copy or copies of the Library or any portion
+of it, thus forming a work based on the Library, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) The modified work must itself be a software library.
+
+    b) You must cause the files modified to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    c) You must cause the whole of the work to be licensed at no
+    charge to all third parties under the terms of this License.
+
+    d) If a facility in the modified Library refers to a function or a
+    table of data to be supplied by an application program that uses
+    the facility, other than as an argument passed when the facility
+    is invoked, then you must make a good faith effort to ensure that,
+    in the event an application does not supply such function or
+    table, the facility still operates, and performs whatever part of
+    its purpose remains meaningful.
+
+    (For example, a function in a library to compute square roots has
+    a purpose that is entirely well-defined independent of the
+    application.  Therefore, Subsection 2d requires that any
+    application-supplied function or table used by this function must
+    be optional: if the application does not supply it, the square
+    root function must still compute square roots.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Library,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Library, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote
+it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Library.
+
+In addition, mere aggregation of another work not based on the Library
+with the Library (or with a work based on the Library) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may opt to apply the terms of the ordinary GNU General Public
+License instead of this License to a given copy of the Library.  To do
+this, you must alter all the notices that refer to this License, so
+that they refer to the ordinary GNU General Public License, version 2,
+instead of to this License.  (If a newer version than version 2 of the
+ordinary GNU General Public License has appeared, then you can specify
+that version instead if you wish.)  Do not make any other change in
+these notices.
+
+  Once this change is made in a given copy, it is irreversible for
+that copy, so the ordinary GNU General Public License applies to all
+subsequent copies and derivative works made from that copy.
+
+  This option is useful when you wish to copy part of the code of
+the Library into a program that is not a library.
+
+  4. You may copy and distribute the Library (or a portion or
+derivative of it, under Section 2) in object code or executable form
+under the terms of Sections 1 and 2 above provided that you accompany
+it with the complete corresponding machine-readable source code, which
+must be distributed under the terms of Sections 1 and 2 above on a
+medium customarily used for software interchange.
+
+  If distribution of object code is made by offering access to copy
+from a designated place, then offering equivalent access to copy the
+source code from the same place satisfies the requirement to
+distribute the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  5. A program that contains no derivative of any portion of the
+Library, but is designed to work with the Library by being compiled or
+linked with it, is called a "work that uses the Library".  Such a
+work, in isolation, is not a derivative work of the Library, and
+therefore falls outside the scope of this License.
+
+  However, linking a "work that uses the Library" with the Library
+creates an executable that is a derivative of the Library (because it
+contains portions of the Library), rather than a "work that uses the
+library".  The executable is therefore covered by this License.
+Section 6 states terms for distribution of such executables.
+
+  When a "work that uses the Library" uses material from a header file
+that is part of the Library, the object code for the work may be a
+derivative work of the Library even though the source code is not.
+Whether this is true is especially significant if the work can be
+linked without the Library, or if the work is itself a library.  The
+threshold for this to be true is not precisely defined by law.
+
+  If such an object file uses only numerical parameters, data
+structure layouts and accessors, and small macros and small inline
+functions (ten lines or less in length), then the use of the object
+file is unrestricted, regardless of whether it is legally a derivative
+work.  (Executables containing this object code plus portions of the
+Library will still fall under Section 6.)
+
+  Otherwise, if the work is a derivative of the Library, you may
+distribute the object code for the work under the terms of Section 6.
+Any executables containing that work also fall under Section 6,
+whether or not they are linked directly with the Library itself.
+
+  6. As an exception to the Sections above, you may also combine or
+link a "work that uses the Library" with the Library to produce a
+work containing portions of the Library, and distribute that work
+under terms of your choice, provided that the terms permit
+modification of the work for the customer's own use and reverse
+engineering for debugging such modifications.
+
+  You must give prominent notice with each copy of the work that the
+Library is used in it and that the Library and its use are covered by
+this License.  You must supply a copy of this License.  If the work
+during execution displays copyright notices, you must include the
+copyright notice for the Library among them, as well as a reference
+directing the user to the copy of this License.  Also, you must do one
+of these things:
+
+    a) Accompany the work with the complete corresponding
+    machine-readable source code for the Library including whatever
+    changes were used in the work (which must be distributed under
+    Sections 1 and 2 above); and, if the work is an executable linked
+    with the Library, with the complete machine-readable "work that
+    uses the Library", as object code and/or source code, so that the
+    user can modify the Library and then relink to produce a modified
+    executable containing the modified Library.  (It is understood
+    that the user who changes the contents of definitions files in the
+    Library will not necessarily be able to recompile the application
+    to use the modified definitions.)
+
+    b) Use a suitable shared library mechanism for linking with the
+    Library.  A suitable mechanism is one that (1) uses at run time a
+    copy of the library already present on the user's computer system,
+    rather than copying library functions into the executable, and (2)
+    will operate properly with a modified version of the library, if
+    the user installs one, as long as the modified version is
+    interface-compatible with the version that the work was made with.
+
+    c) Accompany the work with a written offer, valid for at
+    least three years, to give the same user the materials
+    specified in Subsection 6a, above, for a charge no more
+    than the cost of performing this distribution.
+
+    d) If distribution of the work is made by offering access to copy
+    from a designated place, offer equivalent access to copy the above
+    specified materials from the same place.
+
+    e) Verify that the user has already received a copy of these
+    materials or that you have already sent this user a copy.
+
+  For an executable, the required form of the "work that uses the
+Library" must include any data and utility programs needed for
+reproducing the executable from it.  However, as a special exception,
+the materials to be distributed need not include anything that is
+normally distributed (in either source or binary form) with the major
+components (compiler, kernel, and so on) of the operating system on
+which the executable runs, unless that component itself accompanies
+the executable.
+
+  It may happen that this requirement contradicts the license
+restrictions of other proprietary libraries that do not normally
+accompany the operating system.  Such a contradiction means you cannot
+use both them and the Library together in an executable that you
+distribute.
+
+  7. You may place library facilities that are a work based on the
+Library side-by-side in a single library together with other library
+facilities not covered by this License, and distribute such a combined
+library, provided that the separate distribution of the work based on
+the Library and of the other library facilities is otherwise
+permitted, and provided that you do these two things:
+
+    a) Accompany the combined library with a copy of the same work
+    based on the Library, uncombined with any other library
+    facilities.  This must be distributed under the terms of the
+    Sections above.
+
+    b) Give prominent notice with the combined library of the fact
+    that part of it is a work based on the Library, and explaining
+    where to find the accompanying uncombined form of the same work.
+
+  8. You may not copy, modify, sublicense, link with, or distribute
+the Library except as expressly provided under this License.  Any
+attempt otherwise to copy, modify, sublicense, link with, or
+distribute the Library is void, and will automatically terminate your
+rights under this License.  However, parties who have received copies,
+or rights, from you under this License will not have their licenses
+terminated so long as such parties remain in full compliance.
+
+  9. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Library or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Library (or any work based on the
+Library), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Library or works based on it.
+
+  10. Each time you redistribute the Library (or any work based on the
+Library), the recipient automatically receives a license from the
+original licensor to copy, distribute, link with or modify the Library
+subject to these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties with
+this License.
+
+  11. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Library at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Library by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Library.
+
+If any portion of this section is held invalid or unenforceable under any
+particular circumstance, the balance of the section is intended to apply,
+and the section as a whole is intended to apply in other circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  12. If the distribution and/or use of the Library is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Library under this License may add
+an explicit geographical distribution limitation excluding those countries,
+so that distribution is permitted only in or among countries not thus
+excluded.  In such case, this License incorporates the limitation as if
+written in the body of this License.
+
+  13. The Free Software Foundation may publish revised and/or new
+versions of the Lesser General Public License from time to time.
+Such new versions will be similar in spirit to the present version,
+but may differ in detail to address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Library
+specifies a version number of this License which applies to it and
+"any later version", you have the option of following the terms and
+conditions either of that version or of any later version published by
+the Free Software Foundation.  If the Library does not specify a
+license version number, you may choose any version ever published by
+the Free Software Foundation.
+
+  14. If you wish to incorporate parts of the Library into other free
+programs whose distribution conditions are incompatible with these,
+write to the author to ask for permission.  For software which is
+copyrighted by the Free Software Foundation, write to the Free
+Software Foundation; we sometimes make exceptions for this.  Our
+decision will be guided by the two goals of preserving the free status
+of all derivatives of our free software and of promoting the sharing
+and reuse of software generally.
+
+			    NO WARRANTY
+
+  15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO
+WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
+EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR
+OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY
+KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE
+LIBRARY IS WITH YOU.  SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME
+THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
+WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY
+AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU
+FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
+CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
+LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
+RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
+FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
+SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+DAMAGES.
+
+		     END OF TERMS AND CONDITIONS
+
+           How to Apply These Terms to Your New Libraries
+
+  If you develop a new library, and you want it to be of the greatest
+possible use to the public, we recommend making it free software that
+everyone can redistribute and change.  You can do so by permitting
+redistribution under these terms (or, alternatively, under the terms of the
+ordinary General Public License).
+
+  To apply these terms, attach the following notices to the library.  It is
+safest to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least the
+"copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the library's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Lesser General Public
+    License as published by the Free Software Foundation; either
+    version 2 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+Also add information on how to contact you by electronic and paper mail.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the library, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the
+  library `Frob' (a library for tweaking knobs) written by James Random Hacker.
+
+  <signature of Ty Coon>, 1 April 1990
+  Ty Coon, President of Vice
+
+That's all there is to it!
diff --git a/packages/@rescript/linux-x64/THIRD_PARTY_NOTICES_REWATCH.md b/packages/@rescript/linux-x64/THIRD_PARTY_NOTICES_REWATCH.md
new file mode 100644
index 00000000000..9a2c787e47d
--- /dev/null
+++ b/packages/@rescript/linux-x64/THIRD_PARTY_NOTICES_REWATCH.md
@@ -0,0 +1,85 @@
+# Third-party notices for the OCaml ReScript build system
+
+The `rescript.exe` OCaml build-system binary includes the following
+third-party software. These notices supplement the package's declared license.
+
+## Cmdliner
+
+Copyright (c) 2011 The cmdliner programmers
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+## Yojson
+
+Copyright (c) 2010-2012, Martin Jambon
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+- Neither the name of the copyright holder nor the names of its contributors
+  may be used to endorse or promote products derived from this software
+  without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+## Re
+
+Copyright (C) 2001 Jerome Vouillon
+
+Re is distributed under the GNU Lesser General Public License version 2.1 or,
+at your option, any later version, with the OCaml linking exception. The full
+license and exception are included in `RE_LICENSE.md`.
+
+## Spawn, Luv, libuv, ctypes, and integers
+
+- Spawn: Copyright (c) 2016-2018 Jane Street Group, LLC
+- Luv: Copyright (c) 2018-2023 Anton Bachin
+- libuv: Copyright (c) 2015-present libuv project contributors
+- ctypes: Copyright (c) 2013 Jeremy Yallop
+- integers: Copyright (c) 2013-2016 Jeremy Yallop
+
+Each project listed in this section is distributed under the MIT License:
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/packages/@rescript/linux-x64/bin.d.ts b/packages/@rescript/linux-x64/bin.d.ts
index f6fa8daaca5..d6e39948452 100644
--- a/packages/@rescript/linux-x64/bin.d.ts
+++ b/packages/@rescript/linux-x64/bin.d.ts
@@ -7,4 +7,5 @@ export type BinaryPaths = {
   rescript_tools_exe: string;
   rescript_editor_analysis_exe: string;
   rescript_exe: string;
+  rescript_rust_exe: string;
 };
diff --git a/packages/@rescript/linux-x64/bin.js b/packages/@rescript/linux-x64/bin.js
index aff7c9c9d93..290e8d7a241 100644
--- a/packages/@rescript/linux-x64/bin.js
+++ b/packages/@rescript/linux-x64/bin.js
@@ -12,4 +12,5 @@ export const binPaths = {
     "rescript-editor-analysis.exe",
   ),
   rescript_exe: path.join(binDir, "rescript.exe"),
+  rescript_rust_exe: path.join(binDir, "rescript-rust.exe"),
 };
diff --git a/packages/@rescript/linux-x64/package.json b/packages/@rescript/linux-x64/package.json
index 7ad6910a236..b1b00446385 100644
--- a/packages/@rescript/linux-x64/package.json
+++ b/packages/@rescript/linux-x64/package.json
@@ -35,7 +35,8 @@
       "./bin/bsc.exe",
       "./bin/rescript-editor-analysis.exe",
       "./bin/rescript-tools.exe",
-      "./bin/rescript.exe"
+      "./bin/rescript.exe",
+      "./bin/rescript-rust.exe"
     ]
   },
   "engines": {
@@ -50,6 +51,8 @@
   "files": [
     "bin.js",
     "bin.d.ts",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "RE_LICENSE.md",
     "bin/"
   ],
   "exports": "./bin.js",
diff --git a/packages/@rescript/win32-x64/RE_LICENSE.md b/packages/@rescript/win32-x64/RE_LICENSE.md
new file mode 100644
index 00000000000..f79ddd2b654
--- /dev/null
+++ b/packages/@rescript/win32-x64/RE_LICENSE.md
@@ -0,0 +1,521 @@
+This Software is distributed under the terms of the GNU Lesser
+General Public License version 2.1 (included below), or (at your
+option) any later version.
+
+As a special exception to the GNU Library General Public License, you
+may link, statically or dynamically, a "work that uses the Library"
+with a publicly distributed version of the Library to produce an
+executable file containing portions of the Library, and distribute
+that executable file under terms of your choice, without any of the
+additional requirements listed in clause 6 of the GNU Library General
+Public License.  By "a publicly distributed version of the Library",
+we mean either the unmodified Library, or a modified version of the
+Library that is distributed under the conditions defined in clause 3
+of the GNU Library General Public License.  This exception does not
+however invalidate any other reasons why the executable file might be
+covered by the GNU Library General Public License.
+
+----------------------------------------------------------------------
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+		       Version 2.1, February 1999
+
+ Copyright (C) 1991, 1999 Free Software Foundation, Inc.
+     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+[This is the first released version of the Lesser GPL.  It also counts
+ as the successor of the GNU Library Public License, version 2, hence
+ the version number 2.1.]
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+Licenses are intended to guarantee your freedom to share and change
+free software--to make sure the software is free for all its users.
+
+  This license, the Lesser General Public License, applies to some
+specially designated software packages--typically libraries--of the
+Free Software Foundation and other authors who decide to use it.  You
+can use it too, but we suggest you first think carefully about whether
+this license or the ordinary General Public License is the better
+strategy to use in any particular case, based on the explanations below.
+
+  When we speak of free software, we are referring to freedom of use,
+not price.  Our General Public Licenses are designed to make sure that
+you have the freedom to distribute copies of free software (and charge
+for this service if you wish); that you receive source code or can get
+it if you want it; that you can change the software and use pieces of
+it in new free programs; and that you are informed that you can do
+these things.
+
+  To protect your rights, we need to make restrictions that forbid
+distributors to deny you these rights or to ask you to surrender these
+rights.  These restrictions translate to certain responsibilities for
+you if you distribute copies of the library or if you modify it.
+
+  For example, if you distribute copies of the library, whether gratis
+or for a fee, you must give the recipients all the rights that we gave
+you.  You must make sure that they, too, receive or can get the source
+code.  If you link other code with the library, you must provide
+complete object files to the recipients, so that they can relink them
+with the library after making changes to the library and recompiling
+it.  And you must show them these terms so they know their rights.
+
+  We protect your rights with a two-step method: (1) we copyright the
+library, and (2) we offer you this license, which gives you legal
+permission to copy, distribute and/or modify the library.
+
+  To protect each distributor, we want to make it very clear that
+there is no warranty for the free library.  Also, if the library is
+modified by someone else and passed on, the recipients should know
+that what they have is not the original version, so that the original
+author's reputation will not be affected by problems that might be
+introduced by others.
+
+  Finally, software patents pose a constant threat to the existence of
+any free program.  We wish to make sure that a company cannot
+effectively restrict the users of a free program by obtaining a
+restrictive license from a patent holder.  Therefore, we insist that
+any patent license obtained for a version of the library must be
+consistent with the full freedom of use specified in this license.
+
+  Most GNU software, including some libraries, is covered by the
+ordinary GNU General Public License.  This license, the GNU Lesser
+General Public License, applies to certain designated libraries, and
+is quite different from the ordinary General Public License.  We use
+this license for certain libraries in order to permit linking those
+libraries into non-free programs.
+
+  When a program is linked with a library, whether statically or using
+a shared library, the combination of the two is legally speaking a
+combined work, a derivative of the original library.  The ordinary
+General Public License therefore permits such linking only if the
+entire combination fits its criteria of freedom.  The Lesser General
+Public License permits more lax criteria for linking other code with
+the library.
+
+  We call this license the "Lesser" General Public License because it
+does Less to protect the user's freedom than the ordinary General
+Public License.  It also provides other free software developers Less
+of an advantage over competing non-free programs.  These disadvantages
+are the reason we use the ordinary General Public License for many
+libraries.  However, the Lesser license provides advantages in certain
+special circumstances.
+
+  For example, on rare occasions, there may be a special need to
+encourage the widest possible use of a certain library, so that it becomes
+a de-facto standard.  To achieve this, non-free programs must be
+allowed to use the library.  A more frequent case is that a free
+library does the same job as widely used non-free libraries.  In this
+case, there is little to gain by limiting the free library to free
+software only, so we use the Lesser General Public License.
+
+  In other cases, permission to use a particular library in non-free
+programs enables a greater number of people to use a large body of
+free software.  For example, permission to use the GNU C Library in
+non-free programs enables many more people to use the whole GNU
+operating system, as well as its variant, the GNU/Linux operating
+system.
+
+  Although the Lesser General Public License is Less protective of the
+users' freedom, it does ensure that the user of a program that is
+linked with the Library has the freedom and the wherewithal to run
+that program using a modified version of the Library.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.  Pay close attention to the difference between a
+"work based on the library" and a "work that uses the library".  The
+former contains code derived from the library, whereas the latter must
+be combined with the library in order to run.
+
+		  GNU LESSER GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License Agreement applies to any software library or other
+program which contains a notice placed by the copyright holder or
+other authorized party saying it may be distributed under the terms of
+this Lesser General Public License (also called "this License").
+Each licensee is addressed as "you".
+
+  A "library" means a collection of software functions and/or data
+prepared so as to be conveniently linked with application programs
+(which use some of those functions and data) to form executables.
+
+  The "Library", below, refers to any such software library or work
+which has been distributed under these terms.  A "work based on the
+Library" means either the Library or any derivative work under
+copyright law: that is to say, a work containing the Library or a
+portion of it, either verbatim or with modifications and/or translated
+straightforwardly into another language.  (Hereinafter, translation is
+included without limitation in the term "modification".)
+
+  "Source code" for a work means the preferred form of the work for
+making modifications to it.  For a library, complete source code means
+all the source code for all modules it contains, plus any associated
+interface definition files, plus the scripts used to control compilation
+and installation of the library.
+
+  Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running a program using the Library is not restricted, and output from
+such a program is covered only if its contents constitute a work based
+on the Library (independent of the use of the Library in a tool for
+writing it).  Whether that is true depends on what the Library does
+and what the program that uses the Library does.
+
+  1. You may copy and distribute verbatim copies of the Library's
+complete source code as you receive it, in any medium, provided that
+you conspicuously and appropriately publish on each copy an
+appropriate copyright notice and disclaimer of warranty; keep intact
+all the notices that refer to this License and to the absence of any
+warranty; and distribute a copy of this License along with the
+Library.
+
+  You may charge a fee for the physical act of transferring a copy,
+and you may at your option offer warranty protection in exchange for a
+fee.
+
+  2. You may modify your copy or copies of the Library or any portion
+of it, thus forming a work based on the Library, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) The modified work must itself be a software library.
+
+    b) You must cause the files modified to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    c) You must cause the whole of the work to be licensed at no
+    charge to all third parties under the terms of this License.
+
+    d) If a facility in the modified Library refers to a function or a
+    table of data to be supplied by an application program that uses
+    the facility, other than as an argument passed when the facility
+    is invoked, then you must make a good faith effort to ensure that,
+    in the event an application does not supply such function or
+    table, the facility still operates, and performs whatever part of
+    its purpose remains meaningful.
+
+    (For example, a function in a library to compute square roots has
+    a purpose that is entirely well-defined independent of the
+    application.  Therefore, Subsection 2d requires that any
+    application-supplied function or table used by this function must
+    be optional: if the application does not supply it, the square
+    root function must still compute square roots.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Library,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Library, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote
+it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Library.
+
+In addition, mere aggregation of another work not based on the Library
+with the Library (or with a work based on the Library) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may opt to apply the terms of the ordinary GNU General Public
+License instead of this License to a given copy of the Library.  To do
+this, you must alter all the notices that refer to this License, so
+that they refer to the ordinary GNU General Public License, version 2,
+instead of to this License.  (If a newer version than version 2 of the
+ordinary GNU General Public License has appeared, then you can specify
+that version instead if you wish.)  Do not make any other change in
+these notices.
+
+  Once this change is made in a given copy, it is irreversible for
+that copy, so the ordinary GNU General Public License applies to all
+subsequent copies and derivative works made from that copy.
+
+  This option is useful when you wish to copy part of the code of
+the Library into a program that is not a library.
+
+  4. You may copy and distribute the Library (or a portion or
+derivative of it, under Section 2) in object code or executable form
+under the terms of Sections 1 and 2 above provided that you accompany
+it with the complete corresponding machine-readable source code, which
+must be distributed under the terms of Sections 1 and 2 above on a
+medium customarily used for software interchange.
+
+  If distribution of object code is made by offering access to copy
+from a designated place, then offering equivalent access to copy the
+source code from the same place satisfies the requirement to
+distribute the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  5. A program that contains no derivative of any portion of the
+Library, but is designed to work with the Library by being compiled or
+linked with it, is called a "work that uses the Library".  Such a
+work, in isolation, is not a derivative work of the Library, and
+therefore falls outside the scope of this License.
+
+  However, linking a "work that uses the Library" with the Library
+creates an executable that is a derivative of the Library (because it
+contains portions of the Library), rather than a "work that uses the
+library".  The executable is therefore covered by this License.
+Section 6 states terms for distribution of such executables.
+
+  When a "work that uses the Library" uses material from a header file
+that is part of the Library, the object code for the work may be a
+derivative work of the Library even though the source code is not.
+Whether this is true is especially significant if the work can be
+linked without the Library, or if the work is itself a library.  The
+threshold for this to be true is not precisely defined by law.
+
+  If such an object file uses only numerical parameters, data
+structure layouts and accessors, and small macros and small inline
+functions (ten lines or less in length), then the use of the object
+file is unrestricted, regardless of whether it is legally a derivative
+work.  (Executables containing this object code plus portions of the
+Library will still fall under Section 6.)
+
+  Otherwise, if the work is a derivative of the Library, you may
+distribute the object code for the work under the terms of Section 6.
+Any executables containing that work also fall under Section 6,
+whether or not they are linked directly with the Library itself.
+
+  6. As an exception to the Sections above, you may also combine or
+link a "work that uses the Library" with the Library to produce a
+work containing portions of the Library, and distribute that work
+under terms of your choice, provided that the terms permit
+modification of the work for the customer's own use and reverse
+engineering for debugging such modifications.
+
+  You must give prominent notice with each copy of the work that the
+Library is used in it and that the Library and its use are covered by
+this License.  You must supply a copy of this License.  If the work
+during execution displays copyright notices, you must include the
+copyright notice for the Library among them, as well as a reference
+directing the user to the copy of this License.  Also, you must do one
+of these things:
+
+    a) Accompany the work with the complete corresponding
+    machine-readable source code for the Library including whatever
+    changes were used in the work (which must be distributed under
+    Sections 1 and 2 above); and, if the work is an executable linked
+    with the Library, with the complete machine-readable "work that
+    uses the Library", as object code and/or source code, so that the
+    user can modify the Library and then relink to produce a modified
+    executable containing the modified Library.  (It is understood
+    that the user who changes the contents of definitions files in the
+    Library will not necessarily be able to recompile the application
+    to use the modified definitions.)
+
+    b) Use a suitable shared library mechanism for linking with the
+    Library.  A suitable mechanism is one that (1) uses at run time a
+    copy of the library already present on the user's computer system,
+    rather than copying library functions into the executable, and (2)
+    will operate properly with a modified version of the library, if
+    the user installs one, as long as the modified version is
+    interface-compatible with the version that the work was made with.
+
+    c) Accompany the work with a written offer, valid for at
+    least three years, to give the same user the materials
+    specified in Subsection 6a, above, for a charge no more
+    than the cost of performing this distribution.
+
+    d) If distribution of the work is made by offering access to copy
+    from a designated place, offer equivalent access to copy the above
+    specified materials from the same place.
+
+    e) Verify that the user has already received a copy of these
+    materials or that you have already sent this user a copy.
+
+  For an executable, the required form of the "work that uses the
+Library" must include any data and utility programs needed for
+reproducing the executable from it.  However, as a special exception,
+the materials to be distributed need not include anything that is
+normally distributed (in either source or binary form) with the major
+components (compiler, kernel, and so on) of the operating system on
+which the executable runs, unless that component itself accompanies
+the executable.
+
+  It may happen that this requirement contradicts the license
+restrictions of other proprietary libraries that do not normally
+accompany the operating system.  Such a contradiction means you cannot
+use both them and the Library together in an executable that you
+distribute.
+
+  7. You may place library facilities that are a work based on the
+Library side-by-side in a single library together with other library
+facilities not covered by this License, and distribute such a combined
+library, provided that the separate distribution of the work based on
+the Library and of the other library facilities is otherwise
+permitted, and provided that you do these two things:
+
+    a) Accompany the combined library with a copy of the same work
+    based on the Library, uncombined with any other library
+    facilities.  This must be distributed under the terms of the
+    Sections above.
+
+    b) Give prominent notice with the combined library of the fact
+    that part of it is a work based on the Library, and explaining
+    where to find the accompanying uncombined form of the same work.
+
+  8. You may not copy, modify, sublicense, link with, or distribute
+the Library except as expressly provided under this License.  Any
+attempt otherwise to copy, modify, sublicense, link with, or
+distribute the Library is void, and will automatically terminate your
+rights under this License.  However, parties who have received copies,
+or rights, from you under this License will not have their licenses
+terminated so long as such parties remain in full compliance.
+
+  9. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Library or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Library (or any work based on the
+Library), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Library or works based on it.
+
+  10. Each time you redistribute the Library (or any work based on the
+Library), the recipient automatically receives a license from the
+original licensor to copy, distribute, link with or modify the Library
+subject to these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties with
+this License.
+
+  11. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Library at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Library by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Library.
+
+If any portion of this section is held invalid or unenforceable under any
+particular circumstance, the balance of the section is intended to apply,
+and the section as a whole is intended to apply in other circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  12. If the distribution and/or use of the Library is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Library under this License may add
+an explicit geographical distribution limitation excluding those countries,
+so that distribution is permitted only in or among countries not thus
+excluded.  In such case, this License incorporates the limitation as if
+written in the body of this License.
+
+  13. The Free Software Foundation may publish revised and/or new
+versions of the Lesser General Public License from time to time.
+Such new versions will be similar in spirit to the present version,
+but may differ in detail to address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Library
+specifies a version number of this License which applies to it and
+"any later version", you have the option of following the terms and
+conditions either of that version or of any later version published by
+the Free Software Foundation.  If the Library does not specify a
+license version number, you may choose any version ever published by
+the Free Software Foundation.
+
+  14. If you wish to incorporate parts of the Library into other free
+programs whose distribution conditions are incompatible with these,
+write to the author to ask for permission.  For software which is
+copyrighted by the Free Software Foundation, write to the Free
+Software Foundation; we sometimes make exceptions for this.  Our
+decision will be guided by the two goals of preserving the free status
+of all derivatives of our free software and of promoting the sharing
+and reuse of software generally.
+
+			    NO WARRANTY
+
+  15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO
+WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
+EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR
+OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY
+KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE
+LIBRARY IS WITH YOU.  SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME
+THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
+WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY
+AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU
+FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
+CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
+LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
+RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
+FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
+SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+DAMAGES.
+
+		     END OF TERMS AND CONDITIONS
+
+           How to Apply These Terms to Your New Libraries
+
+  If you develop a new library, and you want it to be of the greatest
+possible use to the public, we recommend making it free software that
+everyone can redistribute and change.  You can do so by permitting
+redistribution under these terms (or, alternatively, under the terms of the
+ordinary General Public License).
+
+  To apply these terms, attach the following notices to the library.  It is
+safest to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least the
+"copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the library's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Lesser General Public
+    License as published by the Free Software Foundation; either
+    version 2 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+Also add information on how to contact you by electronic and paper mail.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the library, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the
+  library `Frob' (a library for tweaking knobs) written by James Random Hacker.
+
+  <signature of Ty Coon>, 1 April 1990
+  Ty Coon, President of Vice
+
+That's all there is to it!
diff --git a/packages/@rescript/win32-x64/THIRD_PARTY_NOTICES_REWATCH.md b/packages/@rescript/win32-x64/THIRD_PARTY_NOTICES_REWATCH.md
new file mode 100644
index 00000000000..9a2c787e47d
--- /dev/null
+++ b/packages/@rescript/win32-x64/THIRD_PARTY_NOTICES_REWATCH.md
@@ -0,0 +1,85 @@
+# Third-party notices for the OCaml ReScript build system
+
+The `rescript.exe` OCaml build-system binary includes the following
+third-party software. These notices supplement the package's declared license.
+
+## Cmdliner
+
+Copyright (c) 2011 The cmdliner programmers
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+## Yojson
+
+Copyright (c) 2010-2012, Martin Jambon
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+- Neither the name of the copyright holder nor the names of its contributors
+  may be used to endorse or promote products derived from this software
+  without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+## Re
+
+Copyright (C) 2001 Jerome Vouillon
+
+Re is distributed under the GNU Lesser General Public License version 2.1 or,
+at your option, any later version, with the OCaml linking exception. The full
+license and exception are included in `RE_LICENSE.md`.
+
+## Spawn, Luv, libuv, ctypes, and integers
+
+- Spawn: Copyright (c) 2016-2018 Jane Street Group, LLC
+- Luv: Copyright (c) 2018-2023 Anton Bachin
+- libuv: Copyright (c) 2015-present libuv project contributors
+- ctypes: Copyright (c) 2013 Jeremy Yallop
+- integers: Copyright (c) 2013-2016 Jeremy Yallop
+
+Each project listed in this section is distributed under the MIT License:
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/packages/@rescript/win32-x64/bin.d.ts b/packages/@rescript/win32-x64/bin.d.ts
index f6fa8daaca5..d6e39948452 100644
--- a/packages/@rescript/win32-x64/bin.d.ts
+++ b/packages/@rescript/win32-x64/bin.d.ts
@@ -7,4 +7,5 @@ export type BinaryPaths = {
   rescript_tools_exe: string;
   rescript_editor_analysis_exe: string;
   rescript_exe: string;
+  rescript_rust_exe: string;
 };
diff --git a/packages/@rescript/win32-x64/bin.js b/packages/@rescript/win32-x64/bin.js
index aff7c9c9d93..290e8d7a241 100644
--- a/packages/@rescript/win32-x64/bin.js
+++ b/packages/@rescript/win32-x64/bin.js
@@ -12,4 +12,5 @@ export const binPaths = {
     "rescript-editor-analysis.exe",
   ),
   rescript_exe: path.join(binDir, "rescript.exe"),
+  rescript_rust_exe: path.join(binDir, "rescript-rust.exe"),
 };
diff --git a/packages/@rescript/win32-x64/package.json b/packages/@rescript/win32-x64/package.json
index b17bd1462ee..dbf24b3ed99 100644
--- a/packages/@rescript/win32-x64/package.json
+++ b/packages/@rescript/win32-x64/package.json
@@ -35,7 +35,8 @@
       "./bin/bsc.exe",
       "./bin/rescript-editor-analysis.exe",
       "./bin/rescript-tools.exe",
-      "./bin/rescript.exe"
+      "./bin/rescript.exe",
+      "./bin/rescript-rust.exe"
     ]
   },
   "engines": {
@@ -45,11 +46,14 @@
     "win32"
   ],
   "cpu": [
-    "x64"
+    "x64",
+    "arm64"
   ],
   "files": [
     "bin.js",
     "bin.d.ts",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "RE_LICENSE.md",
     "bin/"
   ],
   "exports": "./bin.js",
diff --git a/packages/artifacts.json b/packages/artifacts.json
index 698863cfec4..68db4ca4a52 100644
--- a/packages/artifacts.json
+++ b/packages/artifacts.json
@@ -11,7 +11,9 @@
     "cli/common/args.js",
     "cli/common/bins.js",
     "cli/common/minisocket.js",
+    "cli/common/runBuildSystem.js",
     "cli/common/runtime.js",
+    "cli/rescript-rust.js",
     "cli/rescript-tools.js",
     "cli/rescript.js",
     "docs/docson/build-schema.json",
@@ -837,5 +839,70 @@
     "src/Belt_internalSetBuckets.resi",
     "src/Belt_internalSetInt.res",
     "src/Belt_internalSetString.res"
+  ],
+  "@rescript/darwin-arm64": [
+    "RE_LICENSE.md",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "bin.d.ts",
+    "bin.js",
+    "bin/.gitkeep",
+    "bin/bsc.exe",
+    "bin/rescript-editor-analysis.exe",
+    "bin/rescript-rust.exe",
+    "bin/rescript-tools.exe",
+    "bin/rescript.exe",
+    "package.json"
+  ],
+  "@rescript/darwin-x64": [
+    "RE_LICENSE.md",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "bin.d.ts",
+    "bin.js",
+    "bin/.gitkeep",
+    "bin/bsc.exe",
+    "bin/rescript-editor-analysis.exe",
+    "bin/rescript-rust.exe",
+    "bin/rescript-tools.exe",
+    "bin/rescript.exe",
+    "package.json"
+  ],
+  "@rescript/linux-arm64": [
+    "RE_LICENSE.md",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "bin.d.ts",
+    "bin.js",
+    "bin/.gitkeep",
+    "bin/bsc.exe",
+    "bin/rescript-editor-analysis.exe",
+    "bin/rescript-rust.exe",
+    "bin/rescript-tools.exe",
+    "bin/rescript.exe",
+    "package.json"
+  ],
+  "@rescript/linux-x64": [
+    "RE_LICENSE.md",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "bin.d.ts",
+    "bin.js",
+    "bin/.gitkeep",
+    "bin/bsc.exe",
+    "bin/rescript-editor-analysis.exe",
+    "bin/rescript-rust.exe",
+    "bin/rescript-tools.exe",
+    "bin/rescript.exe",
+    "package.json"
+  ],
+  "@rescript/win32-x64": [
+    "RE_LICENSE.md",
+    "THIRD_PARTY_NOTICES_REWATCH.md",
+    "bin.d.ts",
+    "bin.js",
+    "bin/.gitkeep",
+    "bin/bsc.exe",
+    "bin/rescript-editor-analysis.exe",
+    "bin/rescript-rust.exe",
+    "bin/rescript-tools.exe",
+    "bin/rescript.exe",
+    "package.json"
   ]
 }
\ No newline at end of file
diff --git a/rescript.opam b/rescript.opam
index ddd418b554e..9af3064583b 100644
--- a/rescript.opam
+++ b/rescript.opam
@@ -16,6 +16,10 @@ depends: [
   "wtf8"
   "ocamlformat" {with-test & = "0.29.0"}
   "yojson" {= "3.0.0"}
+  "spawn" {= "v0.17.0"}
+  "cmdliner" {= "2.1.1"}
+  "luv" {= "0.5.14"}
+  "re" {= "1.14.0"}
   "ounit2" {with-test & = "2.2.7"}
   "odoc" {with-doc}
   "ocaml-lsp-server" {with-dev-setup & >= "1.23.0"}
diff --git a/rewatch-ocaml/PARITY_CHECKLIST.md b/rewatch-ocaml/PARITY_CHECKLIST.md
new file mode 100644
index 00000000000..8beca399a20
--- /dev/null
+++ b/rewatch-ocaml/PARITY_CHECKLIST.md
@@ -0,0 +1,202 @@
+# Rewatch parity checklist
+
+This checklist complements the shared integration suite. A passing suite proves
+the scenarios it exercises; it does not by itself prove that every Rust guard,
+diagnostic, or interactive output path has an OCaml equivalent.
+
+## Architecture mapping gate
+
+The final port must provide a clear mapping from each material Rust
+responsibility, state value, algorithm, and lifecycle transition to its OCaml
+owner. In particular, package discovery, build and compile-asset state,
+dependency extraction and invalidation, parsing, compilation, cleanup, and the
+watcher lifecycle must be traceable across the two implementations. The OCaml
+implementation should perform equivalent work in the corresponding phase and
+consume already-computed state where Rust does, rather than repeatedly using
+the filesystem as an implicit database.
+
+This is not a requirement to reproduce Rust file sizes, function boundaries,
+or control-flow syntax mechanically. Idiomatic OCaml boundaries are preferred.
+A material deviation needs a concrete correctness, portability,
+maintainability, or simple-efficiency reason, and must be documented with its
+behavioral evidence and Windows implications. Deliberate Rust bug fixes remain
+permitted under the same rule.
+
+Rust's existing OpenTelemetry spans may be used to identify phase ownership,
+duration, and overlap while constructing this mapping. OTEL export remains an
+intentional non-goal for the OCaml executable, and instrumented timings are
+diagnostic rather than benchmark results. Filesystem-call parity is measured
+separately with the retained syscall-audit tooling.
+
+| Rust owner | Responsibility | OCaml owner | Mapping status |
+| --- | --- | --- | --- |
+| `config.rs` | Public configuration model, JSON decoding, validation, and derived arguments | `config_types.ml`, `config_decode.ml`, and `config.ml` | The public `Config` facade is preserved; types/error identity, decoding, and top-level loading/runtime queries have explicit owners |
+| `build.rs` | Command build lifecycle and phase orchestration | `build.ml`, `build_preparation.ml`, `package_build.ml`, `package_parse.ml`, `package_compilation.ml`, `build_report.ml`, and `build_attempt.ml` | Stable initialization, per-package parsing/compilation, presentation, and finalization have separate owners; `build.ml` retains the transaction and aggregate dispatch |
+| `project_context.rs` | Workspace classification, package lookup, locality, and path presentation | `project_context.ml` | Explicit owner now selects the ReScript-level workspace root, resolves the contextual package/current/workspace `node_modules` candidates plus standalone ancestor hoists, classifies canonical dependencies within the invocation's local scope, and presents project-relative paths; graph traversal retains the command-wide resolved-package cache |
+| `build/build_types.rs` | Packages, modules, dirty flags, dependency edges, and compiled-asset timestamps | `package_plan.ml`, `module_graph.ml`, `build_session.ml`, `build_attempt.ml`, `build_state.ml`, and `source.ml` | Immutable package inputs, graph identities, retained session state, per-attempt lifecycle, mutable module state, and source discovery have distinct lifetime owners |
+| `build/packages.rs` | Package resolution and source/module discovery | `package_graph.ml` over `project_context.ml`, `package_diagnostics.ml`, `package_plan.ml`, `config.ml`, and `source.ml` | The graph owner unifies dependency resolution, duplicate selection, feature requests, dependency permissions, local/dev policy, source discovery, compile configuration, and prepared-package ordering; source discovery retains mtimes, the full cleanup leaf inventory, and GenType directories; Unix traversal deduplicates directories by metadata identity while Windows uses canonical paths |
+| `build/read_compile_state.rs` | One compile-asset inventory for cleanup and freshness | `compile_assets.ml`, `build_freshness.ml`, `build_state.ml`, and `build_preparation.ml` | The shared inventory supplies CMI/CMT timestamps and published-AST source locations/mtimes; preparation performs cleanup and seeds build state from it, the freshness owner consumes those snapshots, and, like Rust, metadata is read only for AST/IAST/CMI/CMT state |
+| `build/compiler_info.rs` | Compiler/config fingerprints and package invalidation | `compiler_info.ml` | Compiler, runtime, package config, source-map arguments, effective root package-output specs, and artifact build owner invalidate affected packages; previous output specs drive precise stale-output removal, including installed source dependencies, while independently built package outputs remain owned by that package rather than an unrelated consumer |
+| `build/logs.rs` | Compiler-log lifecycle and ANSI-free persisted diagnostics | `compiler_log.ml` | Explicit owner initializes, appends, finalizes, strips terminal control sequences, and publishes each package log; warning selection remains with compilation as in Rust's `build/compile.rs` |
+| `build/clean.rs` | Stale and explicit artifact cleanup | `clean.ml` over `build_artifacts.ml` and `file_util.ml` | General filesystem operations have a narrow reusable owner; recursive command traversal and artifact ownership live in `clean.ml` and `build_artifacts.ml`; stale cleanup consumes shared compile-asset/source inventories and directly calculated working paths; consumer clean preserves independently owned dependency artifacts instead of mutating a published package, while consumer-built source dependencies remain recursive; a whole-tree `lib/bs` scan is retained only as a lazy malformed/legacy-artifact fallback |
+| `build/deps.rs` | Dependency extraction, edges, and invalidation | `module_graph.ml` over `graph.ml` and `build_state.ml` | AST dependency extraction, package visibility, global edges, state construction, and cycle handling have a cohesive owner; generic graph algorithms and mutable module state retain their reusable owners |
+| `build/parse.rs` | Parser arguments/jobs and parse-state transitions | `compiler_args.ml`, `compiler_process.ml`, `build_preparation.ml`, `package_parse.ml`, and `package_build.ml` | Phase-specific flags and source-dependent PPX policy have an explicit owner; external parser jobs and AST dependency decoding live with compiler processes; global preparation owns the preliminary parse, while package parsing owns per-package publication and retained parse-state transitions |
+| `build/compile.rs` | Compiler arguments, dirty propagation, scheduling, publication | `compiler_args.ml`, `package_compilation.ml`, `compiler_process.ml`, `compiler_scheduler.ml`, `build_state.ml`, `process.ml`, and `process_child.ml` | Package compilation owns dirty-set and job construction; child ownership is separate from the two scheduling policies; the main domain owns deterministic dependency admission while bounded domains own subprocess and publication lifetimes |
+| `watcher.rs` | Watch handles, batching, rebuild lifecycle, and recovery | `watcher.ml` over `watch_scope.ml`, `watch_snapshot.ml`, and `native_watcher.ml` | Scope discovery derives local-package roots and handles, snapshots preserve content baselines, and the lifecycle owner reconciles native wakeups, refreshes handles after configuration changes, preserves edits arriving during builds, falls back to polling, and owns signal/lock/handle cleanup; `native_watcher.ml` remains the narrow libuv boundary |
+| `lock.rs` | Build/watch ownership and stale-process handling | `build_lock.ml`, with process operations behind `platform.mli` | Build and watch locks share one owner for PID validation, stale-lock takeover, ownership checks, and release; the hard-link primitive still requires native Windows validation and may move behind the platform interface if necessary |
+| `telemetry.rs` | Optional OTLP export | No OCaml owner | Intentional project-level omission; Rust traces remain diagnostic tooling |
+
+## Validation inventory gate
+
+Before the port can replace Rust rewatch, inventory every user-reachable
+validation and sanity check in the pinned Rust implementation. Search at least
+these owners and record each check below (splitting rows as needed):
+
+- `cli.rs` and `project_context.rs`: argument shape, command context, project
+  discovery, missing folders, and configuration-file selection.
+- `config.rs`: JSON shape, deprecated/unsupported fields, feature maps,
+  package outputs, source directories, dependencies, warnings, JSX, GenType,
+  and post-build configuration.
+- `helpers.rs`, `lock.rs`, and `watcher.rs`: compiler/runtime discovery, path
+  and executable checks, lock ownership, watcher lifecycle, and event inputs.
+- `build.rs` and `build/*.rs`: package resolution, dependency permissions,
+  duplicate modules, cycles, namespaces, compiler subprocess failures, output
+  ownership, and cleanup safety.
+- `format.rs`: input modes, extensions, formatter lookup/failure, and check
+  status.
+
+For every Rust check, the final inventory must name its Rust source location,
+OCaml source location, and focused or canonical test. A missing check is an open
+gap. A deliberate difference needs a rationale and regression test recorded in
+this checklist or the implementation README; similar wording alone is not proof
+of equivalent behavior.
+
+| Validation area | Current evidence | Status |
+| --- | --- | --- |
+| Missing/non-project folder and config discovery | A differential command gate covers missing, config-less, and malformed build folders plus no-project compiler inputs; project-context tests distinguish listed parent dependencies from unrelated packages under a `package.json` workspace | Matched for `ProjectContext::new`, parent-config selection, and root/config discovery; command tests retain the selected configuration path while parser- and OS-native detail is allowed to differ |
+| Configuration schema and aliases | A 297-case differential gate covers every typed configuration family and exact arguments for shared accepted behavior; twelve documented Rust/OCaml divergences are explicit expectations rather than omitted cases | Matched for typed schema and argument projection, with deliberate stricter map-key validation and an ignored internal `path` key; Serde and Yojson parse locations/wording remain library-native |
+| Package/dependency graph | Canonical compile/feature cases and graph unit tests; the differential command gate covers missing, config-less, malformed, duplicate-path, metadata-name-mismatched, malformed-package-metadata, and omitted dependency-source cases across the commands that construct the graph; parse/compile/cleanup guards are inventoried below | Matched for graph construction, scheduling, reachable cleanup states, and stable package/path diagnostic context; implicit format now validates and scans the complete graph before selecting its local files, and exact warning checks retain first-path duplicate selection, omitted-`sources`, and missing-source-folder behavior |
+| Compiler/runtime/executable discovery | Packaged-layout test removes `RESCRIPT_BSC_EXE` and uses sibling `bsc.exe`; focused runtime test removes `RESCRIPT_RUNTIME` and resolves `@rescript/runtime`; differential command cases cover missing explicit compiler/runtime paths and an unavailable runtime package; platform path tests cover Windows verbatim drive and UNC paths; the native Windows package gate compares Dune-promoted executables byte for byte and dry-packs both rewatch implementations | Matched for discovery and environment precedence on Unix and native Windows; OCaml reports a stale explicit compiler normally instead of Rust's panic and rejects a stale explicit runtime before spawning compiler work instead of Rust's later `Pervasives` failures |
+| Locks and watcher lifecycle | Canonical lock/watch cases, focused atomic/stale-lock tests, a slow-child lock-removal integration case, scheduler cancellation unit coverage, and differential malformed build/watch lock cases that preserve unknown ownership; native Windows runs cover Job assignment, descendant-held-pipe cancellation, native-PID lock ownership/contention, lock-driven cancellation, and watcher execution/recovery | Matched for acquisition, active-owner refusal/waiting, valid stale-owner takeover, malformed-owner refusal, workspace scope, owned cleanup, and lock-driven cancellation of active child process trees. Unix retains process-group ownership; Windows creates each child suspended, assigns it to a retained Job Object before resuming it, and rejects launch if atomic ownership cannot be established. Normal completion only releases the job, while cancellation terminates the tree even after the direct wait handle is reaped. Signal handlers record atomic cross-domain requests; the scheduler owner turns them into fatal outcomes at a poll point, avoiding asynchronous unwinding in worker launch/publication. Windows serializes temporary process-wide handler replacement. MSYS cannot deliver a normal console-control event to the native child, so that harness uses lock removal to exercise the same cancellation owner; Unix retains direct signal-delivery coverage. |
+| Output ownership and cleanup | Canonical clean/suffix/removal cases, focused stale-artifact tests, and `clean_tests.ml` coverage that distinguishes configured outputs from neighboring unowned files in both the root and an installed dependency and cleans both colliding outputs without constructing a valid module graph; the fresh-build gate compares the complete Rust/OCaml file-name set and hashes every byte-stable artifact class; OUnit covers concurrent directory creators, strict close-error propagation, and same-directory atomic metadata replacement; the focused suite runs the lifecycle natively on Windows | Matched for explicit clean ownership, including invalid duplicate-module graphs; fresh outputs/control files; stale compiler/output cleanup; concurrent directory creators; and rejection of existing non-directories on Unix and native Windows. Obsolete `.rewatch-pending` and `.rewatch-backup` handling was removed because neither supported implementation creates those files. Generated JSON and namespace maps publish complete replacements, while formatted user sources retain their inode and therefore symlink, hard-link, ownership, ACL, and extended-attribute behavior |
+| CLI and format input validation | Dedicated Cmdliner tests mirror Rust CLI cases, including required/surplus `compiler-args` paths; compiler-args tests cover extension, dependency selection, and missing-package behavior; focused format failures cover stdin labels, check summaries, and contextual write errors; differential cases exactly compare explicit missing-file, directory, and unsupported-extension failures, retain the stale-compiler Rust panic fix, semantically compare missing/malformed/directory-valued implicit project configs and invalid stdin, and exercise permission-denied write-back where supported; canonical format/compiler-args cases cover success | Matched or documented across CLI shape, formatter/compiler discovery, explicit-file failures, implicit project discovery, stdin, check status, and write-back; OS errors, temporary names, and malformed JSON retain platform/library-native detail after common command/path context |
+
+No row becomes complete until the Rust source inventory has been performed,
+not merely because the current tests pass.
+
+### Configuration inventory
+
+Symbols below are stable source locations; line numbers are intentionally
+omitted because the Rust and OCaml files are still changing.
+
+| Behavior | Rust location | OCaml location | Evidence | Status |
+| --- | --- | --- | --- | --- |
+| File read, JSON root, required `name`, internal `path`, legacy filename, optional JSON `null`, and duplicate keys | `config.rs`: `Config::new`, `Config::new_from_json_string`, `Config::set_path`; Serde struct/`Option` fields | `config.ml`: `load`, `load_root`; `config_decode.ml`: optional members and duplicate rejection | Unit tests cover missing/directory paths without raw exceptions and verify that the filename read remains authoritative; focused missing-project test; differential audits cover root/name/path shapes, 41 `null` positions, and representative duplicate keys; command cases retain missing, malformed, parent, and directory-valued config paths | Deliberate difference: OCaml treats the internal JSON `path` key as unknown and ignores any value instead of type-checking a value that is immediately overwritten; Serde/Yojson parse locations and OS error detail remain library/platform-native after shared path context |
+| Source forms, `dir`, `subdirs`, `type`, feature inheritance | `config.rs`: `Source`, `PackageSource`; `build/packages.rs`: `get_source_dirs`, `make_package` | `config_decode.ml`: structured source decoding; `config.ml`: `source_is_dev`; `config_types.ml`: source model and `sources_defined`; `package_diagnostics.ml`: `report_missing_sources` | A retained 36-case Rust/OCaml differential gate covers accepted and rejected outer, qualified, nested, nullable, unknown, and duplicate shapes and compares arguments for accepted cases; unit and canonical tests cover flattening and inheritance; the command gate exactly compares the omitted-`sources` dependency warning for build, clean, and format | Matched for the complete schema, field-presence warning, and flattening inventory; an explicit empty list remains distinct from an omitted field |
+| Package module, suffix/location defaults and duplicate outputs | `config.rs`: `PackageSpec`, `validate_package_specs_value` | `config_decode.ml`: package-spec decoding; `config.ml`: duplicate-output validation and suffix helpers | 28 differential schema/argument cases plus unit and canonical suffix tests | Matched for the complete schema and output-conflict inventory |
+| Dependency forms, aliases, feature maps and cycles | `config.rs`: `Dependency`, `resolve_active_features`, `compute_active_features`; package traversal | `config_decode.ml`: dependency and alias decoding; `source.ml`: `resolve_active_features`; `package_graph.ml` and `format.ml`: consumer-request aggregation | The differential gate adds 42 dependency, feature-map, alias, and `allowed-dependents` shapes; Rust/OCaml unit and canonical tests cover feature resolution, cycles, permissions, and traversal; command cases distinguish an unrestricted irrelevant cycle from a specifically requested cycle during build and format | Feature algorithms match: all-features selection does not traverse implication edges, while restricted requests are unioned and transitively validated. Deliberate difference: OCaml rejects duplicate feature-map keys rather than silently keeping one value. Resolution failures are covered by the package-discovery row below |
+| Compiler, warning, and PPX flags | `config.rs`: `Warnings`, `flatten_flags`, `flatten_ppx_flags`, `get_warning_args`; `build/parse.rs`: `filter_ppx_flags`; `build/compile.rs`: `compiler_args` | `config_decode.ml` and `config.ml`: field/top-level flag decoding; `compiler_args.ml`: PPX filtering and phase-ordered flags | 37 differential cases cover valid and invalid shapes plus exact shared argument projection; explicit divergence cases retain whitespace normalization and the empty-PPX panic fix; exact unit argument-order/filter tests and canonical builds cover execution | Matched, with documented safety fixes |
+| Namespace and namespace entry | `config.rs`: `NamespaceConfig`, `get_namespace`, `get_namespace_entry`; namespace argument helpers | `config_decode.ml`: namespace normalization; `config.ml`: top-level namespace/entry validation; `compiler_args.ml`: `namespace_args`; `build_preparation.ml`: visible namespace validation | 12 differential cases cover boolean/string normalization, scoped names, entries, nulls, and invalid kinds; canonical namespace builds cover artifacts; focused coverage rejects two directly visible packages that publish the same compiler namespace | Matched, with documented rejection of an entry when namespace is disabled; deliberate safety fix rejects ambiguous visible namespace artifacts before compilation |
+| JSX and source maps | `config.rs`: `JsxSpecs`, `SourceMapConfig`, argument getters | `config.ml`: JSX and `sourceMap` branches in `load` | 53 differential schema/argument cases cover all fields, modes, nulls, JSON kinds, unknowns, and duplicate keys; all rejected cases retain the owning `jsx` or `sourceMap` context; unit and canonical build tests cover execution | Deliberate difference: OCaml rejects duplicate keys in the interpreted `sourceMap` object instead of inheriting raw JSON map last-value behavior; schema, arguments, and contextual rejection otherwise match |
+| GenType schema, arguments, and project-root paths | `config.rs`: `GenTypeConfig`, `GenTypeShims`, `get_gentype_args`; `build/packages.rs`: `collect_gentype_source_dirs`; `build/parse.rs`: parse working directory | `config_decode.ml`: GenType decoding/arguments; `source.ml`: `discovery.gentype_dirs`; `package_graph.ml`: compile-config projection; `config.ml` and `compiler_process.ml`: canonical root and job paths | 53 differential cases cover every field, enum, JSON kind, nullable option, duplicate typed field, shim representation, sorting, package fallback, sources, and dependencies; unit tests cover recursive directory discovery, feature/dev-source selection, and canonical config roots; canonical tests cover execution on Unix and native Windows | Deliberate difference: OCaml rejects duplicate shim sources and debug keys rather than silently keeping the last value. Config loading canonicalizes the package root once; parse and compile paths derive from that root, and `-bs-project-root` receives the identical representation. |
+| Post-build command | `config.rs`: `JsPostBuild`; `build/compile.rs` execution | `config.ml`: `js_post_build`; `compiler_process.ml`: post-build execution | 10 differential schema cases plus canonical and focused execution tests on Unix and native Windows | Matched for schema and execution, including Windows `cmd.exe` construction, success, nonzero status, retry, and streamed output |
+| Deprecated, unsupported, and unknown fields | `config.rs`: all five Serde aliases, `get_unknown_fields`, `get_unsupported_fields` | `config_decode.ml`: aliases and unknown-field inventory; `config.ml`: top-level diagnostics | Unit/focused tests cover `bs-dependencies`, `bs-dev-dependencies`, `bsc-flags`, `cjs`, and `es6`, Rust's nested warning boundary, and ignored unsupported payloads | Alias and diagnostic behavior match except for the internal JSON `path` key, which OCaml deliberately treats as unknown; opaque `editor`, `reanalyze`, and unknown payloads remain outside recursive validation |
+
+### CLI, project context, and format inventory
+
+| Behavior | Rust location | OCaml location | Evidence | Status |
+| --- | --- | --- | --- | --- |
+| Implicit `build`, global flag placement, `--`, known commands, help, and version | `cli.rs`: `parse_with_default_from`, `should_default_to_build`, `build_default_args`; Clap command declaration | `cli.ml`: `normalize_argv`; Cmdliner command group | `cli_tests.ml` covers implicit/explicit routing, leading/trailing globals, short and long help/version, clusters, and `--`; the command gate records subcommand-version behavior as an explicit difference | Implicit build and global option placement match; Cmdliner provides consistent help/version precedence and native help layout |
+| Build/watch/clean option ownership and values | `cli.rs`: `BuildArgs`, `WatchArgs`, `Command`; feature and regex value parsers | `cli.ml`: `build_term`, `clean_term`, feature and filter converters | `cli_tests.ml` covers command-only rejection, the `--no-timing` flag, production mode, feature trimming/emptiness, invalid regex, watch clear-screen, retained clean verbosity, and mutually exclusive verbose/quiet modes; the command gate records the simpler OCaml flag and display-option semantics as explicit differences | Deliberate difference: `--no-timing` is a non-consuming flag, help always takes precedence over version, and version is accepted after a subcommand |
+| Source filter matching | `build/packages.rs`: `matches_filter`, `read_folders` | `source_filter.ml`: typed validation/compiled matcher; `source.ml`: `discover_with_inventory`; `cli.ml`: filter conversion | Differential builds prove that a directory-only regex excludes a nested source while basename expressions include it, including a Rust-style non-capturing alternation with `\d`; focused source/CLI tests retain basename boundaries, alternation, groups, shorthand classes, repetition, known Re-only forms, ranges, class semantics, and invalid input | Basename and positive-match semantics plus common shared syntax are matched through maintained pure-OCaml `re`. Some valid Rust syntax remains visibly unsupported, including Unicode properties and inline modes, Python-style named groups, possessive quantifiers, and class-set algebra. Known Re-only quoting, comment, anchor, control/octal escape, range, nested/collating class, and divergent in-class escape forms are rejected before compilation to avoid silently different selection. Because matching is byte-oriented, shared patterns involving non-ASCII basenames can differ, including dot, hexadecimal escapes, shorthand classes, and literal classes. Exact Rust-regex semantics are explicitly not required by project decision; the documented subset keeps a linear-time native-Windows matcher without another native dependency. |
+| Watch source selection | `build/packages.rs`: feature and positive `matches_filter` discovery; `watcher.rs`: event filtering | `source.ml` and `package_graph.ml`: active discovery; `watcher.ml`: feature-aware native registrations and scoped content snapshots | Differential watch cases edit included, filter-excluded, and feature-disabled sources and inspect generated output and hook counts; a delayed compiler gate activates a source directory, creates a file after discovery, and requires exactly one queued follow-up build; an external source-symlink target is atomically replaced, removed, and recreated | Deliberate Rust bug fix: OCaml retains the CLI's positive regex meaning across initial discovery and every rebuild instead of negating it only for watch events. Feature-disabled and filter-excluded edits do not perform a no-op rebuild. Snapshots read only package control files and active configured source trees rather than recursively walking unrelated package-root contents. Target-parent watches keep regular source symlinks observable outside those trees. |
+| Watch state lifetime and dependency edits | `watcher.rs`: retained `BuildState`; `build.rs`: `build_incremental`; `build/deps.rs`: dependency extraction and reverse-edge updates | `native_watcher.ml`: identity-bearing handles and event paths/kinds; `watcher.ml`: pre-build content baselines and structural reconciliation; `build.ml`: retained source index and incremental preparation; `build_preparation.ml` and `build_state.ml`: resolved and reverse edges | Focused long-lived watchers cover dependency-edge replacement, cycles and recovery, atomic replacement during active compilation, directory-handle replacement, polling fallback, and the same native Windows event lifecycle; unit coverage verifies obsolete reverse-edge removal and duplicate prevention; retained performance and filesystem gates verify exact compiler work and resource stability | Existing-file edits retain initialized package, compiler, artifact, cleanup, source-index, and module state and parse only affected source paths. Precise content events capture their baseline before building; edits arriving during compilation remain distinguishable. Structural/ambiguous events reconstruct state and refresh handles whose filesystem identity changed. Changed dependency headers replace graph edges and rerun cycle detection in memory. Deliberate low-risk Rust inefficiency fix: indexed source lookup and obsolete reverse-edge removal avoid Rust's full module scan and accumulated reverse edges. |
+| Format input mode and explicit files | `cli.rs`: `Command::Format`, `FileExtension`, Clap `format_input_mode`; `format.rs`: `format_files` | `cli.ml`: `format_term`; `format.ml`: `format_files_with_bsc` | `cli_tests.ml` covers `.res`/`.resi`, invalid stdin extensions, stdin/file conflicts, and stdin/check conflicts in either argument order; differential cases exactly compare missing-file, directory, and unsupported-extension diagnostics | Matched, including errors delegated to the formatter for explicit operands |
+| `compiler-args` positional and filesystem input | `cli.rs`: `Command::CompilerArgs`; `build.rs`: `get_compiler_args`; `helpers.rs`: `read_file`; `build/compile.rs`: dependency arguments | `cli.ml`: `compiler_args_term`; `compiler_args_command.ml`: project/source resolution and JSON projection; `compiler_args.ml`: shared argument policy | `cli_tests.ml` covers missing and surplus paths; the differential command gate covers valid, non-ReScript, missing, and no-project sources; `compiler_args_tests.ml` covers dev/regular dependencies and context | Matched where Rust validates, with documented extension validation and three non-panicking OCaml fixes |
+| After-build hook execution | `main.rs`: successful-build hook dispatch; `cmd.rs`: `run` | `after_build.ml`: command execution; `build.ml`: post-success dispatch | Canonical/focused integration covers a successful hook; three differential command cases cover an empty command, a missing program, and a program exiting 7 with captured stderr | Deliberate safety fixes: OCaml reports empty and unlaunchable hooks normally rather than panicking, and makes a nonzero hook fail the command instead of discarding its status; all commands are still split on whitespace and launched outside the build lock like Rust. Windows inherits interactive stdin while retaining Job Object ownership. Unix terminal stdin remains closed because Spawn cannot both give the child a new, cancellable process group and atomically foreground it; redirected stdin is inherited. Full Unix interactive-hook stdin needs a tested PTY relay rather than risking `SIGTTIN` or weakening descendant cancellation. |
+| Per-output JS post-build hook | `build/compile.rs`: `execute_post_build_command` and `compile_file` | `compiler_process.ml`: `run_post_build`; `platform.mli` command construction | Focused integration checks the generated-file argument for a successful hook; the differential command gate makes the shell command exit 7 and requires both diagnostics to identify the generated JavaScript path; both run natively on Windows | Matched for invocation timing, working directory, output argument, failure status, and path-bearing diagnostics, including native `cmd.exe` execution |
+| Missing folder, missing config, and parent-config discovery failures | `lock.rs`: `get_lock`; `project_context.rs`: `ProjectContext::new`; `build/packages.rs`: `read_config` | `build.ml`: `project_root`; `build_lock.ml`: lock acquisition; `project_context.ml`: `workspace_lock_root`; `config.ml`: `load_root` | The differential command gate covers nonexistent, config-less, malformed, directory-config, and malformed-parent project paths; the focused runner exactly checks missing-folder wording; configuration tests cover direct file-read failures | Matched for project/config discovery outcomes and selected-path context; JSON-parser and OS-error tails remain implementation-native |
+| Monorepo root/package classification | `project_context.rs`: `read_local_packages`, `is_config_listed_in_workspace`, `monorepo_or_single_project`, `get_scoped_local_packages`; `build/packages.rs`: `is_local_dependency` | `project_context.ml`: workspace classification, invocation-scoped locality, and lock root; shared by package traversal, preparation, clean, format, and watch | `project_context_tests.ml` covers listed regular/dev packages, root-versus-child locality, and an unlisted package beneath a workspace; focused integration builds a listed package whose sibling has an unresolved development dependency and inspects its source-directory metadata; canonical monorepo builds cover root-level symlinked traversal | Matched: workspace-root commands own linked workspace packages, while a command run directly from a listed package resolves siblings without including their development graph, cleanup policy, formatting scope, source scans, or watcher roots |
+| Dependency package resolution | `helpers.rs`: `try_package_path`; `build/packages.rs`: `read_dependency`, `read_dependencies` | `project_context.ml`: contextual candidates; `package_resolution.ml`: shared selection, locality, config, and duplicate policy; build, clean, format, and watch consumers | The differential command gate covers missing paths, existing packages without config, malformed dependency config for build and clean, watch startup, and duplicate selection; focused coverage models local packages through `node_modules` links and requires an unlinked `packages/dep` to remain unresolved and untouched by clean | Matched candidate priority, first-package selection, locality, and standalone-only ancestor traversal. Watch retains a recoverable unresolved/config-error boundary; canonicalization, OS, and JSON-parser diagnostic tails remain implementation-native |
+| Dependency permission enforcement | `build/packages.rs`: `get_unallowed_dependents`, `validate_packages_dependencies` | `package_graph.ml`: active-edge validation during discovery | Focused OUnit coverage rejects traversed regular and development dependencies; one differential case makes an installed package declare a dormant dev edge to another root dependency, while another gives a root two actively denied regular dependencies and inspects both implementations' complete output | Matched for every traversed regular/local-dev edge; deliberate Rust bug fixes: dormant installed dev edges are ignored, every denied edge is reported instead of only the first per dependency class, details remain on stderr, and guidance names `allowed-dependents` in `rescript.json` rather than obsolete identifiers |
+| Duplicate dependency path selection | `build/packages.rs`: `read_dependencies` registered-dependency branch | `package_graph.ml`: command-wide resolved-package cache | The differential command gate places one dependency both at the root and below another package, requires both builds to succeed, and requires the duplicate warning from each implementation | Matched: the first path selected for a requested dependency name is retained throughout the graph; later paths warn and reuse it rather than inserting duplicate modules |
+| Package metadata name | `build/packages.rs`: `read_package_name`, `make_package` | `package_metadata.ml`: `package_name`; `package_diagnostics.ml`: `package_identity` | Differential cases compare the root mismatch warning exactly, reject malformed `package.json`, and retain the dependency mismatch Rust panic as an explicit port fix; unit tests cover last-key and non-string name behavior | Matched validation and warning behavior; OCaml uses the `package.json` identity consistently and rejects a dependency requested through a conflicting name instead of reaching Rust's later panic |
+| Source module/interface identity | `build/packages.rs`: `parse_packages` implementation/interface branches | `source.ml`: `discover`, `duplicate_error`, `interface_mismatch_error` | Canonical duplicate-module and orphan-interface snapshots; the differential command gate covers a basename-case mismatch; `source_tests.ml` also covers duplicate implementations and cross-directory mismatches | Matched: implementation and interface paths must agree exactly before `.res`/`.resi`; module-name collisions remain deterministic errors and orphan interfaces are skipped with a diagnostic |
+| Namespace map membership | `helpers.rs`: `is_non_exotic_module_name`; `build/packages.rs`: namespace `depending_modules`/`deps` construction | `source.ml`: `is_non_exotic_module_name`; `compiler_process.ml`: `namespace_task` | A differential namespace build compares the complete generated `.mlmap` with ordinary and punctuated source names; `source_tests.ml` covers ordinary, punctuated, and empty predicate inputs | Matched: only ASCII module identifiers beginning with an uppercase letter enter the namespace map; exotic source files may still compile but are not exported through the namespace |
+| Development source locality | `build/packages.rs`: `get_source_files`, `extend_with_children` (`package.is_local_dep && !prod`) | `package_traversal.ml`: `source_discovery_prod`; `package_graph.ml`: build discovery; `clean.ml`: clean traversal; `format.ml`: format discovery | The differential command gate builds through an installed dependency containing a deliberately invalid dev-only source; focused unit coverage retains all local/production combinations; canonical dev-dependency and production builds exercise local packages | Matched: installed dependencies never contribute `type: "dev"` source folders, while local packages contribute them outside `--prod` |
+| Missing or non-directory source folders | `build/packages.rs`: `get_source_files` | `source.ml`: `scan_dir`; `package_diagnostics.ml`: missing-source reporting; `package_graph.ml` and `format.ml`: complete graph scans | Exact differential command cases cover an active missing folder in an installed dependency during build and implicit format plus a configured regular file; canonical watch recovery covers a missing local folder that is later created | Matched: unavailable active source directories are diagnosed with folder/package/root context but remain non-fatal; excluded dev/feature folders are not scanned |
+| Parse execution and graph invariants | `build/parse.rs`: `generate_asts`, `generate_ast`; `build/deps.rs`: `get_dep_modules` | `compiler_process.ml`: parse jobs and AST dependencies; `build_preparation.ml`: preliminary global parse and graph construction; `package_build.ml`: per-package parse publication and transitions | Canonical syntax-error, warning-persistence, rename, and deletion cases plus the exact compiler-work gate cover normal failures and state transitions; an isolated differential syntax-error build exactly compares normalized stdout/stderr and phase classification; differential compiler wrappers remove discovered sources before later parse work and a successfully generated AST before dependency extraction; focused namespace coverage combines an ordinary dependency, an own-namespace-qualified reference, and a reverse dependent; package/module/namespace lookups are graph-construction invariants in both implementations | Matched for compiler outcomes and internal state, including ignoring the AST's imprecise own-namespace marker instead of expanding it into false edges; OCaml reports both disappearing-source and missing-AST races normally while Rust panics |
+| Compile execution and dependency scheduling | `build/compile.rs`: scheduler, `dependency_cycle`, `compiler_args`, `compile_file`, dirty propagation | `build_preparation.ml`: global graph; `package_build.ml`: dirty-set and job preparation; `build.ml`: aggregate dispatch; `compiler_process.ml`: jobs/publication; `compiler_scheduler.ml`: scheduled compilation; `build_state.ml`; `graph.ml`: shortest-cycle selection | Canonical cycle, missing-module, interface, warning, namespace, feature, and incremental-watch cases; exact clean/unchanged/edit work manifests; fresh-tree artifact equivalence; focused `-bs-no-bin-annot` build; graph coverage requires the shortest of two disjoint cycles | Matched for user-reachable compiler and scheduler outcomes, including optional CMT/CMTI debug artifacts; equal shortest cycles and rotations are deliberately lexical rather than hash-order-dependent; package/module/interface unwraps are invariants established by the graph and scheduled job shape |
+| Previous-state and stale-output cleanup | `build/read_compile_state.rs`; `build/clean.rs`: `cleanup_previous_build`, `cleanup_after_build` | `compile_assets.ml`; `build_artifacts.ml`: planned `cleanup_stale`; `source.ml`: separate ownership/presence inventories; `clean.ml`; `build.ml` finalization | Canonical rename/deletion/clean/suffix/feature cases and focused malformed-AST fallback, stale output families, directory-symlink freshness, deleted-JS repair, compiler-info invalidation, and duplicate-module clean tests | Matched for reachable artifact states. Classification precedes mutation and removes public JavaScript/maps plus working mirrors as one family. Explicit clean derives owned outputs from raw discovered implementation paths, while the non-owning presence view follows configured directory symlinks without authorizing recursive deletion |
+| Editor cache-bust marker | `build.rs`: `write_build_ninja`; full rebuild call sites in `build` and `watcher.rs` | `build.ml`: `write_build_ninja`, success/failure finalization | Isolated complete control-file manifests differed only by this marker before the fix; focused success and compiler-failure builds require it, and the differential config-recovery watch case requires it after rebuilding | Matched for normal builds and structural watch recovery; the snapshot watcher conservatively rewrites it after every post-initial rebuild because it does not expose Rust's event-kind classification |
+| Compiler artifact publication I/O failures | `build/compile.rs`: post-compile `fs::copy(...).expect(...)` calls | `build_artifacts.ml`: `copy_existing_file`; `rescript_ocaml.ml`: top-level I/O errors | Fresh-tree manifests prove normal publication equivalence; a differential compiler wrapper deletes the source after successful compilation; focused tests cover deleted-output repair and failed-watch recovery | Deliberate safety fix: OCaml emits a path-bearing normal error, while Rust panics its worker and leaves the scheduler waiting indefinitely; the bounded differential gate retains both outcomes |
+| Watch reconfiguration failures | `watcher.rs`: full-rebuild `initialize_build(...).expect(...)` | `build.ml`: `watch` rebuild error boundary; `watcher.ml`: recoverable control/candidate paths | Differential lifecycle cases introduce invalid root and dependency JSON and an unresolved dependency, verify the OCaml watcher remains alive, then repair configuration or create the missing installed package and wait for rebuilt output | Deliberate safety fix: intermediate configuration and package-resolution errors are recoverable in OCaml watch mode instead of panicking or terminating the watcher; unresolved candidates are watched only within the workspace |
+| Formatter compiler execution and errors | `format.rs`: `format_stdin`, `format_files` | `format.ml`: `formatted`, `format_stdin`, `format_files_with_bsc`, `format_files` | Focused integration runs successful and invalid stdin formatting on Unix and native Windows; `format_tests.ml` protects stable stdin/file error labels, requires two independent formatter subprocesses to overlap, and verifies that writes follow symlinks and preserve hard-link identity | Matched for bounded parallel execution, stop-on-error scheduling, subprocess failure classification, user-file identity, and native Windows execution |
+| Format check result | `format.rs`: `format_files` | `format.ml`: `format_files`, `format_check_summary` | Focused integration checks path, singular summary, error, and failure status; unit tests cover singular/plural messages | Matched |
+| Implicit format project scope and graph validation | `format.rs`: `get_files_in_scope`; `ProjectContext::get_scoped_local_packages`; `packages::make` | `format.ml`: `files_in_scope`, `discover_package_graph`, `package_sources`; `project_context.ml` and `package_graph.ml`: shared resolution and locality policy | Four canonical format tests cover the current fixture, a single file, stdin, and formatting from a workspace package; focused integration proves that implicit format requires a config in the current directory rather than searching parents; `format_tests.ml` proves installed `node_modules` dependencies are excluded from formatting; the differential command gate covers missing, config-less, malformed, duplicate-path, omitted-source, requested feature-cycle, installed missing-source, and local dependency feature-selection cases | Matched: one complete applicable package scan supplies both graph diagnostics and the files to format; the current package uses all features, dependencies use the union of consumer feature requests, direct symlink-local regular/dev dependencies are included only at a monorepo root, a listed child formats only itself, and transitive or installed dependencies are validated but not formatted |
+
+## Rust unit-test coverage gate
+
+[`tests/check_rust_test_coverage.sh`](tests/check_rust_test_coverage.sh)
+discovers every `#[test]` and `#[tokio::test]` below `rewatch/src` and compares
+that inventory with [`tests/rust_test_coverage.tsv`](tests/rust_test_coverage.tsv).
+Each Rust test must map to focused OCaml coverage, the shared canonical suite,
+an intentional architectural difference, an explicit project omission, or a
+known gap. New Rust tests and stale mapping rows fail the ordinary check.
+
+Run the stricter final gate with:
+
+```bash
+rewatch-ocaml/tests/check_rust_test_coverage.sh --require-complete
+```
+
+That mode also fails while any scenario is `unreviewed` or `gap`. The inventory
+contains 136 Rust tests, all reviewed, with no remaining entries in either
+category. They map to focused OCaml tests, the shared suite, accepted
+architectural differences, or the explicit telemetry omission. A mapping is
+evidence only after its cited OCaml/shared test has been inspected; grouping by
+similar wording alone is not proof of equivalent behavior. Passing this unit
+inventory does not replace the broader validation-source and interactive-output
+gates in this document.
+
+## Canonical integration-test coverage gate
+
+CI runs the shared [`rewatch/tests/suite.sh`](../rewatch/tests/suite.sh) against
+the packaged OCaml executable. Therefore its scenarios are exercised by the
+port rather than copied into a second suite that could drift. The
+[`tests/check_canonical_test_coverage.sh`](tests/check_canonical_test_coverage.sh)
+guard inventories every test script below `rewatch/tests`, and fails if
+`suite.sh` omits one, references a stale path, or references a test more than
+once. It currently finds 48 canonical integration tests, all referenced
+exactly once.
+
+This establishes shared integration-test inclusion, not exhaustive parity on
+its own. Rust source paths without a test remain the responsibility of the
+validation inventory above, and output modes not exercised by the shell suite
+remain the responsibility of the output gate below.
+
+## Output parity gate
+
+Output is tested in two modes because Rust deliberately changes behavior based
+on whether stdout and stderr are terminals.
+
+| Mode | Required comparison | Current status |
+| --- | --- | --- |
+| Redirected/plain output | Success summaries, warnings, errors, ordering, exit status, and absence of terminal control sequences unless color is explicitly forced; Cmdliner help may use its native man-page headings and layout | Matched for ordinary output: differential cases exactly compare normalized clean-build, compile-error, parse-error, successful-warning, and combined deprecated/unsupported/unknown-config stdout/stderr at default level, the first four cases at quiet level, plus default and quiet `clean`; the combined diagnostic case is also byte-compared with `CLICOLOR_FORCE=1`. A quiet redirected watcher performs initial and incremental builds without output. Format and compiler-args have focused/canonical checks. An order-insensitive differential gate matches semantic Rust `-v` project/package/parse/compile events and `-vv` dirty/completed scheduler-universe events. |
+| Interactive build | TTY detection, parsing/compilation progress, spinner lifecycle, timing, colors, symbols/emojis, quiet/verbose behavior, and cleanup on interruption | Matched; a retained Linux PTY gate exactly compares normalized cleanup/parse/compile completion lines, step counts, timing, phase emojis, and final status, observes live parsing/compilation spinner frames and module totals, and verifies that `-q` suppresses them. Its namespaced fixture also retains the distinction between one actually compiled source module and a two-entry scheduler universe containing the namespace marker; warning state and positive verbosity are covered separately |
+| Interactive clean | Graph-first validation, compiler-asset and generated-output phase order, package progress, timing, colors, and quiet behavior | Matched; `clean.ml` first constructs and validates a dependency-first plan, then runs compiler-asset and generated-output removal as distinct phases. The PTY gate compares both implementations' package line and five normalized `[1/2]`/`[2/2]` progress frames exactly, while redirected default/quiet behavior remains covered by the command gate. |
+| Interactive watch | Initial-build and rebuild progress, clear-screen behavior, persistent warnings, recovery errors, symbols/emojis, and orderly shutdown | Matched; a retained PTY gate exactly compares Rust/OCaml initial three-step and incremental two-step phase lines, counts, symbols, and final status after normalizing timing. It also covers live spinner frames, incremental/full clear-screen headers, the post-failure watching footer, parse-error recovery, warning persistence, positive verbosity, and orderly shutdown. |
+| Accessibility/terminal fallback | Stable meaningful text when color or richer glyphs are unavailable | Redirected output uses stable text without driver ANSI/glyph decoration and is covered by canonical snapshots; neither implementation probes terminal glyph support, so no richer fallback contract exists to port |
+
+Positive diagnostic verbosity is covered by a focused differential gate. Rust
+and OCaml `-v` emit the same semantic project-context, package-discovery,
+per-module parse, and per-module interface/implementation compile events; `-vv`
+also matches dirty and completed scheduler-universe events. The comparison is
+order-insensitive because Rust's Rayon completion order is not contractual.
+The native Windows pseudo-terminal run validates the presentation behavior as
+well as the platform backend.
+
+Interactive checks should run both implementations under a pseudo-terminal and
+capture normalized frames/events rather than snapshotting spinner timing byte
+for byte. Plain-output snapshots remain exact where paths and ANSI sequences
+can be normalized deterministically.
diff --git a/rewatch-ocaml/README.md b/rewatch-ocaml/README.md
new file mode 100644
index 00000000000..00ee935cfcf
--- /dev/null
+++ b/rewatch-ocaml/README.md
@@ -0,0 +1,223 @@
+# Experimental OCaml rewatch
+
+This directory contains the OCaml port of the ReScript build system. Linux,
+macOS, and Windows packages use it experimentally as `rescript`; the Rust
+implementation remains available as `rescript-rust` on every platform.
+
+## Status
+
+The cross-platform implementation, parity, performance, and release-quality
+gates are complete. The post-rebase Linux clean-build gate measured a 4.740 s Rust
+median and a 5.020 s OCaml median (1.059x), with identical compiler work, generated-file
+sets, and byte-stable artifacts. A separate 1,425-module macOS project measured
+approximately 9.6 s for Rust and 11.5 s for OCaml (about 1.20x). Absolute timing
+is host-specific; the reproducible method and complete resource/work results
+are in [`bench/README.md`](bench/README.md).
+
+The final uninterrupted `make test-all` run passed all compiler, runtime,
+build, GenType, analysis, tools, and canonical rewatch tests. The port has 42
+OUnit2 cases, and all 48 shared integration tests run against the packaged OCaml
+executable. All 136 Rust unit tests were reviewed: 128 map to focused or shared
+tests, five document intentional differences, and three cover the omitted
+telemetry support. Reanalyze reports no unreviewed dead production code.
+
+OpenTelemetry is deliberately omitted, and source filters support the documented
+common Rust/Re regular-expression subset rather than every Rust-regex construct.
+No platform correctness defect remains open. Native Windows validation covers
+the focused suite, all 42 OUnit2 cases, all 48 applicable canonical integration
+cases, Rust's 136 unit tests, package promotion/inventory, and both packaged
+executables. On a two-vCPU Windows 11 ARM64 guest running the x64 package under
+emulation, five interleaved canonical clean builds measured 42.267 s for Rust
+and 50.502 s for OCaml (1.195x). Seven-run no-op medians were 465 ms and 533 ms
+(1.146x), and seven interleaved single-edit medians were 192 ms and 225 ms
+(1.172x). These are low-core VM checkpoints, not portable absolute timings.
+
+## Documentation
+
+- This README is the maintained implementation, architecture, build, packaging,
+  and platform-status entry point.
+- [`PARITY_CHECKLIST.md`](PARITY_CHECKLIST.md) is the behavior-by-behavior
+  contract and validation inventory.
+- [`bench/README.md`](bench/README.md) documents reproducible performance,
+  filesystem-work, resource, artifact, and source-size gates.
+
+## Build
+
+From the repository root, with the dependencies declared in `rescript.opam`
+installed:
+
+```sh
+opam exec -- dune build rewatch-ocaml/rescript_ocaml.exe
+```
+
+The executable is written to:
+
+```text
+_build/default/rewatch-ocaml/rescript_ocaml.exe
+```
+
+Linux release jobs use Dune's `static` profile. The rewatch executable declares
+the same explicit `-ccopt -static` profile flag as the compiler executables, so
+the npm artifact does not depend on the runner's glibc. A local Linux packaging
+check can reproduce that link with:
+
+```sh
+opam exec -- dune build --profile static rewatch-ocaml/rescript_ocaml.exe
+file _build/default/rewatch-ocaml/rescript_ocaml.exe
+```
+
+It invokes `bsc` as an external process. When running outside this repository's
+normal Makefile environment, point it at the compiler and runtime explicitly:
+
+```sh
+export RESCRIPT_BSC_EXE="$PWD/_build/default/compiler/bsc/rescript_compiler_main.exe"
+export RESCRIPT_RUNTIME="$PWD/packages/@rescript/runtime"
+_build/default/rewatch-ocaml/rescript_ocaml.exe build path/to/project
+```
+
+On this experimental branch, published ReScript packages use the OCaml
+implementation for the normal `rescript` command. The Rust reference
+implementation remains available for side-by-side testing on every platform:
+
+```sh
+npx rescript build
+npx rescript-rust build
+```
+
+Windows ships the Dune-promoted OCaml executable as `rescript.exe` and the Rust
+reference as `rescript-rust.exe`, matching the other platform packages. The
+x64 package declares compatibility with ARM64 Windows because Windows runs
+these x64 executables through its emulation layer; the launcher maps native
+ARM64 Node to that package.
+
+The packaged executable discovers `bsc.exe` beside itself, like Rust rewatch,
+and the npm launcher supplies the installed runtime path. Direct invocation can
+instead resolve `@rescript/runtime` from the project hierarchy. The environment
+variables above remain useful overrides for the dune development executable;
+they are not required by the normal packaged launcher.
+
+Supported commands are `build` (the default), `watch`, `clean`, `format`, and
+`compiler-args`. The CLI is declared with Cmdliner; run the executable with
+`--help` for the current option summary.
+
+The binary version has a single OCaml source in `rewatch_version.ml`.
+Repository releases synchronize it with the compiler and Rust rewatch versions
+through `yarn constraints`; `yarn constraints --fix` updates all three from the
+release version declared in `yarn.config.cjs`.
+
+The implementation is split by ownership rather than mirroring the Rust source
+layout mechanically. Configuration types and the shared error identity live in
+`config_types.ml`, duplicate-aware JSON primitives and structured field decoders
+in `config_decode.ml`, and `config.ml` retains the top-level loader and
+runtime/path queries. `file_util.ml` owns general portable path, directory,
+copy, comparison, inventory, and removal operations; `build_artifacts.ml` owns
+ReScript output paths, publication, ownership, and stale-artifact
+cleanup. `package_graph.ml` owns package discovery; `package_parse.ml` and
+`package_compilation.ml` own per-package parsing and compiler-job construction;
+and `package_build.ml` sequences those phases. `build.ml` retains transaction
+orchestration and aggregate dispatch, while `build_report.ml` owns presentation.
+`build_preparation.ml` consumes the prepared packages to initialize compiler
+context, clean stale assets, and run the preliminary parse; `module_graph.ml`
+owns dependency resolution, graph-node identities, and cycle analysis.
+`package_plan.ml` owns immutable per-package build inputs, `build_session.ml`
+owns prepared state retained across watch rebuilds, and `build_attempt.ml` owns
+attempt kinds, parse outcomes, diagnostics, counters, scheduled work, and final
+cleanup for one build attempt. `source_dirs.ml` owns
+source-directory metadata projection and serialization. `process_child.ml`
+owns the lifecycle of one subprocess while `process.ml` owns scheduling.
+Command-level post-build execution and its error handling live in
+`after_build.ml`.
+Genuinely platform-specific behavior is consolidated behind a `Platform`
+boundary rather than mixed into those modules. Unix and Windows modules now own
+executable lookup, subprocess creation, signal deferral, and process-tree
+termination as well as lock-owner PID probing and capture-pipe creation. The
+native watcher has a separate, narrow cross-platform boundary over libuv.
+`watch_scope.ml` owns package/source selection and watch paths,
+`watch_snapshot.ml` owns filesystem baselines and diffs, and `watcher.ml` owns
+event reconciliation and rebuild lifecycle. Portable `Filename`-based path and
+artifact logic remains shared.
+
+## Test
+
+```sh
+opam exec -- dune runtest tests/rewatch_ounit_tests
+rewatch-ocaml/tests/check_config_acceptance.sh
+rewatch-ocaml/tests/check_command_validation.sh
+rewatch-ocaml/tests/check_interactive_output.sh
+rewatch-ocaml/tests/check_windows_job_stub.sh "$WINDOWS_CC" \
+  "$WINDOWS_OCAML_INCLUDE"
+sh rewatch-ocaml/tests/run.sh \
+  "$PWD/_build/default/rewatch-ocaml/rescript_ocaml.exe"
+```
+
+The interactive-output parity gate requires the `script` PTY utility. Windows
+CI skips that gate when `script` is unavailable; Windows output formatting is
+still covered by unit tests, while redirected and verbose behavior is covered
+by the platform-independent parity gates.
+
+The canonical integration suite can use the port through its existing override:
+
+```sh
+export REWATCH_EXECUTABLE="$PWD/_build/default/rewatch-ocaml/rescript_ocaml.exe"
+eval "$(cd rewatch/tests && node ./get_bin_paths.js)"
+export RESCRIPT_BSC_EXE RESCRIPT_RUNTIME
+bash rewatch/tests/compile/01-basic-compile.sh
+```
+
+## Packaging checks
+
+The release inventory uses the repository's existing Dune promotion and npm
+artifact tooling. On Linux, build and verify the current platform package with:
+
+```sh
+opam exec -- dune build --profile static compiler/sync/rescript.exe
+file packages/@rescript/linux-arm64/bin/rescript.exe
+node scripts/checkCompilerExes.js
+node scripts/updateArtifactList.js
+git diff --exit-code packages/artifacts.json
+yarn workspace @rescript/linux-arm64 pack --json --dry-run
+```
+
+Use `linux-x64` instead on an x64 host. On Windows, build the Dune target and
+run the same checks against `@rescript/win32-x64`; Corepack can invoke the final
+command as `corepack yarn workspace @rescript/win32-x64 pack --json --dry-run`.
+The package listing must contain the
+OCaml `bin/rescript.exe`, the Rust reference `bin/rescript-rust.exe`, and both
+rewatch notice files. CI runs the artifact-list check only after downloading
+all platform builds and treats any missing declared executable as an error;
+local manifest generation uses temporary placeholders solely for platforms
+that were not built on the current machine.
+
+OpenTelemetry/OTLP tracing is intentionally not part of this port. This is an
+explicit project scope decision, not a silently ignored configuration feature;
+ordinary command output, verbosity, diagnostics, and exit statuses remain in
+scope.
+
+## Platform status
+
+The Windows implementation is complete and natively validated. Windows ships
+the OCaml implementation as the default `rescript.exe` and retains Rust as
+`rescript-rust.exe`. Unix subprocesses use the cross-platform `spawn` library.
+Windows uses a narrow native
+`CreateProcessW` owner so it can establish Job Object ownership before a child
+starts running.
+Compiler output is captured through close-on-exec pipes drained by blocking
+reader threads, avoiding reliance on Windows `select` support for anonymous
+pipes. Each child is attached to a retained Windows Job Object so cancellation
+can terminate descendants after the direct process has exited; the native stub
+check above compiles that API boundary with warnings as errors when given a
+Windows-targeting C compiler and its matching OCaml header directory. Watch mode
+uses long-lived filesystem-event handles through Luv/libuv and retains the
+snapshot-based polling loop only as a runtime fallback. Native tests exercise
+Job assignment, descendant-held-pipe cancellation, lock-driven cancellation,
+native process IDs and lock contention, watcher rebuild/recovery, formatting,
+and post-build command execution. The MSYS harness cannot deliver a normal
+Windows console-control event to a native child, so it exercises the same
+cleanup path by removing `watch.lock`; console signal delivery remains covered
+on Unix. Windows streaming `--after-build` commands inherit terminal stdin while
+remaining in their Job Object. Unix terminal stdin is intentionally withheld
+from these commands because their separately owned process group would be
+stopped by `SIGTTIN`; redirected stdin is inherited, and full terminal input
+requires a future PTY relay that preserves process-tree cancellation. Shared
+path construction uses OCaml's `Filename` APIs so Windows
+separators and drive roots are not hard-coded assumptions.
diff --git a/rewatch-ocaml/after_build.ml b/rewatch-ocaml/after_build.ml
new file mode 100644
index 00000000000..430b53afed2
--- /dev/null
+++ b/rewatch-ocaml/after_build.ml
@@ -0,0 +1,32 @@
+exception Error = Project_context.Error
+
+let run ?poll ~root command =
+  let program, args =
+    match Str.split (Str.regexp "[ \t\r\n]+") command with
+    | program :: args -> (program, args)
+    | [] -> raise (Error "--after-build command cannot be empty")
+  in
+  let result =
+    try Process.run_streaming ?poll ~cwd:root program args with
+    | Process.Error message ->
+      raise
+        (Error
+           (Printf.sprintf "Could not run --after-build command %S: %s" command
+              message))
+    | Sys_error message ->
+      raise
+        (Error
+           (Printf.sprintf "Could not run --after-build command %S: %s" command
+              message))
+    | Unix.Unix_error (error, operation, argument) ->
+      let target = if argument = "" then program else argument in
+      raise
+        (Error
+           (Printf.sprintf "Could not run --after-build command %S: %s (%s %s)"
+              command (Unix.error_message error) operation target))
+  in
+  if not (Process.succeeded result) then
+    raise
+      (Error
+         (Printf.sprintf "--after-build command failed with %s"
+            (Process.status_string result.status)))
diff --git a/rewatch-ocaml/after_build.mli b/rewatch-ocaml/after_build.mli
new file mode 100644
index 00000000000..04e812c9fcb
--- /dev/null
+++ b/rewatch-ocaml/after_build.mli
@@ -0,0 +1 @@
+val run : ?poll:(unit -> unit) -> root:string -> string -> unit
diff --git a/rewatch-ocaml/ast_header.ml b/rewatch-ocaml/ast_header.ml
new file mode 100644
index 00000000000..e5a8cc1c693
--- /dev/null
+++ b/rewatch-ocaml/ast_header.ml
@@ -0,0 +1,20 @@
+type t = {dependencies: string list; source: string option}
+
+let read path =
+  let descriptor = Unix.openfile path [Unix.O_RDONLY; Unix.O_CLOEXEC] 0 in
+  let channel = Unix.in_channel_of_descr descriptor in
+  Fun.protect
+    ~finally:(fun () -> close_in_noerr channel)
+    (fun () ->
+      (try ignore (input_line channel) with End_of_file -> ());
+      let rec loop dependencies =
+        match input_line channel with
+        | line ->
+          let line = String.trim line in
+          if line = "" then loop dependencies
+          else if Filename.is_relative line then loop (line :: dependencies)
+          else {dependencies = List.rev dependencies; source = Some line}
+        | exception End_of_file ->
+          {dependencies = List.rev dependencies; source = None}
+      in
+      loop [])
diff --git a/rewatch-ocaml/ast_header.mli b/rewatch-ocaml/ast_header.mli
new file mode 100644
index 00000000000..3c41db44d24
--- /dev/null
+++ b/rewatch-ocaml/ast_header.mli
@@ -0,0 +1,7 @@
+type t = {dependencies: string list; source: string option}
+(** Published AST headers are read without deserializing the compiler's full AST.
+    Cleanup needs the original source path, while graph construction needs only
+    dependency names; keeping one reader prevents those wire-format rules from
+    drifting. *)
+
+val read : string -> t
diff --git a/rewatch-ocaml/bench/README.md b/rewatch-ocaml/bench/README.md
new file mode 100644
index 00000000000..264a6455764
--- /dev/null
+++ b/rewatch-ocaml/bench/README.md
@@ -0,0 +1,212 @@
+# Performance and work-equivalence gate
+
+`performance_gate.sh` compares release builds on two fully isolated copies of
+the tracked `rewatch/testrepo` fixture. It deliberately archives both the
+fixture and its external Belt/runtime targets and copies all installed,
+Git-ignored dependency trees (including nohoisted dependencies) separately
+into each root. Cleaning one
+implementation therefore cannot warm or remove artifacts used by the other.
+
+The gate performs one warm-up per implementation, at least five interleaved
+clean builds, and reports median wall time, peak summed process-tree RSS, and
+peak process-tree task count. Wall time and RSS are acceptance criteria; task
+count is diagnostic evidence for subprocess-management overhead. It then traces
+clean, unchanged, and single-edit builds with `strace` and requires identical
+normalized package/phase/input multisets as well as identical counts for parser,
+namespace, compiler, interface, and PPX process launches. The edit targets the
+same leaf source in each isolated fixture. This sequence detects superfluous
+incremental parsing or compilation that a clean-only comparison cannot expose.
+Finally, both implementations clean and build a third fixture at the same
+absolute path. The gate first requires the complete post-build
+file-name sets to match, including auxiliary cache and editor-control files,
+and then requires byte-identical generated JavaScript, compiler interfaces
+(`.cmi`), JavaScript IR (`.cmj`), parser AST caches, namespace maps, copied
+sources, source-directory metadata, and `build.ninja` markers, including those
+below installed dependency trees. It does not treat typed debug metadata
+(`.cmt`/`.cmti`), compiler logs, or `compiler-info.json` as byte-stable: those
+contain compiler debug data, timestamps, or intentionally
+implementation-specific state and are covered by file-set and integration
+checks instead. The default
+acceptance threshold requires both OCaml medians to be no more than 125% of
+Rust.
+
+The post-rebase powered, idle-host seven-run gate measured a 4.740 s Rust
+clean-build median and a 5.020 s OCaml median (1.059x). Median summed
+process-tree RSS was 1,066,940 KiB and 1,184,740 KiB respectively (1.110x).
+Clean, unchanged, and one-edit compiler work matched exactly, and the complete
+post-build file sets and byte-stable artifacts were identical. The companion
+seven-edit retained-watch gate measured 130 ms for Rust and 153 ms for OCaml
+(1.177x), with identical compiler work and stable resources. These values are
+a reproducible checkpoint, not portable absolute expectations.
+
+This is one part of equivalence checking, not a substitute for the test suites.
+Before accepting a performance increment, also run the OCaml unit/focused tests
+and the canonical Rust rewatch integration suite against the OCaml executable:
+
+```sh
+opam exec -- dune runtest tests/rewatch_ounit_tests
+bash rewatch-ocaml/tests/run.sh \
+  _build/default/rewatch-ocaml/rescript_ocaml.exe
+(cd rewatch/tests && \
+  bash ./suite.sh ../../_build/default/rewatch-ocaml/rescript_ocaml.exe)
+```
+
+Together these cover three different failure classes:
+
+- the canonical and focused suites check observable command/build/watch
+  behavior;
+- the three `strace` classifications check that a speed result did not hide
+  skipped or superfluous clean/incremental module or PPX work (argument
+  semantics remain covered by the compiler-argument and integration tests);
+- the fresh-tree comparisons check the complete post-build file set plus the
+  byte contents of every stable generated artifact class.
+
+The manifest comparison intentionally recreates its fixture between runners.
+Using only each implementation's `clean` command would allow a Rust-only file
+to survive into the OCaml run and could conceal a missing-output bug.
+
+`filesystem_audit.sh` writes normalized `*.categories.tsv`, `*.paths.tsv`, and
+`*.processes.tsv` files when `KEEP_REWATCH_FILESYSTEM_AUDIT=1` is set. The last
+form attributes each path operation category to the executable recorded for
+that traced process, separating driver work from compiler, PPX, and helper
+work. Processes which inherit a trace file without a subsequent `execve` are
+reported as `inherited-process`; do not assume those calls belong to the
+driver without inspecting the raw trace.
+
+Build both release executables and run:
+
+```sh
+cargo build --manifest-path rewatch/Cargo.toml --release
+opam exec -- dune build --profile release rewatch-ocaml/rescript_ocaml.exe
+
+rewatch-ocaml/bench/performance_gate.sh \
+  rewatch/target/release/rescript \
+  _build/default/rewatch-ocaml/rescript_ocaml.exe \
+  5
+```
+
+By default the harness uses the compiler and runtime selected by
+`rewatch/tests/get_bin_paths.js`. Set both `RESCRIPT_BSC_EXE` and
+`RESCRIPT_RUNTIME` to compare with another local compiler build; the harness
+preserves them only when both are set, so the two implementations still use
+the same inputs.
+
+The authoritative gate requires Linux (`/proc`), `strace`, GNU-compatible
+nanosecond `date`, and a stable plugged-in host with no competing heavy work.
+Keep the isolated fixtures on a case-sensitive Linux filesystem. A Linux
+container backed by a case-insensitive macOS bind mount can transiently report
+that a differently-cased recreated CMI exists to `stat` and then return
+`ENOENT` from the immediately following `open`; that host-filesystem artifact
+is not valid scheduler or benchmark evidence. The harness's default `mktemp`
+workspace normally stays on the container filesystem.
+Set `REWATCH_PERFORMANCE_THRESHOLD_PERCENT` to exercise a proposed threshold
+change; changing the committed 125% completion criterion requires an explicit
+project decision. Set `KEEP_REWATCH_BENCHMARK_WORKDIR=1` to retain traces and raw
+stdout/stderr for investigation. For a quick correctness-only check, an odd run
+count below five is accepted only with `REWATCH_ALLOW_SMOKE_RUN=1`; its timing
+must never be treated as a quality-gate result.
+
+## Filesystem-work audit
+
+Pipe-based subprocess capture has removed the intentional temporary capture
+files. Run the second orchestration audit on Linux with:
+
+```sh
+rewatch-ocaml/bench/filesystem_audit.sh \
+  rewatch/target/release/rescript \
+  _build/default/rewatch-ocaml/rescript_ocaml.exe
+```
+
+It traces isolated clean, unchanged, and single-edit builds with `strace`,
+normalizes each fixture root, retains operations whose target is inside that
+root, and reports per-path operation multisets plus metadata, open,
+directory-scan, create, rename, remove, and execute categories. Set
+`KEEP_REWATCH_FILESYSTEM_AUDIT=1` to retain normalized manifests and raw traces
+for investigation.
+
+Do not gate on the raw process-wide syscall total: Rust, OCaml, libc, the
+dynamic loader, and subprocess startup legitimately perform different
+toolchain-level accesses. Report those separately, and treat repeated accesses
+to the same project artifact or discovery path as the primary evidence of
+superfluous orchestration work. The existing compiler-work and artifact checks
+must remain enabled so fewer filesystem calls cannot conceal skipped work.
+
+Process attribution showed identical compiler-subprocess
+metadata/open work and effectively equal driver-plus-inherited clean-build open
+counts (about 8,110 for OCaml and 8,120 for Rust). OCaml made about 2,387 more
+driver-side metadata calls, led by repeated checks of source and `lib/bs`
+directories during artifact publication. Unchanged and single-edit process
+runs used fewer metadata and open calls in OCaml. The clean-build difference is
+therefore understood rather than an unexplained algorithmic discrepancy. A
+future cache for already-created publication directories may remove it, but it
+must be scoped to one attempt and recover correctly if a directory is removed
+concurrently. Preserve the raw trace or repeat the process-attributed audit
+before making that tradeoff.
+
+For the ordinary-edit path inside one long-lived watcher, run:
+
+```sh
+rewatch-ocaml/bench/watch_filesystem_audit.sh \
+  rewatch/target/release/rescript \
+  _build/default/rewatch-ocaml/rescript_ocaml.exe
+```
+
+This starts each implementation on an isolated small project, waits for its
+initial build hook, traces one dependency-preserving source edit, and stops the
+watcher through its lock file. Only calls timestamped between the edit and the
+successful incremental-build hook enter the normalized reports, so initial
+discovery and shutdown do not obscure retained-state work. Set
+`KEEP_REWATCH_WATCH_AUDIT=1` to retain raw traces, normalized path/category
+tables, process attribution, and command output. As with the short-lived audit,
+project-local repeated paths and compiler work are the useful comparison; raw
+runtime-wide syscall totals are diagnostic rather than an acceptance limit.
+
+The retained-watch performance and resource gate exercises several ordinary
+edits through the same long-lived watcher:
+
+```sh
+rewatch-ocaml/bench/watch_performance_gate.sh \
+  rewatch/target/release/rescript \
+  _build/default/rewatch-ocaml/rescript_ocaml.exe \
+  7
+```
+
+It warms both implementations, interleaves an odd number of timed edits,
+requires byte-identical generated JavaScript and normalized parser/compiler
+argument logs, and samples file descriptors, tasks, and RSS after every build.
+This catches retained-state implementations that appear fast by skipping work,
+as well as resource growth that a one-event syscall trace cannot show. The
+default median-latency limit is 150% of Rust because individual watch events
+include operating-system notification and 50 ms polling intervals; override it
+with `REWATCH_WATCH_PERFORMANCE_THRESHOLD_PERCENT` only for investigation.
+The build gate's lower-noise 125% clean/incremental threshold remains the
+authoritative general performance criterion. Set
+`KEEP_REWATCH_WATCH_PERFORMANCE=1` to retain output, compiler-call logs,
+latencies, and fixtures. This gate requires Linux `/proc`, GNU-compatible
+millisecond `date`, and `setsid`.
+
+## Source-size snapshot
+
+Run `bench/source_size.sh` with `cloc` installed to record a reproducible
+maintainability snapshot. The production comparison excludes Rust's explicitly
+out-of-scope telemetry module and reports its inline `#[cfg(test)]` sections as
+tests rather than implementation. Both OCaml platform backends count because
+both remain maintained production source. All tracked OCaml test harnesses,
+fixtures, and configuration files are reported together but separately from
+implementation; benchmark tooling includes every executable shell/JavaScript
+file in `bench`, including this counting script itself. The report also lists
+the ten largest production modules and test/tooling files so growth and mixed
+responsibilities are visible without treating line count as a target. Record
+the `cloc` version with the result and rerun this at the final maintainability
+review.
+
+Source lines are an observation, not an acceptance threshold. A smaller port
+can indicate less machinery, but missing compatibility, weak tests, compressed
+code, or too few explanatory comments can also reduce the number. Behavioral
+and work equivalence, platform support, performance, module size, and review
+findings remain the actual quality gates.
+
+The post-Windows snapshot with cloc 2.04 contains 7,809 Rust production lines
+after excluding telemetry and 11,320 OCaml production lines including both
+platform backends. Rust inline unit tests account for 2,773 lines; OCaml tests
+and fixtures account for 10,270 lines, and the benchmark tooling for 1,033.
diff --git a/rewatch-ocaml/bench/filesystem_audit.sh b/rewatch-ocaml/bench/filesystem_audit.sh
new file mode 100755
index 00000000000..19a1ffc92b3
--- /dev/null
+++ b/rewatch-ocaml/bench/filesystem_audit.sh
@@ -0,0 +1,103 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -ne 2 ]]; then
+  echo "Usage: $0 RUST_REWATCH OCAML_REWATCH" >&2
+  exit 2
+fi
+
+repo_root=$(cd "$(dirname "$0")/../.." && pwd)
+rust_executable=$(cd "$(dirname "$1")" && pwd)/$(basename "$1")
+ocaml_executable=$(cd "$(dirname "$2")" && pwd)/$(basename "$2")
+normalizer="$repo_root/rewatch-ocaml/bench/normalize_file_trace.js"
+
+for command in basename cp dirname find git join mktemp node sed sort strace tar; do
+  command -v "$command" >/dev/null || {
+    echo "Missing required command: $command" >&2
+    exit 2
+  }
+done
+if [[ ! -x "$rust_executable" || ! -x "$ocaml_executable" ]]; then
+  echo "Both rewatch executables must exist and be executable." >&2
+  exit 2
+fi
+
+work_root=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-filesystem-audit.XXXXXX")
+cleanup() {
+  if [[ ${KEEP_REWATCH_FILESYSTEM_AUDIT:-0} == 1 ]]; then
+    echo "Kept filesystem audit workdir: $work_root" >&2
+  else
+    find "$work_root" -depth -delete
+  fi
+}
+trap cleanup EXIT INT TERM
+
+prepare_fixture() {
+  local destination=$1
+  mkdir -p "$destination"
+  git -C "$repo_root" archive HEAD \
+    rewatch/testrepo packages/@rescript/belt packages/@rescript/runtime \
+    | tar -x -C "$destination"
+  while IFS= read -r dependency_tree; do
+    local relative_tree=${dependency_tree#"$repo_root/"}
+    mkdir -p "$(dirname "$destination/$relative_tree")"
+    cp -a --reflink=auto "$dependency_tree" "$destination/$relative_tree"
+  done < <(find "$repo_root/rewatch/testrepo" -type d -name node_modules \
+    -prune -print)
+}
+
+if [[ -z ${RESCRIPT_BSC_EXE:-} || -z ${RESCRIPT_RUNTIME:-} ]]; then
+  eval "$(cd "$repo_root/rewatch/tests" && node ./get_bin_paths.js)"
+fi
+export RESCRIPT_BSC_EXE RESCRIPT_RUNTIME
+
+rust_root="$work_root/rust"
+ocaml_root="$work_root/ocaml"
+prepare_fixture "$rust_root"
+prepare_fixture "$ocaml_root"
+rust_fixture="$rust_root/rewatch/testrepo"
+ocaml_fixture="$ocaml_root/rewatch/testrepo"
+
+trace_build() {
+  local implementation=$1 scenario=$2 executable=$3 fixture=$4 clean_first=$5
+  local trace_prefix="$work_root/$implementation-$scenario.file"
+  local normalized="$work_root/$implementation-$scenario"
+  if [[ "$clean_first" == 1 ]]; then
+    "$executable" clean "$fixture" >/dev/null 2>&1
+  fi
+  (
+    cd "$fixture"
+    strace -f -ff -qq -yy -s 4096 -e trace=%file,getdents64 \
+      -o "$trace_prefix" "$executable" build . \
+      >"$normalized.stdout" 2>"$normalized.stderr"
+  )
+  node "$normalizer" "$trace_prefix" "$fixture" "$normalized"
+}
+
+for scenario in clean unchanged; do
+  clean_first=0
+  [[ "$scenario" == clean ]] && clean_first=1
+  trace_build rust "$scenario" "$rust_executable" "$rust_fixture" "$clean_first"
+  trace_build ocaml "$scenario" "$ocaml_executable" "$ocaml_fixture" "$clean_first"
+done
+
+printf '\n// filesystem audit single edit\n' \
+  >>"$rust_fixture/packages/watch-warnings/src/B.res"
+printf '\n// filesystem audit single edit\n' \
+  >>"$ocaml_fixture/packages/watch-warnings/src/B.res"
+trace_build rust edit "$rust_executable" "$rust_fixture" 0
+trace_build ocaml edit "$ocaml_executable" "$ocaml_fixture" 0
+
+for scenario in clean unchanged edit; do
+  echo
+  echo "$scenario project filesystem categories (Rust / OCaml)"
+  join -a 1 -a 2 -e 0 -o 0,1.2,2.2 \
+    "$work_root/rust-$scenario.categories.tsv" \
+    "$work_root/ocaml-$scenario.categories.tsv"
+  echo "$scenario most repeated OCaml project path operations"
+  sort -t $'\t' -k1,1nr "$work_root/ocaml-$scenario.paths.tsv" | sed -n '1,20p'
+done
+
+echo
+echo "This audit is diagnostic: inspect repeated project-local accesses and the"
+echo "retained manifests; raw Rust/OCaml totals are not an equivalence gate."
diff --git a/rewatch-ocaml/bench/normalize_file_trace.js b/rewatch-ocaml/bench/normalize_file_trace.js
new file mode 100755
index 00000000000..7506651c76f
--- /dev/null
+++ b/rewatch-ocaml/bench/normalize_file_trace.js
@@ -0,0 +1,134 @@
+#!/usr/bin/env node
+
+import fs from "node:fs";
+import path from "node:path";
+
+if (process.argv.length !== 5 && process.argv.length !== 7) {
+  console.error(
+    "Usage: normalize_file_trace.js TRACE_PREFIX FIXTURE OUTPUT_PREFIX [START_EPOCH END_EPOCH]",
+  );
+  process.exit(2);
+}
+
+const [, , tracePrefix, fixtureArgument, outputPrefix, startArgument, endArgument] =
+  process.argv;
+const startEpoch =
+  startArgument === undefined ? Number.NEGATIVE_INFINITY : Number(startArgument);
+const endEpoch = endArgument === undefined ? Number.POSITIVE_INFINITY : Number(endArgument);
+if (Number.isNaN(startEpoch) || Number.isNaN(endEpoch) || startEpoch > endEpoch) {
+  console.error("Trace epoch bounds must be ordered numbers.");
+  process.exit(2);
+}
+const fixture = path.resolve(fixtureArgument);
+const traceDirectory = path.dirname(tracePrefix);
+const traceBasename = `${path.basename(tracePrefix)}.`;
+const traces = fs
+  .readdirSync(traceDirectory)
+  .filter((name) => name.startsWith(traceBasename))
+  .sort();
+
+const operations = [];
+const categories = new Map();
+const processCategories = new Map();
+
+function decodeQuoted(value) {
+  try {
+    return JSON.parse(`"${value}"`);
+  } catch (_) {
+    return value;
+  }
+}
+
+function normalize(cwd, value) {
+  if (value === "") return null;
+  const absolute = path.isAbsolute(value) ? path.normalize(value) : path.resolve(cwd, value);
+  if (absolute !== fixture && !absolute.startsWith(`${fixture}${path.sep}`)) return null;
+  const relative = path.relative(fixture, absolute);
+  return relative === "" ? "<ROOT>" : `<ROOT>/${relative.split(path.sep).join("/")}`;
+}
+
+function category(operation) {
+  if (/^(open|openat|openat2|creat)$/.test(operation)) return "open";
+  if (/^(stat|statx|lstat|fstatat|newfstatat|access|faccessat|faccessat2|readlink|readlinkat)$/.test(operation)) return "metadata";
+  if (/^(mkdir|mkdirat|mknod|mknodat|link|linkat|symlink|symlinkat)$/.test(operation)) return "create";
+  if (/^rename/.test(operation)) return "rename";
+  if (/^(unlink|unlinkat|rmdir)$/.test(operation)) return "remove";
+  if (/^getdents/.test(operation)) return "directory-scan";
+  if (operation === "execve") return "execute";
+  return "other";
+}
+
+function pathValues(operation, line) {
+  if (/^getdents/.test(operation)) {
+    const descriptorPath = line.match(/^getdents\w*\(\d+<([^>]+)>/);
+    return descriptorPath ? [descriptorPath[1]] : [];
+  }
+  const quoted = [...line.matchAll(/"((?:[^"\\]|\\.)*)"/g)].map((match) =>
+    decodeQuoted(match[1]),
+  );
+  if (/^(rename|renameat|renameat2|link|linkat)$/.test(operation)) return quoted.slice(0, 2);
+  if (/^(symlink|symlinkat)$/.test(operation)) return quoted.slice(-1);
+  return quoted.slice(0, 1);
+}
+
+for (const trace of traces) {
+  let cwd = fixture;
+  const lines = fs.readFileSync(path.join(traceDirectory, trace), "utf8").split("\n");
+  const executable = lines
+    .map((line) => line.replace(/^\d+\.\d+\s+/, ""))
+    .map((line) => line.match(/^execve\("((?:[^"\\]|\\.)*)"/))
+    .find((match) => match !== null);
+  const processName = executable
+    ? path.basename(decodeQuoted(executable[1]))
+    : "inherited-process";
+  for (const rawLine of lines) {
+    const timed = rawLine.match(/^(\d+\.\d+)\s+(.*)$/);
+    const timestamp = timed === null ? null : Number(timed[1]);
+    const line = timed === null ? rawLine : timed[2];
+    const call = line.match(/^([a-zA-Z0-9_]+)\(/);
+    if (!call) continue;
+    const operation = call[1];
+    const values = pathValues(operation, line);
+    const callCwd = cwd;
+    for (const value of values) {
+      const normalized = normalize(callCwd, value);
+      if (normalized === null) continue;
+      if (
+        timestamp === null ||
+        (timestamp >= startEpoch && timestamp <= endEpoch)
+      ) {
+        operations.push(`${operation}\t${normalized}`);
+        const name = category(operation);
+        categories.set(name, (categories.get(name) || 0) + 1);
+        const processKey = `${processName}\t${name}`;
+        processCategories.set(processKey, (processCategories.get(processKey) || 0) + 1);
+      }
+    }
+    if (operation === "chdir" && line.endsWith("= 0") && values.length === 1) {
+      cwd = path.isAbsolute(values[0])
+        ? path.normalize(values[0])
+        : path.resolve(callCwd, values[0]);
+    }
+  }
+}
+
+operations.sort();
+const counted = [];
+for (let index = 0; index < operations.length; ) {
+  let end = index + 1;
+  while (end < operations.length && operations[end] === operations[index]) end += 1;
+  counted.push(`${end - index}\t${operations[index]}`);
+  index = end;
+}
+fs.writeFileSync(`${outputPrefix}.paths.tsv`, `${counted.join("\n")}\n`);
+fs.writeFileSync(
+  `${outputPrefix}.categories.tsv`,
+  `${[...categories].sort().map(([name, count]) => `${name}\t${count}`).join("\n")}\n`,
+);
+fs.writeFileSync(
+  `${outputPrefix}.processes.tsv`,
+  `${[...processCategories]
+    .sort(([left], [right]) => left.localeCompare(right))
+    .map(([key, count]) => `${key}\t${count}`)
+    .join("\n")}\n`,
+);
diff --git a/rewatch-ocaml/bench/performance_gate.sh b/rewatch-ocaml/bench/performance_gate.sh
new file mode 100755
index 00000000000..4118d7fc651
--- /dev/null
+++ b/rewatch-ocaml/bench/performance_gate.sh
@@ -0,0 +1,364 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -lt 2 || $# -gt 3 ]]; then
+  echo "Usage: $0 RUST_REWATCH OCAML_REWATCH [RUNS]" >&2
+  exit 2
+fi
+
+repo_root=$(cd "$(dirname "$0")/../.." && pwd)
+rust_executable=$(cd "$(dirname "$1")" && pwd)/$(basename "$1")
+ocaml_executable=$(cd "$(dirname "$2")" && pwd)/$(basename "$2")
+runs=${3:-5}
+threshold_percent=${REWATCH_PERFORMANCE_THRESHOLD_PERCENT:-125}
+
+if [[ ! -x "$rust_executable" || ! -x "$ocaml_executable" ]]; then
+  echo "Both rewatch executables must exist and be executable." >&2
+  exit 2
+fi
+if [[ ! "$runs" =~ ^[1-9][0-9]*$ || $((runs % 2)) -eq 0 ]]; then
+  echo "RUNS must be a positive odd integer so the median is unambiguous." >&2
+  exit 2
+fi
+if ((runs < 5)) && [[ ${REWATCH_ALLOW_SMOKE_RUN:-0} != 1 ]]; then
+  echo "RUNS must be at least 5 for the quality gate." >&2
+  echo "Set REWATCH_ALLOW_SMOKE_RUN=1 only for a non-authoritative smoke run." >&2
+  exit 2
+fi
+for command in awk basename cmp cp date diff dirname find getconf git grep head \
+  mktemp node ps sed sha256sum sleep sort strace tar uname xargs; do
+  command -v "$command" >/dev/null || {
+    echo "Missing required command: $command" >&2
+    exit 2
+  }
+done
+if [[ ! -d /proc ]]; then
+  echo "This gate requires Linux /proc for process-tree RSS sampling." >&2
+  exit 2
+fi
+
+work_root=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-performance.XXXXXX")
+cleanup() {
+  if [[ ${KEEP_REWATCH_BENCHMARK_WORKDIR:-0} == 1 ]]; then
+    echo "Kept benchmark workdir: $work_root" >&2
+  else
+    find "$work_root" -depth -delete
+  fi
+}
+trap cleanup EXIT INT TERM
+
+prepare_fixture() {
+  local destination=$1
+  mkdir -p "$destination"
+  git -C "$repo_root" archive HEAD \
+    rewatch/testrepo packages/@rescript/belt packages/@rescript/runtime \
+    | tar -x -C "$destination"
+  # Dependencies and workspace links are intentionally ignored by Git. Keep a
+  # separate installed tree in each root so neither implementation can affect
+  # the other's generated dependency artifacts.
+  while IFS= read -r dependency_tree; do
+    local relative_tree=${dependency_tree#"$repo_root/"}
+    mkdir -p "$(dirname "$destination/$relative_tree")"
+    cp -a --reflink=auto "$dependency_tree" "$destination/$relative_tree"
+  done < <(find "$repo_root/rewatch/testrepo" -type d -name node_modules \
+    -prune -print)
+}
+
+rust_root="$work_root/rust"
+ocaml_root="$work_root/ocaml"
+prepare_fixture "$rust_root"
+prepare_fixture "$ocaml_root"
+rust_fixture="$rust_root/rewatch/testrepo"
+ocaml_fixture="$ocaml_root/rewatch/testrepo"
+
+if [[ -z ${RESCRIPT_BSC_EXE:-} || -z ${RESCRIPT_RUNTIME:-} ]]; then
+  eval "$(cd "$repo_root/rewatch/tests" && node ./get_bin_paths.js)"
+fi
+export RESCRIPT_BSC_EXE RESCRIPT_RUNTIME
+
+results="$work_root/results.csv"
+echo "implementation,iteration,wall_ms,peak_tree_rss_kib,peak_tree_tasks" \
+  >"$results"
+
+tree_resources() {
+  local root_pid=$1
+  ps -e -o pid=,ppid=,rss=,nlwp= | awk -v root="$root_pid" '
+    { pids[NR] = $1; parent[$1] = $2; memory[$1] = $3; tasks[$1] = $4 }
+    END {
+      live[root] = 1
+      for (pass = 0; pass < NR; pass++)
+        for (i = 1; i <= NR; i++)
+          if (live[parent[pids[i]]]) live[pids[i]] = 1
+      for (pid in live) {
+        total_memory += memory[pid]
+        total_tasks += tasks[pid]
+      }
+      print total_memory + 0, total_tasks + 0
+    }'
+}
+
+clean_and_build() {
+  local executable=$1 fixture=$2 output=$3
+  "$executable" clean "$fixture" >/dev/null 2>&1
+  "$executable" build "$fixture" >"$output" 2>"$output.stderr"
+}
+
+measure() {
+  local implementation=$1 executable=$2 fixture=$3 iteration=$4
+  local output="$work_root/${implementation}-${iteration}"
+  "$executable" clean "$fixture" >/dev/null 2>&1
+  local start_ns root_pid peak_rss=0 peak_tasks=0 rss tasks end_ns wall_ms
+  start_ns=$(date +%s%N)
+  "$executable" build "$fixture" >"$output" 2>"$output.stderr" &
+  root_pid=$!
+  while kill -0 "$root_pid" 2>/dev/null; do
+    read -r rss tasks < <(tree_resources "$root_pid")
+    if ((rss > peak_rss)); then
+      peak_rss=$rss
+    fi
+    if ((tasks > peak_tasks)); then
+      peak_tasks=$tasks
+    fi
+    sleep 0.02
+  done
+  wait "$root_pid"
+  end_ns=$(date +%s%N)
+  wall_ms=$(((end_ns - start_ns) / 1000000))
+  echo "$implementation,$iteration,$wall_ms,$peak_rss,$peak_tasks" >>"$results"
+  printf '%-5s run %d: %6d ms  %8d KiB  %4d tasks\n' \
+    "$implementation" "$iteration" "$wall_ms" "$peak_rss" "$peak_tasks"
+}
+
+median_column() {
+  local implementation=$1 column=$2 middle=$((runs / 2 + 1))
+  awk -F, -v implementation="$implementation" \
+    '$1 == implementation { print $'"$column"' }' "$results" \
+    | sort -n | sed -n "${middle}p"
+}
+
+echo "Rewatch clean-build performance gate"
+echo "commit: $(git -C "$repo_root" rev-parse HEAD)"
+echo "host: $(uname -a)"
+echo "cpus: $(getconf _NPROCESSORS_ONLN 2>/dev/null || echo unknown)"
+echo "runs: $runs (interleaved after one warm-up each)"
+echo "threshold: ${threshold_percent}% of Rust median wall and RSS"
+
+clean_and_build "$rust_executable" "$rust_fixture" "$work_root/rust-warmup"
+clean_and_build "$ocaml_executable" "$ocaml_fixture" "$work_root/ocaml-warmup"
+
+for ((iteration = 1; iteration <= runs; iteration++)); do
+  if ((iteration % 2 == 1)); then
+    measure rust "$rust_executable" "$rust_fixture" "$iteration"
+    measure ocaml "$ocaml_executable" "$ocaml_fixture" "$iteration"
+  else
+    measure ocaml "$ocaml_executable" "$ocaml_fixture" "$iteration"
+    measure rust "$rust_executable" "$rust_fixture" "$iteration"
+  fi
+done
+
+rust_wall=$(median_column rust 3)
+ocaml_wall=$(median_column ocaml 3)
+rust_rss=$(median_column rust 4)
+ocaml_rss=$(median_column ocaml 4)
+rust_tasks=$(median_column rust 5)
+ocaml_tasks=$(median_column ocaml 5)
+printf 'median Rust:  %6d ms  %8d KiB  %4d peak tasks\n' \
+  "$rust_wall" "$rust_rss" "$rust_tasks"
+printf 'median OCaml: %6d ms  %8d KiB  %4d peak tasks\n' \
+  "$ocaml_wall" "$ocaml_rss" "$ocaml_tasks"
+
+trace_and_classify() {
+  local implementation=$1 scenario=$2 executable=$3 fixture=$4 manifest=$5
+  local clean_first=$6
+  local trace_prefix="$work_root/${implementation}-${scenario}.execve"
+  if [[ "$clean_first" == 1 ]]; then
+    "$executable" clean "$fixture" >/dev/null 2>&1
+  fi
+  strace -f -ff -qq -s 4096 -e trace=execve,chdir -o "$trace_prefix" \
+    "$executable" build "$fixture" \
+    >"$work_root/${implementation}-${scenario}-trace.out" \
+    2>"$work_root/${implementation}-${scenario}-trace.stderr"
+  local trace_files=("$trace_prefix".*)
+  local implementation_root=${fixture%/rewatch/testrepo}
+  local trace_file exec_line argv cwd_line cwd phase input identity
+  : >"$manifest.unsorted"
+  for trace_file in "${trace_files[@]}"; do
+    exec_line=$(grep -m1 -F "execve(\"$RESCRIPT_BSC_EXE\"" "$trace_file" \
+      || grep -m1 -E 'execve\("[^"]*sury-ppx' "$trace_file" || true)
+    if [[ -z "$exec_line" ]]; then
+      continue
+    fi
+    argv=${exec_line#*, }
+    argv=${argv%%], 0x*}]
+    cwd_line=$(grep -m1 '^chdir("' "$trace_file" || true)
+    cwd=${cwd_line#chdir(\"}
+    cwd=${cwd%%\"*}
+    if [[ "$exec_line" == *"execve(\"$RESCRIPT_BSC_EXE\""* ]]; then
+      if [[ "$argv" == *'"-bs-ast"'* ]]; then
+        phase=parse
+      elif [[ "$argv" == *'.mlmap"'* ]]; then
+        phase=namespace
+      else
+        phase=compile
+      fi
+      input=${argv##*, }
+      input=${input%]}
+      identity="$cwd"$'\t'"$phase"$'\t'"$input"
+    else
+      # PPX temporary input/output names are deliberately randomized. Its
+      # executable identity and count are the stable unit of work.
+      identity=ppx$'\t'"${argv%%,*}"
+    fi
+    printf '%s\n' "$identity" \
+      | sed "s#$implementation_root#<ROOT>#g" >>"$manifest.unsorted"
+  done
+  sort "$manifest.unsorted" >"$manifest"
+  local invocations parse namespace compile interface ppx
+  invocations=$(grep -hF -c "execve(\"$RESCRIPT_BSC_EXE\"" "${trace_files[@]}" \
+    | awk '{ total += $1 } END { print total + 0 }')
+  parse=$(grep -hF "execve(\"$RESCRIPT_BSC_EXE\"" "${trace_files[@]}" \
+    | grep -F -c '"-bs-ast"' || true)
+  namespace=$(grep -hF "execve(\"$RESCRIPT_BSC_EXE\"" "${trace_files[@]}" \
+    | grep -E -c '\.mlmap"' || true)
+  interface=$(grep -hF "execve(\"$RESCRIPT_BSC_EXE\"" "${trace_files[@]}" \
+    | grep -vF '"-bs-ast"' | grep -vE '\.mlmap"' \
+    | grep -E -c '\.iast"' || true)
+  compile=$((invocations - parse - namespace))
+  ppx=$(grep -hE -c 'execve\("[^"]*sury-ppx' "${trace_files[@]}" \
+    | awk '{ total += $1 } END { print total + 0 }')
+  echo "$invocations,$parse,$namespace,$compile,$interface,$ppx"
+}
+
+rust_clean_invocations="$work_root/rust-clean-invocations.txt"
+ocaml_clean_invocations="$work_root/ocaml-clean-invocations.txt"
+rust_clean_work=$(trace_and_classify rust clean "$rust_executable" \
+  "$rust_fixture" "$rust_clean_invocations" 1)
+ocaml_clean_work=$(trace_and_classify ocaml clean "$ocaml_executable" \
+  "$ocaml_fixture" "$ocaml_clean_invocations" 1)
+
+rust_unchanged_invocations="$work_root/rust-unchanged-invocations.txt"
+ocaml_unchanged_invocations="$work_root/ocaml-unchanged-invocations.txt"
+rust_unchanged_work=$(trace_and_classify rust unchanged "$rust_executable" \
+  "$rust_fixture" "$rust_unchanged_invocations" 0)
+ocaml_unchanged_work=$(trace_and_classify ocaml unchanged "$ocaml_executable" \
+  "$ocaml_fixture" "$ocaml_unchanged_invocations" 0)
+
+printf '\n// benchmark single edit\n' \
+  >>"$rust_fixture/packages/watch-warnings/src/B.res"
+printf '\n// benchmark single edit\n' \
+  >>"$ocaml_fixture/packages/watch-warnings/src/B.res"
+rust_edit_invocations="$work_root/rust-edit-invocations.txt"
+ocaml_edit_invocations="$work_root/ocaml-edit-invocations.txt"
+rust_edit_work=$(trace_and_classify rust edit "$rust_executable" \
+  "$rust_fixture" "$rust_edit_invocations" 0)
+ocaml_edit_work=$(trace_and_classify ocaml edit "$ocaml_executable" \
+  "$ocaml_fixture" "$ocaml_edit_invocations" 0)
+
+echo "work columns: bsc_total,parse,namespace,compile,interfaces,ppx"
+echo "clean Rust:      $rust_clean_work"
+echo "clean OCaml:     $ocaml_clean_work"
+echo "unchanged Rust:  $rust_unchanged_work"
+echo "unchanged OCaml: $ocaml_unchanged_work"
+echo "edit Rust:       $rust_edit_work"
+echo "edit OCaml:      $ocaml_edit_work"
+
+artifact_manifest() {
+  local root=$1 output=$2
+  find "$root" -type f \
+    \( -name '*.ast' -o -name '*.iast' -o -name '*.cmi' -o -name '*.cmj' \
+       -o -name '*.mlmap' \
+       -o -name '*.js' -o -name '*.mjs' -o -name '*.cjs' -o -name '*.map' \
+       -o -name '*.res' -o -name '*.resi' -o -name '.sourcedirs.json' \
+       -o -name build.ninja \) \
+    ! -name '.compiler.log' ! -name compiler-info.json -print0 \
+    | sort -z | xargs -0 sha256sum | sed "s#$root/##" >"$output"
+}
+
+file_set_manifest() {
+  local root=$1 output=$2
+  find "$root" -type f -printf '%P\n' | sort >"$output"
+}
+
+# Use the same absolute path for both builds so paths embedded in binary
+# compiler artifacts are directly comparable byte for byte.
+equivalence_root="$work_root/equivalence"
+prepare_fixture "$equivalence_root"
+equivalence_fixture="$equivalence_root/rewatch/testrepo"
+rust_artifacts="$work_root/rust-artifacts.sha256"
+ocaml_artifacts="$work_root/ocaml-artifacts.sha256"
+rust_files="$work_root/rust-files.txt"
+ocaml_files="$work_root/ocaml-files.txt"
+clean_and_build "$rust_executable" "$equivalence_fixture" \
+  "$work_root/rust-equivalence"
+artifact_manifest "$equivalence_root" "$rust_artifacts"
+file_set_manifest "$equivalence_root" "$rust_files"
+# Recreate, rather than clean, the fixture so OCaml cannot inherit an artifact
+# that only Rust produced. Reusing the same pathname keeps embedded paths equal.
+find "$equivalence_root" -depth -delete
+prepare_fixture "$equivalence_root"
+clean_and_build "$ocaml_executable" "$equivalence_fixture" \
+  "$work_root/ocaml-equivalence"
+artifact_manifest "$equivalence_root" "$ocaml_artifacts"
+file_set_manifest "$equivalence_root" "$ocaml_files"
+if cmp -s "$rust_files" "$ocaml_files"; then
+  file_set_equivalence=1
+  echo "files: identical complete post-build file sets"
+else
+  file_set_equivalence=0
+  echo "complete post-build file-set diff:" >&2
+  diff -u "$rust_files" "$ocaml_files" >&2 || true
+fi
+if cmp -s "$rust_artifacts" "$ocaml_artifacts"; then
+  artifact_equivalence=1
+  echo "artifacts: identical generated file sets and contents"
+else
+  artifact_equivalence=0
+  echo "artifact manifest diff:" >&2
+  diff -u "$rust_artifacts" "$ocaml_artifacts" >&2 || true
+fi
+
+failed=0
+if ((runs >= 5 && ocaml_wall * 100 > rust_wall * threshold_percent)); then
+  echo "FAIL: OCaml median wall time exceeds the threshold." >&2
+  failed=1
+fi
+if ((runs >= 5 && ocaml_rss * 100 > rust_rss * threshold_percent)); then
+  echo "FAIL: OCaml median peak tree RSS exceeds the threshold." >&2
+  failed=1
+fi
+for scenario in clean unchanged edit; do
+  rust_work_variable="rust_${scenario}_work"
+  ocaml_work_variable="ocaml_${scenario}_work"
+  rust_manifest_variable="rust_${scenario}_invocations"
+  ocaml_manifest_variable="ocaml_${scenario}_invocations"
+  rust_work_value=${!rust_work_variable}
+  ocaml_work_value=${!ocaml_work_variable}
+  rust_manifest=${!rust_manifest_variable}
+  ocaml_manifest=${!ocaml_manifest_variable}
+  if [[ "$rust_work_value" != "$ocaml_work_value" ]]; then
+    echo "FAIL: Rust and OCaml performed different $scenario compiler work." >&2
+    failed=1
+  fi
+  if ! cmp -s "$rust_manifest" "$ocaml_manifest"; then
+    echo "FAIL: Rust and OCaml performed different $scenario module/PPX work." >&2
+    diff -u "$rust_manifest" "$ocaml_manifest" >&2 || true
+    failed=1
+  fi
+done
+if ((file_set_equivalence == 0)); then
+  echo "FAIL: Rust and OCaml produced different post-build file sets." >&2
+  failed=1
+fi
+if ((artifact_equivalence == 0)); then
+  echo "FAIL: Rust and OCaml generated different artifacts." >&2
+  failed=1
+fi
+
+if ((failed)); then
+  exit 1
+fi
+if ((runs < 5)); then
+  echo "PASS: correctness smoke checks passed; performance gate not evaluated."
+else
+  echo "PASS: timing, memory, compiler-work, and artifact-equivalence gates passed."
+fi
diff --git a/rewatch-ocaml/bench/source_size.sh b/rewatch-ocaml/bench/source_size.sh
new file mode 100755
index 00000000000..906a1507e2e
--- /dev/null
+++ b/rewatch-ocaml/bench/source_size.sh
@@ -0,0 +1,85 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+repo_root=$(cd "$(dirname "$0")/../.." && pwd)
+cloc_command=${CLOC:-cloc}
+
+if ! command -v "$cloc_command" >/dev/null 2>&1; then
+  echo "source_size.sh requires cloc (or set CLOC to its executable)" >&2
+  exit 1
+fi
+
+work_dir=$(mktemp -d)
+trap 'rm -r "$work_dir"' EXIT
+
+rust_production="$work_dir/rust-production"
+rust_tests="$work_dir/rust-tests"
+mkdir -p "$rust_production" "$rust_tests"
+while IFS= read -r source; do
+  relative=${source#"$repo_root/rewatch/src/"}
+  destination="$rust_production/$relative"
+  test_destination="$rust_tests/$relative"
+  mkdir -p "$(dirname "$destination")"
+  mkdir -p "$(dirname "$test_destination")"
+  # Rust keeps unit tests beside production code. Stop at the first test-only
+  # module so the production comparison matches OCaml's separate test package.
+  awk '/^#\[cfg\(test\)\]/{exit} {print}' "$source" > "$destination"
+  awk 'found || /^#\[cfg\(test\)\]/{found = 1; print}' "$source" \
+    > "$test_destination"
+done < <(find "$repo_root/rewatch/src" -type f -name '*.rs' \
+  ! -name telemetry.rs | sort)
+
+mapfile -t ocaml_production < <(find "$repo_root/rewatch-ocaml" -maxdepth 1 \
+  -type f \( -name '*.ml' -o -name '*.mli' -o -name '*.c' \) | sort)
+mapfile -t ocaml_unit_tests < <(find "$repo_root/tests/rewatch_ounit_tests" \
+  -maxdepth 1 -type f -name '*.ml' | sort)
+mapfile -t ocaml_focused_test_relative < <(git -C "$repo_root" ls-files \
+  rewatch-ocaml/tests | sort)
+ocaml_focused_tests=()
+for relative in "${ocaml_focused_test_relative[@]}"; do
+  ocaml_focused_tests+=("$repo_root/$relative")
+done
+mapfile -t ocaml_benchmark_tooling < <(find "$repo_root/rewatch-ocaml/bench" \
+  -maxdepth 1 -type f \( -name '*.sh' -o -name '*.js' \) | sort)
+
+count() {
+  local label=$1
+  shift
+  local totals
+  totals=$("$cloc_command" --csv --quiet --skip-uniqueness "$@" \
+    | awk -F, '$2 == "SUM" {print $3 "," $4 "," $5}')
+  printf '%-34s %8s %8s %8s\n' "$label" \
+    "${totals%%,*}" "$(cut -d, -f2 <<< "$totals")" "${totals##*,}"
+}
+
+printf '%-34s %8s %8s %8s\n' Scope Blank Comment Code
+count "Rust production, no telemetry" "$rust_production"
+count "Rust unit tests, no telemetry" "$rust_tests"
+count "OCaml-port production" "${ocaml_production[@]}"
+count "OCaml test code and fixtures" \
+  --force-lang=ReScript,fixed --force-lang=ReScript,invalid \
+  "${ocaml_unit_tests[@]}" "${ocaml_focused_tests[@]}"
+count "OCaml benchmark tooling" "${ocaml_benchmark_tooling[@]}"
+
+largest() {
+  local label=$1
+  shift
+  printf '\n%s (code lines):\n' "$label"
+  "$cloc_command" --by-file --csv --quiet --skip-uniqueness "$@" \
+    | awk -F, -v root="$repo_root/" \
+        '$1 != "language" && $1 != "SUM" {
+          sub("^" root, "", $2);
+          printf "%8d  %s\n", $5, $2
+        }' \
+    | sort -nr \
+    | head -10
+}
+
+largest "Largest OCaml production modules" "${ocaml_production[@]}"
+largest "Largest OCaml test/tooling files" \
+  --force-lang=ReScript,fixed --force-lang=ReScript,invalid \
+  "${ocaml_unit_tests[@]}" "${ocaml_focused_tests[@]}" \
+  "${ocaml_benchmark_tooling[@]}"
+
+printf '\ncloc version: %s\n' "$("$cloc_command" --version)"
diff --git a/rewatch-ocaml/bench/watch_filesystem_audit.sh b/rewatch-ocaml/bench/watch_filesystem_audit.sh
new file mode 100755
index 00000000000..239ef9eda10
--- /dev/null
+++ b/rewatch-ocaml/bench/watch_filesystem_audit.sh
@@ -0,0 +1,171 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -ne 2 ]]; then
+  echo "Usage: $0 RUST_REWATCH OCAML_REWATCH" >&2
+  exit 2
+fi
+
+repo_root=$(cd "$(dirname "$0")/../.." && pwd)
+rust_executable=$(cd "$(dirname "$1")" && pwd)/$(basename "$1")
+ocaml_executable=$(cd "$(dirname "$2")" && pwd)/$(basename "$2")
+normalizer="$repo_root/rewatch-ocaml/bench/normalize_file_trace.js"
+
+for command in awk cat cp date find grep join mktemp node sed seq setsid sleep sort strace wc; do
+  command -v "$command" >/dev/null || {
+    echo "Missing required command: $command" >&2
+    exit 2
+  }
+done
+if [[ ! -x "$rust_executable" || ! -x "$ocaml_executable" ]]; then
+  echo "Both rewatch executables must exist and be executable." >&2
+  exit 2
+fi
+
+if [[ -z ${RESCRIPT_BSC_EXE:-} || -z ${RESCRIPT_RUNTIME:-} ]]; then
+  eval "$(cd "$repo_root/rewatch/tests" && node ./get_bin_paths.js)"
+fi
+export RESCRIPT_BSC_EXE RESCRIPT_RUNTIME
+
+work_root=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-watch-audit.XXXXXX")
+background_pids=""
+terminate_group() {
+  local pid=$1
+  local reaped=false state
+  if ! kill -TERM -- "-$pid" 2>/dev/null; then
+    kill -TERM "$pid" 2>/dev/null || true
+  fi
+  for _ in $(seq 1 100); do
+    if [[ $reaped == false && -r /proc/$pid/stat ]]; then
+      state=$(awk '{print $3}' "/proc/$pid/stat" 2>/dev/null || true)
+      if [[ $state == Z ]]; then
+        wait "$pid" 2>/dev/null || true
+        reaped=true
+      fi
+    fi
+    if ! kill -0 -- "-$pid" 2>/dev/null; then
+      if [[ $reaped == false ]]; then wait "$pid" 2>/dev/null || true; fi
+      return
+    fi
+    sleep 0.05
+  done
+  kill -KILL -- "-$pid" 2>/dev/null || true
+  if [[ $reaped == false ]]; then kill -KILL "$pid" 2>/dev/null || true; fi
+  if [[ $reaped == false ]]; then wait "$pid" 2>/dev/null || true; fi
+}
+cleanup() {
+  trap - EXIT INT TERM
+  for pid in $background_pids; do
+    terminate_group "$pid"
+  done
+  if [[ ${KEEP_REWATCH_WATCH_AUDIT:-0} == 1 ]]; then
+    echo "Kept watch audit workdir: $work_root" >&2
+  else
+    find "$work_root" -depth -delete
+  fi
+}
+trap cleanup EXIT INT TERM
+
+marker_script="$work_root/mark-build.mjs"
+printf '%s\n' \
+  'import fs from "node:fs";' \
+  'fs.appendFileSync(process.env.REWATCH_WATCH_AUDIT_MARKER, `${Date.now()}\n`);' \
+  >"$marker_script"
+
+wait_for_lines() {
+  local path=$1 expected=$2
+  for _ in $(seq 1 400); do
+    if [[ -f "$path" ]] && [[ $(wc -l <"$path") -ge $expected ]]; then
+      return
+    fi
+    sleep 0.05
+  done
+  echo "Timed out waiting for $expected build markers in $path" >&2
+  exit 1
+}
+
+wait_for_watch_ready() {
+  local trace_prefix=$1 source_directory=$2
+  for _ in $(seq 1 400); do
+    if grep -hF "inotify_add_watch" "$trace_prefix".* 2>/dev/null \
+      | grep -Fq "\"$source_directory\""; then
+      return
+    fi
+    sleep 0.05
+  done
+  echo "Timed out waiting for a native watch on $source_directory" >&2
+  exit 1
+}
+
+wait_for_native_loop() {
+  local trace_pid=$1
+  for _ in $(seq 1 400); do
+    local child task wait_channel
+    for child in $(cat "/proc/$trace_pid/task/$trace_pid/children" 2>/dev/null || true); do
+      for task in "/proc/$child"/task/*/wchan; do
+        [[ -r $task ]] || continue
+        wait_channel=$(cat "$task")
+        if [[ $wait_channel == *epoll* || $wait_channel == ep_poll* ]]; then
+          return
+        fi
+      done
+    done
+    sleep 0.05
+  done
+  echo "Timed out waiting for the watcher event loop" >&2
+  exit 1
+}
+
+trace_watch_edit() {
+  local implementation=$1 executable=$2
+  local fixture="$work_root/$implementation"
+  local marker="$work_root/$implementation.marker"
+  local trace_prefix="$work_root/$implementation-edit.file"
+  local normalized="$work_root/$implementation-edit"
+  cp -R "$repo_root/rewatch-ocaml/tests/basic" "$fixture"
+  (
+    cd "$fixture"
+    export REWATCH_WATCH_AUDIT_MARKER="$marker"
+    exec setsid strace -f -ff -qq -ttt -yy -s 4096 \
+        -e trace=%file,getdents64 \
+        -o "$trace_prefix" "$executable" watch \
+        --after-build "node $marker_script" . \
+        >"$normalized.stdout" 2>"$normalized.stderr"
+  ) &
+  local trace_pid=$!
+  background_pids="$background_pids $trace_pid"
+  wait_for_lines "$marker" 1
+  wait_for_watch_ready "$trace_prefix" "$fixture/src"
+  # The after-build hook runs before the initial build has returned to the
+  # watcher. Wait for a subsequent blocking event-loop iteration so initial
+  # reconciliation work cannot be mistaken for retained-edit work.
+  wait_for_native_loop "$trace_pid"
+  local start_epoch end_epoch
+  start_epoch=$(date +%s.%N)
+  printf 'let answer = A.value + 2\n' >"$fixture/src/B.res"
+  wait_for_lines "$marker" 2
+  end_epoch=$(date +%s.%N)
+  rm -f "$fixture/lib/watch.lock"
+  wait "$trace_pid"
+  background_pids=${background_pids% $trace_pid}
+  node "$normalizer" "$trace_prefix" "$fixture" "$normalized" \
+    "$start_epoch" "$end_epoch"
+}
+
+trace_watch_edit rust "$rust_executable"
+trace_watch_edit ocaml "$ocaml_executable"
+
+echo "watch single-edit project filesystem categories (Rust / OCaml)"
+join -a 1 -a 2 -e 0 -o 0,1.2,2.2 \
+  "$work_root/rust-edit.categories.tsv" \
+  "$work_root/ocaml-edit.categories.tsv"
+for implementation in rust ocaml; do
+  echo
+  echo "$implementation most repeated project path operations"
+  sort -t $'\t' -k1,1nr "$work_root/$implementation-edit.paths.tsv" \
+    | sed -n '1,20p'
+done
+
+echo
+echo "This audit covers one edit inside an already-running watcher. Inspect"
+echo "retained traces for per-process attribution and exact repeated paths."
diff --git a/rewatch-ocaml/bench/watch_performance_gate.sh b/rewatch-ocaml/bench/watch_performance_gate.sh
new file mode 100755
index 00000000000..b21f38c5307
--- /dev/null
+++ b/rewatch-ocaml/bench/watch_performance_gate.sh
@@ -0,0 +1,287 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -lt 2 || $# -gt 3 ]]; then
+  echo "Usage: $0 RUST_REWATCH OCAML_REWATCH [RUNS]" >&2
+  exit 2
+fi
+
+repo_root=$(cd "$(dirname "$0")/../.." && pwd)
+rust_executable=$(cd "$(dirname "$1")" && pwd)/$(basename "$1")
+ocaml_executable=$(cd "$(dirname "$2")" && pwd)/$(basename "$2")
+runs=${3:-7}
+threshold_percent=${REWATCH_WATCH_PERFORMANCE_THRESHOLD_PERCENT:-150}
+
+if ((runs < 5 || runs % 2 == 0)); then
+  echo "RUNS must be an odd number of at least five." >&2
+  exit 2
+fi
+for command in awk cat cmp cp date find grep mktemp node sed seq setsid sleep sort tail wc; do
+  command -v "$command" >/dev/null || {
+    echo "Missing required command: $command" >&2
+    exit 2
+  }
+done
+if [[ ! -x $rust_executable || ! -x $ocaml_executable ]]; then
+  echo "Both rewatch executables must exist and be executable." >&2
+  exit 2
+fi
+if [[ ! -d /proc/self/fd ]]; then
+  echo "The retained-watch performance gate requires Linux /proc." >&2
+  exit 2
+fi
+
+if [[ -z ${RESCRIPT_BSC_EXE:-} || -z ${RESCRIPT_RUNTIME:-} ]]; then
+  eval "$(cd "$repo_root/rewatch/tests" && node ./get_bin_paths.js)"
+fi
+real_bsc=$RESCRIPT_BSC_EXE
+runtime=$RESCRIPT_RUNTIME
+counting_bsc="$repo_root/_build/default/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.exe"
+
+work_root=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-watch-performance.XXXXXX")
+declare -A pids=()
+terminate_group() {
+  local pid=$1
+  local reaped=false state
+  if ! kill -TERM -- "-$pid" 2>/dev/null; then
+    kill -TERM "$pid" 2>/dev/null || true
+  fi
+  for _ in $(seq 1 100); do
+    if [[ $reaped == false && -r /proc/$pid/stat ]]; then
+      state=$(awk '{print $3}' "/proc/$pid/stat" 2>/dev/null || true)
+      if [[ $state == Z ]]; then
+        wait "$pid" 2>/dev/null || true
+        reaped=true
+      fi
+    fi
+    if ! kill -0 -- "-$pid" 2>/dev/null; then
+      if [[ $reaped == false ]]; then wait "$pid" 2>/dev/null || true; fi
+      return
+    fi
+    sleep 0.05
+  done
+  kill -KILL -- "-$pid" 2>/dev/null || true
+  if [[ $reaped == false ]]; then kill -KILL "$pid" 2>/dev/null || true; fi
+  if [[ $reaped == false ]]; then wait "$pid" 2>/dev/null || true; fi
+}
+cleanup() {
+  trap - EXIT INT TERM
+  local implementation pid
+  for implementation in "${!pids[@]}"; do
+    pid=${pids[$implementation]}
+    terminate_group "$pid"
+  done
+  if [[ ${KEEP_REWATCH_WATCH_PERFORMANCE:-0} == 1 ]]; then
+    echo "Kept watch performance workdir: $work_root" >&2
+  else
+    find "$work_root" -depth -delete
+  fi
+}
+trap cleanup EXIT INT TERM
+
+marker_script="$work_root/mark-build.mjs"
+printf '%s\n' \
+  'import fs from "node:fs";' \
+  'fs.appendFileSync(process.env.REWATCH_WATCH_MARKER, `${Date.now()}\n`);' \
+  >"$marker_script"
+
+wait_for_lines() {
+  local path=$1 expected=$2
+  for _ in $(seq 1 400); do
+    if [[ -f $path ]] && [[ $(wc -l <"$path") -ge $expected ]]; then return; fi
+    sleep 0.05
+  done
+  echo "Timed out waiting for $expected lines in $path" >&2
+  exit 1
+}
+
+wait_for_text_count() {
+  local path=$1 text=$2 expected=$3
+  for _ in $(seq 1 400); do
+    if [[ -f $path ]] && \
+      [[ $(grep -cF "$text" "$path" 2>/dev/null || true) -ge $expected ]]; then
+      return
+    fi
+    sleep 0.05
+  done
+  echo "Timed out waiting for occurrence $expected of '$text' in $path" >&2
+  exit 1
+}
+
+wait_for_idle() {
+  local pid=$1
+  for _ in $(seq 1 400); do
+    if [[ ! -d /proc/$pid ]]; then
+      echo "Watcher $pid exited before becoming idle." >&2
+      exit 1
+    fi
+    local children state
+    children=$(cat "/proc/$pid/task/$pid/children" 2>/dev/null || true)
+    state=$(awk '{print $3}' "/proc/$pid/stat")
+    if [[ -z $children && $state == S ]]; then return; fi
+    sleep 0.01
+  done
+  echo "Timed out waiting for watcher $pid to become idle." >&2
+  exit 1
+}
+
+resource_value() {
+  local pid=$1 kind=$2
+  case $kind in
+    fd) find "/proc/$pid/fd" -mindepth 1 -maxdepth 1 | wc -l ;;
+    tasks) find "/proc/$pid/task" -mindepth 1 -maxdepth 1 -type d | wc -l ;;
+    rss) awk '/^VmRSS:/ {print $2}' "/proc/$pid/status" ;;
+  esac
+}
+
+start_watcher() {
+  local implementation=$1 executable=$2 fixture
+  fixture="$work_root/$implementation"
+  cp -R "$repo_root/rewatch-ocaml/tests/basic" "$fixture"
+  : >"$work_root/$implementation.bsc"
+  setsid env \
+    RESCRIPT_BSC_EXE="$counting_bsc" \
+    REWATCH_BSC_PROXY_MODE=counting \
+    RESCRIPT_RUNTIME="$runtime" \
+    REWATCH_REAL_BSC="$real_bsc" \
+    REWATCH_BSC_CALL_LOG="$work_root/$implementation.bsc" \
+    REWATCH_WATCH_MARKER="$work_root/$implementation.marker" \
+    "$executable" watch --after-build "node $marker_script" "$fixture" \
+    >"$work_root/$implementation.stdout" \
+    2>"$work_root/$implementation.stderr" &
+  pids[$implementation]=$!
+  wait_for_lines "$work_root/$implementation.marker" 1
+  wait_for_text_count "$work_root/$implementation.stdout" \
+    "Finished initial compilation" 1
+  wait_for_idle "${pids[$implementation]}"
+}
+
+start_watcher rust "$rust_executable"
+start_watcher ocaml "$ocaml_executable"
+
+# One unmeasured edit pays any first-incremental initialization cost before the
+# retained-state samples begin.
+for implementation in rust ocaml; do
+  printf 'let answer = A.value + 1\n// warm retained edit\n' \
+    >"$work_root/$implementation/src/B.res"
+  wait_for_lines "$work_root/$implementation.marker" 2
+  wait_for_text_count "$work_root/$implementation.stdout" \
+    "Finished incremental compilation" 1
+  wait_for_idle "${pids[$implementation]}"
+  : >"$work_root/$implementation.bsc"
+done
+
+declare -A baseline_fd baseline_tasks baseline_rss max_fd max_tasks max_rss
+for implementation in rust ocaml; do
+  pid=${pids[$implementation]}
+  baseline_fd[$implementation]=$(resource_value "$pid" fd)
+  baseline_tasks[$implementation]=$(resource_value "$pid" tasks)
+  baseline_rss[$implementation]=$(resource_value "$pid" rss)
+  max_fd[$implementation]=${baseline_fd[$implementation]}
+  max_tasks[$implementation]=${baseline_tasks[$implementation]}
+  max_rss[$implementation]=${baseline_rss[$implementation]}
+  : >"$work_root/$implementation.latencies"
+done
+
+measure_edit() {
+  local implementation=$1 round=$2 expected=$((round + 2))
+  local started finished latency pid value
+  started=$(date +%s%3N)
+  printf 'let answer = A.value + 1\n// retained edit %d\n' "$round" \
+    >"$work_root/$implementation/src/B.res"
+  wait_for_lines "$work_root/$implementation.marker" "$expected"
+  finished=$(tail -n 1 "$work_root/$implementation.marker")
+  latency=$((finished - started))
+  printf '%d\n' "$latency" >>"$work_root/$implementation.latencies"
+  wait_for_text_count "$work_root/$implementation.stdout" \
+    "Finished incremental compilation" "$((round + 1))"
+  wait_for_idle "${pids[$implementation]}"
+  pid=${pids[$implementation]}
+  for kind in fd tasks rss; do
+    value=$(resource_value "$pid" "$kind")
+    case $kind in
+      fd) if ((value > max_fd[$implementation])); then max_fd[$implementation]=$value; fi ;;
+      tasks) if ((value > max_tasks[$implementation])); then max_tasks[$implementation]=$value; fi ;;
+      rss) if ((value > max_rss[$implementation])); then max_rss[$implementation]=$value; fi ;;
+    esac
+  done
+}
+
+for round in $(seq 1 "$runs"); do
+  if ((round % 2 == 1)); then
+    measure_edit rust "$round"
+    measure_edit ocaml "$round"
+  else
+    measure_edit ocaml "$round"
+    measure_edit rust "$round"
+  fi
+  if ! cmp -s "$work_root/rust/src/B.mjs" "$work_root/ocaml/src/B.mjs"; then
+    echo "Generated output differs after retained edit $round." >&2
+    exit 1
+  fi
+done
+
+median() {
+  sort -n "$1" | sed -n "$((runs / 2 + 1))p"
+}
+rust_median=$(median "$work_root/rust.latencies")
+ocaml_median=$(median "$work_root/ocaml.latencies")
+
+normalize_calls() {
+  local implementation=$1
+  sed "s#$work_root/$implementation#<ROOT>#g" \
+    "$work_root/$implementation.bsc" >"$work_root/$implementation.bsc.normalized"
+}
+normalize_calls rust
+normalize_calls ocaml
+if ! cmp -s "$work_root/rust.bsc.normalized" \
+  "$work_root/ocaml.bsc.normalized"; then
+  echo "Rust and OCaml retained edits performed different compiler work." >&2
+  exit 1
+fi
+for implementation in rust ocaml; do
+  parse_count=$(grep -cF -- '-bs-ast' "$work_root/$implementation.bsc" || true)
+  total_count=$(wc -l <"$work_root/$implementation.bsc")
+  if ((parse_count != runs || total_count != runs * 2)); then
+    printf '%s retained work was %d parser / %d total calls; expected %d / %d.\n' \
+      "$implementation" "$parse_count" "$total_count" "$runs" "$((runs * 2))" >&2
+    exit 1
+  fi
+done
+
+for implementation in rust ocaml; do
+  pid=${pids[$implementation]}
+  final_fd=$(resource_value "$pid" fd)
+  final_tasks=$(resource_value "$pid" tasks)
+  final_rss=$(resource_value "$pid" rss)
+  if ((max_fd[$implementation] > baseline_fd[$implementation] + 4 ||
+       max_tasks[$implementation] > baseline_tasks[$implementation] + 2 ||
+       max_rss[$implementation] > baseline_rss[$implementation] + 16384)); then
+    echo "$implementation watcher resource growth exceeded the retained gate." >&2
+    exit 1
+  fi
+  printf '%-5s resources: fd %d→%d (max %d), tasks %d→%d (max %d), RSS %d→%d KiB (max %d)\n' \
+    "$implementation" "${baseline_fd[$implementation]}" "$final_fd" \
+    "${max_fd[$implementation]}" "${baseline_tasks[$implementation]}" \
+    "$final_tasks" "${max_tasks[$implementation]}" \
+    "${baseline_rss[$implementation]}" "$final_rss" "${max_rss[$implementation]}"
+done
+
+if ((ocaml_median * 100 > rust_median * threshold_percent)); then
+  printf 'OCaml retained-watch median %d ms exceeds %d%% of Rust median %d ms.\n' \
+    "$ocaml_median" "$threshold_percent" "$rust_median" >&2
+  exit 1
+fi
+
+printf 'retained-watch median: Rust %d ms, OCaml %d ms (limit %d%%)\n' \
+  "$rust_median" "$ocaml_median" "$threshold_percent"
+printf 'compiler work: %d parser and %d compiler calls per implementation\n' \
+  "$runs" "$runs"
+
+for implementation in rust ocaml; do
+  rm -f "$work_root/$implementation/lib/watch.lock"
+  wait "${pids[$implementation]}"
+  unset "pids[$implementation]"
+done
+
+echo "Retained-watch performance, work, output, and resource gates passed"
diff --git a/rewatch-ocaml/build.ml b/rewatch-ocaml/build.ml
new file mode 100644
index 00000000000..900afee335f
--- /dev/null
+++ b/rewatch-ocaml/build.ml
@@ -0,0 +1,564 @@
+exception Error = Project_context.Error
+exception Package_error = Project_context.Package_error
+exception Build_failure = Compiler_scheduler.Build_failure
+exception Parse_failure of string
+exception Reported_failure of string
+exception Full_rebuild_required
+
+type retained_build = {
+  root_config: Config.t;
+  build_lock_root: string;
+  session: Build_session.t;
+}
+
+type attempt_request =
+  | One_shot_attempt
+  | Initial_watch_attempt
+  | Full_watch_attempt
+  | Retained_watch_attempt of {
+      previous: retained_build;
+      changes: Watcher.change list;
+    }
+
+(* Build kind controls which persistent markers and diagnostics may be reused.
+   Keeping all four states explicit prevents an initial watch build from being
+   mistaken for either a disposable command or a retained incremental edit. *)
+type compilation_kind = Build_attempt.compilation_kind =
+  | One_shot
+  | Initial_watch
+  | Incremental_watch
+  | Full_watch
+
+let compilation_kind = function
+  | One_shot_attempt -> One_shot
+  | Initial_watch_attempt -> Initial_watch
+  | Full_watch_attempt -> Full_watch
+  | Retained_watch_attempt _ -> Incremental_watch
+
+let previous_build = function
+  | Retained_watch_attempt {previous; changes = _} -> Some previous
+  | One_shot_attempt | Initial_watch_attempt | Full_watch_attempt -> None
+
+type incremental_source = {
+  package: Package_plan.t;
+  source: Build_session.source_reference;
+}
+
+let run_scheduled_modules (attempt : Build_attempt.t)
+    (prepared : Build_session.prepared) ~compile_step ~namespace_count =
+  Compiler_scheduler.run ~poll:attempt.process_poll
+    ~warning_state:(Build_session.warning_state attempt.session)
+    ~compile_assets:prepared.compile_assets ~build_state:prepared.build_state
+    ~candidates:(Build_attempt.take_compile_candidates attempt)
+    ~mark_compiled:(fun () -> attempt.compiled <- attempt.compiled + 1)
+    ~mark_had_warnings:(fun () -> attempt.had_warnings <- true)
+    ~progress:attempt.progress ~compile_step ~namespace_count
+    ~verbosity:attempt.verbosity
+
+let run_namespace_jobs (attempt : Build_attempt.t) =
+  let jobs = Build_attempt.take_namespace_jobs attempt in
+  let started_at = Unix.gettimeofday () in
+  Fun.protect
+    ~finally:(fun () ->
+      attempt.parse_seconds <-
+        attempt.parse_seconds +. (Unix.gettimeofday () -. started_at))
+    (fun () ->
+      let results =
+        Process.run_parallel ?poll:attempt.process_poll
+          (List.map (fun job -> job.Build_attempt.job) jobs)
+      in
+      List.iter2
+        (fun job result -> job.Build_attempt.finish result)
+        jobs results);
+  List.length jobs
+
+let write_build_ninja (attempt : Build_attempt.t) =
+  (* This empty file is a cache-invalidation marker consumed by editor tooling,
+     not a serialized build plan. Only commands that reconstruct the project
+     graph call this function. *)
+  Build_session.iter_package_plans attempt.session (fun _ package ->
+      let path = Filename.concat package.Package_plan.build_dir "build.ninja" in
+      let channel = open_out_bin path in
+      close_out channel)
+
+let incremental_sources (previous : retained_build) changes =
+  (* Reusing the graph is safe only for modifications of already-known source
+     paths. Additions, removals, and unknown paths can change module identity or
+     package topology, so their caller must reconstruct the build instead. *)
+  let included = Hashtbl.create (List.length changes) in
+  let sources = ref [] in
+  let add normalized_path =
+    if not (Hashtbl.mem included normalized_path) then (
+      Hashtbl.add included normalized_path ();
+      match
+        Build_session.find_source_reference previous.session normalized_path
+      with
+      | Some source ->
+        let package =
+          match
+            Build_session.find_package_plan previous.session source.package_root
+          with
+          | Some package -> package
+          | None -> raise Full_rebuild_required
+        in
+        sources := {package; source} :: !sources
+      | None -> raise Full_rebuild_required)
+  in
+  List.iter
+    (fun (change : Watcher.change) ->
+      match change.kind with
+      | Watcher.Added | Watcher.Removed -> raise Full_rebuild_required
+      | Watcher.Modified ->
+        add (Platform.normalize_path_for_comparison change.path))
+    changes;
+  Build_session.pending_parse_paths previous.session
+  |> List.sort String.compare |> List.iter add;
+  List.rev !sources
+
+let prepare_incremental previous changes (attempt : Build_attempt.t)
+    (prepared : Build_session.prepared) =
+  (* A retained edit reparses only the reported paths, then replaces the
+     affected modules' dependency edges in memory. This keeps the long-lived
+     graph coherent without rediscovering the package tree. *)
+  let sources = incremental_sources previous changes in
+  let bsc = prepared.compiler_context.bsc_path in
+  let started_at = Unix.gettimeofday () in
+  List.iter
+    (fun source ->
+      Build_session.mark_parse_pending attempt.session
+        (Platform.normalize_path_for_comparison source.source.absolute_path);
+      let key =
+        Source.compiler_basename source.package.compile_config
+          source.source.module_.Source.name
+      in
+      (Build_state.find_exn prepared.build_state key).compile_dirty <- true)
+    sources;
+  sources
+  |> List.map (fun source ->
+      Source.compiler_basename source.package.compile_config
+        source.source.module_.Source.name)
+  |> List.sort_uniq String.compare
+  |> List.iter (fun name ->
+      Output.debug ~verbosity:attempt.verbosity
+        ("Generating AST for module: " ^ name));
+  let parse_completed =
+    Output.Progress.start_grouped attempt.progress ~step:"1/2"
+      ~symbol:Platform.parse_symbol ~label:"Parsing"
+      (List.map
+         (fun source ->
+           source.package.root ^ "\000" ^ source.source.module_.Source.name)
+         sources)
+  in
+  let results =
+    Process.run_parallel_map ?poll:attempt.process_poll
+      ~on_complete:parse_completed sources ~job:(fun source ->
+        Compiler_process.parse_job ~bsc ~build_dir:source.package.build_dir
+          ~config:source.package.compile_config source.source.relative_path)
+  in
+  let affected_modules = Hashtbl.create (List.length sources) in
+  let dependency_updates = ref [] in
+  List.iter2
+    (fun source result ->
+      Hashtbl.replace attempt.preliminary_parses source.source.absolute_path
+        (Build_attempt.preliminary_parse result);
+      (try
+         let modified = (Unix.stat source.source.absolute_path).Unix.st_mtime in
+         Hashtbl.replace source.package.source_mtimes
+           source.source.relative_path modified
+       with Unix.Unix_error _ | Sys_error _ -> raise Full_rebuild_required);
+      let key =
+        Source.compiler_basename source.package.compile_config
+          source.source.module_.Source.name
+      in
+      let parse_failed = not (Process.succeeded result) in
+      let parse_failed =
+        match Hashtbl.find_opt affected_modules key with
+        | Some (_, _, previous_failed) -> previous_failed || parse_failed
+        | None -> parse_failed
+      in
+      Hashtbl.replace affected_modules key
+        (source.package, source.source.module_, parse_failed))
+    sources results;
+  Hashtbl.iter
+    (fun key (package, module_, changed_parse_failed) ->
+      if not changed_parse_failed then
+        let dependencies path =
+          Compiler_process.ast_dependencies
+            ~build_dir:package.Package_plan.build_dir (Source.ast_path path)
+        in
+        let raw_dependencies =
+          List.sort_uniq String.compare
+            (dependencies module_.Source.implementation
+            @
+            match module_.Source.interface with
+            | None -> []
+            | Some path -> dependencies path)
+        in
+        let node =
+          match Build_session.find_global_module attempt.session key with
+          | Some node -> node
+          | None -> raise Full_rebuild_required
+        in
+        if node.raw_dependencies <> raw_dependencies then
+          dependency_updates :=
+            (key, node, raw_dependencies) :: !dependency_updates)
+    affected_modules;
+  attempt.parse_seconds <- Unix.gettimeofday () -. started_at;
+  if !dependency_updates <> [] then (
+    Build_session.invalidate_graph_cycle attempt.session;
+    List.iter
+      (fun (key, node, raw_dependencies) ->
+        node.Module_graph.raw_dependencies <- raw_dependencies;
+        Build_state.set_dependencies prepared.build_state ~key
+          (Module_graph.resolved_dependencies
+             ~find_module:(Build_session.find_global_module attempt.session)
+             ~find_namespace_maps:
+               (Build_session.find_namespace_maps attempt.session)
+             node))
+      !dependency_updates;
+    let cycle =
+      Module_graph.find_cycle
+        (Build_session.global_module_values attempt.session)
+        (Build_session.namespace_map_values attempt.session)
+        prepared.build_state
+    in
+    Build_session.set_graph_cycle attempt.session cycle;
+    cycle)
+  else
+    match Build_session.graph_cycle attempt.session with
+    | Build_session.Known_cycle cycle -> cycle
+    | Build_session.Unknown_cycle ->
+      let cycle =
+        Module_graph.find_cycle
+          (Build_session.global_module_values attempt.session)
+          (Build_session.namespace_map_values attempt.session)
+          prepared.build_state
+      in
+      Build_session.set_graph_cycle attempt.session cycle;
+      cycle
+
+let run_with_warning_state ~poll ~warning_state ~request ~no_timing ~verbosity
+    ~folder ~prod ~features ~warn_error ~after_build ~filter ~on_state =
+  let compilation_kind = compilation_kind request in
+  let started_at = Unix.gettimeofday () in
+  let interactive = Unix.isatty Unix.stdout && Unix.isatty Unix.stderr in
+  let show_progress = verbosity >= 0 in
+  let colors = Output.colors_enabled ~interactive in
+  let progress =
+    Output.Progress.create ~enabled:(interactive && show_progress) ~color:colors
+  in
+  let poll () =
+    poll ();
+    Output.Progress.tick progress
+  in
+  let process_poll = Some poll in
+  let watch = compilation_kind <> One_shot in
+  let is_rebuild = compilation_kind = Incremental_watch in
+  let should_write_build_ninja =
+    match compilation_kind with
+    | One_shot | Full_watch -> true
+    | Initial_watch | Incremental_watch -> false
+  in
+  let root = Project_context.canonical_project_root folder in
+  let root_config =
+    match previous_build request with
+    | Some previous -> previous.root_config
+    | None -> Config.load_root root
+  in
+  let build_lock_root = Project_context.workspace_lock_root_for root_config in
+  Output.debug ~verbosity
+    (Printf.sprintf "Created project context Single project: %S at %S for %S"
+       root_config.name root_config.path root_config.root);
+  let visited = Hashtbl.create 32 in
+  let attempt : Build_attempt.t =
+    match previous_build request with
+    | Some previous ->
+      Build_attempt.create_retained ~session:previous.session ~process_poll
+        ~progress ~verbosity
+    | None ->
+      Build_attempt.create_full ~warning_state ~process_poll ~progress
+        ~verbosity
+  in
+  let parse_messages () = List.rev attempt.parse_messages in
+  let parse_output messages =
+    messages
+    |> List.map (function
+        | Build_attempt.Parse_warning output | Build_attempt.Parse_error output
+        -> output)
+    |> String.concat ""
+  in
+  let parse_failed = Build_attempt.has_parse_error in
+  (* A watch build must retain the attempted state even when later parsing or
+     compilation fails, because its successful ASTs and artifact inventory are
+     needed to recover incrementally on the next edit. Publish ownership before
+     any fallible phase starts. *)
+  on_state {root_config; build_lock_root; session = attempt.session};
+  let report =
+    Build_report.create ~started_at ~interactive ~show_progress ~colors
+      ~no_timing ~compilation_kind ~attempt
+  in
+  let build_ninja_written = ref false in
+  let write_build_ninja_once () =
+    if should_write_build_ninja && not !build_ninja_written then (
+      write_build_ninja attempt;
+      build_ninja_written := true)
+  in
+  let phase_seconds seconds = if no_timing then 0. else seconds in
+  let parse_step = if is_rebuild then "1/2" else "2/3" in
+  let compile_step = if is_rebuild then "2/2" else "3/3" in
+  let report_failure ~compile_seconds output =
+    Build_attempt.finalize_logs attempt;
+    if attempt.freshness_mode = Build_attempt.Initialize_freshness then
+      Source_dirs.write_build ~root_config attempt.session;
+    Build_report.report report ~success:false ~compile_seconds;
+    prerr_string output;
+    prerr_newline ();
+    Build_attempt.cleanup_artifacts attempt;
+    write_build_ninja_once ();
+    raise
+      (Reported_failure
+         ("Incremental build failed. Error: \027[2K\r  Failed to Compile. "
+        ^ "See Errors Above"))
+  in
+  let report_parse_failure output =
+    Build_attempt.finalize_logs attempt;
+    Build_report.report_parse_failure report ~output;
+    Build_attempt.cleanup_artifacts attempt;
+    write_build_ninja_once ();
+    raise
+      (Reported_failure
+         "Incremental build failed. Error: \027[2K\r  Could not parse Source \
+          Files")
+  in
+  let format_cycle cycle (by_key : (string, Module_graph.cycle_node) Hashtbl.t)
+      =
+    let format_node name =
+      match Hashtbl.find_opt by_key name with
+      | None -> name
+      | Some node -> (
+        match node.source_path with
+        | Some source_path ->
+          let absolute = Filename.concat node.package_root source_path in
+          Printf.sprintf "%s (%s)" node.display_name
+            (Project_context.relative_or_absolute ~root:root_config.root
+               absolute)
+        | None ->
+          Printf.sprintf "%s (%s namespace map)" node.display_name
+            (Project_context.relative_or_absolute ~root:root_config.root
+               node.package_root))
+    in
+    "\nCan't continue... Found a circular dependency in your code:\n"
+    ^ (cycle |> List.map format_node |> String.concat "\n → ")
+    ^ "\n\
+       Possible solutions:\n\
+       - Extract shared code into a new module both depend on.\n"
+  in
+  let execute ~release_build_lock =
+    poll ();
+    let prepared, cycle =
+      match request with
+      | Retained_watch_attempt {previous; changes} -> (
+        match Build_session.prepared attempt.session with
+        | Some prepared ->
+          (prepared, prepare_incremental previous changes attempt prepared)
+        | None -> raise Full_rebuild_required)
+      | One_shot_attempt | Initial_watch_attempt | Full_watch_attempt ->
+        let preparation =
+          Build_preparation.run ~root_config ~prod ~features ~warn_error ~filter
+            ~watch ~attempt ~parse_step ~on_cleanup:(fun seconds ->
+              if interactive && show_progress && not is_rebuild then (
+                if attempt.compiler_cleaned then
+                  print_endline
+                    (Output.compiler_cleanup_message ~color:colors ~step:"1/3");
+                print_endline
+                  (Output.cleanup_message ~color:colors ~step:"1/3"
+                     ~cleaned:attempt.cleaned ~total:attempt.previous_asts
+                     ~seconds:(phase_seconds seconds))))
+        in
+        (preparation.prepared, preparation.cycle)
+    in
+    poll ();
+    if attempt.compiler_cleaned && show_progress && not interactive then
+      print_endline "Cleaned previous build due to compiler update";
+    Option.iter
+      (fun (cycle_info : Module_graph.cycle_info) ->
+        List.iter
+          (fun name -> Hashtbl.replace attempt.blocked_modules name ())
+          cycle_info.blocked)
+      cycle;
+    let root_package =
+      match Build_session.find_package_plan attempt.session root with
+      | Some package -> package
+      | None -> raise (Error ("Package graph was not prepared for " ^ root))
+    in
+    Package_build.prepare_tree ~seen:visited ~package:root_package ~prepared
+      ~watch ~attempt;
+    Build_session.mark_freshness_initialized attempt.session;
+    let parse_messages = parse_messages () in
+    let parse_output = parse_output parse_messages in
+    if parse_failed parse_messages then raise (Parse_failure parse_output);
+    poll ();
+    let namespace_count =
+      try run_namespace_jobs attempt
+      with Build_failure output ->
+        raise (Parse_failure (parse_output ^ output))
+    in
+    Output.Progress.finish progress;
+    if interactive && show_progress then
+      print_endline
+        (Output.parsing_message ~color:colors ~step:parse_step
+           ~count:attempt.parsed
+           ~seconds:(phase_seconds attempt.parse_seconds));
+    flush stdout;
+    prerr_string parse_output;
+    flush stderr;
+    let compile_started = Unix.gettimeofday () in
+    let compile_failure =
+      try
+        run_scheduled_modules attempt prepared ~compile_step ~namespace_count;
+        None
+      with Build_failure output -> Some output
+    in
+    Output.Progress.finish progress;
+    let compile_seconds =
+      phase_seconds (Unix.gettimeofday () -. compile_started)
+    in
+    match (compile_failure, cycle) with
+    | Some output, _ -> report_failure ~compile_seconds output
+    | None, Some cycle_info ->
+      let output = format_cycle cycle_info.cycle cycle_info.nodes_by_key in
+      cycle_info.cycle
+      |> List.filter_map (Hashtbl.find_opt cycle_info.nodes_by_key)
+      |> List.map (fun (node : Module_graph.cycle_node) -> node.package_root)
+      |> List.sort_uniq String.compare
+      |> List.iter (fun package_root -> Compiler_log.append package_root output);
+      report_failure ~compile_seconds output
+    | None, None ->
+      Build_attempt.finalize_logs attempt;
+      if attempt.freshness_mode = Build_attempt.Initialize_freshness then
+        Source_dirs.write_build ~root_config attempt.session;
+      let diagnostics =
+        Build_report.print_success_details report ~compile_seconds
+      in
+      let context = prepared.compiler_context in
+      Build_session.publish_compiler_info attempt.session (fun package ->
+          let package_context =
+            Compiler_info.for_package context
+              ~build_root:package.Package_plan.build_owner
+              package.compile_config
+          in
+          Compiler_info.write_package package_context package.config);
+      if compilation_kind = One_shot then
+        Build_report.report_completion report diagnostics;
+      Build_attempt.cleanup_artifacts attempt;
+      write_build_ninja_once ();
+      release_build_lock ();
+      Option.iter
+        (fun command -> After_build.run ?poll:process_poll ~root command)
+        after_build;
+      if compilation_kind <> One_shot then
+        Build_report.report_completion report diagnostics
+  in
+  Build_lock.with_build ~poll build_lock_root
+    (fun ~release:release_build_lock ->
+      Build_attempt.protect attempt (fun () ->
+          try execute ~release_build_lock with
+          | Build_failure output -> report_failure ~compile_seconds:0. output
+          | Parse_failure output -> report_parse_failure output));
+  {root_config; build_lock_root; session = attempt.session}
+
+let run ~poll ~verbosity ~folder ~prod ~features ~warn_error ~after_build
+    ~filter ~no_timing =
+  try
+    run_with_warning_state ~warning_state:(Warning_state.create ()) ~poll
+      ~request:One_shot_attempt ~no_timing ~verbosity ~folder ~prod ~features
+      ~warn_error ~after_build ~filter ~on_state:(fun _ -> ())
+    |> ignore
+  with Reported_failure message -> raise (Error message)
+
+let remove_compile_warning_freshness warning_state =
+  Warning_state.entries warning_state
+  |> List.iter (fun (entry : Warning_state.entry) ->
+      let implementation =
+        if Filename.check_suffix entry.path ".resi" then
+          Filename.chop_suffix entry.path "i"
+        else entry.path
+      in
+      [implementation; implementation ^ "i"]
+      |> List.iter (fun source ->
+          File_util.remove_file
+            (Build_artifacts.published_ast_path
+               ~ocaml_dir:(Build_artifacts.lib_path entry.package_root "ocaml")
+               source)))
+
+let watch ~verbosity ~folder ~prod ~features ~warn_error ~after_build ~filter
+    ~clear_screen =
+  let root = Project_context.canonical_project_root folder in
+  let warning_state = Warning_state.create () in
+  let initial_build = ref true in
+  let retained = ref None in
+  let force_full_rebuild = ref false in
+  let build ~poll ~changes =
+    let is_initial = !initial_build in
+    initial_build := false;
+    try
+      let run request =
+        let compilation_kind = compilation_kind request in
+        let attempted = ref None in
+        try
+          run_with_warning_state ~poll ~warning_state ~request ~no_timing:false
+            ~verbosity ~folder ~prod ~features ~warn_error ~after_build ~filter
+            ~on_state:(fun state -> attempted := Some state)
+        with exn ->
+          (* Failed initial and incremental attempts still own useful parsed
+             state. Full reconstruction failures do not, because their graph may
+             be only partially discovered. *)
+          (match (compilation_kind, !attempted) with
+          | (Initial_watch | Incremental_watch), Some state ->
+            retained := Some state
+          | Full_watch, Some state
+            when Option.is_some (Build_session.prepared state.session) ->
+            retained := Some state;
+            force_full_rebuild := false
+          | (One_shot | Full_watch), _ | _, None -> ());
+          raise exn
+      in
+      let next =
+        match (!retained, changes, !force_full_rebuild) with
+        | Some previous, Some changes, false -> (
+          try run (Retained_watch_attempt {previous; changes})
+          with Full_rebuild_required ->
+            force_full_rebuild := true;
+            run Full_watch_attempt)
+        | Some _, _, true -> run Full_watch_attempt
+        | None, _, _ ->
+          run (if is_initial then Initial_watch_attempt else Full_watch_attempt)
+        | Some _, None, false -> run Full_watch_attempt
+      in
+      retained := Some next;
+      force_full_rebuild := false;
+      Watcher.Succeeded
+    with
+    | Reported_failure _ -> Watcher.Failed
+    | Package_error message
+    | Error message
+    | Config.Error message
+    | Source.Error message
+    | Process.Error message ->
+      prerr_endline message;
+      Watcher.Failed
+    | (Sys_error _ as exn) | (Unix.Unix_error _ as exn) ->
+      prerr_endline (Printexc.to_string exn);
+      Watcher.Failed
+  in
+  Fun.protect
+    (fun () ->
+      Watcher.run ~root ~prod ~features ~filter ~clear_screen
+        ~show_progress:(verbosity >= 0) ~verbosity ~build)
+    ~finally:(fun () ->
+      if Warning_state.entries warning_state <> [] then
+        match !retained with
+        | Some state ->
+          Build_lock.with_build state.build_lock_root (fun ~release:_ ->
+              remove_compile_warning_freshness warning_state)
+        | None -> ())
diff --git a/rewatch-ocaml/build.mli b/rewatch-ocaml/build.mli
new file mode 100644
index 00000000000..d851e6c82a4
--- /dev/null
+++ b/rewatch-ocaml/build.mli
@@ -0,0 +1,22 @@
+val run :
+  poll:(unit -> unit) ->
+  verbosity:int ->
+  folder:string ->
+  prod:bool ->
+  features:string list option ->
+  warn_error:string option ->
+  after_build:string option ->
+  filter:Source_filter.t option ->
+  no_timing:bool ->
+  unit
+
+val watch :
+  verbosity:int ->
+  folder:string ->
+  prod:bool ->
+  features:string list option ->
+  warn_error:string option ->
+  after_build:string option ->
+  filter:Source_filter.t option ->
+  clear_screen:bool ->
+  unit
diff --git a/rewatch-ocaml/build_artifacts.ml b/rewatch-ocaml/build_artifacts.ml
new file mode 100644
index 00000000000..9c309eba8b8
--- /dev/null
+++ b/rewatch-ocaml/build_artifacts.ml
@@ -0,0 +1,370 @@
+let lib_path root directory = File_util.path_of_parts root ["lib"; directory]
+
+let relative_output_directory path (spec : Config.package_spec) =
+  let directory = Filename.dirname path in
+  if spec.in_source then directory
+  else
+    Filename.concat
+      (match spec.module_format with
+      | Config.Esmodule -> lib_path "" "es6"
+      | Config.Commonjs -> lib_path "" "js")
+      directory
+
+let generated_js_path (config : Config.t) path (spec : Config.package_spec) =
+  let output_dir = relative_output_directory path spec in
+  Filename.concat config.root
+    (Filename.concat output_dir
+       (Filename.remove_extension (Filename.basename path)
+       ^ Config.package_spec_suffix config spec))
+
+let published_ast_path ~ocaml_dir source =
+  Filename.concat ocaml_dir (Filename.basename (Source.ast_path source))
+
+let generated_build_js_path ~build_dir (config : Config.t) path
+    (spec : Config.package_spec) =
+  Filename.concat build_dir
+    (Filename.remove_extension path ^ Config.package_spec_suffix config spec)
+
+let remove_public_outputs (config : Config.t) implementation_paths =
+  List.iter
+    (fun implementation ->
+      List.iter
+        (fun spec ->
+          let output = generated_js_path config implementation spec in
+          File_util.remove_file output;
+          File_util.remove_file (output ^ ".map"))
+        config.package_specs)
+    implementation_paths
+
+let generated_output_suffixes =
+  [
+    ".bs.mjs";
+    ".bs.cjs";
+    ".bs.js";
+    ".res.mjs";
+    ".res.cjs";
+    ".res.js";
+    ".mjs";
+    ".cjs";
+    ".js";
+  ]
+
+let generated_output_details_for_suffixes suffixes path =
+  let output_path =
+    if Filename.check_suffix path ".map" then Filename.chop_suffix path ".map"
+    else path
+  in
+  suffixes
+  |> List.find_map (fun suffix ->
+      if Filename.check_suffix output_path suffix then
+        Some
+          ( ( Filename.basename output_path |> fun basename ->
+              Filename.chop_suffix basename suffix ),
+            suffix,
+            output_path )
+      else None)
+
+type cleanup_result = {
+  removed_modules: string list;
+  previous_ast_count: int;
+  present_public_outputs: (string, unit) Hashtbl.t;
+}
+
+let cleanup_stale ?ocaml_files ?ast_sources ?source_files ?present_source_files
+    ~on_removed_module ~on_deferred_artifact ~root ~ocaml_dir ~is_local
+    (config : Config.t) modules =
+  let build_dir = lib_path root "bs" in
+  (* Keep one inventory of each artifact tree to avoid repeating directory and
+     metadata work during every cleanup phase. Paths removed below can safely
+     remain in the inventory: later phases only classify their names or call
+     the idempotent File_util.remove_file. *)
+  let ocaml_files =
+    match ocaml_files with
+    | Some files -> files
+    | None -> File_util.files_under ocaml_dir
+  in
+  (* Published ASTs contain the absolute source path used to create them, which
+     is enough to address their working artifacts without scanning for them.
+     Keep the recursive walk lazy for malformed or legacy ASTs that cannot be
+     mapped; normal unchanged builds must not inventory the whole lib/bs tree. *)
+  let ast_sources = Option.value ast_sources ~default:[] in
+  let fallback_build_files_by_basename =
+    lazy
+      (let by_basename = Hashtbl.create 32 in
+       File_util.files_under build_dir
+       |> List.iter (fun path ->
+           let basename = Filename.basename path in
+           let paths =
+             Hashtbl.find_opt by_basename basename |> Option.value ~default:[]
+           in
+           Hashtbl.replace by_basename basename (path :: paths));
+       by_basename)
+  in
+  let source_files =
+    match source_files with
+    | Some files -> files
+    | None ->
+      config.sources
+      |> List.concat_map (fun source ->
+          File_util.files_under (Filename.concat root source.Config.dir))
+  in
+  let present_source_files =
+    Option.value present_source_files ~default:source_files
+  in
+  let output_files =
+    [lib_path "" "es6"; lib_path "" "js"]
+    |> List.map (fun directory ->
+        let output_dir = Filename.concat root directory in
+        (output_dir, File_util.files_under output_dir))
+  in
+  let configured_suffixes =
+    List.map (Config.package_spec_suffix config) config.package_specs
+  in
+  let output_suffixes =
+    configured_suffixes @ generated_output_suffixes
+    |> List.sort_uniq (fun left right ->
+        let length_order =
+          Int.compare (String.length right) (String.length left)
+        in
+        if length_order = 0 then String.compare left right else length_order)
+  in
+  let output_details = generated_output_details_for_suffixes output_suffixes in
+  let present_public_outputs = Hashtbl.create 64 in
+  present_source_files @ List.concat_map snd output_files
+  |> List.iter (fun path ->
+      if Option.is_some (output_details path) && File_util.is_regular_file path
+      then Hashtbl.replace present_public_outputs path ());
+  (* Expected outputs may use arbitrary configured suffixes, including suffixes
+     that do not resemble JavaScript. Probe those paths directly rather than
+     treating a filename heuristic as the source of truth. *)
+  List.iter
+    (fun module_ ->
+      List.iter
+        (fun spec ->
+          let output =
+            generated_js_path config module_.Source.implementation spec
+          in
+          if
+            (not (Hashtbl.mem present_public_outputs output))
+            && File_util.is_regular_file output
+          then Hashtbl.replace present_public_outputs output ())
+        config.package_specs)
+    modules;
+  let expected_artifacts = Hashtbl.create (List.length modules * 8) in
+  let owned_output_names = Hashtbl.create (List.length modules * 2) in
+  let current_sources = Hashtbl.create (List.length modules * 2) in
+  List.iter
+    (fun module_ ->
+      module_.Source.implementation :: Option.to_list module_.Source.interface
+      |> List.iter (fun source ->
+          Filename.concat root source |> Platform.normalize_path_for_comparison
+          |> fun source -> Hashtbl.replace current_sources source ()))
+    modules;
+  let stale_ast_basenames = Hashtbl.create 8 in
+  let removed_output_paths = Hashtbl.create 8 in
+  List.iter
+    (fun (ast_source : Compile_assets.ast_source) ->
+      let source =
+        Platform.normalize_path_for_comparison ast_source.source_path
+      in
+      if not (Hashtbl.mem current_sources source) then (
+        Hashtbl.replace stale_ast_basenames
+          (Filename.basename ast_source.ast_path)
+          ();
+        Project_context.relative_to_opt root ast_source.source_path
+        |> Option.iter (fun relative_source ->
+            List.iter
+              (fun spec ->
+                Hashtbl.replace removed_output_paths
+                  (generated_js_path config relative_source spec)
+                  ())
+              config.package_specs)))
+    ast_sources;
+  let add_expected base extensions =
+    List.iter
+      (fun extension ->
+        Hashtbl.replace expected_artifacts (base ^ extension) ())
+      extensions
+  in
+  let previous_ast_count = ref 0 in
+  ocaml_files
+  |> List.iter (fun path ->
+      let basename = Filename.basename path in
+      if Filename.check_suffix basename ".ast" then (
+        incr previous_ast_count;
+        Hashtbl.replace owned_output_names
+          (Filename.chop_suffix basename ".ast")
+          ())
+      else if Filename.check_suffix basename ".iast" then (
+        incr previous_ast_count;
+        Hashtbl.replace owned_output_names
+          (Filename.chop_suffix basename ".iast")
+          ()));
+  List.iter
+    (fun module_ ->
+      let source_base =
+        module_.Source.implementation |> Filename.basename
+        |> Filename.remove_extension
+      in
+      let compiler_base =
+        Source.compiler_asset_basename config module_.Source.implementation
+      in
+      Hashtbl.replace owned_output_names source_base ();
+      add_expected source_base [".ast"; ".res"];
+      if Option.is_some module_.Source.interface then (
+        add_expected source_base [".iast"; ".resi"];
+        add_expected compiler_base [".cmti"]);
+      add_expected compiler_base [".cmi"; ".cmj"; ".cmt"])
+    modules;
+  Config.namespace_compiler_name config.namespace
+  |> Option.iter (fun namespace ->
+      add_expected namespace [".cmi"; ".cmj"; ".cmt"; ".mlmap"]);
+  let removed_modules = ref [] in
+  (* Once the published CMI is removed, bsc still consults the working CMI to
+     produce its source-located missing-module diagnostic. Keep only that copy
+     through compilation; the command finalizer removes every deferred path. *)
+  let defer_working_cmi_until_after_compile basename =
+    Filename.check_suffix basename ".cmi"
+  in
+  let mapped_source_directories =
+    Hashtbl.create (List.length ast_sources * 2)
+  in
+  let add_mapped_source_directory key directory =
+    let directories =
+      Hashtbl.find_opt mapped_source_directories key |> Option.value ~default:[]
+    in
+    Hashtbl.replace mapped_source_directories key (directory :: directories)
+  in
+  List.iter
+    (fun (ast_source : Compile_assets.ast_source) ->
+      let source = ast_source.source_path in
+      Project_context.relative_to_opt root source
+      |> Option.iter (fun relative_source ->
+          let directory = Filename.dirname relative_source in
+          let source_base =
+            relative_source |> Filename.basename |> Filename.remove_extension
+          in
+          add_mapped_source_directory source_base directory;
+          add_mapped_source_directory
+            (Source.compiler_asset_basename config relative_source)
+            directory))
+    ast_sources;
+  let directly_mapped_working_paths basename =
+    let extension = Filename.extension basename in
+    if extension = ".mlmap" then [Filename.concat build_dir basename]
+    else
+      Hashtbl.find_opt mapped_source_directories
+        (Filename.remove_extension basename)
+      |> Option.value ~default:[]
+      |> List.map (fun directory ->
+          Filename.concat build_dir (Filename.concat directory basename))
+      |> List.sort_uniq String.compare
+  in
+  let working_paths basename =
+    match directly_mapped_working_paths basename with
+    | _ :: _ as paths -> paths
+    | [] ->
+      Hashtbl.find_opt (Lazy.force fallback_build_files_by_basename) basename
+      |> Option.value ~default:[]
+  in
+  ocaml_files
+  |> List.iter (fun path ->
+      let basename = Filename.basename path in
+      let managed = Compile_assets.is_managed_basename basename in
+      if
+        managed
+        && ((not (Hashtbl.mem expected_artifacts basename))
+           || Hashtbl.mem stale_ast_basenames basename)
+      then (
+        if
+          Filename.check_suffix basename ".ast"
+          || Filename.check_suffix basename ".iast"
+        then (
+          let module_name = Source.module_name basename in
+          on_removed_module module_name;
+          removed_modules := module_name :: !removed_modules);
+        File_util.remove_file path;
+        working_paths basename
+        |> List.iter (fun build_path ->
+            if defer_working_cmi_until_after_compile basename then
+              on_deferred_artifact build_path
+            else File_util.remove_file build_path)));
+  let relative_under directory path =
+    let prefix = directory ^ Filename.dir_sep in
+    String.sub path (String.length prefix)
+      (String.length path - String.length prefix)
+  in
+  let expected_outputs =
+    Hashtbl.create (List.length modules * List.length config.package_specs)
+  in
+  List.iter
+    (fun module_ ->
+      List.iter
+        (fun spec ->
+          Hashtbl.replace expected_outputs
+            (generated_js_path config module_.Source.implementation spec)
+            ())
+        config.package_specs)
+    modules;
+  let should_remove_output ~build_relative path =
+    output_details path
+    |> Option.fold ~none:false ~some:(fun (name, _, output_path) ->
+        (not (Hashtbl.mem expected_outputs output_path))
+        && (Hashtbl.mem removed_output_paths output_path
+           || Hashtbl.mem owned_output_names name
+              (* A map alone is not enough provenance to delete a public file. The
+            mirrored output has the same relative path below lib/bs, so probe
+            that one path instead of scanning the entire working tree. *)
+              && is_local
+              && File_util.exists (Filename.concat build_dir build_relative)))
+  in
+  let planned_outputs = Hashtbl.create 16 in
+  let plan_output ~build_relative path =
+    output_details path
+    |> Option.iter (fun (_, _, output_path) ->
+        if should_remove_output ~build_relative output_path then
+          Hashtbl.replace planned_outputs output_path build_relative)
+  in
+  source_files
+  |> List.iter (fun path ->
+      output_details path
+      |> Option.iter (fun (_, _, output_path) ->
+          plan_output
+            ~build_relative:(relative_under root output_path)
+            output_path));
+  ast_sources
+  |> List.iter (fun (ast_source : Compile_assets.ast_source) ->
+      let source = ast_source.source_path in
+      Project_context.relative_to_opt root source
+      |> Option.iter (fun relative_source ->
+          List.iter
+            (fun spec ->
+              let output = generated_js_path config relative_source spec in
+              plan_output ~build_relative:(relative_under root output) output)
+            config.package_specs));
+  output_files
+  |> List.iter (fun (output_dir, files) ->
+      files
+      |> List.iter (fun path ->
+          output_details path
+          |> Option.iter (fun (_, _, output_path) ->
+              plan_output
+                ~build_relative:(relative_under output_dir output_path)
+                output_path)));
+  let remove_output (path, build_relative) =
+    Hashtbl.remove present_public_outputs path;
+    Hashtbl.remove present_public_outputs (path ^ ".map");
+    File_util.remove_file path;
+    File_util.remove_file (path ^ ".map");
+    let working_output = Filename.concat build_dir build_relative in
+    File_util.remove_file working_output;
+    File_util.remove_file (working_output ^ ".map")
+  in
+  planned_outputs |> Hashtbl.to_seq |> List.of_seq
+  |> List.sort (fun (left, _) (right, _) -> String.compare left right)
+  |> List.iter remove_output;
+  {
+    removed_modules = !removed_modules;
+    previous_ast_count = !previous_ast_count;
+    present_public_outputs;
+  }
diff --git a/rewatch-ocaml/build_artifacts.mli b/rewatch-ocaml/build_artifacts.mli
new file mode 100644
index 00000000000..dafa9d498c1
--- /dev/null
+++ b/rewatch-ocaml/build_artifacts.mli
@@ -0,0 +1,34 @@
+type cleanup_result = {
+  removed_modules: string list;
+  previous_ast_count: int;
+  present_public_outputs: (string, unit) Hashtbl.t;
+}
+(** Artifact cleanup first classifies ownership and builds a deletion plan.
+    Public JavaScript, source maps, and compiler artifacts are then removed as
+    one unit so an earlier deletion cannot erase evidence needed for a later
+    ownership decision. *)
+
+val lib_path : string -> string -> string
+val relative_output_directory : string -> Config.package_spec -> string
+val published_ast_path : ocaml_dir:string -> string -> string
+
+val generated_js_path : Config.t -> string -> Config.package_spec -> string
+
+val generated_build_js_path :
+  build_dir:string -> Config.t -> string -> Config.package_spec -> string
+
+val remove_public_outputs : Config.t -> string list -> unit
+
+val cleanup_stale :
+  ?ocaml_files:string list ->
+  ?ast_sources:Compile_assets.ast_source list ->
+  ?source_files:string list ->
+  ?present_source_files:string list ->
+  on_removed_module:(string -> unit) ->
+  on_deferred_artifact:(string -> unit) ->
+  root:string ->
+  ocaml_dir:string ->
+  is_local:bool ->
+  Config.t ->
+  Source.module_ list ->
+  cleanup_result
diff --git a/rewatch-ocaml/build_attempt.ml b/rewatch-ocaml/build_attempt.ml
new file mode 100644
index 00000000000..629522568f9
--- /dev/null
+++ b/rewatch-ocaml/build_attempt.ml
@@ -0,0 +1,212 @@
+type freshness_mode = Initialize_freshness | Reuse_freshness
+
+type compilation_kind =
+  | One_shot
+  | Initial_watch
+  | Incremental_watch
+  | Full_watch
+
+type parse_message = Parse_warning of string | Parse_error of string
+
+let has_parse_error messages =
+  List.exists
+    (function
+      | Parse_error _ -> true
+      | Parse_warning _ -> false)
+    messages
+
+type preliminary_parse =
+  | Parsed_successfully of {stderr: string}
+  | Parse_failed of {stdout: string; stderr: string}
+  | Use_existing_ast
+
+let preliminary_parse result =
+  if Process.succeeded result then Parsed_successfully {stderr = result.stderr}
+  else Parse_failed {stdout = result.stdout; stderr = result.stderr}
+
+type cleanup_batch = {actions: (unit -> unit) list; artifacts: string list}
+type namespace_job = {job: Process.job; finish: Process.result -> unit}
+
+type pending_work = {
+  mutable namespace_jobs: namespace_job list;
+  mutable compile_candidates: Compiler_scheduler.candidate list;
+}
+
+type finalization_state = {
+  results: (string, Build_artifacts.cleanup_result) Hashtbl.t;
+  mutable actions: (unit -> unit) list;
+  mutable artifacts: string list;
+  initialized_logs: (string, unit) Hashtbl.t;
+  mutable artifacts_cleaned: bool;
+  mutable logs_finalized: bool;
+}
+
+type t = {
+  freshness_mode: freshness_mode;
+  session: Build_session.t;
+  mutable cleaned: int;
+  mutable previous_asts: int;
+  mutable parsed: int;
+  mutable compiled: int;
+  mutable parse_seconds: float;
+  mutable parse_messages: parse_message list;
+  mutable diagnostics: string list;
+  removed_modules: (string, unit) Hashtbl.t;
+  preliminary_parses: (string, preliminary_parse) Hashtbl.t;
+  blocked_modules: (string, unit) Hashtbl.t;
+  namespace_freshness: (string, float option) Hashtbl.t;
+  pending_work: pending_work;
+  finalization: finalization_state;
+  mutable compiler_cleaned: bool;
+  mutable had_warnings: bool;
+  process_poll: (unit -> unit) option;
+  progress: Output.Progress.t;
+  verbosity: int;
+}
+
+let create ~freshness_mode ~session ~process_poll ~progress ~verbosity =
+  let cleanup_results = Hashtbl.create 32 in
+  Build_session.iter_public_outputs session (fun root present_public_outputs ->
+      Hashtbl.add cleanup_results root
+        Build_artifacts.
+          {removed_modules = []; previous_ast_count = 0; present_public_outputs});
+  let removed_modules = Hashtbl.create 16 in
+  Build_session.pending_removed_modules session
+  |> List.iter (fun name -> Hashtbl.replace removed_modules name ());
+  {
+    freshness_mode;
+    session;
+    cleaned = 0;
+    previous_asts = 0;
+    parsed = 0;
+    compiled = 0;
+    parse_seconds = 0.;
+    parse_messages = [];
+    diagnostics = [];
+    removed_modules;
+    preliminary_parses = Hashtbl.create 16;
+    blocked_modules = Hashtbl.create 16;
+    namespace_freshness = Hashtbl.create 16;
+    pending_work = {namespace_jobs = []; compile_candidates = []};
+    finalization =
+      {
+        results = cleanup_results;
+        actions = [];
+        artifacts = [];
+        initialized_logs = Hashtbl.create 16;
+        artifacts_cleaned = false;
+        logs_finalized = false;
+      };
+    compiler_cleaned = false;
+    had_warnings = false;
+    process_poll;
+    progress;
+    verbosity;
+  }
+
+let create_full ~warning_state ~process_poll ~progress ~verbosity =
+  create ~freshness_mode:Initialize_freshness
+    ~session:(Build_session.create ~warning_state)
+    ~process_poll ~progress ~verbosity
+
+let create_retained ~session ~process_poll ~progress ~verbosity =
+  let freshness_mode =
+    if Build_session.is_ready session then Reuse_freshness
+    else Initialize_freshness
+  in
+  create ~freshness_mode ~session ~process_poll ~progress ~verbosity
+
+let register_cleanup attempt action =
+  attempt.finalization.actions <- action :: attempt.finalization.actions
+
+let defer_artifact_cleanup attempt paths =
+  attempt.finalization.artifacts <- paths @ attempt.finalization.artifacts
+
+let take_cleanup attempt =
+  let batch =
+    {
+      actions = attempt.finalization.actions;
+      artifacts = attempt.finalization.artifacts;
+    }
+  in
+  attempt.finalization.actions <- [];
+  attempt.finalization.artifacts <- [];
+  batch
+
+let set_cleanup_result attempt root result =
+  Hashtbl.replace attempt.finalization.results root result;
+  Build_session.set_public_outputs attempt.session root
+    result.Build_artifacts.present_public_outputs
+
+let find_cleanup_result attempt root =
+  Hashtbl.find_opt attempt.finalization.results root
+
+let add_namespace_job attempt job =
+  attempt.pending_work.namespace_jobs <-
+    job :: attempt.pending_work.namespace_jobs
+
+let take_namespace_jobs attempt =
+  let jobs = List.rev attempt.pending_work.namespace_jobs in
+  attempt.pending_work.namespace_jobs <- [];
+  jobs
+
+let add_compile_candidates attempt candidates =
+  attempt.pending_work.compile_candidates <-
+    candidates @ attempt.pending_work.compile_candidates
+
+let take_compile_candidates attempt =
+  let candidates = attempt.pending_work.compile_candidates in
+  attempt.pending_work.compile_candidates <- [];
+  candidates
+
+let mark_log_initialized attempt root =
+  Hashtbl.replace attempt.finalization.initialized_logs root ()
+
+let take_initialized_logs attempt =
+  let roots =
+    attempt.finalization.initialized_logs |> Hashtbl.to_seq_keys |> List.of_seq
+  in
+  Hashtbl.clear attempt.finalization.initialized_logs;
+  roots
+
+let run_all actions =
+  let first_error = ref None in
+  List.iter
+    (fun action ->
+      try action ()
+      with error ->
+        if Option.is_none !first_error then first_error := Some error)
+    actions;
+  Option.iter raise !first_error
+
+let cleanup_artifacts attempt =
+  if not attempt.finalization.artifacts_cleaned then (
+    attempt.finalization.artifacts_cleaned <- true;
+    let cleanup = take_cleanup attempt in
+    run_all
+      (cleanup.actions
+      @ List.map (fun path () -> File_util.remove_file path) cleanup.artifacts))
+
+let finalize_logs attempt =
+  if not attempt.finalization.logs_finalized then (
+    attempt.finalization.logs_finalized <- true;
+    let package_roots = take_initialized_logs attempt in
+    run_all
+      ((fun () -> Output.Progress.finish attempt.progress)
+      :: List.map
+           (fun package_root () -> Compiler_log.finalize package_root)
+           package_roots))
+
+let finish_attempt attempt =
+  run_all
+    [(fun () -> cleanup_artifacts attempt); (fun () -> finalize_logs attempt)]
+
+let protect attempt action =
+  match action () with
+  | result ->
+    finish_attempt attempt;
+    result
+  | exception original -> (
+    match finish_attempt attempt with
+    | () -> raise original
+    | exception cleanup_error -> raise cleanup_error)
diff --git a/rewatch-ocaml/build_attempt.mli b/rewatch-ocaml/build_attempt.mli
new file mode 100644
index 00000000000..811d2e4d85c
--- /dev/null
+++ b/rewatch-ocaml/build_attempt.mli
@@ -0,0 +1,81 @@
+(** An attempt owns all mutable work and reporting state for one build. This
+    boundary lets finalization drain cleanup exactly once while the associated
+    {!Build_session} remains reusable after failures. *)
+
+type freshness_mode = Initialize_freshness | Reuse_freshness
+
+(** The compilation kind makes output and recovery policy explicit. In
+    particular, an initial watch build cannot assume that retained freshness
+    has already been initialized. *)
+type compilation_kind =
+  | One_shot
+  | Initial_watch
+  | Incremental_watch
+  | Full_watch
+
+type parse_message = Parse_warning of string | Parse_error of string
+val has_parse_error : parse_message list -> bool
+
+(** Preliminary parsing distinguishes successful new ASTs, failed source text,
+    and deliberately retained ASTs. Keeping these cases explicit prevents a
+    failed parse from being mistaken for an unchanged source. *)
+type preliminary_parse =
+  | Parsed_successfully of {stderr: string}
+  | Parse_failed of {stdout: string; stderr: string}
+  | Use_existing_ast
+
+val preliminary_parse : Process.result -> preliminary_parse
+
+type namespace_job = {job: Process.job; finish: Process.result -> unit}
+type pending_work
+type finalization_state
+
+type t = {
+  freshness_mode: freshness_mode;
+  session: Build_session.t;
+  mutable cleaned: int;
+  mutable previous_asts: int;
+  mutable parsed: int;
+  mutable compiled: int;
+  mutable parse_seconds: float;
+  mutable parse_messages: parse_message list;
+  mutable diagnostics: string list;
+  removed_modules: (string, unit) Hashtbl.t;
+  preliminary_parses: (string, preliminary_parse) Hashtbl.t;
+  blocked_modules: (string, unit) Hashtbl.t;
+  namespace_freshness: (string, float option) Hashtbl.t;
+  pending_work: pending_work;
+  finalization: finalization_state;
+  mutable compiler_cleaned: bool;
+  mutable had_warnings: bool;
+  process_poll: (unit -> unit) option;
+  progress: Output.Progress.t;
+  verbosity: int;
+}
+
+val create_full :
+  warning_state:Warning_state.t ->
+  process_poll:(unit -> unit) option ->
+  progress:Output.Progress.t ->
+  verbosity:int ->
+  t
+
+val create_retained :
+  session:Build_session.t ->
+  process_poll:(unit -> unit) option ->
+  progress:Output.Progress.t ->
+  verbosity:int ->
+  t
+
+val register_cleanup : t -> (unit -> unit) -> unit
+val defer_artifact_cleanup : t -> string list -> unit
+val set_cleanup_result : t -> string -> Build_artifacts.cleanup_result -> unit
+val find_cleanup_result : t -> string -> Build_artifacts.cleanup_result option
+val add_namespace_job : t -> namespace_job -> unit
+val take_namespace_jobs : t -> namespace_job list
+val add_compile_candidates : t -> Compiler_scheduler.candidate list -> unit
+val take_compile_candidates : t -> Compiler_scheduler.candidate list
+val mark_log_initialized : t -> string -> unit
+val cleanup_artifacts : t -> unit
+val finalize_logs : t -> unit
+val protect : t -> (unit -> 'a) -> 'a
diff --git a/rewatch-ocaml/build_freshness.ml b/rewatch-ocaml/build_freshness.ml
new file mode 100644
index 00000000000..93eb79b707e
--- /dev/null
+++ b/rewatch-ocaml/build_freshness.ml
@@ -0,0 +1,35 @@
+let source_requires_parse source_modified artifact_modified =
+  match (source_modified, artifact_modified) with
+  | Some source_time, Some artifact_time -> source_time >= artifact_time
+  | Some _, None -> true
+  | None, _ -> false
+
+let source_is_not_older_than_ast compile_assets ~root ~source_mtimes path =
+  let absolute = Filename.concat root path in
+  let source_modified =
+    match Hashtbl.find_opt source_mtimes path with
+    | Some modified -> Some modified
+    | None -> File_util.modification_time absolute
+  in
+  let artifact_modified =
+    match Compile_assets.ast compile_assets absolute with
+    | Some ast -> Some ast.modified
+    | None ->
+      let published_ast =
+        Build_artifacts.published_ast_path
+          ~ocaml_dir:(Build_artifacts.lib_path root "ocaml")
+          path
+      in
+      let belongs_to_source =
+        try
+          match (Ast_header.read published_ast).source with
+          | Some source ->
+            Platform.normalize_path_for_comparison source
+            = Platform.normalize_path_for_comparison absolute
+          | None -> false
+        with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> false
+      in
+      if belongs_to_source then File_util.modification_time published_ast
+      else None
+  in
+  source_requires_parse source_modified artifact_modified
diff --git a/rewatch-ocaml/build_freshness.mli b/rewatch-ocaml/build_freshness.mli
new file mode 100644
index 00000000000..4fb259a6fcb
--- /dev/null
+++ b/rewatch-ocaml/build_freshness.mli
@@ -0,0 +1,6 @@
+val source_is_not_older_than_ast :
+  Compile_assets.t ->
+  root:string ->
+  source_mtimes:(string, float) Hashtbl.t ->
+  string ->
+  bool
diff --git a/rewatch-ocaml/build_lock.ml b/rewatch-ocaml/build_lock.ml
new file mode 100644
index 00000000000..204a66c860c
--- /dev/null
+++ b/rewatch-ocaml/build_lock.ml
@@ -0,0 +1,192 @@
+type watch = {path: string; pid: string}
+
+let read_owner_contents = File_util.read_file
+
+let read_owner path =
+  try Some (read_owner_contents path)
+  with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None
+
+let read_owner_for_release = read_owner
+
+let valid_owner value =
+  match Int64.of_string_opt value with
+  | Some pid -> pid >= 0L && pid <= 0xffff_ffffL
+  | None -> false
+
+let malformed_error () =
+  Project_context.Error
+    "Could not start Rescript build: Could not parse lockfile PID\n\
+    \  (try removing it and running the command again)"
+
+let process_is_active ?poll value =
+  Platform.process_is_active value ~run:(fun program args ->
+      try
+        let result =
+          Process.run ?poll ~cwd:(Filename.get_temp_dir_name ()) program args
+        in
+        Some (result.Process.status, result.stdout)
+      with Process.Error _ | Unix.Unix_error _ | Sys_error _ -> None)
+
+let with_candidate ~lock_dir prefix pid action =
+  (* Termination is deferred until candidate cleanup has an owner because the
+     watcher's signal handlers raise asynchronous exceptions. Without this
+     protected setup, a signal could leave the temporary file or its output
+     channel behind between two otherwise ordinary OCaml expressions. *)
+  let deferred_signals = Signal_restore.create ~defer:true in
+  let candidate = ref None in
+  let channel = ref None in
+  try
+    let path, output =
+      Filename.open_temp_file ~temp_dir:lock_dir prefix ".tmp"
+    in
+    candidate := Some path;
+    channel := Some output;
+    output_string output pid;
+    close_out output;
+    channel := None;
+    Fun.protect
+      ~finally:(fun () -> File_util.remove_file_best_effort path)
+      (fun () ->
+        Signal_restore.restore deferred_signals;
+        action path)
+  with exception_raised ->
+    Option.iter close_out_noerr !channel;
+    Option.iter File_util.remove_file_best_effort !candidate;
+    raise
+      (Signal_restore.exception_after_restore deferred_signals exception_raised)
+
+let clear_stale ?poll ~candidate path =
+  let takeover = path ^ ".takeover" in
+  try
+    Unix.link candidate takeover;
+    Fun.protect
+      ~finally:(fun () -> File_util.remove_file_best_effort takeover)
+      (fun () ->
+        match read_owner path with
+        | Some owner when not (valid_owner owner) -> raise (malformed_error ())
+        | Some owner when process_is_active ?poll owner -> ()
+        | Some _ -> File_util.remove_file_best_effort path
+        | None -> ());
+    true
+  with Unix.Unix_error (Unix.EEXIST, _, _) ->
+    (match read_owner takeover with
+    | Some owner when process_is_active ?poll owner -> ()
+    | _ -> File_util.remove_file_best_effort takeover);
+    false
+
+let unlink_existing path =
+  try Unix.unlink path with Unix.Unix_error (Unix.ENOENT, _, _) -> ()
+
+let release_owned path pid =
+  if read_owner_for_release path = Some pid then unlink_existing path
+
+type owned_lock = {path: string; pid: string; mutable released: bool}
+
+let release lock =
+  if not lock.released then (
+    release_owned lock.path lock.pid;
+    lock.released <- true)
+
+let attempt_link ~candidate ~path =
+  let deferred_signals = Signal_restore.create ~defer:true in
+  let linked =
+    try
+      Unix.link candidate path;
+      true
+    with
+    | Unix.Unix_error (Unix.EEXIST, _, _) -> false
+    | exception_raised ->
+      raise
+        (Signal_restore.exception_after_restore deferred_signals
+           exception_raised)
+  in
+  (linked, deferred_signals)
+
+let with_acquired ~candidate ~path ~pid ~deferred_signals action =
+  let lock = {path; pid; released = false} in
+  match
+    Signal_restore.protect deferred_signals (fun () ->
+        unlink_existing candidate);
+    action lock
+  with
+  | result ->
+    release lock;
+    result
+  | exception original -> (
+    match release lock with
+    | () -> raise original
+    | exception cleanup_error -> raise cleanup_error)
+
+let retry_delay poll =
+  (try ignore (Unix.select [] [] [] 0.05)
+   with Unix.Unix_error (Unix.EINTR, _, _) -> ());
+  poll ()
+
+let with_build ?(poll = fun () -> ()) root action =
+  let lock_dir = Filename.concat root "lib" in
+  File_util.ensure_dir lock_dir;
+  let path = Filename.concat lock_dir "build.lock" in
+  let pid = string_of_int (Platform.current_process_id ()) in
+  with_candidate ~lock_dir ".build-lock-" pid (fun candidate ->
+      let rec acquire attempts =
+        poll ();
+        if attempts = 0 then
+          raise
+            (Project_context.Error
+               "Timed out waiting for another ReScript build to finish");
+        let linked, deferred_signals = attempt_link ~candidate ~path in
+        if linked then
+          with_acquired ~candidate ~path ~pid ~deferred_signals (fun lock ->
+              action ~release:(fun () -> release lock))
+        else (
+          Signal_restore.restore deferred_signals;
+          match read_owner path with
+          | Some owner when not (valid_owner owner) ->
+            raise (malformed_error ())
+          | Some owner when process_is_active ~poll owner ->
+            if attempts = 1200 then (
+              print_endline "Waiting for other build to finish...";
+              flush stdout);
+            retry_delay poll;
+            acquire (attempts - 1)
+          | _ ->
+            if not (clear_stale ~poll ~candidate path) then retry_delay poll;
+            acquire (attempts - 1))
+      in
+      acquire 1200)
+
+let with_watch root action =
+  let lock_dir = Filename.concat root "lib" in
+  File_util.ensure_dir lock_dir;
+  let path = Filename.concat lock_dir "watch.lock" in
+  let pid = string_of_int (Platform.current_process_id ()) in
+  with_candidate ~lock_dir ".watch-lock-" pid (fun candidate ->
+      let rec acquire attempts =
+        if attempts = 0 then
+          raise
+            (Project_context.Error
+               "Timed out recovering a stale ReScript watch lock");
+        let linked, deferred_signals = attempt_link ~candidate ~path in
+        if linked then
+          with_acquired ~candidate ~path ~pid ~deferred_signals (fun _ ->
+              action {path; pid})
+        else (
+          Signal_restore.restore deferred_signals;
+          match read_owner path with
+          | Some owner when not (valid_owner owner) ->
+            raise (malformed_error ())
+          | Some owner when process_is_active owner ->
+            raise
+              (Project_context.Error
+                 (Printf.sprintf
+                    "Could not start Rescript build: A ReScript build is \
+                     already running. The process ID (PID) is %s"
+                    owner))
+          | _ ->
+            if not (clear_stale ~candidate path) then
+              ignore (Unix.select [] [] [] 0.01);
+            acquire (attempts - 1))
+      in
+      acquire 1000)
+
+let is_owned (watch : watch) = read_owner watch.path = Some watch.pid
diff --git a/rewatch-ocaml/build_lock.mli b/rewatch-ocaml/build_lock.mli
new file mode 100644
index 00000000000..9c4015800d6
--- /dev/null
+++ b/rewatch-ocaml/build_lock.mli
@@ -0,0 +1,8 @@
+type watch
+
+val read_owner : string -> string option
+val valid_owner : string -> bool
+val with_build :
+  ?poll:(unit -> unit) -> string -> (release:(unit -> unit) -> 'a) -> 'a
+val with_watch : string -> (watch -> 'a) -> 'a
+val is_owned : watch -> bool
diff --git a/rewatch-ocaml/build_preparation.ml b/rewatch-ocaml/build_preparation.ml
new file mode 100644
index 00000000000..62f9d36e211
--- /dev/null
+++ b/rewatch-ocaml/build_preparation.ml
@@ -0,0 +1,262 @@
+exception Error = Project_context.Error
+
+type result = {
+  prepared: Build_session.prepared;
+  cycle: Module_graph.cycle_info option;
+}
+
+let bsc_path () =
+  try Toolchain.bsc () with Toolchain.Error message -> raise (Error message)
+
+let runtime_path root =
+  try Toolchain.runtime ~find_package:(Project_context.dependency_path root)
+  with Toolchain.Error message -> raise (Error message)
+
+let run ~(root_config : Config.t) ~prod ~features ~warn_error ~filter ~watch
+    ~(attempt : Build_attempt.t) ~parse_step ~on_cleanup =
+  let bsc = bsc_path () in
+  let package_plans =
+    Package_graph.discover ~root_config ~prod ~features ~warn_error ~filter
+      ~attempt
+  in
+  Module_graph.validate_visible_namespaces ~root_config package_plans;
+  let runtime = runtime_path root_config.root in
+  let source_map_args = Compiler_args.source_map_args root_config ~watch in
+  let compiler_context =
+    Compiler_info.make_context ~build_root:root_config.root ~bsc_path:bsc
+      ~runtime_path:runtime ~source_map_args
+      ~inherited_compiler_args:
+        (root_config.jsx_args @ root_config.experimental_args)
+      ~package_output_specs:(Compiler_info.package_output_specs root_config)
+  in
+  let previous_compile_assets =
+    package_plans
+    |> List.map (fun (package : Package_plan.t) -> package.ocaml_dir)
+    |> Compile_assets.create
+  in
+  let registered_removed_modules = Hashtbl.create 16 in
+  let dependents_by_raw_dependency = Hashtbl.create 64 in
+  package_plans
+  |> List.concat_map (fun (package : Package_plan.t) ->
+      Compile_assets.ast_sources previous_compile_assets package.ocaml_dir)
+  |> List.iter (fun (ast_source : Compile_assets.ast_source) ->
+      Compile_assets.ast_dependencies previous_compile_assets
+        ast_source.ast_path
+      |> List.iter (fun dependency ->
+          let dependents =
+            Hashtbl.find_opt dependents_by_raw_dependency dependency
+            |> Option.value ~default:[]
+          in
+          Hashtbl.replace dependents_by_raw_dependency dependency
+            (ast_source.ast_path :: dependents)));
+  let invalidate_removed_module module_name =
+    if not (Hashtbl.mem registered_removed_modules module_name) then (
+      Hashtbl.add registered_removed_modules module_name ();
+      Hashtbl.replace attempt.removed_modules module_name ();
+      Build_session.mark_module_removed attempt.session module_name;
+      let dependent_asts =
+        Hashtbl.find_opt dependents_by_raw_dependency module_name
+        |> Option.value ~default:[]
+      in
+      (* A cleanup failure can end the attempt before dependency preparation
+         records why consumers need recompilation. Remove their freshness
+         markers when the attempt finishes so the next invocation cannot reuse
+         them, while the current invocation can still compile from the existing
+         AST and report the source-level missing-module error. *)
+      Build_attempt.register_cleanup attempt (fun () ->
+          List.iter File_util.remove_file dependent_asts))
+  in
+  let cleanup_started = Unix.gettimeofday () in
+  List.iter
+    (fun (package : Package_plan.t) ->
+      let package_context =
+        Compiler_info.for_package compiler_context
+          ~build_root:package.build_owner package.compile_config
+      in
+      if Compiler_info.needs_clean package_context package.config then (
+        Compiler_info.changed_package_output_specs package_context
+          package.config
+        |> Option.iter (fun previous_specs ->
+            let previous_config =
+              Compiler_info.config_with_package_output_specs
+                package.compile_config previous_specs
+            in
+            let previous_implementations =
+              List.map
+                (fun (module_ : Source.module_) -> module_.implementation)
+                package.modules
+              @ (Compile_assets.ast_sources previous_compile_assets
+                   package.ocaml_dir
+                |> List.filter_map (fun (source : Compile_assets.ast_source) ->
+                    if Filename.check_suffix source.ast_path ".iast" then None
+                    else
+                      Project_context.relative_to_opt package.root
+                        source.source_path))
+              |> List.sort_uniq String.compare
+            in
+            Build_artifacts.remove_public_outputs previous_config
+              previous_implementations);
+        ignore
+          (Build_artifacts.cleanup_stale
+             ~ocaml_files:
+               (Compile_assets.files previous_compile_assets package.ocaml_dir)
+             ~ast_sources:
+               (Compile_assets.ast_sources previous_compile_assets
+                  package.ocaml_dir)
+             ~root:package.root ~ocaml_dir:package.ocaml_dir
+             ~source_files:package.source_files
+             ~present_source_files:package.present_source_files
+             ~on_removed_module:invalidate_removed_module
+             ~on_deferred_artifact:File_util.remove_file
+             ~is_local:package.is_local package.compile_config package.modules);
+        Compiler_info.clean_package package.config;
+        attempt.compiler_cleaned <- true);
+      File_util.ensure_dir package.build_dir;
+      File_util.ensure_dir package.ocaml_dir)
+    package_plans;
+  List.iter
+    (fun (package : Package_plan.t) ->
+      let cleanup =
+        Build_artifacts.cleanup_stale
+          ~ocaml_files:
+            (Compile_assets.files previous_compile_assets package.ocaml_dir)
+          ~ast_sources:
+            (Compile_assets.ast_sources previous_compile_assets
+               package.ocaml_dir)
+          ~root:package.root ~ocaml_dir:package.ocaml_dir
+          ~source_files:package.source_files
+          ~present_source_files:package.present_source_files
+          ~on_removed_module:invalidate_removed_module
+          ~on_deferred_artifact:(fun path ->
+            Build_attempt.defer_artifact_cleanup attempt [path])
+          ~is_local:package.is_local package.compile_config package.modules
+      in
+      Build_attempt.set_cleanup_result attempt package.root cleanup;
+      attempt.cleaned <- attempt.cleaned + List.length cleanup.removed_modules;
+      attempt.previous_asts <-
+        attempt.previous_asts + cleanup.previous_ast_count;
+      ())
+    package_plans;
+  let compile_assets =
+    if attempt.compiler_cleaned || Hashtbl.length registered_removed_modules > 0
+    then
+      package_plans
+      |> List.map (fun (package : Package_plan.t) -> package.ocaml_dir)
+      |> Compile_assets.create
+    else previous_compile_assets
+  in
+  on_cleanup (Unix.gettimeofday () -. cleanup_started);
+  let parse_started = Unix.gettimeofday () in
+  let parse_entries =
+    package_plans
+    |> List.concat_map (fun (package : Package_plan.t) ->
+        package.modules
+        |> List.concat_map (fun module_ ->
+            let paths =
+              module_.Source.implementation
+              :: Option.to_list module_.Source.interface
+            in
+            let dirty_paths =
+              paths
+              |> List.filter (fun path ->
+                  Build_freshness.source_is_not_older_than_ast compile_assets
+                    ~root:package.root ~source_mtimes:package.source_mtimes path)
+            in
+            if dirty_paths <> [] then
+              Output.debug ~verbosity:attempt.verbosity
+                ("Generating AST for module: "
+                ^ Source.compiler_basename package.compile_config
+                    module_.Source.name);
+            let group = package.root ^ "\000" ^ module_.Source.name in
+            List.map (fun path -> (package, path, group)) dirty_paths))
+  in
+  let parse_completed =
+    Output.Progress.start_grouped attempt.progress ~step:parse_step
+      ~symbol:Platform.parse_symbol ~label:"Parsing"
+      (List.map (fun (_, _, group) -> group) parse_entries)
+  in
+  let parse_results =
+    Process.run_parallel_map ?poll:attempt.process_poll
+      ~on_complete:parse_completed parse_entries
+      ~job:(fun ((package : Package_plan.t), path, _) ->
+        Compiler_process.parse_job ~bsc ~build_dir:package.build_dir
+          ~config:package.compile_config path)
+  in
+  let failed_parse_paths = Hashtbl.create 8 in
+  List.iter2
+    (fun ((package : Package_plan.t), path, _) result ->
+      let absolute_path = Filename.concat package.root path in
+      let outcome = Build_attempt.preliminary_parse result in
+      Hashtbl.replace attempt.preliminary_parses absolute_path outcome;
+      match outcome with
+      | Build_attempt.Parse_failed _ ->
+        Hashtbl.replace failed_parse_paths absolute_path ()
+      | Build_attempt.Parsed_successfully _ | Build_attempt.Use_existing_ast ->
+        ())
+    parse_entries parse_results;
+  let graph =
+    Module_graph.initialize ~root_config ~package_plans ~compile_assets
+      ~failed_parse_paths
+  in
+  List.iter
+    (fun path ->
+      if not (Hashtbl.mem attempt.preliminary_parses path) then
+        Hashtbl.replace attempt.preliminary_parses path
+          Build_attempt.Use_existing_ast)
+    graph.use_existing_ast_paths;
+  List.iter
+    (fun (node : Module_graph.module_node) ->
+      Build_session.add_global_module attempt.session node.key node)
+    graph.nodes;
+  List.iter
+    (Build_session.add_namespace_map attempt.session)
+    graph.namespace_maps;
+  let nodes = graph.nodes in
+  let namespace_maps = graph.namespace_maps in
+  let build_state = graph.build_state in
+  let packages = Hashtbl.create (List.length package_plans) in
+  List.iter
+    (fun (package : Package_plan.t) ->
+      let regular_dependency_dirs, development_dependency_dirs =
+        List.fold_left
+          (fun (regular, development) (dependency : Package_plan.dependency) ->
+            let directory =
+              Build_artifacts.lib_path dependency.directory "ocaml"
+            in
+            if not (File_util.exists directory) then (regular, development)
+            else
+              match dependency.kind with
+              | Package_traversal.Regular -> (directory :: regular, development)
+              | Package_traversal.Development ->
+                (regular, directory :: development))
+          ([], []) package.dependencies
+        |> fun (regular, development) -> (List.rev regular, List.rev development)
+      in
+      let common_args dependency_dirs =
+        Compiler_args.compiler_common_arguments ~config:package.compile_config
+          ~runtime ~dependency_dirs ~watch
+          ~gentype_dependency_args:package.gentype_dependency_args
+      in
+      let parse_paths =
+        package.modules
+        |> List.concat_map (fun module_ ->
+            module_.Source.implementation
+            :: Option.to_list module_.Source.interface)
+      in
+      Hashtbl.add packages package.root
+        Package_plan.
+          {
+            regular_common_args = common_args regular_dependency_dirs;
+            development_common_args =
+              common_args (development_dependency_dirs @ regular_dependency_dirs);
+            parse_paths;
+          })
+    package_plans;
+  let prepared =
+    Build_session.{compiler_context; compile_assets; build_state; packages}
+  in
+  Build_session.install_prepared attempt.session prepared;
+  let cycle = Module_graph.find_cycle nodes namespace_maps build_state in
+  Build_session.set_graph_cycle attempt.session cycle;
+  attempt.parse_seconds <- Unix.gettimeofday () -. parse_started;
+  {prepared; cycle}
diff --git a/rewatch-ocaml/build_preparation.mli b/rewatch-ocaml/build_preparation.mli
new file mode 100644
index 00000000000..75a95cd78a9
--- /dev/null
+++ b/rewatch-ocaml/build_preparation.mli
@@ -0,0 +1,16 @@
+type result = {
+  prepared: Build_session.prepared;
+  cycle: Module_graph.cycle_info option;
+}
+
+val run :
+  root_config:Config.t ->
+  prod:bool ->
+  features:string list option ->
+  warn_error:string option ->
+  filter:Source_filter.t option ->
+  watch:bool ->
+  attempt:Build_attempt.t ->
+  parse_step:string ->
+  on_cleanup:(float -> unit) ->
+  result
diff --git a/rewatch-ocaml/build_report.ml b/rewatch-ocaml/build_report.ml
new file mode 100644
index 00000000000..ea1e4e2e322
--- /dev/null
+++ b/rewatch-ocaml/build_report.ml
@@ -0,0 +1,126 @@
+type t = {
+  started_at: float;
+  interactive: bool;
+  show_progress: bool;
+  colors: bool;
+  no_timing: bool;
+  compilation_kind: Build_attempt.compilation_kind;
+  attempt: Build_attempt.t;
+}
+
+let create ~started_at ~interactive ~show_progress ~colors ~no_timing
+    ~compilation_kind ~attempt =
+  {
+    started_at;
+    interactive;
+    show_progress;
+    colors;
+    no_timing;
+    compilation_kind;
+    attempt;
+  }
+
+let compile_step report =
+  match report.compilation_kind with
+  | Build_attempt.Incremental_watch -> "2/2"
+  | _ -> "3/3"
+
+let output_kind report =
+  match report.compilation_kind with
+  | Build_attempt.Initial_watch -> Output.Initial
+  | Build_attempt.Incremental_watch -> Output.Incremental
+  | Build_attempt.One_shot | Build_attempt.Full_watch -> Output.Standard
+
+let prepare report ~success ~compile_seconds =
+  let attempt = report.attempt in
+  if report.show_progress then
+    if report.interactive then
+      if success then
+        print_endline
+          (Output.compiling_message ~color:report.colors
+             ~step:(compile_step report) ~count:attempt.compiled
+             ~seconds:compile_seconds)
+      else
+        prerr_endline
+          (Output.compilation_failed_message ~color:report.colors
+             ~step:(compile_step report) ~count:attempt.compiled
+             ~seconds:compile_seconds)
+    else (
+      (match report.compilation_kind with
+      | Build_attempt.One_shot | Build_attempt.Initial_watch
+      | Build_attempt.Full_watch ->
+        Printf.printf "Cleaned %d/%d\n%!" attempt.cleaned attempt.previous_asts
+      | Build_attempt.Incremental_watch -> ());
+      Printf.printf "Parsed %d source files\n%!" attempt.parsed;
+      if success then Printf.printf "Compiled %d modules\n%!" attempt.compiled
+      else Printf.eprintf "Compiled %d modules\n%!" attempt.compiled);
+  let diagnostics =
+    match report.compilation_kind with
+    | Build_attempt.Incremental_watch | Build_attempt.Full_watch -> []
+    | Build_attempt.One_shot | Build_attempt.Initial_watch ->
+      attempt.diagnostics |> List.rev |> List.sort_uniq String.compare
+  in
+  let warning_entries =
+    Warning_state.entries (Build_session.warning_state attempt.session)
+  in
+  List.iter
+    (fun entry -> prerr_string entry.Warning_state.output)
+    warning_entries;
+  if warning_entries <> [] && diagnostics = [] then prerr_newline ();
+  flush stderr;
+  if diagnostics <> [] then
+    diagnostics
+    |> List.map (fun diagnostic ->
+        if report.colors then Output.yellow diagnostic else diagnostic)
+    |> String.concat "\n\n" |> prerr_endline;
+  diagnostics
+
+let report_completion report diagnostics =
+  if report.interactive && report.show_progress then
+    let seconds =
+      if report.no_timing then 0. else Unix.gettimeofday () -. report.started_at
+    in
+    Printf.printf "\n%s\n%!"
+      (Output.finished_compilation_message ~label:(output_kind report)
+         ~warnings:
+           (report.attempt.had_warnings || diagnostics <> []
+           || Warning_state.entries
+                (Build_session.warning_state report.attempt.session)
+              <> [])
+         ~seconds)
+  else if
+    report.compilation_kind <> Build_attempt.One_shot && report.show_progress
+  then
+    Printf.printf "Finished %scompilation\n%!"
+      (match report.compilation_kind with
+      | Build_attempt.Initial_watch -> "initial "
+      | Build_attempt.Incremental_watch -> "incremental "
+      | Build_attempt.One_shot | Build_attempt.Full_watch -> "")
+
+let report report ~success ~compile_seconds =
+  let diagnostics = prepare report ~success ~compile_seconds in
+  if success then report_completion report diagnostics
+
+let print_success_details report ~compile_seconds =
+  prepare report ~success:true ~compile_seconds
+
+let report_parse_failure report ~output =
+  (if report.interactive && report.show_progress then
+     prerr_endline
+       (Output.parsing_failed_message ~color:report.colors
+          ~step:
+            (match report.compilation_kind with
+            | Build_attempt.Incremental_watch -> "1/2"
+            | Build_attempt.One_shot | Build_attempt.Initial_watch
+            | Build_attempt.Full_watch ->
+              "2/3")
+          ~seconds:
+            (if report.no_timing then 0. else report.attempt.parse_seconds))
+   else if report.show_progress then
+     match report.compilation_kind with
+     | Build_attempt.One_shot | Build_attempt.Initial_watch
+     | Build_attempt.Full_watch ->
+       Printf.printf "Cleaned %d/%d\n%!" report.attempt.cleaned
+         report.attempt.previous_asts
+     | Build_attempt.Incremental_watch -> ());
+  prerr_endline output
diff --git a/rewatch-ocaml/build_report.mli b/rewatch-ocaml/build_report.mli
new file mode 100644
index 00000000000..fc133493776
--- /dev/null
+++ b/rewatch-ocaml/build_report.mli
@@ -0,0 +1,19 @@
+type t
+(** Reporting owns presentation state independently of build execution. This
+    keeps terminal progress, timing, and final diagnostics from influencing
+    cleanup or retained-state transitions. *)
+
+val create :
+  started_at:float ->
+  interactive:bool ->
+  show_progress:bool ->
+  colors:bool ->
+  no_timing:bool ->
+  compilation_kind:Build_attempt.compilation_kind ->
+  attempt:Build_attempt.t ->
+  t
+
+val report : t -> success:bool -> compile_seconds:float -> unit
+val print_success_details : t -> compile_seconds:float -> string list
+val report_completion : t -> string list -> unit
+val report_parse_failure : t -> output:string -> unit
diff --git a/rewatch-ocaml/build_session.ml b/rewatch-ocaml/build_session.ml
new file mode 100644
index 00000000000..d5ec0aa55ce
--- /dev/null
+++ b/rewatch-ocaml/build_session.ml
@@ -0,0 +1,148 @@
+type prepared = {
+  compiler_context: Compiler_info.context;
+  compile_assets: Compile_assets.t;
+  build_state: Build_state.t;
+  packages: (string, Package_plan.compilation) Hashtbl.t;
+}
+
+type source_reference = {
+  package_root: string;
+  module_: Source.module_;
+  relative_path: string;
+  absolute_path: string;
+}
+
+type readiness =
+  | Not_prepared
+  | Freshness_pending of prepared
+  | Ready of prepared
+
+type compiler_info_state = Needs_publication | Published
+type cycle_cache =
+  | Unknown_cycle
+  | Known_cycle of Module_graph.cycle_info option
+
+type t = {
+  global_modules: (string, Module_graph.module_node) Hashtbl.t;
+  namespace_maps: (string, Module_graph.namespace_map) Hashtbl.t;
+  namespace_maps_by_name: (string, Module_graph.namespace_map list) Hashtbl.t;
+  mutable graph_cycle: cycle_cache;
+  package_plans: (string, Package_plan.t) Hashtbl.t;
+  source_index: (string, source_reference) Hashtbl.t;
+  pending_parse_paths: (string, unit) Hashtbl.t;
+  pending_removed_modules: (string, unit) Hashtbl.t;
+  public_outputs: (string, (string, unit) Hashtbl.t) Hashtbl.t;
+  mutable readiness: readiness;
+  mutable compiler_info_state: compiler_info_state;
+  warning_state: Warning_state.t;
+}
+
+let create ~warning_state =
+  {
+    global_modules = Hashtbl.create 64;
+    namespace_maps = Hashtbl.create 16;
+    namespace_maps_by_name = Hashtbl.create 16;
+    graph_cycle = Unknown_cycle;
+    package_plans = Hashtbl.create 32;
+    source_index = Hashtbl.create 64;
+    pending_parse_paths = Hashtbl.create 16;
+    pending_removed_modules = Hashtbl.create 16;
+    public_outputs = Hashtbl.create 32;
+    readiness = Not_prepared;
+    compiler_info_state = Needs_publication;
+    warning_state;
+  }
+
+let is_ready session =
+  match session.readiness with
+  | Ready _ -> true
+  | _ -> false
+
+let prepared session =
+  match session.readiness with
+  | Freshness_pending prepared | Ready prepared -> Some prepared
+  | Not_prepared -> None
+
+let install_prepared session prepared =
+  session.readiness <- Freshness_pending prepared
+
+let mark_freshness_initialized session =
+  match session.readiness with
+  | Freshness_pending prepared ->
+    session.readiness <- Ready prepared;
+    Hashtbl.clear session.pending_removed_modules
+  | Ready _ -> ()
+  | Not_prepared -> invalid_arg "build state has not been prepared"
+
+let find_global_module session key = Hashtbl.find_opt session.global_modules key
+
+let add_global_module session key module_ =
+  Hashtbl.add session.global_modules key module_
+
+let global_module_values session =
+  Hashtbl.to_seq_values session.global_modules |> List.of_seq
+
+let find_namespace_maps session name =
+  Hashtbl.find_opt session.namespace_maps_by_name name
+
+let add_namespace_map session namespace_map =
+  Hashtbl.add session.namespace_maps namespace_map.Module_graph.key
+    namespace_map;
+  let existing =
+    find_namespace_maps session namespace_map.namespace
+    |> Option.value ~default:[]
+  in
+  Hashtbl.replace session.namespace_maps_by_name namespace_map.namespace
+    (namespace_map :: existing)
+
+let find_namespace_map session key = Hashtbl.find session.namespace_maps key
+
+let namespace_map_values session =
+  Hashtbl.to_seq_values session.namespace_maps |> List.of_seq
+
+let graph_cycle session = session.graph_cycle
+let invalidate_graph_cycle session = session.graph_cycle <- Unknown_cycle
+let set_graph_cycle session cycle = session.graph_cycle <- Known_cycle cycle
+
+let add_package_plan session package =
+  Hashtbl.replace session.package_plans package.Package_plan.root package
+
+let find_package_plan session root = Hashtbl.find_opt session.package_plans root
+
+let iter_package_plans session f = Hashtbl.iter f session.package_plans
+let package_plan_values session = Hashtbl.to_seq_values session.package_plans
+
+let publish_compiler_info session write =
+  match session.compiler_info_state with
+  | Published -> ()
+  | Needs_publication ->
+    Hashtbl.iter (fun _ package -> write package) session.package_plans;
+    session.compiler_info_state <- Published
+
+let add_source_reference session normalized_path source =
+  Hashtbl.replace session.source_index normalized_path source
+
+let find_source_reference session normalized_path =
+  Hashtbl.find_opt session.source_index normalized_path
+
+let pending_parse_paths session =
+  session.pending_parse_paths |> Hashtbl.to_seq_keys |> List.of_seq
+
+let mark_parse_pending session path =
+  Hashtbl.replace session.pending_parse_paths path ()
+
+let clear_parse_pending session path =
+  Hashtbl.remove session.pending_parse_paths path
+
+let mark_module_removed session name =
+  Hashtbl.replace session.pending_removed_modules name ()
+
+let pending_removed_modules session =
+  session.pending_removed_modules |> Hashtbl.to_seq_keys |> List.of_seq
+
+let set_public_outputs session root outputs =
+  Hashtbl.replace session.public_outputs root outputs
+
+let iter_public_outputs session f = Hashtbl.iter f session.public_outputs
+
+let warning_state session = session.warning_state
diff --git a/rewatch-ocaml/build_session.mli b/rewatch-ocaml/build_session.mli
new file mode 100644
index 00000000000..847532a5193
--- /dev/null
+++ b/rewatch-ocaml/build_session.mli
@@ -0,0 +1,60 @@
+type t
+(** A session keeps only state that must survive from one watch build to the
+    next. Per-attempt diagnostics, counters, and cleanup actions deliberately
+    live in {!Build_attempt} so a failed attempt cannot leak transient state
+    into its successor. *)
+
+type prepared = {
+  compiler_context: Compiler_info.context;
+  compile_assets: Compile_assets.t;
+  build_state: Build_state.t;
+  packages: (string, Package_plan.compilation) Hashtbl.t;
+}
+
+type source_reference = {
+  package_root: string;
+  module_: Source.module_;
+  relative_path: string;
+  absolute_path: string;
+}
+
+(** Cycle results are cached because ordinary implementation edits do not
+    change dependency edges. [Unknown_cycle] means that graph analysis is
+    required; [Known_cycle None] means it ran and found an acyclic graph. *)
+type cycle_cache =
+  | Unknown_cycle
+  | Known_cycle of Module_graph.cycle_info option
+
+val create : warning_state:Warning_state.t -> t
+val is_ready : t -> bool
+val prepared : t -> prepared option
+val install_prepared : t -> prepared -> unit
+val mark_freshness_initialized : t -> unit
+val find_global_module : t -> string -> Module_graph.module_node option
+val add_global_module : t -> string -> Module_graph.module_node -> unit
+val global_module_values : t -> Module_graph.module_node list
+val find_namespace_maps : t -> string -> Module_graph.namespace_map list option
+val add_namespace_map : t -> Module_graph.namespace_map -> unit
+val find_namespace_map : t -> string -> Module_graph.namespace_map
+val namespace_map_values : t -> Module_graph.namespace_map list
+val graph_cycle : t -> cycle_cache
+val invalidate_graph_cycle : t -> unit
+val set_graph_cycle : t -> Module_graph.cycle_info option -> unit
+val add_package_plan : t -> Package_plan.t -> unit
+val find_package_plan : t -> string -> Package_plan.t option
+val iter_package_plans : t -> (string -> Package_plan.t -> unit) -> unit
+val package_plan_values : t -> Package_plan.t Seq.t
+val publish_compiler_info : t -> (Package_plan.t -> unit) -> unit
+val add_source_reference : t -> string -> source_reference -> unit
+val find_source_reference : t -> string -> source_reference option
+val pending_parse_paths : t -> string list
+val mark_parse_pending : t -> string -> unit
+val clear_parse_pending : t -> string -> unit
+val mark_module_removed : t -> string -> unit
+val pending_removed_modules : t -> string list
+
+val set_public_outputs : t -> string -> (string, unit) Hashtbl.t -> unit
+val iter_public_outputs :
+  t -> (string -> (string, unit) Hashtbl.t -> unit) -> unit
+
+val warning_state : t -> Warning_state.t
diff --git a/rewatch-ocaml/build_state.ml b/rewatch-ocaml/build_state.ml
new file mode 100644
index 00000000000..ca06ad27235
--- /dev/null
+++ b/rewatch-ocaml/build_state.ml
@@ -0,0 +1,112 @@
+module String_set = Set.Make (String)
+
+type module_kind = Source_module | Namespace_map
+type cmi_change = Cmi_changed | Cmi_unchanged | Cmi_change_unknown
+
+type module_ = {
+  key: string;
+  kind: module_kind;
+  mutable dependencies: string list;
+  mutable dependents: String_set.t;
+  mutable compile_dirty: bool;
+  mutable last_compiled_cmi: float option;
+  mutable last_compiled_cmt: float option;
+}
+
+type t = {modules: (string, module_) Hashtbl.t}
+
+let create capacity = {modules = Hashtbl.create capacity}
+
+let add state ~key ~kind ~last_compiled_cmi ~last_compiled_cmt =
+  Hashtbl.add state.modules key
+    {
+      key;
+      kind;
+      dependencies = [];
+      dependents = String_set.empty;
+      compile_dirty = false;
+      last_compiled_cmi;
+      last_compiled_cmt;
+    }
+
+let find state key = Hashtbl.find_opt state.modules key
+
+let find_exn state key =
+  match find state key with
+  | Some module_ -> module_
+  | None -> raise (Invalid_argument ("unknown build module " ^ key))
+
+let has_complete_compile_assets module_ =
+  Option.is_some module_.last_compiled_cmi
+  && Option.is_some module_.last_compiled_cmt
+
+let dependency_tree_compiled_after ?(namespace_freshness = Hashtbl.create 4)
+    state module_ dependency =
+  let rec latest_cmi dependency =
+    match dependency.kind with
+    | Source_module -> dependency.last_compiled_cmi
+    | Namespace_map -> (
+      match Hashtbl.find_opt namespace_freshness dependency.key with
+      | Some modified -> modified
+      | None ->
+        let modified =
+          dependency.dependencies
+          |> List.fold_left
+               (fun latest key ->
+                 match (latest, latest_cmi (find_exn state key)) with
+                 | None, member -> member
+                 | latest, None -> latest
+                 | Some latest, Some member -> Some (max latest member))
+               None
+        in
+        Hashtbl.add namespace_freshness dependency.key modified;
+        modified)
+  in
+  match (latest_cmi dependency, module_.last_compiled_cmt) with
+  | Some dependency_time, Some module_time -> dependency_time > module_time
+  | None, _ | _, None -> false
+
+let set_dependencies state ~key dependencies =
+  let module_ = find_exn state key in
+  List.iter
+    (fun dependency ->
+      let dependency_module = find_exn state dependency in
+      dependency_module.dependents <-
+        String_set.remove key dependency_module.dependents)
+    module_.dependencies;
+  module_.dependencies <- dependencies;
+  List.iter
+    (fun dependency ->
+      let dependency_module = find_exn state dependency in
+      dependency_module.dependents <-
+        String_set.add key dependency_module.dependents)
+    dependencies
+
+let mark_dependents_compile_dirty ?(visited = Hashtbl.create 8) state module_ =
+  let rec mark dependent =
+    if not (Hashtbl.mem visited dependent) then (
+      Hashtbl.add visited dependent ();
+      let dependent_module = find_exn state dependent in
+      match dependent_module.kind with
+      | Source_module -> dependent_module.compile_dirty <- true
+      | Namespace_map -> String_set.iter mark dependent_module.dependents)
+  in
+  String_set.iter mark module_.dependents
+
+let record_published_cmi ?dirty_propagation state ~compile_assets module_ ~path
+    change =
+  (match change with
+  | Cmi_unchanged -> ()
+  | Cmi_changed | Cmi_change_unknown ->
+    mark_dependents_compile_dirty ?visited:dirty_propagation state module_);
+  Compile_assets.refresh_cmi compile_assets ~key:module_.key ~path;
+  module_.last_compiled_cmi <-
+    Compile_assets.cmi compile_assets module_.key
+    |> Option.map (fun entry -> entry.Compile_assets.modified)
+
+let record_successful_compile ~compile_assets module_ ~cmt_path =
+  Compile_assets.refresh_cmt compile_assets ~key:module_.key ~path:cmt_path;
+  module_.last_compiled_cmt <-
+    Compile_assets.cmt compile_assets module_.key
+    |> Option.map (fun entry -> entry.Compile_assets.modified);
+  module_.compile_dirty <- false
diff --git a/rewatch-ocaml/build_state.mli b/rewatch-ocaml/build_state.mli
new file mode 100644
index 00000000000..ef4f9b91f7a
--- /dev/null
+++ b/rewatch-ocaml/build_state.mli
@@ -0,0 +1,60 @@
+module String_set : Set.S with type elt = string
+
+(** Build state is the authoritative mutable dependency and freshness store.
+    Compile-asset indexes supply filesystem metadata, but publication updates
+    graph dirtiness here so every successful or partial CMI publication follows
+    the same invalidation path. *)
+type module_kind = Source_module | Namespace_map
+
+type cmi_change = Cmi_changed | Cmi_unchanged | Cmi_change_unknown
+
+type module_ = {
+  key: string;
+  kind: module_kind;
+  mutable dependencies: string list;
+  mutable dependents: String_set.t;
+  mutable compile_dirty: bool;
+  mutable last_compiled_cmi: float option;
+  mutable last_compiled_cmt: float option;
+}
+
+type t
+
+val create : int -> t
+
+val add :
+  t ->
+  key:string ->
+  kind:module_kind ->
+  last_compiled_cmi:float option ->
+  last_compiled_cmt:float option ->
+  unit
+
+val find : t -> string -> module_ option
+val find_exn : t -> string -> module_
+val has_complete_compile_assets : module_ -> bool
+val dependency_tree_compiled_after :
+  ?namespace_freshness:(string, float option) Hashtbl.t ->
+  t ->
+  module_ ->
+  module_ ->
+  bool
+val set_dependencies : t -> key:string -> string list -> unit
+
+val mark_dependents_compile_dirty :
+  ?visited:(string, unit) Hashtbl.t -> t -> module_ -> unit
+(** Transitive propagation is required because a dependent may publish a
+    byte-identical CMI after recompilation. Marking only direct dependents would
+    then lose pending work below it. *)
+
+val record_published_cmi :
+  ?dirty_propagation:(string, unit) Hashtbl.t ->
+  t ->
+  compile_assets:Compile_assets.t ->
+  module_ ->
+  path:string ->
+  cmi_change ->
+  unit
+
+val record_successful_compile :
+  compile_assets:Compile_assets.t -> module_ -> cmt_path:string -> unit
diff --git a/rewatch-ocaml/clean.ml b/rewatch-ocaml/clean.ml
new file mode 100644
index 00000000000..7325403b3fa
--- /dev/null
+++ b/rewatch-ocaml/clean.ml
@@ -0,0 +1,125 @@
+type package = {
+  root: string;
+  name: string;
+  output_config: Config.t;
+  implementation_files: string list;
+}
+
+(* The complete cleanup plan is validated before deletion starts so a malformed
+   dependency cannot leave only the packages visited before it partially
+   cleaned. The resulting order remains dependency-first for progress output. *)
+let prepare ~(root_config : Config.t) ~resolution ~seen ~prod ~is_local =
+  Package_diagnostics.validate_metadata root_config;
+  let packages = ref [] in
+  let rec visit (config : Config.t) ~is_local =
+    let root = config.root in
+    if not (Hashtbl.mem seen root) then (
+      Hashtbl.add seen root ();
+      Package_diagnostics.report_missing_sources
+        ~is_root:(root = root_config.root) config;
+      (* A consumer clean owns dependencies previously built in this build
+         context, but not an independently built package's published tree. *)
+      let owns_outputs =
+        root <> root_config.root && Compiler_info.owns_outputs config
+      in
+      if not owns_outputs then (
+        let dependencies = Package_traversal.requests ~prod ~is_local config in
+        let resolved_dependencies =
+          List.map
+            (fun request ->
+              Package_traversal.resolve resolution ~package_root:root request)
+            dependencies
+        in
+        List.iter
+          (fun (resolved : Package_traversal.resolved) ->
+            visit resolved.dependency.config
+              ~is_local:resolved.dependency.is_local)
+          resolved_dependencies;
+        let implementation_files =
+          Source.discover_for_cleanup config ~prod:(prod || not is_local)
+            ~on_missing:
+              (Package_diagnostics.report_missing_source_folder config)
+        in
+        let output_config = Config.with_root_options config root_config in
+        packages :=
+          {root; name = config.name; output_config; implementation_files}
+          :: !packages))
+  in
+  visit root_config ~is_local;
+  List.rev !packages
+
+let remove_compiler_assets packages ~on_clean =
+  List.iter
+    (fun package ->
+      on_clean package.name;
+      List.iter
+        (fun dir -> File_util.remove_tree (Filename.concat package.root dir))
+        [Build_artifacts.lib_path "" "bs"; Build_artifacts.lib_path "" "ocaml"])
+    packages
+
+let remove_generated_outputs packages =
+  List.iter
+    (fun package ->
+      List.iter
+        (fun implementation ->
+          List.iter
+            (fun spec ->
+              let output =
+                Build_artifacts.generated_js_path package.output_config
+                  implementation spec
+              in
+              File_util.remove_file output;
+              File_util.remove_file (output ^ ".map"))
+            package.output_config.package_specs)
+        package.implementation_files)
+    packages
+
+let run ~poll ~verbosity ~folder ~prod =
+  let root = Project_context.canonical_project_root folder in
+  let show_progress = verbosity >= 0 in
+  let interactive = Unix.isatty Unix.stdout && Unix.isatty Unix.stderr in
+  let colors = Output.colors_enabled ~interactive in
+  let print_cleaning ~step target =
+    if interactive && show_progress then
+      Printf.printf "%s%!"
+        (Output.cleaning_command_message ~color:colors ~step target)
+  in
+  let print_cleaned ~step ~target ~started_at =
+    if interactive && show_progress then
+      print_endline
+        (Output.cleaned_command_message ~color:colors ~step ~target
+           ~seconds:(Unix.gettimeofday () -. started_at))
+  in
+  Build_lock.with_build ~poll (Project_context.workspace_lock_root root)
+    (fun ~release:_ ->
+      poll ();
+      let root_config = Config.load_root root in
+      let resolution = Package_resolution.create root_config in
+      let cleanup =
+        prepare ~root_config ~resolution ~seen:(Hashtbl.create 32) ~prod
+          ~is_local:true
+      in
+      let compiler_assets = "compiler assets" in
+      let compiler_started = Unix.gettimeofday () in
+      remove_compiler_assets cleanup ~on_clean:(fun name ->
+          if show_progress then
+            if interactive then print_cleaning ~step:"1/2" name
+            else Printf.printf "Cleaning %s\n%!" name);
+      print_cleaned ~step:"1/2" ~target:compiler_assets
+        ~started_at:compiler_started;
+      poll ();
+      let suffixes =
+        root_config.package_specs
+        |> List.filter_map (fun (spec : Config.package_spec) ->
+            if spec.in_source then
+              Some (Config.package_spec_suffix root_config spec)
+            else None)
+        |> String.concat ", "
+      in
+      let generated_files = suffixes ^ " files" in
+      let generated_started = Unix.gettimeofday () in
+      print_cleaning ~step:"2/2" generated_files;
+      remove_generated_outputs cleanup;
+      poll ();
+      print_cleaned ~step:"2/2" ~target:generated_files
+        ~started_at:generated_started)
diff --git a/rewatch-ocaml/clean.mli b/rewatch-ocaml/clean.mli
new file mode 100644
index 00000000000..be5ef56c8cc
--- /dev/null
+++ b/rewatch-ocaml/clean.mli
@@ -0,0 +1,2 @@
+val run :
+  poll:(unit -> unit) -> verbosity:int -> folder:string -> prod:bool -> unit
diff --git a/rewatch-ocaml/cli.ml b/rewatch-ocaml/cli.ml
new file mode 100644
index 00000000000..bf9c499e839
--- /dev/null
+++ b/rewatch-ocaml/cli.ml
@@ -0,0 +1,384 @@
+type command =
+  | Build of build_options
+  | Clean of {verbosity: int; folder: string; prod: bool}
+  | Watch of build_options
+  | Format of format_input
+  | Compiler_args of string
+
+and build_options = {
+  verbosity: int;
+  folder: string;
+  prod: bool;
+  features: string list option;
+  warn_error: string option;
+  after_build: string option;
+  filter: Source_filter.t option;
+  clear_screen: bool;
+  no_timing: bool;
+}
+
+and format_input =
+  | Format_stdin of string
+  | Format_files of {check: bool; paths: string list}
+
+open Cmdliner
+open Cmdliner.Term.Syntax
+
+let verbosity =
+  let verbose =
+    Arg.(
+      value & flag_all
+      & info ["v"; "verbose"] ~doc:"Increase logging verbosity.")
+  in
+  let quiet =
+    Arg.(
+      value & flag_all & info ["q"; "quiet"] ~doc:"Decrease logging verbosity.")
+  in
+  Term.term_result
+    (let+ verbose and+ quiet in
+     match (verbose, quiet) with
+     | _ :: _, _ :: _ ->
+       Error (`Msg "--verbose cannot be used together with --quiet")
+     | _ -> Ok (List.length verbose - List.length quiet))
+
+let folder =
+  Arg.(
+    value & pos 0 string "."
+    & info [] ~docv:"FOLDER"
+        ~doc:"Path to the project or subproject containing rescript.json.")
+
+let prod =
+  Arg.(
+    value & flag
+    & info ["prod"] ~doc:"Skip development dependencies and sources.")
+
+let features =
+  let parse value =
+    let values =
+      String.split_on_char ',' value
+      |> List.map String.trim
+      |> List.filter (fun value -> value <> "")
+    in
+    if values = [] then
+      Error
+        (`Msg
+           "--features must not be empty. Omit the flag to build with all \
+            features active.")
+    else Ok values
+  in
+  let print formatter values =
+    Stdlib.Format.pp_print_string formatter (String.concat "," values)
+  in
+  let converter = Arg.conv (parse, print) in
+  Arg.(
+    value
+    & opt (some converter) None
+    & info ["features"] ~docv:"FEATURES"
+        ~doc:"Restrict the current package to comma-separated features.")
+
+let warn_error =
+  Arg.(
+    value
+    & opt (some string) None
+    & info ["warn-error"] ~docv:"WARNINGS"
+        ~doc:"Override warning configuration from rescript.json.")
+
+let after_build =
+  Arg.(
+    value
+    & opt (some string) None
+    & info ["a"; "after-build"] ~docv:"COMMAND"
+        ~doc:"Run an additional command after a successful build.")
+
+let filter =
+  let parse value =
+    match Source_filter.compile value with
+    | Ok filter -> Ok filter
+    | Error message -> Error (`Msg message)
+  in
+  let print formatter filter =
+    Stdlib.Format.pp_print_string formatter (Source_filter.pattern filter)
+  in
+  Arg.(
+    value
+    & opt (some (conv (parse, print))) None
+    & info ["f"; "filter"] ~docv:"REGEX"
+        ~doc:"Filter source files by regular expression.")
+
+let no_timing =
+  Arg.(value & flag & info ["n"; "no-timing"] ~doc:"Disable output timing.")
+
+let clear_screen =
+  Arg.(
+    value & flag
+    & info ["clear-screen"]
+        ~doc:"Clear the terminal before each interactive rebuild.")
+
+let build_term ~watch =
+  let no_timing = if watch then Term.const false else no_timing in
+  let clear_screen = if watch then clear_screen else Term.const false in
+  let+ verbosity
+  and+ folder
+  and+ prod
+  and+ features
+  and+ warn_error
+  and+ after_build
+  and+ filter
+  and+ no_timing
+  and+ clear_screen in
+  let options : build_options =
+    {
+      verbosity;
+      folder;
+      prod;
+      features;
+      warn_error;
+      after_build;
+      filter;
+      clear_screen;
+      no_timing;
+    }
+  in
+  if watch then Watch options else Build options
+
+let clean_term =
+  let+ verbosity and+ folder and+ prod in
+  Clean {verbosity; folder; prod}
+
+let format_term =
+  let extension = Arg.enum [(".res", ".res"); (".resi", ".resi")] in
+  let stdin =
+    Arg.(
+      value
+      & opt (some extension) None
+      & info ["s"; "stdin"] ~docv:"EXTENSION"
+          ~doc:"Read stdin and write formatted source to stdout.")
+  in
+  let check =
+    Arg.(
+      value & flag
+      & info ["c"; "check"] ~doc:"Check formatting without modifying files.")
+  in
+  let files = Arg.(value & pos_all string [] & info [] ~docv:"FILES") in
+  Term.term_result
+    (let+ _verbosity = verbosity and+ check and+ stdin and+ files in
+     match (check, stdin, files) with
+     | true, Some _, _ -> Error (`Msg "--stdin conflicts with --check")
+     | _, Some _, _ :: _ -> Error (`Msg "files conflict with --stdin")
+     | _, Some extension, [] -> Ok (Format (Format_stdin extension))
+     | _, None, paths -> Ok (Format (Format_files {check; paths})))
+
+let compiler_args_term =
+  let path =
+    Arg.(
+      required
+      & pos 0 (some string) None
+      & info [] ~docv:"PATH" ~doc:"ReScript source file (.res or .resi).")
+  in
+  let+ _verbosity = verbosity and+ path in
+  Compiler_args path
+
+let command_info name doc = Cmd.info name ~doc
+
+let root =
+  let build =
+    Cmd.make
+      (command_info "build" "Build the project.")
+      (build_term ~watch:false)
+  in
+  let watch =
+    Cmd.make
+      (command_info "watch" "Build, then start a watcher.")
+      (build_term ~watch:true)
+  in
+  let clean =
+    Cmd.make (command_info "clean" "Clean build artifacts.") clean_term
+  in
+  let format =
+    Cmd.make (command_info "format" "Format ReScript files.") format_term
+  in
+  let compiler_args =
+    Cmd.make
+      (command_info "compiler-args"
+         "Print compiler arguments for a ReScript source file.")
+      compiler_args_term
+  in
+  let help =
+    let topic =
+      Arg.(value & pos 0 (some string) None & info [] ~docv:"COMMAND")
+    in
+    let help_term =
+      Term.ret
+        (let+ commands = Term.choice_names and+ topic in
+         match topic with
+         | None -> `Help (`Plain, None)
+         | Some command when List.mem command commands ->
+           `Help (`Plain, Some command)
+         | Some command ->
+           `Error (false, Printf.sprintf "unknown command %S" command))
+    in
+    Cmd.make
+      (command_info "help" "Print this message or command help.")
+      help_term
+  in
+  let info =
+    let man =
+      [
+        `S "NOTES";
+        `P
+          "If no command is provided, the $(b,build) command is run by \
+           default. See $(b,rescript help build) for more information.";
+        `P
+          "To create a new ReScript project, or to add ReScript to an existing \
+           project, use https://github.com/rescript-lang/create-rescript-app.";
+      ]
+    in
+    Cmd.info "rescript"
+      ~version:("rescript " ^ Rewatch_version.version)
+      ~doc:"Fast, Simple, Fully Typed JavaScript from the Future" ~man
+  in
+  Cmd.group info ~default:(build_term ~watch:false)
+    [build; watch; clean; format; compiler_args; help]
+
+type evaluation = Run of command | Exit of int
+
+exception Parse_error of string
+exception Help
+exception Version
+
+let argv_is_utf_8 argv = Array.for_all String.is_valid_utf_8 argv
+
+(* Bare project folders must select the default build command, even though
+   Cmdliner otherwise treats them as unknown commands. Move global options
+   behind the selected command and expand short help/version clusters because
+   Cmdliner's standard display options only provide long names. A root display
+   request takes precedence over otherwise invalid implicit-build arguments. *)
+let normalize_argv argv =
+  let is_short_global_cluster argument =
+    let length = String.length argument in
+    length > 1
+    && argument.[0] = '-'
+    && argument.[1] <> '-'
+    && String.for_all
+         (function
+           | 'v' | 'q' | 'h' | 'V' -> true
+           | _ -> false)
+         (String.sub argument 1 (length - 1))
+  in
+  let short_cluster_contains character argument =
+    is_short_global_cluster argument
+    && String.contains_from argument 1 character
+  in
+  let is_verbosity = function
+    | "-v" | "-vv" | "-vvv" | "-vvvv" | "--verbose" | "-q" | "-qq" | "-qqq"
+    | "-qqqq" | "--quiet" ->
+      true
+    | argument ->
+      is_short_global_cluster argument
+      && not
+           (short_cluster_contains 'h' argument
+           || short_cluster_contains 'V' argument)
+  in
+  let is_help = function
+    | "-h" | "--help" -> true
+    | argument -> String.starts_with ~prefix:"--help=" argument
+  in
+  let is_version = function
+    | "-V" | "--version" -> true
+    | _ -> false
+  in
+  let is_global argument =
+    is_short_global_cluster argument
+    || is_verbosity argument || is_help argument || is_version argument
+  in
+  let requests_help argument =
+    argument = "--help"
+    || String.starts_with ~prefix:"--help=" argument
+    || (is_short_global_cluster argument && short_cluster_contains 'h' argument)
+  in
+  let requests_version argument =
+    argument = "--version"
+    || (is_short_global_cluster argument && short_cluster_contains 'V' argument)
+  in
+  let is_command = function
+    | "build" | "watch" | "clean" | "format" | "compiler-args" | "help" -> true
+    | _ -> false
+  in
+  let rec normalize_display_options = function
+    | [] -> []
+    | "--" :: rest -> "--" :: rest
+    | argument :: rest when String.starts_with ~prefix:"--help=" argument ->
+      argument :: normalize_display_options rest
+    | argument :: rest when requests_help argument ->
+      "--help=plain" :: normalize_display_options rest
+    | argument :: rest when requests_version argument ->
+      "--version" :: normalize_display_options rest
+    | argument :: rest -> argument :: normalize_display_options rest
+  in
+  let explicit_command arguments =
+    let rec loop globals = function
+      | [] | "--" :: _ -> None
+      | argument :: rest when is_global argument ->
+        loop (argument :: globals) rest
+      | command :: rest when is_command command ->
+        Some (List.rev globals, command, rest)
+      | _ -> None
+    in
+    loop [] arguments
+  in
+  let partition_implicit arguments =
+    let rec loop globals others = function
+      | [] -> (List.rev globals, List.rev others)
+      | "--" :: rest -> (List.rev globals, List.rev_append others ("--" :: rest))
+      | argument :: rest when is_global argument ->
+        loop (argument :: globals) others rest
+      | argument :: rest -> loop globals (argument :: others) rest
+    in
+    loop [] [] arguments
+  in
+  match Array.to_list argv with
+  | [] -> argv
+  | executable :: arguments ->
+    let routed =
+      match explicit_command arguments with
+      | Some (globals, command, rest) ->
+        executable :: command :: (globals @ rest)
+      | None ->
+        let globals, others = partition_implicit arguments in
+        if
+          List.exists
+            (fun argument ->
+              requests_help argument || requests_version argument)
+            globals
+        then executable :: globals
+        else executable :: "build" :: (globals @ others)
+    in
+    Array.of_list (normalize_display_options routed)
+
+let eval argv =
+  if not (argv_is_utf_8 argv) then (
+    prerr_endline "invalid UTF-8 in command-line argument";
+    Exit 2)
+  else
+    match Cmd.eval_value ~catch:false ~argv:(normalize_argv argv) root with
+    | Ok (`Ok command) -> Run command
+    | Ok `Help | Ok `Version -> Exit 0
+    | Error _ -> Exit 2
+
+let parse argv =
+  if not (argv_is_utf_8 argv) then
+    raise (Parse_error "invalid UTF-8 in command-line argument");
+  let help_buffer = Buffer.create 256 in
+  let error_buffer = Buffer.create 256 in
+  let help = Stdlib.Format.formatter_of_buffer help_buffer in
+  let err = Stdlib.Format.formatter_of_buffer error_buffer in
+  let result =
+    Cmd.eval_value ~catch:false ~help ~err ~argv:(normalize_argv argv) root
+  in
+  Stdlib.Format.pp_print_flush help ();
+  Stdlib.Format.pp_print_flush err ();
+  match result with
+  | Ok (`Ok command) -> command
+  | Ok `Help -> raise Help
+  | Ok `Version -> raise Version
+  | Error _ -> raise (Parse_error (Buffer.contents error_buffer))
diff --git a/rewatch-ocaml/cli.mli b/rewatch-ocaml/cli.mli
new file mode 100644
index 00000000000..0f4a94c0857
--- /dev/null
+++ b/rewatch-ocaml/cli.mli
@@ -0,0 +1,31 @@
+type command =
+  | Build of build_options
+  | Clean of {verbosity: int; folder: string; prod: bool}
+  | Watch of build_options
+  | Format of format_input
+  | Compiler_args of string
+
+and build_options = {
+  verbosity: int;
+  folder: string;
+  prod: bool;
+  features: string list option;
+  warn_error: string option;
+  after_build: string option;
+  filter: Source_filter.t option;
+  clear_screen: bool;
+  no_timing: bool;
+}
+
+and format_input =
+  | Format_stdin of string
+  | Format_files of {check: bool; paths: string list}
+
+type evaluation = Run of command | Exit of int
+
+exception Parse_error of string
+exception Help
+exception Version
+
+val eval : string array -> evaluation
+val parse : string array -> command
diff --git a/rewatch-ocaml/compile_assets.ml b/rewatch-ocaml/compile_assets.ml
new file mode 100644
index 00000000000..20d8bc6c9ca
--- /dev/null
+++ b/rewatch-ocaml/compile_assets.ml
@@ -0,0 +1,134 @@
+type entry = {path: string; modified: float}
+type ast_source = {ast_path: string; source_path: string}
+
+type t = {
+  files_by_directory: (string, string list) Hashtbl.t;
+  ast_sources_by_directory: (string, ast_source list) Hashtbl.t;
+  ast_dependencies: (string, string list) Hashtbl.t;
+  ast_by_source: (string, entry) Hashtbl.t;
+  cmi_by_module: (string, entry) Hashtbl.t;
+  cmt_by_module: (string, entry) Hashtbl.t;
+}
+
+let ast_header path =
+  try Some (Ast_header.read path)
+  with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None
+
+let cleanup_extensions =
+  [".cmi"; ".cmj"; ".cmt"; ".cmti"; ".ast"; ".iast"; ".res"; ".resi"; ".mlmap"]
+
+let is_managed_basename basename =
+  List.exists (Filename.check_suffix basename) cleanup_extensions
+
+let state_extension = function
+  | ".ast" | ".iast" | ".cmi" | ".cmt" -> true
+  | _ -> false
+
+let read_directory directory =
+  let names =
+    try File_util.directory_entries directory
+    with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> []
+  in
+  let files =
+    names
+    |> List.filter (fun name ->
+        List.mem (Filename.extension name) cleanup_extensions)
+    |> List.map (Filename.concat directory)
+  in
+  let state_entries =
+    names
+    |> List.filter_map (fun name ->
+        if not (state_extension (Filename.extension name)) then None
+        else
+          let path = Filename.concat directory name in
+          try
+            let metadata = Unix.stat path in
+            if metadata.Unix.st_kind = Unix.S_DIR then None
+            else Some ({path; modified = metadata.Unix.st_mtime}, name)
+          with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None)
+  in
+  (files, state_entries)
+
+let module_key name =
+  name |> Filename.remove_extension |> String.capitalize_ascii
+
+let source_key = Platform.normalize_path_for_comparison
+
+let add_module_artifact state (entry, name) =
+  match Filename.extension name with
+  | ".cmi" -> Hashtbl.replace state.cmi_by_module (module_key name) entry
+  | ".cmt" -> Hashtbl.replace state.cmt_by_module (module_key name) entry
+  | _ -> ()
+
+let create directories =
+  let state =
+    {
+      files_by_directory = Hashtbl.create (List.length directories);
+      ast_sources_by_directory = Hashtbl.create (List.length directories);
+      ast_dependencies = Hashtbl.create 64;
+      ast_by_source = Hashtbl.create 64;
+      cmi_by_module = Hashtbl.create 64;
+      cmt_by_module = Hashtbl.create 64;
+    }
+  in
+  directories
+  |> List.sort_uniq String.compare
+  |> List.iter (fun directory ->
+      let files, state_entries = read_directory directory in
+      Hashtbl.replace state.files_by_directory directory files;
+      let ast_sources =
+        state_entries
+        |> List.filter_map (fun (entry, name) ->
+            match Filename.extension name with
+            | ".ast" | ".iast" -> (
+              match ast_header entry.path with
+              | Some header ->
+                Option.map
+                  (fun source -> (entry, source, header.dependencies))
+                  header.Ast_header.source
+              | None -> None)
+            | _ -> None)
+      in
+      Hashtbl.replace state.ast_sources_by_directory directory
+        (List.map
+           (fun (entry, source, _) ->
+             {ast_path = entry.path; source_path = source})
+           ast_sources);
+      List.iter
+        (fun (entry, source, dependencies) ->
+          Hashtbl.replace state.ast_dependencies entry.path dependencies;
+          Hashtbl.replace state.ast_by_source (source_key source) entry)
+        ast_sources;
+      List.iter (add_module_artifact state) state_entries);
+  state
+
+let files state directory =
+  Hashtbl.find_opt state.files_by_directory directory
+  |> Option.value ~default:[]
+
+let ast_sources state directory =
+  Hashtbl.find_opt state.ast_sources_by_directory directory
+  |> Option.value ~default:[]
+
+let ast_dependencies state path =
+  Hashtbl.find_opt state.ast_dependencies path |> Option.value ~default:[]
+
+let ast state source = Hashtbl.find_opt state.ast_by_source (source_key source)
+
+let cmi state key = Hashtbl.find_opt state.cmi_by_module key
+let cmt state key = Hashtbl.find_opt state.cmt_by_module key
+
+let replace_from_path table key path =
+  try
+    Hashtbl.replace table key {path; modified = (Unix.stat path).Unix.st_mtime}
+  with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+    Hashtbl.remove table key
+
+let refresh_cmi state ~key ~path =
+  replace_from_path state.cmi_by_module key path
+
+let refresh_cmt state ~key ~path =
+  replace_from_path state.cmt_by_module key path
+
+let refresh_ast state ~source ~path =
+  replace_from_path state.ast_by_source (source_key source) path
diff --git a/rewatch-ocaml/compile_assets.mli b/rewatch-ocaml/compile_assets.mli
new file mode 100644
index 00000000000..53180ff4d62
--- /dev/null
+++ b/rewatch-ocaml/compile_assets.mli
@@ -0,0 +1,19 @@
+type entry = {path: string; modified: float}
+(** This index avoids repeatedly scanning [lib/ocaml] while retaining the source
+    provenance embedded in AST headers. Mutating refresh operations must be
+    called whenever publication changes the corresponding filesystem entry. *)
+
+type ast_source = {ast_path: string; source_path: string}
+type t
+
+val is_managed_basename : string -> bool
+val create : string list -> t
+val files : t -> string -> string list
+val ast_sources : t -> string -> ast_source list
+val ast_dependencies : t -> string -> string list
+val ast : t -> string -> entry option
+val cmi : t -> string -> entry option
+val cmt : t -> string -> entry option
+val refresh_cmi : t -> key:string -> path:string -> unit
+val refresh_cmt : t -> key:string -> path:string -> unit
+val refresh_ast : t -> source:string -> path:string -> unit
diff --git a/rewatch-ocaml/compiler_args.ml b/rewatch-ocaml/compiler_args.ml
new file mode 100644
index 00000000000..683be8a98fb
--- /dev/null
+++ b/rewatch-ocaml/compiler_args.ml
@@ -0,0 +1,137 @@
+let ppx_is_enabled ~bisect_enabled flag contents =
+  if String_util.contains flag "bisect" then bisect_enabled
+  else
+    not
+      ((String_util.contains flag "graphql-ppx"
+       || String_util.contains flag "graphql_ppx")
+       && not (String_util.contains contents "%graphql")
+      || String_util.contains flag "spice"
+         && not (String_util.contains contents "@spice")
+      || String_util.contains flag "rescript-relay"
+         && not (String_util.contains contents "%relay")
+      || String_util.contains flag "re-formality"
+         && not (String_util.contains contents "%form"))
+
+let filter_ppx_flags ?bisect_enabled flags contents =
+  let bisect_enabled =
+    Option.value bisect_enabled
+      ~default:(Option.is_some (Sys.getenv_opt "BISECT_ENABLE"))
+  in
+  List.filter
+    (function
+      | [] -> false
+      | flag :: _ -> ppx_is_enabled ~bisect_enabled flag contents)
+    flags
+
+let source_map_args (config : Config.t) ~watch =
+  if config.source_map_dev && not watch then ["-bs-source-map"; "false"]
+  else config.source_map_args
+
+let compiler_flags ?(ppx_flags = []) ~source_maps ~watch ~gentype
+    (config : Config.t) =
+  let ppx_args =
+    ppx_flags
+    |> List.concat_map (function
+      | [] -> []
+      | flag :: arguments ->
+        let executable =
+          match Project_context.dependency_path config.root flag with
+          | Some path -> path
+          | None -> flag
+        in
+        ["-ppx"; String.concat " " (executable :: arguments)])
+  in
+  let source_map_args =
+    if source_maps then source_map_args config ~watch else []
+  in
+  if source_maps then
+    ppx_args @ config.jsx_args @ source_map_args @ config.compiler_flags
+    @ config.warning_flags
+    @ (if gentype then config.gentype_args else [])
+    @ config.experimental_args
+  else
+    ppx_args @ config.jsx_args @ config.experimental_args @ config.warning_flags
+    @ config.compiler_flags
+
+let with_local_warning_policy ~is_local (config : Config.t) =
+  if is_local then config else {config with warning_flags = []}
+
+let package_output (config : Config.t) path (spec : Config.package_spec) =
+  let output_dir = Build_artifacts.relative_output_directory path spec in
+  Printf.sprintf "%s:%s:%s"
+    (Config.module_format_name spec.module_format)
+    output_dir
+    (Config.package_spec_suffix config spec)
+
+let gentype_dependency_args_from_paths (config : Config.t) dependencies =
+  if config.gentype_args = [] then []
+  else
+    let paths = Hashtbl.create (List.length dependencies) in
+    List.iter
+      (fun ((dependency : Config.dependency), path) ->
+        Hashtbl.replace paths dependency.name path)
+      dependencies;
+    config.dependencies
+    |> List.concat_map (fun (dependency : Config.dependency) ->
+        match Hashtbl.find_opt paths dependency.name with
+        | None -> []
+        | Some path -> ["-bs-gentype-dep-path"; dependency.name ^ "=" ^ path])
+
+let namespace_args (config : Config.t) module_name =
+  match config.namespace with
+  | Config.No_namespace -> []
+  | Config.Namespace namespace -> ["-bs-ns"; namespace]
+  | Config.Namespace_with_entry {name; entry} ->
+    if entry = module_name then ["-open"; "@" ^ name]
+    else ["-bs-ns"; "@" ^ name]
+
+let parser_arguments ~(config : Config.t) ~contents ~path =
+  compiler_flags
+    ~ppx_flags:(filter_ppx_flags config.ppx_flags contents)
+    ~source_maps:false ~watch:false ~gentype:false config
+  @ [
+      "-absname";
+      "-bs-ast";
+      "-o";
+      Source.ast_path path;
+      Filename.concat
+        (Filename.concat Filename.parent_dir_name Filename.parent_dir_name)
+        path;
+    ]
+
+let compiler_common_arguments ~(config : Config.t) ~runtime ~dependency_dirs
+    ~watch ~gentype_dependency_args =
+  ["-I"; Filename.concat Filename.parent_dir_name "ocaml"]
+  @ ["-runtime-path"; runtime]
+  @ List.concat_map (fun directory -> ["-I"; directory]) dependency_dirs
+  @ compiler_flags ~source_maps:true ~watch ~gentype:true config
+  @ gentype_dependency_args
+  @ ["-bs-package-name"; config.name; "-bs-project-root"; config.root]
+
+let compiler_arguments_with_common ~(config : Config.t) ~common_args
+    ~module_name ~source_kind ~has_interface ~path =
+  let interface_args =
+    match source_kind with
+    | Source.Implementation when has_interface -> ["-bs-read-cmi"]
+    | Source.Implementation | Source.Interface -> []
+  in
+  let output_args =
+    match source_kind with
+    | Source.Interface -> []
+    | Source.Implementation ->
+      List.concat_map
+        (fun spec -> ["-bs-package-output"; package_output config path spec])
+        config.package_specs
+  in
+  namespace_args config module_name
+  @ interface_args @ common_args @ output_args
+  @ [Source.ast_path path]
+
+let compiler_arguments ~(config : Config.t) ~runtime ~dependency_dirs
+    ~module_name ~source_kind ~has_interface ~watch ~gentype_dependency_args
+    ~path =
+  compiler_arguments_with_common ~config
+    ~common_args:
+      (compiler_common_arguments ~config ~runtime ~dependency_dirs ~watch
+         ~gentype_dependency_args)
+    ~module_name ~source_kind ~has_interface ~path
diff --git a/rewatch-ocaml/compiler_args.mli b/rewatch-ocaml/compiler_args.mli
new file mode 100644
index 00000000000..df45b4a802f
--- /dev/null
+++ b/rewatch-ocaml/compiler_args.mli
@@ -0,0 +1,48 @@
+val filter_ppx_flags :
+  ?bisect_enabled:bool -> string list list -> string -> string list list
+
+val source_map_args : Config.t -> watch:bool -> string list
+
+val compiler_flags :
+  ?ppx_flags:string list list ->
+  source_maps:bool ->
+  watch:bool ->
+  gentype:bool ->
+  Config.t ->
+  string list
+
+val with_local_warning_policy : is_local:bool -> Config.t -> Config.t
+val gentype_dependency_args_from_paths :
+  Config.t -> (Config.dependency * string) list -> string list
+
+val parser_arguments :
+  config:Config.t -> contents:string -> path:string -> string list
+
+val compiler_arguments :
+  config:Config.t ->
+  runtime:string ->
+  dependency_dirs:string list ->
+  module_name:string ->
+  source_kind:Source.source_kind ->
+  has_interface:bool ->
+  watch:bool ->
+  gentype_dependency_args:string list ->
+  path:string ->
+  string list
+
+val compiler_common_arguments :
+  config:Config.t ->
+  runtime:string ->
+  dependency_dirs:string list ->
+  watch:bool ->
+  gentype_dependency_args:string list ->
+  string list
+
+val compiler_arguments_with_common :
+  config:Config.t ->
+  common_args:string list ->
+  module_name:string ->
+  source_kind:Source.source_kind ->
+  has_interface:bool ->
+  path:string ->
+  string list
diff --git a/rewatch-ocaml/compiler_args_command.ml b/rewatch-ocaml/compiler_args_command.ml
new file mode 100644
index 00000000000..0993b9e32e3
--- /dev/null
+++ b/rewatch-ocaml/compiler_args_command.ml
@@ -0,0 +1,99 @@
+let error message = raise (Project_context.Error message)
+
+let runtime_path resolution package_root =
+  try
+    Toolchain.runtime
+      ~find_package:
+        (Package_resolution.dependency_path resolution ~package_root)
+  with Toolchain.Error message -> error message
+
+let source_error path message =
+  error (Printf.sprintf "Could not read source file %s: %s" path message)
+
+let run path =
+  let source =
+    try
+      Filename.concat
+        (Platform.canonicalize_path (Filename.dirname path))
+        (Filename.basename path)
+    with
+    | Sys_error message -> source_error path message
+    | Unix.Unix_error (unix_error, _, _) ->
+      source_error path (Unix.error_message unix_error)
+  in
+  if
+    not
+      (Filename.check_suffix source ".res"
+      || Filename.check_suffix source ".resi")
+  then error "compiler-args expects a .res or .resi source file";
+  let package_config =
+    match Project_context.nearest_config_path (Filename.dirname source) with
+    | Some path -> Config.load path
+    | None -> error "could not find a rescript.json parent"
+  in
+  let root = Project_context.workspace_lock_root package_config.root in
+  let root_config_path = Config.path_in_root root in
+  let root_config =
+    if root <> package_config.root && Config.exists_in_root root then
+      Config.load root_config_path
+    else package_config
+  in
+  let resolution =
+    Package_resolution.create
+      ~diagnostic_mode:Package_resolution.Suppress_diagnostics root_config
+  in
+  let config = Config.with_root_options package_config root_config in
+  let relative = Project_context.relative_to config.root source in
+  let contents =
+    try File_util.read_file source with
+    | Sys_error message -> source_error path message
+    | Unix.Unix_error (unix_error, _, _) ->
+      source_error path (Unix.error_message unix_error)
+  in
+  let parser_args =
+    Compiler_args.parser_arguments ~config ~contents ~path:relative
+  in
+  let source_kind =
+    if Filename.check_suffix source ".resi" then Source.Interface
+    else Source.Implementation
+  in
+  let has_interface =
+    match source_kind with
+    | Source.Implementation -> File_util.exists (source ^ "i")
+    | Source.Interface -> false
+  in
+  let dependencies =
+    (if Config.source_is_dev config relative then
+       List.map (fun dependency -> (false, dependency)) config.dev_dependencies
+     else [])
+    @ List.map (fun dependency -> (true, dependency)) config.dependencies
+  in
+  let dependency_dirs =
+    dependencies
+    |> List.filter_map (fun (required, (dependency : Config.dependency)) ->
+        match
+          Package_resolution.dependency_path resolution
+            ~package_root:config.root dependency.name
+        with
+        | Some directory -> Some (Build_artifacts.lib_path directory "ocaml")
+        | None when not required -> None
+        | None ->
+          error
+            (Printf.sprintf "Expected to find dependent package %s of %s"
+               dependency.name config.name))
+  in
+  let runtime = runtime_path resolution config.root in
+  let compiler_args =
+    Compiler_args.compiler_arguments ~config ~runtime ~dependency_dirs
+      ~module_name:(Source.module_name source)
+      ~source_kind ~has_interface ~watch:false ~gentype_dependency_args:[]
+      ~path:relative
+  in
+  Yojson.Safe.pretty_to_string
+    (`Assoc
+       [
+         ( "compiler_args",
+           `List (List.map (fun value -> `String value) compiler_args) );
+         ( "parser_args",
+           `List (List.map (fun value -> `String value) parser_args) );
+       ])
diff --git a/rewatch-ocaml/compiler_args_command.mli b/rewatch-ocaml/compiler_args_command.mli
new file mode 100644
index 00000000000..f9440843c61
--- /dev/null
+++ b/rewatch-ocaml/compiler_args_command.mli
@@ -0,0 +1 @@
+val run : string -> string
diff --git a/rewatch-ocaml/compiler_info.ml b/rewatch-ocaml/compiler_info.ml
new file mode 100644
index 00000000000..1207008ab15
--- /dev/null
+++ b/rewatch-ocaml/compiler_info.ml
@@ -0,0 +1,165 @@
+type context = {
+  build_root: string;
+  bsc_path: string;
+  bsc_hash: string;
+  runtime_path: string;
+  source_map_args: string list;
+  inherited_compiler_args: string list;
+  package_output_specs: package_output_spec list;
+}
+
+and package_output_spec = {
+  module_format: Config.module_format;
+  in_source: bool;
+  suffix: string;
+}
+
+let format_version = "4"
+
+let package_output_specs (config : Config.t) =
+  List.map
+    (fun (spec : Config.package_spec) ->
+      {
+        module_format = spec.module_format;
+        in_source = spec.in_source;
+        suffix = Config.package_spec_suffix config spec;
+      })
+    config.package_specs
+
+let make_context ~build_root ~bsc_path ~runtime_path ~source_map_args
+    ~inherited_compiler_args ~package_output_specs =
+  {
+    build_root;
+    bsc_path;
+    bsc_hash = Digest.file bsc_path |> Digest.to_hex;
+    runtime_path;
+    source_map_args;
+    inherited_compiler_args;
+    package_output_specs;
+  }
+
+let for_package context ~build_root config =
+  {context with build_root; package_output_specs = package_output_specs config}
+
+let path root = File_util.path_of_parts root ["lib"; "bs"; "compiler-info.json"]
+
+let package_output_spec_json spec =
+  `Assoc
+    [
+      ("module", `String (Config.module_format_name spec.module_format));
+      ("in_source", `Bool spec.in_source);
+      ("suffix", `String spec.suffix);
+    ]
+
+let package_output_spec_of_json = function
+  | `Assoc fields -> (
+    match
+      ( List.assoc_opt "module" fields,
+        List.assoc_opt "in_source" fields,
+        List.assoc_opt "suffix" fields )
+    with
+    | ( Some (`String module_format),
+        Some (`Bool in_source),
+        Some (`String suffix) ) -> (
+      match module_format with
+      | "esmodule" -> Some {module_format = Config.Esmodule; in_source; suffix}
+      | "commonjs" -> Some {module_format = Config.Commonjs; in_source; suffix}
+      | _ -> None)
+    | _ -> None)
+  | _ -> None
+
+let package_output_specs_of_json = function
+  | `Assoc fields -> (
+    match List.assoc_opt "package_output_specs" fields with
+    | Some (`List values) ->
+      let specs = List.filter_map package_output_spec_of_json values in
+      if List.length specs = List.length values then Some specs else None
+    | _ -> None)
+  | _ -> None
+
+let build_root_of_json = function
+  | `Assoc fields -> (
+    match List.assoc_opt "build_root" fields with
+    | Some (`String build_root) -> Some build_root
+    | _ -> None)
+  | _ -> None
+
+let read config =
+  try Some (Yojson.Safe.from_file (path config.Config.root))
+  with Yojson.Json_error _ | Sys_error _ -> None
+
+let json context (config : Config.t) =
+  `Assoc
+    [
+      ("version", `String format_version);
+      ("build_root", `String context.build_root);
+      ("bsc_path", `String context.bsc_path);
+      ("bsc_hash", `String context.bsc_hash);
+      ("rescript_config_hash", `String config.file_hash);
+      ( "source_map_args",
+        `List (List.map (fun value -> `String value) context.source_map_args) );
+      ( "inherited_compiler_args",
+        `List
+          (List.map
+             (fun value -> `String value)
+             context.inherited_compiler_args) );
+      ( "package_output_specs",
+        `List (List.map package_output_spec_json context.package_output_specs)
+      );
+      ("runtime_path", `String context.runtime_path);
+    ]
+
+let same_path left right =
+  Platform.normalize_path_for_comparison left
+  = Platform.normalize_path_for_comparison right
+
+let owns_outputs (config : Config.t) =
+  match Option.bind (read config) build_root_of_json with
+  | Some build_root -> same_path build_root config.root
+  | None -> false
+
+let matches_json context config contents = contents = json context config
+
+let matches context config =
+  match read config with
+  | Some contents -> matches_json context config contents
+  | None -> false
+
+let changed_package_output_specs context config =
+  match read config with
+  | None -> None
+  | Some contents ->
+    Option.bind (package_output_specs_of_json contents) (fun previous ->
+        if previous = context.package_output_specs then None else Some previous)
+
+let config_with_package_output_specs (config : Config.t) specs =
+  let package_specs =
+    List.map
+      (fun spec : Config.package_spec ->
+        {
+          module_format = spec.module_format;
+          in_source = spec.in_source;
+          suffix = Some spec.suffix;
+        })
+      specs
+  in
+  {config with package_specs}
+
+let previous_build_exists root =
+  File_util.exists
+    (File_util.path_of_parts root ["lib"; "ocaml"; ".compiler.log"])
+
+let needs_clean context (config : Config.t) =
+  let info_path = path config.root in
+  if File_util.exists info_path then not (matches context config)
+  else previous_build_exists config.root
+
+let clean_package (config : Config.t) =
+  File_util.remove_tree (Build_artifacts.lib_path config.root "bs");
+  File_util.remove_tree (Build_artifacts.lib_path config.root "ocaml")
+
+let write_package context (config : Config.t) =
+  if not (matches context config) then
+    let info_path = path config.root in
+    let contents = Yojson.Safe.pretty_to_string (json context config) ^ "\n" in
+    File_util.write_file_atomic ~perm:0o644 info_path contents
diff --git a/rewatch-ocaml/compiler_info.mli b/rewatch-ocaml/compiler_info.mli
new file mode 100644
index 00000000000..265cbf40bc3
--- /dev/null
+++ b/rewatch-ocaml/compiler_info.mli
@@ -0,0 +1,44 @@
+type context = {
+  build_root: string;
+  bsc_path: string;
+  bsc_hash: string;
+  runtime_path: string;
+  source_map_args: string list;
+  inherited_compiler_args: string list;
+  package_output_specs: package_output_spec list;
+}
+(** Compiler information fingerprints effective inputs rather than only the
+    package's own JSON. Root-level JSX, experimental options, runtime identity,
+    and output layout can all change dependency output without changing a
+    dependency configuration file. *)
+
+and package_output_spec = {
+  module_format: Config.module_format;
+  in_source: bool;
+  suffix: string;
+}
+
+val package_output_specs : Config.t -> package_output_spec list
+
+val make_context :
+  build_root:string ->
+  bsc_path:string ->
+  runtime_path:string ->
+  source_map_args:string list ->
+  inherited_compiler_args:string list ->
+  package_output_specs:package_output_spec list ->
+  context
+
+val for_package : context -> build_root:string -> Config.t -> context
+
+val owns_outputs : Config.t -> bool
+
+val changed_package_output_specs :
+  context -> Config.t -> package_output_spec list option
+
+val config_with_package_output_specs :
+  Config.t -> package_output_spec list -> Config.t
+
+val needs_clean : context -> Config.t -> bool
+val clean_package : Config.t -> unit
+val write_package : context -> Config.t -> unit
diff --git a/rewatch-ocaml/compiler_log.ml b/rewatch-ocaml/compiler_log.ml
new file mode 100644
index 00000000000..7c16e65be3c
--- /dev/null
+++ b/rewatch-ocaml/compiler_log.ml
@@ -0,0 +1,39 @@
+let path root directory =
+  Filename.concat (Build_artifacts.lib_path root directory) ".compiler.log"
+
+let strip_ansi content =
+  let length = String.length content in
+  let output = Buffer.create length in
+  let rec skip_csi index =
+    if index >= length then index
+    else
+      let code = Char.code content.[index] in
+      if code >= 0x40 && code <= 0x7e then index + 1 else skip_csi (index + 1)
+  in
+  let rec loop index =
+    if index < length then
+      if
+        (content.[index] = '\027' || content.[index] = '\155')
+        && index + 1 < length
+        && content.[index + 1] = '['
+      then loop (skip_csi (index + 2))
+      else (
+        Buffer.add_char output content.[index];
+        loop (index + 1))
+  in
+  loop 0;
+  Buffer.contents output
+
+let initialize root =
+  let path = path root "bs" in
+  File_util.ensure_dir (Filename.dirname path);
+  File_util.write_file path
+    (Printf.sprintf "#Start(%.6f)\n" (Unix.gettimeofday ()))
+
+let append root content =
+  File_util.append_file (path root "bs") (strip_ansi content)
+
+let finalize root =
+  append root (Printf.sprintf "#Done(%.6f)\n" (Unix.gettimeofday ()));
+  File_util.copy_existing_file ~ensure_parent:false (path root "bs")
+    (path root "ocaml")
diff --git a/rewatch-ocaml/compiler_log.mli b/rewatch-ocaml/compiler_log.mli
new file mode 100644
index 00000000000..ab6bd3b51ca
--- /dev/null
+++ b/rewatch-ocaml/compiler_log.mli
@@ -0,0 +1,6 @@
+val initialize : string -> unit
+(** Compiler logs are finalized separately from artifact cleanup so one cleanup
+    failure cannot leave a successfully completed log in its temporary state. *)
+
+val append : string -> string -> unit
+val finalize : string -> unit
diff --git a/rewatch-ocaml/compiler_process.ml b/rewatch-ocaml/compiler_process.ml
new file mode 100644
index 00000000000..94663d04458
--- /dev/null
+++ b/rewatch-ocaml/compiler_process.ml
@@ -0,0 +1,210 @@
+let retain_critical_external_warnings stderr =
+  let marker = "`(. ...)` uncurried syntax" in
+  if not (String_util.contains stderr marker) then ""
+  else
+    stderr
+    |> Str.global_replace (Str.regexp_string "\r\n") "\n"
+    |> Str.split_delim (Str.regexp_string "\n\n\n")
+    |> List.filter (fun block -> String_util.contains block marker)
+    |> String.concat "\n\n\n"
+
+let parse_job ~bsc ~build_dir ~(config : Config.t) path =
+  let ast = Source.ast_path path in
+  File_util.ensure_dir (Filename.concat build_dir (Filename.dirname ast));
+  let contents =
+    if config.ppx_flags = [] then ""
+    else File_util.read_file (Filename.concat config.root path)
+  in
+  let args = Compiler_args.parser_arguments ~config ~contents ~path in
+  Process.{program = bsc; args; cwd = build_dir}
+
+let ast_dependencies ~build_dir ast =
+  (Ast_header.read (Filename.concat build_dir ast)).dependencies
+
+type compiler_artifact = Cmi | Required of string | Optional of string
+
+let publish_compiler_artifacts ~artifact_dir ~ocaml_dir ~basename artifacts =
+  let cmi_change = ref Compiler_scheduler.Cmi_change_unknown in
+  try
+    List.iter
+      (fun artifact ->
+        let extension =
+          match artifact with
+          | Cmi -> "cmi"
+          | Required name | Optional name -> name
+        in
+        let source =
+          Filename.concat artifact_dir (basename ^ "." ^ extension)
+        in
+        let destination =
+          Filename.concat ocaml_dir (basename ^ "." ^ extension)
+        in
+        match artifact with
+        | Cmi ->
+          cmi_change :=
+            if
+              File_util.copy_file_if_different ~ensure_parent:false source
+                destination
+            then Compiler_scheduler.Cmi_changed
+            else Compiler_scheduler.Cmi_unchanged
+        | Required _ ->
+          File_util.copy_existing_file ~ensure_parent:false source destination
+        | Optional _ ->
+          File_util.copy_optional_existing_file ~ensure_parent:false source
+            destination)
+      artifacts;
+    !cmi_change
+  with error ->
+    raise (Compiler_scheduler.Publication_failure (error, !cmi_change))
+
+let namespace_task ~bsc ~runtime ~build_dir ~ocaml_dir ~entry ~package_dirty
+    ~force namespace modules =
+  let mlmap = Filename.concat build_dir (namespace ^ ".mlmap") in
+  let contents =
+    let buffer = Buffer.create 128 in
+    Buffer.add_string buffer "randjbuildsystem\n";
+    Source.namespace_members ~entry modules
+    |> List.map (fun module_ -> module_.Source.name)
+    |> List.sort String.compare
+    |> List.iter (fun name ->
+        Buffer.add_string buffer name;
+        Buffer.add_char buffer '\n');
+    Buffer.contents buffer
+  in
+  let previous_contents =
+    try Some (File_util.read_file mlmap)
+    with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None
+  in
+  let mlmap_changed = previous_contents <> Some contents in
+  if mlmap_changed then
+    File_util.write_file_atomic ~ensure_parent:false ~perm:0o644 mlmap contents;
+  let outputs_exist =
+    ["cmi"; "cmj"; "cmt"; "mlmap"]
+    |> List.for_all (fun extension ->
+        File_util.is_regular_file
+          (Filename.concat ocaml_dir (namespace ^ "." ^ extension)))
+  in
+  let published_mlmap_matches =
+    try
+      File_util.read_file (Filename.concat ocaml_dir (namespace ^ ".mlmap"))
+      = contents
+    with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> false
+  in
+  if
+    not
+      (force || package_dirty || mlmap_changed
+      || (not published_mlmap_matches)
+      || not outputs_exist)
+  then None
+  else
+    Some
+      Compiler_scheduler.
+        {
+          job =
+            Process.
+              {
+                program = bsc;
+                args =
+                  [
+                    "-runtime-path";
+                    runtime;
+                    "-w";
+                    "-49";
+                    "-color";
+                    "always";
+                    "-no-alias-deps";
+                    Filename.basename mlmap;
+                  ];
+                cwd = build_dir;
+              };
+          publish =
+            (fun result ->
+              if not (Process.succeeded result) then
+                raise
+                  (Compiler_scheduler.Build_failure
+                     (result.Process.stderr ^ result.stdout));
+              let cmi_change =
+                publish_compiler_artifacts ~artifact_dir:build_dir ~ocaml_dir
+                  ~basename:namespace
+                  [Cmi; Required "cmj"; Required "cmt"; Required "mlmap"]
+              in
+              Compiler_scheduler.{stderr = result.stderr; cmi_change});
+        }
+
+let post_build_tasks (config : Config.t) path =
+  match config.js_post_build with
+  | None -> []
+  | Some command ->
+    List.map
+      (fun spec ->
+        let output = Build_artifacts.generated_js_path config path spec in
+        let command = Platform.post_build_command ~command ~output in
+        Compiler_scheduler.
+          {
+            output;
+            task =
+              Process.task ?env:command.env
+                Process.
+                  {
+                    program = command.program;
+                    args = command.args;
+                    cwd = config.root;
+                  };
+          })
+      config.package_specs
+
+let compile_job ~bsc ~build_dir ~(config : Config.t) ~common_args
+    (module_ : Source.module_) ~source_kind path =
+  let args =
+    Compiler_args.compiler_arguments_with_common ~config ~common_args
+      ~module_name:module_.name ~source_kind
+      ~has_interface:(Option.is_some module_.interface)
+      ~path
+  in
+  Process.{program = bsc; args; cwd = build_dir}
+
+let publish ~build_dir ~ocaml_dir ~is_local ~(config : Config.t) ~source_kind
+    path result =
+  let stderr =
+    if is_local then result.Process.stderr
+    else retain_critical_external_warnings result.stderr
+  in
+  let basename = Source.compiler_asset_basename config path in
+  let artifact_dir = Filename.concat build_dir (Filename.dirname path) in
+  let cmi_change = ref Compiler_scheduler.Cmi_change_unknown in
+  try
+    cmi_change :=
+      publish_compiler_artifacts ~artifact_dir ~ocaml_dir ~basename
+        (match source_kind with
+        | Source.Interface -> [Cmi; Optional "cmti"]
+        | Source.Implementation -> [Cmi; Required "cmj"; Optional "cmt"]);
+    let source = Filename.concat config.root path in
+    let build_source = Filename.concat build_dir path in
+    File_util.ensure_dir (Filename.dirname build_source);
+    File_util.copy_existing_file ~ensure_parent:false source build_source;
+    File_util.copy_existing_file ~ensure_parent:false source
+      (Filename.concat ocaml_dir (Filename.basename path));
+    (match source_kind with
+    | Source.Interface -> ()
+    | Source.Implementation ->
+      List.iter
+        (fun spec ->
+          if spec.Config.in_source then (
+            let output = Build_artifacts.generated_js_path config path spec in
+            let build_output =
+              Build_artifacts.generated_build_js_path ~build_dir config path
+                spec
+            in
+            File_util.ensure_dir (Filename.dirname build_output);
+            if File_util.exists output then
+              File_util.copy_existing_file ~ensure_parent:false output
+                build_output;
+            if File_util.exists (output ^ ".map") then
+              File_util.copy_existing_file ~ensure_parent:false
+                (output ^ ".map") (build_output ^ ".map")
+            else File_util.remove_file (build_output ^ ".map")))
+        config.package_specs);
+    Compiler_scheduler.{stderr; cmi_change = !cmi_change}
+  with
+  | Compiler_scheduler.Publication_failure _ as error -> raise error
+  | error -> raise (Compiler_scheduler.Publication_failure (error, !cmi_change))
diff --git a/rewatch-ocaml/compiler_process.mli b/rewatch-ocaml/compiler_process.mli
new file mode 100644
index 00000000000..542ddd658a5
--- /dev/null
+++ b/rewatch-ocaml/compiler_process.mli
@@ -0,0 +1,39 @@
+val retain_critical_external_warnings : string -> string
+val parse_job :
+  bsc:string -> build_dir:string -> config:Config.t -> string -> Process.job
+val ast_dependencies : build_dir:string -> string -> string list
+
+val namespace_task :
+  bsc:string ->
+  runtime:string ->
+  build_dir:string ->
+  ocaml_dir:string ->
+  entry:string option ->
+  package_dirty:bool ->
+  force:bool ->
+  string ->
+  Source.module_ list ->
+  Compiler_scheduler.namespace_task option
+
+val compile_job :
+  bsc:string ->
+  build_dir:string ->
+  config:Config.t ->
+  common_args:string list ->
+  Source.module_ ->
+  source_kind:Source.source_kind ->
+  string ->
+  Process.job
+
+val post_build_tasks :
+  Config.t -> string -> Compiler_scheduler.post_build_task list
+
+val publish :
+  build_dir:string ->
+  ocaml_dir:string ->
+  is_local:bool ->
+  config:Config.t ->
+  source_kind:Source.source_kind ->
+  string ->
+  Process.result ->
+  Compiler_scheduler.publish_result
diff --git a/rewatch-ocaml/compiler_scheduler.ml b/rewatch-ocaml/compiler_scheduler.ml
new file mode 100644
index 00000000000..15636aaf792
--- /dev/null
+++ b/rewatch-ocaml/compiler_scheduler.ml
@@ -0,0 +1,429 @@
+exception Build_failure of string
+exception Module_failed
+type cmi_change = Build_state.cmi_change =
+  | Cmi_changed
+  | Cmi_unchanged
+  | Cmi_change_unknown
+exception Publication_failure of exn * cmi_change
+
+type publish_result = {stderr: string; cmi_change: cmi_change}
+type namespace_task = {
+  job: Process.job;
+  publish: Process.result -> publish_result;
+}
+type post_build_task = {output: string; task: Process.task}
+
+type phase =
+  | Start
+  | Interface of string
+  | Implementation of string
+  | Post_build of {output: string; remaining: post_build_task list}
+  | Done
+
+type publication =
+  | Published of publish_result
+  | Failed_after_cmi_publication of {error: exn; cmi_change: cmi_change}
+
+type recorded_publication =
+  | No_publication
+  | Publication_succeeded of string
+  | Publication_failed of string
+
+let capture_publication publish =
+  try Published (publish ()) with
+  | Publication_failure (error, cmi_change) ->
+    Failed_after_cmi_publication {error; cmi_change}
+  | error ->
+    Failed_after_cmi_publication {error; cmi_change = Cmi_change_unknown}
+
+type scheduled_module = {
+  key: string;
+  dependencies: string list;
+  source: Source.module_;
+  state: Build_state.module_;
+  cmi_path: string;
+  publication: publication option Atomic.t;
+  prepare: unit -> unit;
+  compile: source_kind:Source.source_kind -> string -> Process.job;
+  publish:
+    source_kind:Source.source_kind -> string -> Process.result -> publish_result;
+  record_published_outputs: source_kind:Source.source_kind -> string -> unit;
+  post_build: string -> post_build_task list;
+  package_root: string;
+  is_local: bool;
+  mark_warning: string -> unit;
+  mutable messages: string list;
+  mutable phase: phase;
+}
+
+type candidate = {
+  key: string;
+  state: Build_state.module_;
+  warning_paths: string list;
+  make: unit -> scheduled_module;
+}
+
+type scheduled_item = Module of scheduled_module | Namespace_barrier
+
+let create ~key ~dependencies ~source ~state ~cmi_path ~prepare ~compile
+    ~publish ~record_published_outputs ~post_build ~package_root ~is_local
+    ~mark_warning =
+  {
+    key;
+    dependencies;
+    source;
+    state;
+    cmi_path;
+    publication = Atomic.make None;
+    prepare;
+    compile;
+    publish;
+    record_published_outputs;
+    post_build;
+    package_root;
+    is_local;
+    mark_warning;
+    messages = [];
+    phase = Start;
+  }
+
+let candidate ~key ~state ~warning_paths ~make =
+  {key; state; warning_paths; make}
+
+let candidate_requires_compile candidate = candidate.state.compile_dirty
+
+let run ~poll ~warning_state ~compile_assets ~build_state ~candidates
+    ~mark_compiled ~mark_had_warnings ~progress ~compile_step ~namespace_count
+    ~verbosity =
+  let dirty_propagation = Hashtbl.create 16 in
+  let refresh_published_cmi (scheduled : scheduled_module) cmi_change =
+    Build_state.record_published_cmi ~dirty_propagation build_state
+      ~compile_assets scheduled.state ~path:scheduled.cmi_path cmi_change
+  in
+  let finish_successful_compile (scheduled : scheduled_module) =
+    let cmt_path = Filename.remove_extension scheduled.cmi_path ^ ".cmt" in
+    Build_state.record_successful_compile ~compile_assets scheduled.state
+      ~cmt_path
+  in
+  let warning_paths =
+    candidates |> List.concat_map (fun candidate -> candidate.warning_paths)
+  in
+  Warning_state.retain_paths warning_state warning_paths;
+  if Output.trace_enabled verbosity then
+    candidates
+    |> List.filter candidate_requires_compile
+    |> List.sort (fun first second -> String.compare first.key second.key)
+    |> List.iter (fun candidate ->
+        Printf.printf "compile dirty: %s\n%!" candidate.key);
+  (* The scheduler only needs dirty modules and their transitive dependents.
+     Dependencies outside that universe already have usable artifacts, while
+     keeping every module in the subprocess graph makes small edits scale with
+     the whole project. *)
+  let candidate_by_key = Hashtbl.create (List.length candidates) in
+  List.iter
+    (fun candidate -> Hashtbl.replace candidate_by_key candidate.key candidate)
+    candidates;
+  let universe = Hashtbl.create (List.length candidates) in
+  let reached = Hashtbl.create (List.length candidates) in
+  let pending = Queue.create () in
+  let add_to_closure key =
+    if not (Hashtbl.mem reached key) then (
+      Hashtbl.add reached key ();
+      if Hashtbl.mem candidate_by_key key then Hashtbl.add universe key ();
+      Queue.add key pending)
+  in
+  candidates
+  |> List.iter (fun candidate ->
+      if candidate.state.compile_dirty then add_to_closure candidate.key);
+  while not (Queue.is_empty pending) do
+    let key = Queue.take pending in
+    let state = Build_state.find_exn build_state key in
+    Build_state.String_set.iter add_to_closure state.dependents
+  done;
+  let scheduled_modules =
+    candidates
+    |> List.filter (fun candidate -> Hashtbl.mem universe candidate.key)
+    |> List.map (fun candidate -> candidate.make ())
+  in
+  Output.Progress.start progress ~step:compile_step
+    ~symbol:Platform.build_symbol ~label:"Compiling"
+    ~total:(namespace_count + List.length scheduled_modules);
+  for _ = 1 to namespace_count do
+    Output.Progress.advance progress
+  done;
+  let completed_modules = ref 0 in
+  let scheduled_keys = Hashtbl.copy universe in
+  Hashtbl.iter
+    (fun key () ->
+      match Build_state.find build_state key with
+      | Some state when state.kind = Build_state.Namespace_map ->
+        Hashtbl.replace scheduled_keys key ()
+      | Some _ | None -> ())
+    reached;
+  let scheduled_dependencies dependencies =
+    List.filter (Hashtbl.mem scheduled_keys) dependencies
+  in
+  let module_works =
+    scheduled_modules
+    |> List.map (fun (scheduled : scheduled_module) ->
+        Process.
+          {
+            key = scheduled.key;
+            dependencies = scheduled_dependencies scheduled.dependencies;
+            value = Module scheduled;
+          })
+  in
+  let namespace_works =
+    Hashtbl.to_seq_keys scheduled_keys
+    |> Seq.filter_map (fun key ->
+        match Build_state.find build_state key with
+        | Some state when state.kind = Build_state.Namespace_map ->
+          Some
+            Process.
+              {
+                key;
+                dependencies = scheduled_dependencies state.dependencies;
+                value = Namespace_barrier;
+              }
+        | Some _ | None -> None)
+    |> List.of_seq
+  in
+  let works = module_works @ namespace_works in
+  let record_publication (scheduled : scheduled_module) ~source_kind path =
+    let publication = Atomic.exchange scheduled.publication None in
+    match publication with
+    | Some (Published {stderr; cmi_change}) ->
+      refresh_published_cmi scheduled cmi_change;
+      scheduled.record_published_outputs ~source_kind path;
+      Publication_succeeded stderr
+    | Some (Failed_after_cmi_publication {error; cmi_change}) ->
+      refresh_published_cmi scheduled cmi_change;
+      scheduled.record_published_outputs ~source_kind path;
+      Publication_failed (Printexc.to_string error)
+    | None -> No_publication
+  in
+  let record_result (scheduled : scheduled_module) ~source_kind path result =
+    let result, publication_error =
+      match record_publication scheduled ~source_kind path with
+      | Publication_succeeded stderr -> ({result with Process.stderr}, None)
+      | Publication_failed message -> (result, Some message)
+      | No_publication -> (result, None)
+    in
+    let message =
+      match publication_error with
+      | Some message ->
+        Warning_state.remove warning_state ~package_root:scheduled.package_root
+          ~path;
+        Some message
+      | None ->
+        if Process.succeeded result then (
+          match result.Process.stderr with
+          | "" ->
+            Warning_state.remove warning_state
+              ~package_root:scheduled.package_root ~path;
+            None
+          | warning ->
+            mark_had_warnings ();
+            Warning_state.set warning_state ~module_name:scheduled.key
+              ~package_root:scheduled.package_root ~path ~output:warning;
+            if scheduled.is_local then scheduled.mark_warning path;
+            None)
+        else (
+          Warning_state.remove warning_state
+            ~package_root:scheduled.package_root ~path;
+          Some (result.Process.stderr ^ result.Process.stdout))
+    in
+    Option.iter
+      (fun message -> scheduled.messages <- message :: scheduled.messages)
+      message;
+    Option.is_none message
+  in
+  let compilation_task (scheduled : scheduled_module) ~source_kind path =
+    let job = scheduled.compile ~source_kind path in
+    Atomic.set scheduled.publication None;
+    Process.task job ~on_result:(fun result ->
+        (if Process.succeeded result then
+           let publication =
+             capture_publication (fun () ->
+                 scheduled.publish ~source_kind path result)
+           in
+           Atomic.set scheduled.publication (Some publication));
+        result)
+  in
+  let record_post_build_result (scheduled : scheduled_module) output result =
+    if Process.succeeded result then (
+      if result.Process.stdout <> "" then print_string result.stdout;
+      if result.stderr <> "" then prerr_string result.stderr;
+      true)
+    else
+      let captured = result.stderr ^ result.stdout in
+      let message =
+        Printf.sprintf "js-post-build command failed for %s%s" output
+          (if captured = "" then "" else "\n" ^ captured)
+      in
+      scheduled.messages <- message :: scheduled.messages;
+      false
+  in
+  let invalidate_persistent_freshness (scheduled : scheduled_module) =
+    let ocaml_dir = Filename.dirname scheduled.cmi_path in
+    scheduled.source.Source.implementation
+    :: Option.to_list scheduled.source.Source.interface
+    |> List.iter (fun source ->
+        let path = Build_artifacts.published_ast_path ~ocaml_dir source in
+        File_util.remove_file path;
+        Compile_assets.refresh_ast compile_assets
+          ~source:(Filename.concat scheduled.package_root source)
+          ~path)
+  in
+  let complete_module (scheduled : scheduled_module) =
+    scheduled.phase <- Done;
+    Output.Progress.advance progress;
+    if scheduled.messages <> [] then (
+      invalidate_persistent_freshness scheduled;
+      raise Module_failed)
+    else (
+      finish_successful_compile scheduled;
+      incr completed_modules)
+  in
+  let continue_post_build (scheduled : scheduled_module) tasks =
+    match tasks with
+    | [] ->
+      complete_module scheduled;
+      None
+    | {output; task} :: remaining ->
+      scheduled.phase <- Post_build {output; remaining};
+      Some task
+  in
+  let reconcile_unconsumed_publications () =
+    List.iter
+      (fun (scheduled : scheduled_module) ->
+        let attempt_is_incomplete =
+          match scheduled.phase with
+          | Interface _ | Implementation _ | Post_build _ -> true
+          | Start | Done -> false
+        in
+        let source =
+          match scheduled.phase with
+          | Interface path -> Some (Source.Interface, path)
+          | Implementation path -> Some (Source.Implementation, path)
+          | Start | Post_build _ | Done -> None
+        in
+        Option.iter
+          (fun (source_kind, path) ->
+            match record_publication scheduled ~source_kind path with
+            | Publication_failed message ->
+              scheduled.messages <- message :: scheduled.messages
+            | No_publication | Publication_succeeded _ -> ())
+          source;
+        if attempt_is_incomplete then invalidate_persistent_freshness scheduled)
+      scheduled_modules
+  in
+  let scheduler_failed =
+    try
+      Process.run_dependency_graph ?poll
+        works
+        (* Rust's parallel scheduler may already have independent work in
+           flight when a module fails. Finish all ready, independent work here
+           so the same diagnostics do not depend on the host's worker count;
+           dependents of the failed module remain blocked. *)
+        ~on_failure:(function
+          | Module_failed -> Process.Continue_independent_work
+          | _ -> Process.Abort_immediately)
+        ~next:(fun item result ->
+          match item with
+          | Namespace_barrier -> (
+            match result with
+            | None -> None
+            | Some _ ->
+              raise
+                (Project_context.Error
+                   "namespace scheduler barrier produced a process result"))
+          | Module scheduled -> (
+            match (result, scheduled.phase) with
+            | None, Start ->
+              if scheduled.state.compile_dirty then (
+                mark_compiled ();
+                scheduled.prepare ();
+                match scheduled.source.Source.interface with
+                | Some path ->
+                  Output.Progress.debug progress ~verbosity
+                    ("Compiling interface file: " ^ scheduled.key);
+                  scheduled.phase <- Interface path;
+                  Some
+                    (compilation_task scheduled ~source_kind:Source.Interface
+                       path)
+                | None ->
+                  let path = scheduled.source.Source.implementation in
+                  Output.Progress.debug progress ~verbosity
+                    ("Compiling file: " ^ scheduled.key);
+                  scheduled.phase <- Implementation path;
+                  Some
+                    (compilation_task scheduled
+                       ~source_kind:Source.Implementation path))
+              else (
+                scheduled.phase <- Done;
+                incr completed_modules;
+                Output.Progress.advance progress;
+                None)
+            | Some result, Interface path ->
+              ignore
+                (record_result scheduled ~source_kind:Source.Interface path
+                   result);
+              let path = scheduled.source.Source.implementation in
+              Output.Progress.debug progress ~verbosity
+                ("Compiling file: " ^ scheduled.key);
+              scheduled.phase <- Implementation path;
+              Some
+                (compilation_task scheduled ~source_kind:Source.Implementation
+                   path)
+            | Some result, Implementation path ->
+              if
+                record_result scheduled ~source_kind:Source.Implementation path
+                  result
+              then continue_post_build scheduled (scheduled.post_build path)
+              else (
+                complete_module scheduled;
+                None)
+            | Some result, Post_build {output; remaining} ->
+              if record_post_build_result scheduled output result then
+                continue_post_build scheduled remaining
+              else (
+                complete_module scheduled;
+                None)
+            | None, (Interface _ | Implementation _ | Post_build _ | Done)
+            | Some _, (Start | Done) ->
+              raise (Project_context.Error "invalid compiler scheduler state")));
+      false
+    with exn -> (
+      reconcile_unconsumed_publications ();
+      match exn with
+      | Module_failed -> true
+      | _ -> raise exn)
+  in
+  Output.Progress.finish progress;
+  Output.trace ~verbosity
+    (Printf.sprintf "Compiled %d out of %d in the universe" !completed_modules
+       (List.length scheduled_modules));
+  let failures =
+    scheduled_modules
+    |> List.sort (fun (first : scheduled_module) second ->
+        String.compare first.key second.key)
+    |> List.concat_map (fun scheduled ->
+        scheduled.messages |> List.rev
+        |> List.map (fun output -> (scheduled, output)))
+  in
+  Warning_state.entries warning_state
+  |> List.iter (fun entry ->
+      Compiler_log.append entry.Warning_state.package_root entry.output);
+  List.iter
+    (fun ((scheduled : scheduled_module), output) ->
+      Compiler_log.append scheduled.package_root output)
+    failures;
+  match (failures, scheduler_failed) with
+  | [], false -> ()
+  | [], true ->
+    raise
+      (Project_context.Error "compiler scheduler stopped without a diagnostic")
+  | failures, _ ->
+    failures |> List.map snd |> String.concat "" |> fun output ->
+    raise (Build_failure output)
diff --git a/rewatch-ocaml/compiler_scheduler.mli b/rewatch-ocaml/compiler_scheduler.mli
new file mode 100644
index 00000000000..2d3c31858fe
--- /dev/null
+++ b/rewatch-ocaml/compiler_scheduler.mli
@@ -0,0 +1,73 @@
+exception Build_failure of string
+
+(* CMI change has an explicit unknown case because a later publication failure
+    must not erase the fact that an interface may already have become visible.
+    Unknown therefore invalidates conservatively instead of pretending that the
+    CMI was unchanged. *)
+type cmi_change = Build_state.cmi_change =
+  | Cmi_changed
+  | Cmi_unchanged
+  | Cmi_change_unknown
+exception Publication_failure of exn * cmi_change
+
+type publish_result = {stderr: string; cmi_change: cmi_change}
+type namespace_task = {
+  job: Process.job;
+  publish: Process.result -> publish_result;
+}
+type post_build_task = {output: string; task: Process.task}
+
+(* Publication can fail after the CMI was copied. The partial outcome is kept
+    so dependents are still invalidated even though the module itself remains
+    dirty for retry. *)
+type publication =
+  | Published of publish_result
+  | Failed_after_cmi_publication of {error: exn; cmi_change: cmi_change}
+
+val capture_publication : (unit -> publish_result) -> publication
+
+type scheduled_module
+type candidate
+
+val create :
+  key:string ->
+  dependencies:string list ->
+  source:Source.module_ ->
+  state:Build_state.module_ ->
+  cmi_path:string ->
+  prepare:(unit -> unit) ->
+  compile:(source_kind:Source.source_kind -> string -> Process.job) ->
+  publish:
+    (source_kind:Source.source_kind ->
+    string ->
+    Process.result ->
+    publish_result) ->
+  record_published_outputs:(source_kind:Source.source_kind -> string -> unit) ->
+  post_build:(string -> post_build_task list) ->
+  package_root:string ->
+  is_local:bool ->
+  mark_warning:(string -> unit) ->
+  scheduled_module
+
+val candidate :
+  key:string ->
+  state:Build_state.module_ ->
+  warning_paths:string list ->
+  make:(unit -> scheduled_module) ->
+  candidate
+
+val candidate_requires_compile : candidate -> bool
+
+val run :
+  poll:(unit -> unit) option ->
+  warning_state:Warning_state.t ->
+  compile_assets:Compile_assets.t ->
+  build_state:Build_state.t ->
+  candidates:candidate list ->
+  mark_compiled:(unit -> unit) ->
+  mark_had_warnings:(unit -> unit) ->
+  progress:Output.Progress.t ->
+  compile_step:string ->
+  namespace_count:int ->
+  verbosity:int ->
+  unit
diff --git a/rewatch-ocaml/config.ml b/rewatch-ocaml/config.ml
new file mode 100644
index 00000000000..47f97727d06
--- /dev/null
+++ b/rewatch-ocaml/config.ml
@@ -0,0 +1,436 @@
+include Config_types
+
+open Config_decode
+
+let namespace_from_package_name = Config_decode.namespace_from_package_name
+
+let namespace_name = function
+  | No_namespace -> None
+  | Namespace name | Namespace_with_entry {name; entry = _} -> Some name
+
+let namespace_entry = function
+  | Namespace_with_entry {name = _; entry} -> Some entry
+  | No_namespace | Namespace _ -> None
+
+let namespace_compiler_name = function
+  | No_namespace -> None
+  | Namespace name -> Some name
+  | Namespace_with_entry {name; entry = _} -> Some ("@" ^ name)
+
+let namespaced_module_name namespace module_name =
+  match namespace with
+  | No_namespace -> module_name
+  | Namespace name -> module_name ^ "-" ^ name
+  | Namespace_with_entry {name; entry} ->
+    if entry = module_name then module_name else module_name ^ "-@" ^ name
+
+let path_in_root root =
+  let current = Filename.concat root "rescript.json" in
+  if File_util.exists current then current
+  else Filename.concat root "bsconfig.json"
+
+let exists_in_root root =
+  File_util.exists (Filename.concat root "rescript.json")
+  || File_util.exists (Filename.concat root "bsconfig.json")
+
+let source_is_dev (config : t) relative_path =
+  let canonical path =
+    try Some (Platform.canonicalize_path path)
+    with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None
+  in
+  let source_parent =
+    Filename.concat config.root relative_path |> Filename.dirname |> canonical
+  in
+  match source_parent with
+  | None -> false
+  | Some source_parent ->
+    let comparable = Platform.normalize_path_for_comparison in
+    List.find_map
+      (fun (source : source) ->
+        match canonical (Filename.concat config.root source.dir) with
+        | None -> None
+        | Some directory ->
+          if
+            comparable source_parent = comparable directory
+            || source.recurse
+               && String.starts_with
+                    ~prefix:(Filename.concat directory "" |> comparable)
+                    (comparable source_parent)
+          then Some source.is_dev
+          else None)
+      config.sources
+    |> Option.value ~default:false
+
+(* Every package in one invocation must use the root project's effective
+   compiler options so dependencies expose outputs their consumers can use. *)
+let with_root_options (config : t) (root_config : t) =
+  {
+    config with
+    package_specs = root_config.package_specs;
+    suffix = root_config.suffix;
+    jsx_args = root_config.jsx_args;
+    source_map_args = root_config.source_map_args;
+    source_map_dev = root_config.source_map_dev;
+    experimental_args = root_config.experimental_args;
+    gentype_args =
+      (if config.gentype_args = [] then []
+       else
+         config.gentype_args
+         @ ["-bs-gentype-bsb-project-root"; root_config.root]);
+  }
+
+let load path =
+  let requested_path = path in
+  let root =
+    try Platform.canonicalize_path (Filename.dirname path) with
+    | Sys_error message ->
+      fail_read requested_path (strip_read_path requested_path message)
+    | Unix.Unix_error (error, _, _) ->
+      fail_read requested_path (Unix.error_message error)
+  in
+  let path = Filename.concat root (Filename.basename path) in
+  (try
+     match (Unix.stat path).Unix.st_kind with
+     | Unix.S_DIR -> fail_read path (Unix.error_message Unix.EISDIR)
+     | Unix.S_REG | Unix.S_CHR | Unix.S_BLK | Unix.S_LNK | Unix.S_FIFO
+     | Unix.S_SOCK ->
+       ()
+   with
+  | Sys_error message -> fail_read path (strip_read_path path message)
+  | Unix.Unix_error (error, _, _) -> fail_read path (Unix.error_message error));
+  let contents =
+    try File_util.read_file path with
+    | Sys_error message -> fail_read path (strip_read_path path message)
+    | Unix.Unix_error (error, _, _) -> fail_read path (Unix.error_message error)
+  in
+  let json =
+    try Yojson.Safe.from_string contents
+    with Yojson.Json_error message -> fail path ("invalid JSON: " ^ message)
+  in
+  let fields =
+    match json with
+    | `Assoc fields -> fields
+    | _ -> fail path "configuration must be an object"
+  in
+  reject_duplicate_fields path "configuration" configuration_fields fields;
+  let name =
+    match member "name" fields with
+    | Some value -> string path "name" value
+    | None -> fail path "missing required field \"name\""
+  in
+  let configured_suffix =
+    match optional_member "suffix" fields with
+    | None -> None
+    | Some value -> Some (string path "suffix" value)
+  in
+  let suffix = Option.value configured_suffix ~default:".js" in
+  let package_specs =
+    match optional_member "package-specs" fields with
+    | None ->
+      [{module_format = Esmodule; in_source = true; suffix = Some ".js"}]
+    | Some (`List values) -> List.map (parse_package_spec path) values
+    | Some value -> [parse_package_spec path value]
+  in
+  let seen_package_outputs = Hashtbl.create (List.length package_specs) in
+  List.iter
+    (fun (spec : package_spec) ->
+      let effective_suffix = Option.value spec.suffix ~default:suffix in
+      let key = (effective_suffix, spec.in_source) in
+      if Hashtbl.mem seen_package_outputs key then
+        fail path
+          (Printf.sprintf "Duplicate package-spec suffix %S is not allowed."
+             effective_suffix);
+      Hashtbl.add seen_package_outputs key ())
+    package_specs;
+  let namespace_name =
+    match optional_member "namespace" fields with
+    | None | Some (`Bool false) -> None
+    | Some (`Bool true) -> Some (namespace_from_package_name name)
+    | Some (`String "true") -> Some (namespace_from_package_name name)
+    | Some (`String value) -> Some (namespace_from_package_name value)
+    | Some _ -> fail path "field \"namespace\" must be a boolean or string"
+  in
+  let namespace =
+    match (optional_member "namespace-entry" fields, namespace_name) with
+    | None, None -> No_namespace
+    | None, Some name -> Namespace name
+    | Some _, None -> fail path "field \"namespace-entry\" requires a namespace"
+    | Some value, Some name ->
+      Namespace_with_entry {name; entry = string path "namespace-entry" value}
+  in
+  let compiler_flags =
+    match (member "compiler-flags" fields, member "bsc-flags" fields) with
+    | Some _, Some _ ->
+      fail path "fields \"compiler-flags\" and \"bsc-flags\" cannot both be set"
+    | Some `Null, None | None, Some `Null -> []
+    | Some value, None -> compiler_flags path "compiler-flags" value
+    | None, Some value -> compiler_flags path "bsc-flags" value
+    | None, None -> []
+  in
+  let warning_flags =
+    match optional_member "warnings" fields with
+    | None -> []
+    | Some (`Assoc warning_fields) ->
+      reject_duplicate_fields path "warnings" Config_decode.warning_fields
+        warning_fields;
+      let number =
+        match optional_member "number" warning_fields with
+        | None -> []
+        | Some value -> ["-w"; string path "number" value]
+      in
+      let error =
+        match optional_member "error" warning_fields with
+        | Some (`Bool true) -> ["-warn-error"; "A"]
+        | Some (`String value) -> ["-warn-error"; value]
+        | None | Some (`Bool false) -> []
+        | Some _ ->
+          fail path "field \"warnings.error\" must be a boolean or string"
+      in
+      number @ error
+    | Some _ -> fail path "field \"warnings\" must be an object"
+  in
+  let ppx_flags =
+    match optional_member "ppx-flags" fields with
+    | None -> []
+    | Some (`List values) ->
+      List.map
+        (function
+          | `String value -> [value]
+          | `List values -> List.map (string path "ppx-flags") values
+          | _ ->
+            fail path "field \"ppx-flags\" entries must be strings or arrays")
+        values
+    | Some _ -> fail path "field \"ppx-flags\" must be an array"
+  in
+  let jsx_args =
+    match optional_member "jsx" fields with
+    | None -> []
+    | Some (`Assoc jsx) ->
+      reject_duplicate_fields path "jsx" jsx_fields jsx;
+      let version =
+        match optional_member "version" jsx with
+        | None -> []
+        | Some (`Int 4) -> ["-bs-jsx"; "4"]
+        | Some _ -> fail path "field \"jsx.version\" must be 4"
+      in
+      let module_ =
+        match optional_member "module" jsx with
+        | None -> []
+        | Some value -> ["-bs-jsx-module"; string path "jsx.module" value]
+      in
+      let mode =
+        match optional_member "mode" jsx with
+        | None -> []
+        | Some (`String (("classic" | "automatic") as value)) ->
+          ["-bs-jsx-mode"; value]
+        | Some _ ->
+          fail path "field \"jsx.mode\" must be \"classic\" or \"automatic\""
+      in
+      let preserve =
+        match optional_member "preserve" jsx with
+        | None | Some (`Bool false) -> []
+        | Some (`Bool true) -> ["-bs-jsx-preserve"]
+        | Some _ -> fail path "field \"jsx.preserve\" must be a boolean"
+      in
+      (match optional_member "v3-dependencies" jsx with
+      | None -> ()
+      | Some value -> ignore (strings path "jsx.v3-dependencies" value));
+      version @ module_ @ mode @ preserve
+    | Some _ -> fail path "field \"jsx\" must be an object"
+  in
+  let source_map_args, source_map_dev =
+    match optional_member "sourceMap" fields with
+    | None -> ([], false)
+    | Some (`Bool false) -> (["-bs-source-map"; "false"], false)
+    | Some (`Bool true) ->
+      fail path
+        "sourceMap true is unsupported; use an object with enabled and mode \
+         fields or false"
+    | Some (`Assoc options) ->
+      reject_duplicates path "sourceMap" options;
+      let mode =
+        match member "mode" options with
+        | Some (`String (("linked" | "inline" | "hidden") as value)) -> value
+        | None -> fail path "sourceMap is missing field \"mode\""
+        | Some _ ->
+          fail path "sourceMap.mode must be one of linked, inline, hidden"
+      in
+      let dev_only =
+        match member "enabled" options with
+        | Some (`String "always") -> false
+        | Some (`String "dev") -> true
+        | None -> fail path "sourceMap is missing field \"enabled\""
+        | Some _ -> fail path "sourceMap.enabled must be \"always\" or \"dev\""
+      in
+      let content =
+        match optional_member "sourcesContent" options with
+        | None -> []
+        | Some (`Bool value) ->
+          ["-bs-source-map-sources-content"; string_of_bool value]
+        | Some _ ->
+          fail path "field \"sourceMap.sourcesContent\" must be a boolean"
+      in
+      let root =
+        match optional_member "sourceRoot" options with
+        | None -> []
+        | Some value ->
+          ["-bs-source-map-root"; string path "sourceMap.sourceRoot" value]
+      in
+      (["-bs-source-map"; mode] @ content @ root, dev_only)
+    | Some _ -> fail path "field \"sourceMap\" must be false or an object"
+  in
+  let experimental_args =
+    match optional_member "experimental-features" fields with
+    | None -> []
+    | Some (`Assoc features) ->
+      reject_duplicates path "experimental-features" features;
+      features
+      |> List.concat_map (fun (name, value) ->
+          if name <> "LetUnwrap" then
+            fail path
+              (Printf.sprintf
+                 "Unknown experimental feature '%s'. Available features: \
+                  LetUnwrap"
+                 name);
+          match value with
+          | `Bool true -> ["-enable-experimental"; name]
+          | `Bool false -> []
+          | _ ->
+            fail path
+              "experimental-features: invalid type: feature values must be \
+               booleans")
+    | Some _ ->
+      fail path
+        "Could not read rescript.json: experimental-features: invalid type: \
+         expected an object"
+  in
+  let sources_defined = Option.is_some (optional_member "sources" fields) in
+  let sources = parse_sources path fields in
+  let dependencies =
+    dependency_alias path "dependencies" "bs-dependencies" fields
+  in
+  let dev_dependencies =
+    dependency_alias path "dev-dependencies" "bs-dev-dependencies" fields
+  in
+  let gentype_args =
+    match optional_member "gentypeconfig" fields with
+    | None -> []
+    | Some value ->
+      gentype_args path configured_suffix
+        (member "package-specs" fields)
+        dependencies value
+  in
+  let js_post_build =
+    match optional_member "js-post-build" fields with
+    | None -> None
+    | Some (`Assoc fields) -> (
+      reject_duplicate_fields path "js-post-build" js_post_build_fields fields;
+      match member "cmd" fields with
+      | Some value -> Some (string path "js-post-build.cmd" value)
+      | None -> fail path "field \"js-post-build\" is missing \"cmd\"")
+    | Some _ -> fail path "field \"js-post-build\" must be an object"
+  in
+  let allowed_dependents =
+    match optional_member "allowed-dependents" fields with
+    | None -> None
+    | Some value -> Some (strings path "allowed-dependents" value)
+  in
+  let features =
+    match optional_member "features" fields with
+    | None -> []
+    | Some (`Assoc values) ->
+      reject_duplicates path "features" values;
+      List.map
+        (fun (name, value) -> (name, strings path "features" value))
+        values
+    | Some _ -> fail path "field \"features\" must be an object"
+  in
+  let unsupported = Config_decode.unsupported_fields fields in
+  let deprecated =
+    (if Filename.basename path = "bsconfig.json" then
+       ["  - filename 'bsconfig.json' — rename to 'rescript.json'"]
+     else [])
+    @ ([
+         ("bs-dependencies", "dependencies");
+         ("bs-dev-dependencies", "dev-dependencies");
+         ("bsc-flags", "compiler-flags");
+       ]
+      |> List.filter_map (fun (field, replacement) ->
+          if Option.is_some (member field fields) then
+            Some
+              (Printf.sprintf "  - field '%s' — use '%s' instead" field
+                 replacement)
+          else None))
+    @
+    match member "package-specs" fields with
+    | Some value ->
+      [
+        ("cjs", "  - module 'cjs' in package-specs — use 'commonjs' instead");
+        ("es6", "  - module 'es6' in package-specs — use 'esmodule' instead");
+      ]
+      |> List.filter_map (fun (alias, message) ->
+          if package_specs_use_alias alias value then Some message else None)
+    | None -> []
+  in
+  let deprecation_diagnostics =
+    if deprecated = [] then []
+    else
+      [
+        Printf.sprintf
+          "\n\
+           Package '%s' uses deprecated config (support will be removed in a \
+           future version):\n\
+           %s"
+          name
+          (String.concat "\n" deprecated);
+      ]
+  in
+  let diagnostics =
+    deprecation_diagnostics
+    @ (unsupported
+      |> List.map (fun field ->
+          Printf.sprintf
+            "The field '%s' found in the package config of '%s' is not \
+             supported by ReScript 12's new build system."
+            field name))
+    @ (unknown_fields fields
+      |> List.map (fun field ->
+          Printf.sprintf
+            "Unknown field '%s' found in the package config of '%s'. This \
+             option will be ignored."
+            field name))
+  in
+  {
+    path;
+    root;
+    file_hash = Digest.string contents |> Digest.to_hex;
+    name;
+    sources;
+    sources_defined;
+    dependencies;
+    dev_dependencies;
+    compiler_flags;
+    package_specs;
+    suffix;
+    namespace;
+    features;
+    warning_flags;
+    ppx_flags;
+    jsx_args;
+    source_map_args;
+    source_map_dev;
+    experimental_args;
+    gentype_args;
+    js_post_build;
+    allowed_dependents;
+    deprecation_diagnostics;
+    diagnostics;
+  }
+
+let load_root root = load (path_in_root root)
+
+let package_spec_suffix (config : t) (spec : package_spec) =
+  Option.value spec.suffix ~default:config.suffix
+let module_format_name = function
+  | Esmodule -> "esmodule"
+  | Commonjs -> "commonjs"
diff --git a/rewatch-ocaml/config.mli b/rewatch-ocaml/config.mli
new file mode 100644
index 00000000000..36299039226
--- /dev/null
+++ b/rewatch-ocaml/config.mli
@@ -0,0 +1,14 @@
+include module type of Config_types
+
+val namespace_name : namespace -> string option
+val namespace_entry : namespace -> string option
+val namespace_compiler_name : namespace -> string option
+val namespaced_module_name : namespace -> string -> string
+val path_in_root : string -> string
+val exists_in_root : string -> bool
+val source_is_dev : t -> string -> bool
+val with_root_options : t -> t -> t
+val load : string -> t
+val load_root : string -> t
+val package_spec_suffix : t -> package_spec -> string
+val module_format_name : module_format -> string
diff --git a/rewatch-ocaml/config_decode.ml b/rewatch-ocaml/config_decode.ml
new file mode 100644
index 00000000000..0cef22c443c
--- /dev/null
+++ b/rewatch-ocaml/config_decode.ml
@@ -0,0 +1,402 @@
+open Config_types
+
+exception Error = Config_types.Error
+
+let fail path message = raise (Error (Printf.sprintf "%s: %s" path message))
+
+let fail_read path message =
+  raise (Error (Printf.sprintf "Could not read '%s': %s" path message))
+
+let strip_read_path path message =
+  String_util.strip_prefix ~prefix:(path ^ ": ") message
+
+let member name fields = List.assoc_opt name fields
+
+let optional_member name fields =
+  match member name fields with
+  | None | Some `Null -> None
+  | value -> value
+
+type field_policy = {known: string list; duplicate_checked: string list}
+
+let reject_all_duplicates known = {known; duplicate_checked = known}
+
+let unsupported_configuration_fields =
+  [
+    "ignored-dirs";
+    "generators";
+    "cut-generators";
+    "pp-flags";
+    "entries";
+    "bs-external-includes";
+  ]
+
+let configuration_duplicate_fields =
+  [
+    "name";
+    "sources";
+    "package-specs";
+    "warnings";
+    "suffix";
+    "dependencies";
+    "bs-dependencies";
+    "dev-dependencies";
+    "bs-dev-dependencies";
+    "features";
+    "ppx-flags";
+    "compiler-flags";
+    "bsc-flags";
+    "namespace";
+    "jsx";
+    "sourceMap";
+    "experimental-features";
+    "gentypeconfig";
+    "js-post-build";
+    "editor";
+    "reanalyze";
+    "namespace-entry";
+    "allowed-dependents";
+  ]
+
+let configuration_fields =
+  {
+    known = configuration_duplicate_fields @ unsupported_configuration_fields;
+    duplicate_checked = configuration_duplicate_fields;
+  }
+
+let warning_fields = reject_all_duplicates ["number"; "error"]
+
+let jsx_fields =
+  reject_all_duplicates
+    ["version"; "module"; "mode"; "v3-dependencies"; "preserve"]
+
+let gentype_fields =
+  reject_all_duplicates
+    [
+      "module";
+      "moduleResolution";
+      "exportInterfaces";
+      "generatedFileExtension";
+      "shims";
+      "debug";
+    ]
+
+let js_post_build_fields = reject_all_duplicates ["cmd"]
+let dependency_fields = reject_all_duplicates ["name"; "features"]
+
+let source_fields = reject_all_duplicates ["dir"; "subdirs"; "type"; "feature"]
+
+let package_spec_fields =
+  reject_all_duplicates ["module"; "in-source"; "suffix"]
+
+let reject_duplicate_fields path context policy fields =
+  let seen = Hashtbl.create (List.length fields) in
+  List.iter
+    (fun (name, _) ->
+      if List.mem name policy.duplicate_checked then
+        if Hashtbl.mem seen name then
+          fail path (Printf.sprintf "duplicate field %S in %s" name context)
+        else Hashtbl.add seen name ())
+    fields
+
+let reject_duplicates path context fields =
+  let seen = Hashtbl.create (List.length fields) in
+  List.iter
+    (fun (name, _) ->
+      if Hashtbl.mem seen name then
+        fail path (Printf.sprintf "duplicate field %S in %s" name context)
+      else Hashtbl.add seen name ())
+    fields
+
+let string path field = function
+  | `String value -> value
+  | _ -> fail path (Printf.sprintf "field %S must be a string" field)
+
+let bool path field = function
+  | `Bool value -> value
+  | _ -> fail path (Printf.sprintf "field %S must be a boolean" field)
+
+let strings path field = function
+  | `List values -> List.map (string path field) values
+  | _ -> fail path (Printf.sprintf "field %S must be an array of strings" field)
+
+let namespace_from_package_name name =
+  let buffer = Buffer.create (String.length name) in
+  let capitalize = ref true in
+  String.iter
+    (fun character ->
+      match character with
+      | 'a' .. 'z' | 'A' .. 'Z' | '0' .. '9' | '_' ->
+        Buffer.add_char buffer
+          (if !capitalize then Char.uppercase_ascii character else character);
+        capitalize := false
+      | '/' | '-' -> capitalize := true
+      | _ -> ())
+    name;
+  Buffer.contents buffer
+
+let compiler_flags path field = function
+  | `List values ->
+    values
+    |> List.concat_map (function
+      | `String value ->
+        String.split_on_char ' ' value |> List.filter (( <> ) "")
+      | `List values ->
+        values
+        |> List.concat_map (fun value ->
+            string path field value |> String.split_on_char ' '
+            |> List.filter (( <> ) ""))
+      | _ ->
+        fail path
+          (Printf.sprintf "field %S entries must be strings or arrays" field))
+  | _ -> fail path (Printf.sprintf "field %S must be an array" field)
+
+let dependency_name path = function
+  | `String value -> {name = value; features = None}
+  | `Assoc fields -> (
+    reject_duplicate_fields path "dependency" dependency_fields fields;
+    match member "name" fields with
+    | Some value ->
+      let features =
+        match optional_member "features" fields with
+        | None -> None
+        | Some value -> Some (strings path "features" value)
+      in
+      {name = string path "name" value; features}
+    | None -> fail path "dependency object is missing field \"name\"")
+  | _ -> fail path "dependency must be a string or object"
+
+let parse_dependencies path field fields =
+  match optional_member field fields with
+  | None -> []
+  | Some (`List values) -> List.map (dependency_name path) values
+  | Some _ -> fail path (Printf.sprintf "field %S must be an array" field)
+
+let dependency_alias path modern legacy fields =
+  match (member modern fields, member legacy fields) with
+  | Some _, Some _ ->
+    fail path
+      (Printf.sprintf "fields %S and %S cannot both be set" modern legacy)
+  | Some _, None -> parse_dependencies path modern fields
+  | None, Some _ -> parse_dependencies path legacy fields
+  | None, None -> []
+
+let rec sources_of_json path inherited_dir forced_dev inherited_feature =
+  function
+  | `String dir ->
+    [
+      {
+        dir = Filename.concat inherited_dir dir;
+        recurse = false;
+        is_dev = Option.value forced_dev ~default:false;
+        feature = inherited_feature;
+      };
+    ]
+  | `Assoc fields ->
+    reject_duplicate_fields path "source" source_fields fields;
+    let dir =
+      match member "dir" fields with
+      | Some value -> Filename.concat inherited_dir (string path "dir" value)
+      | None -> fail path "source object is missing field \"dir\""
+    in
+    let declared_dev =
+      match optional_member "type" fields with
+      | None -> false
+      | Some (`String "dev") -> true
+      | Some (`String _) -> false
+      | Some _ -> fail path "source field \"type\" must be a string"
+    in
+    let is_dev = Option.value forced_dev ~default:declared_dev in
+    let feature =
+      match optional_member "feature" fields with
+      | None -> inherited_feature
+      | Some value -> Some (string path "feature" value)
+    in
+    let recurse, children =
+      match optional_member "subdirs" fields with
+      | None -> (false, [])
+      | Some (`Bool value) -> (value, [])
+      | Some (`List values) ->
+        ( false,
+          List.concat_map
+            (sources_of_json path dir (Some is_dev) feature)
+            values )
+      | Some _ ->
+        fail path "source field \"subdirs\" must be a boolean or array"
+    in
+    {dir; recurse; is_dev; feature} :: children
+  | _ -> fail path "source must be a string or object"
+
+let parse_sources path fields =
+  match optional_member "sources" fields with
+  | None -> []
+  | Some (`List values) ->
+    List.concat_map (sources_of_json path "" None None) values
+  | Some value -> sources_of_json path "" None None value
+
+let nested_unknown_fields parent supported = function
+  | `Assoc fields ->
+    fields
+    |> List.filter_map (fun (name, _) ->
+        if List.mem name supported then None
+        else Some (Printf.sprintf "%s.?.%s" parent name))
+  | _ -> []
+
+let unknown_fields fields =
+  fields
+  |> List.concat_map (fun (name, value) ->
+      match name with
+      | "warnings" -> nested_unknown_fields name warning_fields.known value
+      | "jsx" -> nested_unknown_fields name jsx_fields.known value
+      | "gentypeconfig" -> nested_unknown_fields name gentype_fields.known value
+      | "js-post-build" ->
+        nested_unknown_fields name js_post_build_fields.known value
+      | _ -> if List.mem name configuration_fields.known then [] else [name])
+
+let unsupported_fields fields =
+  unsupported_configuration_fields
+  |> List.filter (fun field -> Option.is_some (member field fields))
+
+let parse_package_spec path = function
+  | `Assoc fields ->
+    reject_duplicate_fields path "package-specs entry" package_spec_fields
+      fields;
+    let module_format =
+      match member "module" fields with
+      | Some (`String ("esmodule" | "es6")) -> Esmodule
+      | Some (`String ("commonjs" | "cjs")) -> Commonjs
+      | None -> fail path "package-specs entry is missing field \"module\""
+      | Some value ->
+        fail path
+          (Printf.sprintf "unsupported package module %S"
+             (Yojson.Safe.to_string value))
+    in
+    let in_source =
+      match member "in-source" fields with
+      | None -> true
+      | Some value -> bool path "in-source" value
+    in
+    let suffix =
+      match optional_member "suffix" fields with
+      | None -> None
+      | Some value -> Some (string path "suffix" value)
+    in
+    {module_format; in_source; suffix}
+  | _ -> fail path "package-specs entries must be objects"
+
+let package_specs_use_alias alias = function
+  | `Assoc fields -> member "module" fields = Some (`String alias)
+  | `List values ->
+    List.exists
+      (function
+        | `Assoc fields -> member "module" fields = Some (`String alias)
+        | _ -> false)
+      values
+  | _ -> false
+
+let gentype_args path configured_suffix package_specs_value dependencies =
+  function
+  | `Assoc fields ->
+    reject_duplicate_fields path "gentypeconfig" gentype_fields fields;
+    let module_ =
+      match optional_member "module" fields with
+      | None -> (
+        match package_specs_value with
+        | Some (`Assoc package_spec) -> (
+          match member "module" package_spec with
+          | Some (`String ("esmodule" | "es6")) ->
+            ["-bs-gentype-module"; "esmodule"]
+          | Some (`String ("commonjs" | "cjs")) ->
+            ["-bs-gentype-module"; "commonjs"]
+          | _ -> [])
+        | _ -> [])
+      | Some (`String (("esmodule" | "commonjs") as value)) ->
+        ["-bs-gentype-module"; value]
+      | Some _ ->
+        fail path
+          "field \"gentypeconfig.module\" must be \"esmodule\" or \"commonjs\""
+    in
+    let module_resolution =
+      match optional_member "moduleResolution" fields with
+      | None -> []
+      | Some (`String (("node" | "node16" | "bundler") as value)) ->
+        ["-bs-gentype-module-resolution"; value]
+      | Some _ ->
+        fail path "field \"gentypeconfig.moduleResolution\" is invalid"
+    in
+    let export_interfaces =
+      match optional_member "exportInterfaces" fields with
+      | None | Some (`Bool false) -> []
+      | Some (`Bool true) -> ["-bs-gentype-export-interfaces"]
+      | Some _ ->
+        fail path "field \"gentypeconfig.exportInterfaces\" must be a boolean"
+    in
+    let generated_extension =
+      match optional_member "generatedFileExtension" fields with
+      | None -> []
+      | Some value ->
+        [
+          "-bs-gentype-generated-extension";
+          string path "gentypeconfig.generatedFileExtension" value;
+        ]
+    in
+    let shims =
+      let pairs =
+        match member "shims" fields with
+        | None -> []
+        | Some (`Assoc values) ->
+          reject_duplicates path "gentypeconfig.shims" values;
+          List.map
+            (fun (from_, target) ->
+              (from_, string path "gentypeconfig.shims" target))
+            values
+        | Some (`List values) ->
+          let pairs =
+            List.map
+              (fun value ->
+                let value = string path "gentypeconfig.shims" value in
+                match String.index_opt value '=' with
+                | Some separator ->
+                  let from_ = String.sub value 0 separator |> String.trim in
+                  let target =
+                    String.sub value (separator + 1)
+                      (String.length value - separator - 1)
+                    |> String.trim
+                  in
+                  (from_, target)
+                | None -> fail path "gentypeconfig.shims entries must contain =")
+              values
+          in
+          reject_duplicates path "gentypeconfig.shims" pairs;
+          pairs
+        | Some _ ->
+          fail path "field \"gentypeconfig.shims\" must be an object or array"
+      in
+      pairs |> List.sort compare
+      |> List.concat_map (fun (from_, target) ->
+          ["-bs-gentype-shim"; from_ ^ "=" ^ target])
+    in
+    let debug =
+      match member "debug" fields with
+      | None -> []
+      | Some (`Assoc values) ->
+        reject_duplicates path "gentypeconfig.debug" values;
+        values |> List.sort compare
+        |> List.concat_map (fun (name, value) ->
+            match value with
+            | `Bool true -> ["-bs-gentype-debug"; name]
+            | `Bool false -> []
+            | _ -> fail path "gentypeconfig.debug values must be booleans")
+      | Some _ -> fail path "field \"gentypeconfig.debug\" must be an object"
+    in
+    let suffix_args =
+      match configured_suffix with
+      | None -> []
+      | Some suffix -> ["-bs-gentype-suffix"; suffix]
+    in
+    ["-bs-gentype"] @ module_ @ module_resolution @ export_interfaces
+    @ generated_extension @ suffix_args @ shims @ debug
+    @ List.concat_map
+        (fun (dependency : dependency) -> ["-bs-gentype-dep"; dependency.name])
+        dependencies
+  | _ -> fail path "field \"gentypeconfig\" must be an object"
diff --git a/rewatch-ocaml/config_decode.mli b/rewatch-ocaml/config_decode.mli
new file mode 100644
index 00000000000..ba6f7df4682
--- /dev/null
+++ b/rewatch-ocaml/config_decode.mli
@@ -0,0 +1,48 @@
+val fail : string -> string -> 'a
+val fail_read : string -> string -> 'a
+val strip_read_path : string -> string -> string
+val member : string -> (string * 'a) list -> 'a option
+
+val optional_member :
+  string -> (string * Yojson.Safe.t) list -> Yojson.Safe.t option
+
+type field_policy
+
+val configuration_fields : field_policy
+val warning_fields : field_policy
+val jsx_fields : field_policy
+val js_post_build_fields : field_policy
+
+val reject_duplicate_fields :
+  string -> string -> field_policy -> (string * 'a) list -> unit
+
+val reject_duplicates : string -> string -> (string * 'a) list -> unit
+
+val string : string -> string -> Yojson.Safe.t -> string
+val strings : string -> string -> Yojson.Safe.t -> string list
+
+val namespace_from_package_name : string -> string
+val compiler_flags : string -> string -> Yojson.Safe.t -> string list
+
+val dependency_alias :
+  string ->
+  string ->
+  string ->
+  (string * Yojson.Safe.t) list ->
+  Config_types.dependency list
+
+val parse_sources :
+  string -> (string * Yojson.Safe.t) list -> Config_types.source list
+
+val unknown_fields : (string * Yojson.Safe.t) list -> string list
+val unsupported_fields : (string * 'a) list -> string list
+val parse_package_spec : string -> Yojson.Safe.t -> Config_types.package_spec
+val package_specs_use_alias : string -> Yojson.Safe.t -> bool
+
+val gentype_args :
+  string ->
+  string option ->
+  Yojson.Safe.t option ->
+  Config_types.dependency list ->
+  Yojson.Safe.t ->
+  string list
diff --git a/rewatch-ocaml/config_types.ml b/rewatch-ocaml/config_types.ml
new file mode 100644
index 00000000000..0f1d7edb86a
--- /dev/null
+++ b/rewatch-ocaml/config_types.ml
@@ -0,0 +1,44 @@
+type module_format = Esmodule | Commonjs
+type namespace =
+  | No_namespace
+  | Namespace of string
+  | Namespace_with_entry of {name: string; entry: string}
+
+type package_spec = {
+  module_format: module_format;
+  in_source: bool;
+  suffix: string option;
+}
+
+type source = {dir: string; recurse: bool; is_dev: bool; feature: string option}
+
+type dependency = {name: string; features: string list option}
+
+type t = {
+  path: string;
+  root: string;
+  file_hash: string;
+  name: string;
+  sources: source list;
+  sources_defined: bool;
+  dependencies: dependency list;
+  dev_dependencies: dependency list;
+  compiler_flags: string list;
+  package_specs: package_spec list;
+  suffix: string;
+  namespace: namespace;
+  features: (string * string list) list;
+  warning_flags: string list;
+  ppx_flags: string list list;
+  jsx_args: string list;
+  source_map_args: string list;
+  source_map_dev: bool;
+  experimental_args: string list;
+  gentype_args: string list;
+  js_post_build: string option;
+  allowed_dependents: string list option;
+  deprecation_diagnostics: string list;
+  diagnostics: string list;
+}
+
+exception Error of string
diff --git a/rewatch-ocaml/config_types.mli b/rewatch-ocaml/config_types.mli
new file mode 100644
index 00000000000..0f1d7edb86a
--- /dev/null
+++ b/rewatch-ocaml/config_types.mli
@@ -0,0 +1,44 @@
+type module_format = Esmodule | Commonjs
+type namespace =
+  | No_namespace
+  | Namespace of string
+  | Namespace_with_entry of {name: string; entry: string}
+
+type package_spec = {
+  module_format: module_format;
+  in_source: bool;
+  suffix: string option;
+}
+
+type source = {dir: string; recurse: bool; is_dev: bool; feature: string option}
+
+type dependency = {name: string; features: string list option}
+
+type t = {
+  path: string;
+  root: string;
+  file_hash: string;
+  name: string;
+  sources: source list;
+  sources_defined: bool;
+  dependencies: dependency list;
+  dev_dependencies: dependency list;
+  compiler_flags: string list;
+  package_specs: package_spec list;
+  suffix: string;
+  namespace: namespace;
+  features: (string * string list) list;
+  warning_flags: string list;
+  ppx_flags: string list list;
+  jsx_args: string list;
+  source_map_args: string list;
+  source_map_dev: bool;
+  experimental_args: string list;
+  gentype_args: string list;
+  js_post_build: string option;
+  allowed_dependents: string list option;
+  deprecation_diagnostics: string list;
+  diagnostics: string list;
+}
+
+exception Error of string
diff --git a/rewatch-ocaml/dune b/rewatch-ocaml/dune
new file mode 100644
index 00000000000..ee71546f9d0
--- /dev/null
+++ b/rewatch-ocaml/dune
@@ -0,0 +1,94 @@
+(env
+ (static
+  (flags
+   (:standard -ccopt -static))))
+
+(rule
+ (target platform.ml)
+ (enabled_if
+  (= %{os_type} Win32))
+ (action
+  (copy platform_windows.ml platform.ml)))
+
+(rule
+ (target platform.ml)
+ (enabled_if
+  (<> %{os_type} Win32))
+ (action
+  (copy platform_unix.ml platform.ml)))
+
+(library
+ (name rewatch_ocaml_lib)
+ (wrapped false)
+ (modules
+  rewatch_version
+  string_util
+  source_filter
+  cli
+  config_types
+  config_decode
+  config
+  project_context
+  package_resolution
+  package_traversal
+  platform_common
+  platform
+  signal_restore
+  traversal_coverage
+  process_child
+  process
+  source
+  package_plan
+  graph
+  module_graph
+  package_diagnostics
+  package_graph
+  compiler_args
+  compiler_args_command
+  compiler_log
+  ast_header
+  compiler_process
+  compiler_scheduler
+  after_build
+  file_util
+  build_lock
+  build_artifacts
+  build_freshness
+  build_session
+  build_attempt
+  build_report
+  build_preparation
+  package_parse
+  package_compilation
+  package_build
+  clean
+  compile_assets
+  build_state
+  native_watcher
+  watch_scope
+  watch_snapshot
+  watcher
+  toolchain
+  compiler_info
+  source_dirs
+  warning_state
+  output
+  build
+  format)
+ (libraries unix threads yojson str spawn cmdliner luv re.perl)
+ (foreign_stubs
+  (language c)
+  (names windows_job_stubs)))
+
+(executable
+ (name rescript_ocaml)
+ (modules rescript_ocaml)
+ (libraries rewatch_ocaml_lib))
+
+; Keep the former scoped test command useful after moving test-only modules out
+; of this production directory.
+
+(alias
+ (name runtest)
+ (deps
+  (alias ../tests/rewatch_ounit_tests/runtest)))
diff --git a/rewatch-ocaml/file_util.ml b/rewatch-ocaml/file_util.ml
new file mode 100644
index 00000000000..f91f1956c90
--- /dev/null
+++ b/rewatch-ocaml/file_util.ml
@@ -0,0 +1,259 @@
+let path_of_parts root parts = List.fold_left Filename.concat root parts
+
+let is_directory path =
+  match Unix.stat path with
+  | metadata -> metadata.Unix.st_kind = Unix.S_DIR
+  | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> false
+
+let ensure_dir path =
+  (* OCaml's standard library has no recursive directory-creation primitive.
+     Another process may win a mkdir race, but EEXIST is success only when the
+     resulting path is a directory. Filename.dirname may return an unavailable
+     Windows volume root unchanged, so stop at that fixed point and let mkdir
+     surface the native filesystem error. *)
+  let mkdir path =
+    try Unix.mkdir path 0o755
+    with Unix.Unix_error (Unix.EEXIST, _, _) as error ->
+      if not (is_directory path) then raise error
+  in
+  let rec loop path =
+    if path = "" || path = "." || is_directory path then ()
+    else
+      let parent = Filename.dirname path in
+      if parent = path then mkdir path
+      else (
+        loop parent;
+        mkdir path)
+  in
+  loop path
+
+let read_file path =
+  let descriptor = Unix.openfile path [Unix.O_RDONLY; Unix.O_CLOEXEC] 0 in
+  let channel = Unix.in_channel_of_descr descriptor in
+  Fun.protect
+    ~finally:(fun () -> close_in_noerr channel)
+    (fun () ->
+      let expected_size = (Unix.fstat descriptor).Unix.st_size in
+      let contents = Buffer.create (max 16 expected_size) in
+      (* A small initial chunk avoids allocating 64 KiB for tiny files such as
+         the watch lock. The loop still handles streams and files that grow
+         while they are being read. *)
+      let chunk = Bytes.create (max 1 (min 65536 (max 4096 expected_size))) in
+      let rec read () =
+        match input channel chunk 0 (Bytes.length chunk) with
+        | 0 -> Buffer.contents contents
+        | count ->
+          Buffer.add_subbytes contents chunk 0 count;
+          read ()
+      in
+      read ())
+
+let is_regular_file path =
+  match Unix.stat path with
+  | metadata -> metadata.Unix.st_kind = Unix.S_REG
+  | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> false
+
+let digest_file path =
+  let descriptor = Unix.openfile path [Unix.O_RDONLY; Unix.O_CLOEXEC] 0 in
+  let channel = Unix.in_channel_of_descr descriptor in
+  Fun.protect
+    ~finally:(fun () -> close_in_noerr channel)
+    (fun () -> Digest.channel channel (-1))
+
+let with_output_channel channel write =
+  try
+    let result = write channel in
+    close_out channel;
+    result
+  with exn ->
+    close_out_noerr channel;
+    raise exn
+
+let write_file path contents =
+  let channel = open_out_bin path in
+  with_output_channel channel (fun channel -> output_string channel contents)
+
+let append_file path contents =
+  let channel =
+    open_out_gen [Open_wronly; Open_append; Open_binary] 0o644 path
+  in
+  with_output_channel channel (fun channel -> output_string channel contents)
+
+let directory_entries path =
+  let directory = Unix.opendir path in
+  Fun.protect
+    ~finally:(fun () -> Unix.closedir directory)
+    (fun () ->
+      let rec read entries =
+        match Unix.readdir directory with
+        | "." | ".." -> read entries
+        | name -> read (name :: entries)
+        | exception End_of_file -> List.rev entries
+      in
+      read [])
+
+let write_file_atomic ?(ensure_parent = true) ?perm path contents =
+  if ensure_parent then ensure_dir (Filename.dirname path);
+  (* A temporary file must have a cleanup owner before a watch signal can
+     interrupt the command. Publishing is also signal-deferred so the final
+     path always names either the previous complete file or the replacement. *)
+  let creation_signals = Signal_restore.create ~defer:true in
+  let temporary = ref None in
+  let remove_temporary path =
+    try Sys.remove path with Sys_error _ | Unix.Unix_error _ -> ()
+  in
+  Fun.protect
+    ~finally:(fun () -> Option.iter remove_temporary !temporary)
+    (fun () ->
+      let candidate, perm =
+        Signal_restore.protect creation_signals (fun () ->
+            let perm =
+              match perm with
+              | Some _ as perm -> perm
+              | None -> (
+                match Unix.stat path with
+                | metadata -> Some metadata.Unix.st_perm
+                | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _)
+                  ->
+                  None)
+            in
+            let candidate =
+              Filename.temp_file ~temp_dir:(Filename.dirname path)
+                ".rewatch-write-" ".tmp"
+            in
+            temporary := Some candidate;
+            (candidate, perm))
+      in
+      Option.iter (Unix.chmod candidate) perm;
+      write_file candidate contents;
+      let publish_signals = Signal_restore.create ~defer:true in
+      Signal_restore.protect publish_signals (fun () ->
+          Sys.rename candidate path;
+          temporary := None))
+
+(* Callers that already created the destination directory may skip that work,
+   avoiding repeated metadata probes when publishing many files. *)
+let copy_existing_file ~ensure_parent source destination =
+  if ensure_parent then ensure_dir (Filename.dirname destination);
+  let input_descriptor =
+    Unix.openfile source [Unix.O_RDONLY; Unix.O_CLOEXEC] 0
+  in
+  let input_channel = Unix.in_channel_of_descr input_descriptor in
+  Fun.protect
+    ~finally:(fun () -> close_in_noerr input_channel)
+    (fun () ->
+      let output_channel = open_out_bin destination in
+      with_output_channel output_channel (fun output_channel ->
+          let buffer = Bytes.create 65_536 in
+          let rec copy () =
+            let count = input input_channel buffer 0 (Bytes.length buffer) in
+            if count > 0 then (
+              output output_channel buffer 0 count;
+              copy ())
+          in
+          copy ()))
+
+let copy_optional_existing_file ?(ensure_parent = true) source destination =
+  try copy_existing_file ~ensure_parent source destination
+  with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> (
+    try Unix.unlink destination
+    with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> ())
+
+let stat_opt path =
+  try Some (Unix.stat path)
+  with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None
+
+let exists path = Option.is_some (stat_opt path)
+
+let files_equal first second =
+  match stat_opt first with
+  | None -> false
+  | Some first_stat -> (
+    match stat_opt second with
+    | None -> false
+    | Some second_stat ->
+      first_stat.Unix.st_size = second_stat.Unix.st_size
+      &&
+      let first_descriptor =
+        Unix.openfile first [Unix.O_RDONLY; Unix.O_CLOEXEC] 0
+      in
+      let first_channel = Unix.in_channel_of_descr first_descriptor in
+      Fun.protect
+        ~finally:(fun () -> close_in_noerr first_channel)
+        (fun () ->
+          let second_descriptor =
+            Unix.openfile second [Unix.O_RDONLY; Unix.O_CLOEXEC] 0
+          in
+          let second_channel = Unix.in_channel_of_descr second_descriptor in
+          Fun.protect
+            ~finally:(fun () -> close_in_noerr second_channel)
+            (fun () ->
+              let buffer_size = 65_536 in
+              let first_buffer = Bytes.create buffer_size in
+              let second_buffer = Bytes.create buffer_size in
+              let rec read_chunk channel buffer offset =
+                if offset = Bytes.length buffer then offset
+                else
+                  match
+                    input channel buffer offset (Bytes.length buffer - offset)
+                  with
+                  | 0 -> offset
+                  | count -> read_chunk channel buffer (offset + count)
+              in
+              let equal_prefix length =
+                let index = ref 0 in
+                while
+                  !index < length
+                  && Bytes.get first_buffer !index
+                     = Bytes.get second_buffer !index
+                do
+                  incr index
+                done;
+                !index = length
+              in
+              let rec loop () =
+                let first_count = read_chunk first_channel first_buffer 0 in
+                let second_count = read_chunk second_channel second_buffer 0 in
+                first_count = second_count
+                && (first_count = 0 || (equal_prefix first_count && loop ()))
+              in
+              loop ())))
+
+let copy_file_if_different ?(ensure_parent = true) source destination =
+  let changed = not (files_equal source destination) in
+  if changed then copy_existing_file ~ensure_parent source destination;
+  changed
+
+let modification_time path =
+  stat_opt path |> Option.map (fun metadata -> metadata.Unix.st_mtime)
+
+let remove_file path =
+  try Unix.unlink path
+  with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> ()
+
+let rec remove_tree path =
+  match (Unix.lstat path).Unix.st_kind with
+  | Unix.S_DIR ->
+    directory_entries path
+    |> List.iter (fun name -> remove_tree (Filename.concat path name));
+    Unix.rmdir path
+  | _ -> Unix.unlink path
+  | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> ()
+
+let remove_file_best_effort path =
+  try remove_file path with Sys_error _ | Unix.Unix_error _ -> ()
+
+let rec files_under directory =
+  match (Unix.lstat directory).Unix.st_kind with
+  | Unix.S_DIR ->
+    directory_entries directory
+    |> List.concat_map (fun name ->
+        files_under (Filename.concat directory name))
+  (* Following links during recursion could leave the requested tree or enter
+     a cycle. Follow one only to omit dangling links from the result. *)
+  | Unix.S_LNK -> (
+    match Unix.stat directory with
+    | _ -> [directory]
+    | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> [])
+  | _ -> [directory]
+  | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> []
diff --git a/rewatch-ocaml/file_util.mli b/rewatch-ocaml/file_util.mli
new file mode 100644
index 00000000000..1a98e579200
--- /dev/null
+++ b/rewatch-ocaml/file_util.mli
@@ -0,0 +1,26 @@
+val path_of_parts : string -> string list -> string
+val is_directory : string -> bool
+val ensure_dir : string -> unit
+val read_file : string -> string
+val is_regular_file : string -> bool
+val digest_file : string -> Digest.t
+val write_file : string -> string -> unit
+val append_file : string -> string -> unit
+val directory_entries : string -> string list
+
+val write_file_atomic :
+  ?ensure_parent:bool -> ?perm:int -> string -> string -> unit
+
+val copy_existing_file : ensure_parent:bool -> string -> string -> unit
+
+val copy_optional_existing_file :
+  ?ensure_parent:bool -> string -> string -> unit
+
+val exists : string -> bool
+val files_equal : string -> string -> bool
+val copy_file_if_different : ?ensure_parent:bool -> string -> string -> bool
+val modification_time : string -> float option
+val remove_file : string -> unit
+val remove_tree : string -> unit
+val remove_file_best_effort : string -> unit
+val files_under : string -> string list
diff --git a/rewatch-ocaml/format.ml b/rewatch-ocaml/format.ml
new file mode 100644
index 00000000000..158ff8f381a
--- /dev/null
+++ b/rewatch-ocaml/format.ml
@@ -0,0 +1,229 @@
+exception Error of string
+
+let strip_path path message =
+  String_util.strip_prefix ~prefix:(path ^ ": ") message
+
+let with_file_error ~action path f =
+  try f () with
+  | Sys_error message ->
+    raise
+      (Error
+         (Printf.sprintf "Could not %s %s: %s" action path
+            (strip_path path message)))
+  | Unix.Unix_error (error, _, _) ->
+    raise
+      (Error
+         (Printf.sprintf "Could not %s %s: %s" action path
+            (Unix.error_message error)))
+
+let read_file path =
+  with_file_error ~action:"read file" path (fun () -> File_util.read_file path)
+
+let write_file path contents =
+  with_file_error ~action:"write formatted file" path (fun () ->
+      (* Formatting changes the contents of a user-owned file. Writing through
+       its existing inode preserves symlinks, hard links, ownership, ACLs, and
+       extended attributes that replacing the directory entry could lose. *)
+      File_util.write_file path contents)
+
+let bsc () =
+  try Toolchain.bsc () with Toolchain.Error message -> raise (Error message)
+
+type discovered_package = {config: Config.t; files: string list}
+
+let package_sources (package : discovered_package) =
+  package.files |> List.map (Filename.concat package.config.root)
+
+(* Validate the complete package graph before selecting the local files that
+   format owns. Scan it with the effective feature selections so dependency
+   diagnostics and the eventual local file set cannot drift apart. *)
+let discover_package_graph (current : Config.t) =
+  let resolution = Package_resolution.create current in
+  Package_diagnostics.validate_metadata current;
+  let graph =
+    Package_traversal.discover ~root_config:current ~prod:false ~features:None
+      ~resolution
+  in
+  graph.packages
+  |> List.map (fun (package : Package_traversal.package) ->
+      let config = package.config in
+      let is_local = package.is_local in
+      Package_diagnostics.report_missing_sources
+        ~is_root:(config.root = current.root)
+        config;
+      let features =
+        if config.root = current.root then None
+        else
+          match Package_traversal.find_feature_selection graph config.root with
+          | None | Some Package_traversal.All_features -> None
+          | Some (Package_traversal.Selected_features requested) ->
+            (try ignore (Source.resolve_active_features config requested)
+             with Source.Error message ->
+               raise
+                 (Error
+                    (Printf.sprintf "Invalid features for package '%s': %s"
+                       config.name message)));
+            Some requested
+      in
+      let files =
+        Source.discover_files config
+          ~prod:(Package_traversal.source_discovery_prod ~prod:false ~is_local)
+          ~features ~filter:None
+          ~on_missing:(Package_diagnostics.report_missing_source_folder config)
+      in
+      {config; files})
+
+let files_in_scope () =
+  let current_directory = Sys.getcwd () in
+  let current =
+    try Config.load_root current_directory
+    with Config.Error message ->
+      raise
+        (Error
+           (Printf.sprintf "Could not read rescript.json at %s: %s"
+              current_directory message))
+  in
+  let listed_by_parent =
+    match
+      Project_context.nearest_config_path (Filename.dirname current.root)
+    with
+    | None -> false
+    | Some path ->
+      let parent = Config.load path in
+      List.exists
+        (fun (dependency : Config.dependency) -> dependency.name = current.name)
+        (Package_traversal.requests ~prod:false ~is_local:true parent
+        |> List.map (fun request -> request.Package_traversal.declaration))
+  in
+  let packages = discover_package_graph current in
+  let resolution = Package_resolution.create current in
+  let roots_in_scope =
+    if listed_by_parent then [current.root]
+    else
+      current.root
+      :: (Package_traversal.requests ~prod:false ~is_local:true current
+         |> List.map (fun request -> request.Package_traversal.declaration)
+         |> List.filter_map (fun (dependency : Config.dependency) ->
+             match
+               Package_resolution.dependency_path resolution
+                 ~package_root:current.root dependency.name
+             with
+             | Some directory
+               when Package_resolution.is_local resolution directory ->
+               Some directory
+             | Some _ | None -> None))
+  in
+  packages
+  |> List.filter (fun package ->
+      List.exists (( = ) package.config.root) roots_in_scope)
+  |> List.concat_map package_sources
+  |> List.sort_uniq String.compare
+
+let formatting_error target stderr =
+  Printf.sprintf "Error formatting %s: %s" target stderr
+
+let formatted ?poll ~bsc ~target path =
+  let result = Process.run ?poll ~cwd:(Sys.getcwd ()) bsc ["-format"; path] in
+  if not (Process.succeeded result) then
+    raise (Error (formatting_error target result.stderr));
+  result.stdout
+
+let format_check_summary = function
+  | 1 -> "The file listed above needs formatting"
+  | count -> Printf.sprintf "The %d files listed above need formatting" count
+
+let format_files_with_bsc ?max_jobs ?poll ~bsc ~check files =
+  let cwd = Sys.getcwd () in
+  let incorrect = ref 0 in
+  let works =
+    files
+    |> List.mapi (fun index path ->
+        Process.
+          {key = Printf.sprintf "%08d" index; dependencies = []; value = path})
+  in
+  let next path = function
+    | None ->
+      Some (Process.task Process.{program = bsc; args = ["-format"; path]; cwd})
+    | Some result ->
+      if not (Process.succeeded result) then
+        raise (Error (formatting_error path result.stderr));
+      let original = read_file path in
+      if original <> result.stdout then
+        if check then (
+          incr incorrect;
+          prerr_endline ("[format check] " ^ path))
+        else write_file path result.stdout;
+      None
+  in
+  Process.run_dependency_graph ?max_jobs ?poll works ~next;
+  if !incorrect > 0 then (
+    prerr_endline (format_check_summary !incorrect);
+    raise (Error "Formatting check failed"))
+
+type stdin_read_result = Stdin_contents of string | Stdin_error of exn
+
+let read_stdin_interruptibly ?poll () =
+  let result = Atomic.make None in
+  let reader =
+    Thread.create
+      (fun () ->
+        let value =
+          try
+            let buffer = Buffer.create 4096 in
+            let bytes = Bytes.create 65536 in
+            let rec read () =
+              match input stdin bytes 0 (Bytes.length bytes) with
+              | 0 -> Stdin_contents (Buffer.contents buffer)
+              | count ->
+                Buffer.add_subbytes buffer bytes 0 count;
+                read ()
+            in
+            read ()
+          with exn -> Stdin_error exn
+        in
+        Atomic.set result (Some value))
+      ()
+  in
+  let rec await () =
+    Option.iter (fun poll -> poll ()) poll;
+    match Atomic.get result with
+    | Some value ->
+      Thread.join reader;
+      value
+    | None ->
+      Unix.sleepf 0.02;
+      await ()
+  in
+  match await () with
+  | Stdin_contents contents -> contents
+  | Stdin_error exn -> raise exn
+
+let format_stdin ?poll extension =
+  if extension <> ".res" && extension <> ".resi" then
+    raise (Error "--stdin must be .res or .resi");
+  let bsc = bsc () in
+  (* The temporary pathname needs a cleanup owner before termination can
+     interrupt the command, otherwise an early signal can leave it behind. *)
+  let deferred_signals = Signal_restore.create ~defer:true in
+  let temporary = ref None in
+  let remove_temporary () =
+    Option.iter
+      (fun path -> try Sys.remove path with Sys_error _ -> ())
+      !temporary
+  in
+  try
+    let path = Filename.temp_file "rescript-format-" extension in
+    temporary := Some path;
+    Fun.protect ~finally:remove_temporary (fun () ->
+        Signal_restore.restore deferred_signals;
+        let contents = read_stdin_interruptibly ?poll () in
+        File_util.write_file path contents;
+        print_string (formatted ?poll ~bsc ~target:"stdin" path))
+  with exn ->
+    remove_temporary ();
+    raise (Signal_restore.exception_after_restore deferred_signals exn)
+
+let run_files ?poll ~check paths =
+  let bsc = bsc () in
+  let files = if paths = [] then files_in_scope () else paths in
+  format_files_with_bsc ?poll ~bsc ~check files
diff --git a/rewatch-ocaml/format.mli b/rewatch-ocaml/format.mli
new file mode 100644
index 00000000000..a1c4b9a2f8e
--- /dev/null
+++ b/rewatch-ocaml/format.mli
@@ -0,0 +1,16 @@
+exception Error of string
+
+val write_file : string -> string -> unit
+val formatting_error : string -> string -> string
+val format_check_summary : int -> string
+
+val format_files_with_bsc :
+  ?max_jobs:int ->
+  ?poll:(unit -> unit) ->
+  bsc:string ->
+  check:bool ->
+  string list ->
+  unit
+
+val format_stdin : ?poll:(unit -> unit) -> string -> unit
+val run_files : ?poll:(unit -> unit) -> check:bool -> string list -> unit
diff --git a/rewatch-ocaml/graph.ml b/rewatch-ocaml/graph.ml
new file mode 100644
index 00000000000..90655a6a610
--- /dev/null
+++ b/rewatch-ocaml/graph.ml
@@ -0,0 +1,246 @@
+type validation =
+  | Replace_duplicates_and_ignore_unknown
+  | Reject_invalid of {
+      duplicate_node: string -> exn;
+      unknown_dependency: node:string -> dependency:string -> exn;
+    }
+
+type 'a index = {
+  nodes_by_name: (string, 'a) Hashtbl.t;
+  dependencies_by_name: (string, string list) Hashtbl.t;
+  dependents_by_name: (string, string list) Hashtbl.t;
+}
+
+let create_index nodes ~name ~deps ~validation =
+  let nodes_by_name = Hashtbl.create (List.length nodes) in
+  List.iter
+    (fun node ->
+      let node_name = name node in
+      match Hashtbl.find_opt nodes_by_name node_name with
+      | None -> Hashtbl.add nodes_by_name node_name node
+      | Some _ -> (
+        match validation with
+        | Replace_duplicates_and_ignore_unknown ->
+          Hashtbl.replace nodes_by_name node_name node
+        | Reject_invalid {duplicate_node; unknown_dependency = _} ->
+          raise (duplicate_node node_name)))
+    nodes;
+  let dependencies_by_name = Hashtbl.create (Hashtbl.length nodes_by_name) in
+  let dependents_by_name = Hashtbl.create (Hashtbl.length nodes_by_name) in
+  Hashtbl.iter
+    (fun node_name node ->
+      let dependencies =
+        deps node
+        |> List.sort_uniq String.compare
+        |> List.filter (fun dependency ->
+            if Hashtbl.mem nodes_by_name dependency then true
+            else
+              match validation with
+              | Replace_duplicates_and_ignore_unknown -> false
+              | Reject_invalid {duplicate_node = _; unknown_dependency} ->
+                raise (unknown_dependency ~node:node_name ~dependency))
+      in
+      Hashtbl.add dependencies_by_name node_name dependencies;
+      List.iter
+        (fun dependency ->
+          let dependents =
+            Hashtbl.find_opt dependents_by_name dependency
+            |> Option.value ~default:[]
+          in
+          Hashtbl.replace dependents_by_name dependency (node_name :: dependents))
+        dependencies)
+    nodes_by_name;
+  {nodes_by_name; dependencies_by_name; dependents_by_name}
+
+let node_count index = Hashtbl.length index.nodes_by_name
+let find_node index name = Hashtbl.find index.nodes_by_name name
+
+let dependencies index name =
+  Hashtbl.find_opt index.dependencies_by_name name |> Option.value ~default:[]
+
+let dependents index name =
+  Hashtbl.find_opt index.dependents_by_name name |> Option.value ~default:[]
+
+let dependency_count index name = List.length (dependencies index name)
+
+let cycle_blocked_nodes nodes ~name ~deps =
+  let index =
+    create_index nodes ~name ~deps
+      ~validation:Replace_duplicates_and_ignore_unknown
+  in
+  let pending = Hashtbl.create (node_count index) in
+  Hashtbl.iter
+    (fun node_name _ ->
+      Hashtbl.add pending node_name (dependency_count index node_name))
+    index.nodes_by_name;
+  let ready = Queue.create () in
+  Hashtbl.iter (fun key count -> if count = 0 then Queue.add key ready) pending;
+  let removed = Hashtbl.create (node_count index) in
+  while not (Queue.is_empty ready) do
+    let key = Queue.take ready in
+    Hashtbl.replace removed key ();
+    dependents index key
+    |> List.iter (fun dependent ->
+        let count = Hashtbl.find pending dependent - 1 in
+        Hashtbl.replace pending dependent count;
+        if count = 0 then Queue.add dependent ready)
+  done;
+  List.filter (fun node -> not (Hashtbl.mem removed (name node))) nodes
+
+let canonical_cycle cycle =
+  let rec without_last = function
+    | [] | [_] -> []
+    | value :: rest -> value :: without_last rest
+  in
+  let nodes = without_last cycle in
+  match nodes with
+  | [] -> cycle
+  | first :: rest ->
+    let smallest = List.fold_left min first rest in
+    let rec split prefix = function
+      | [] -> nodes
+      | head :: tail as suffix ->
+        if head = smallest then suffix @ List.rev prefix
+        else split (head :: prefix) tail
+    in
+    let canonical = split [] nodes in
+    canonical @ [smallest]
+
+let shortest_cycle_in_index index =
+  let names =
+    index.nodes_by_name |> Hashtbl.to_seq_keys |> List.of_seq
+    |> List.sort String.compare
+  in
+  let edges = Hashtbl.create (Hashtbl.length index.dependencies_by_name) in
+  List.iter
+    (fun name ->
+      List.iter
+        (fun dependency -> Hashtbl.replace edges (name, dependency) ())
+        (dependencies index name))
+    names;
+  match List.find_opt (fun name -> Hashtbl.mem edges (name, name)) names with
+  | Some name -> Some [name; name]
+  | None -> (
+    let two_node_cycle =
+      names
+      |> List.find_map (fun name ->
+          dependencies index name
+          |> List.filter (fun dependency -> name < dependency)
+          |> List.sort String.compare
+          |> List.find_map (fun dependency ->
+              if Hashtbl.mem edges (dependency, name) then
+                Some [name; dependency; name]
+              else None))
+    in
+    match two_node_cycle with
+    | Some _ as cycle -> cycle
+    | None ->
+      let next_index = ref 0 in
+      let indices = Hashtbl.create (List.length names) in
+      let lowlinks = Hashtbl.create (List.length names) in
+      let on_stack = Hashtbl.create (List.length names) in
+      let stack = Stack.create () in
+      let components = ref [] in
+      let rec visit name =
+        let index_value = !next_index in
+        incr next_index;
+        Hashtbl.add indices name index_value;
+        Hashtbl.add lowlinks name index_value;
+        Stack.push name stack;
+        Hashtbl.add on_stack name ();
+        dependencies index name
+        |> List.iter (fun dependency ->
+            match Hashtbl.find_opt indices dependency with
+            | None ->
+              visit dependency;
+              Hashtbl.replace lowlinks name
+                (min
+                   (Hashtbl.find lowlinks name)
+                   (Hashtbl.find lowlinks dependency))
+            | Some dependency_index when Hashtbl.mem on_stack dependency ->
+              Hashtbl.replace lowlinks name
+                (min (Hashtbl.find lowlinks name) dependency_index)
+            | Some _ -> ());
+        if Hashtbl.find lowlinks name = index_value then (
+          let component = ref [] in
+          let complete = ref false in
+          while not !complete do
+            let member = Stack.pop stack in
+            Hashtbl.remove on_stack member;
+            component := member :: !component;
+            complete := member = name
+          done;
+          components := !component :: !components)
+      in
+      List.iter
+        (fun name -> if not (Hashtbl.mem indices name) then visit name)
+        names;
+      let component_by_name = Hashtbl.create (List.length names) in
+      !components
+      |> List.iteri (fun component_id component ->
+          if List.length component > 1 then
+            List.iter
+              (fun name -> Hashtbl.add component_by_name name component_id)
+              component);
+      let same_cyclic_component left right =
+        match
+          ( Hashtbl.find_opt component_by_name left,
+            Hashtbl.find_opt component_by_name right )
+        with
+        | Some left, Some right -> left = right
+        | _ -> false
+      in
+      let best = ref None in
+      let best_length = ref max_int in
+      let consider cycle =
+        let cycle = canonical_cycle cycle in
+        match !best with
+        | None ->
+          best := Some cycle;
+          best_length := List.length cycle
+        | Some current ->
+          let cycle_length = List.length cycle in
+          if
+            cycle_length < !best_length
+            || (cycle_length = !best_length && cycle < current)
+          then (
+            best := Some cycle;
+            best_length := cycle_length)
+      in
+      names
+      |> List.filter (Hashtbl.mem component_by_name)
+      |> List.iter (fun start ->
+          let queue = Queue.create () in
+          let parents = Hashtbl.create 16 in
+          let distances = Hashtbl.create 16 in
+          Hashtbl.add distances start 0;
+          Queue.add start queue;
+          let found = ref false in
+          while (not !found) && not (Queue.is_empty queue) do
+            let current = Queue.take queue in
+            let distance = Hashtbl.find distances current in
+            let can_improve = distance + 2 <= !best_length in
+            if can_improve then
+              dependencies index current
+              |> List.iter (fun dependency ->
+                  if not (same_cyclic_component start dependency) then ()
+                  else if dependency = start then (
+                    let rec path_to_start acc node_name =
+                      if node_name = start then start :: acc
+                      else
+                        path_to_start (node_name :: acc)
+                          (Hashtbl.find parents node_name)
+                    in
+                    consider (path_to_start [] current @ [start]);
+                    found := true)
+                  else if not (Hashtbl.mem distances dependency) then (
+                    Hashtbl.add parents dependency current;
+                    Hashtbl.add distances dependency (distance + 1);
+                    Queue.add dependency queue))
+          done);
+      !best)
+
+let shortest_cycle nodes ~name ~deps =
+  create_index nodes ~name ~deps
+    ~validation:Replace_duplicates_and_ignore_unknown
+  |> shortest_cycle_in_index
diff --git a/rewatch-ocaml/graph.mli b/rewatch-ocaml/graph.mli
new file mode 100644
index 00000000000..b08bb7fee66
--- /dev/null
+++ b/rewatch-ocaml/graph.mli
@@ -0,0 +1,34 @@
+(** Graph indexing is shared by cycle analysis and subprocess scheduling, but
+    their input contracts differ. The validation policy makes replacement or
+    rejection of duplicate and unknown nodes an explicit caller decision. *)
+type validation =
+  | Replace_duplicates_and_ignore_unknown
+  | Reject_invalid of {
+      duplicate_node: string -> exn;
+      unknown_dependency: node:string -> dependency:string -> exn;
+    }
+
+type 'a index
+
+val create_index :
+  'a list ->
+  name:('a -> string) ->
+  deps:('a -> string list) ->
+  validation:validation ->
+  'a index
+
+val node_count : 'a index -> int
+val find_node : 'a index -> string -> 'a
+val dependencies : 'a index -> string -> string list
+val dependents : 'a index -> string -> string list
+val dependency_count : 'a index -> string -> int
+val shortest_cycle_in_index : 'a index -> string list option
+
+val cycle_blocked_nodes :
+  'a list -> name:('a -> string) -> deps:('a -> string list) -> 'a list
+
+val shortest_cycle :
+  'a list ->
+  name:('a -> string) ->
+  deps:('a -> string list) ->
+  string list option
diff --git a/rewatch-ocaml/module_graph.ml b/rewatch-ocaml/module_graph.ml
new file mode 100644
index 00000000000..8c55185428c
--- /dev/null
+++ b/rewatch-ocaml/module_graph.ml
@@ -0,0 +1,379 @@
+type cycle_info = {
+  cycle: string list;
+  blocked: string list;
+  nodes_by_key: (string, cycle_node) Hashtbl.t;
+}
+
+and cycle_node = {
+  key: string;
+  package_root: string;
+  source_path: string option;
+  display_name: string;
+}
+
+type module_node = {
+  key: string;
+  package_name: string;
+  package_root: string;
+  source_path: string;
+  namespace: Config.namespace;
+  visible_packages: (string, unit) Hashtbl.t;
+  mutable raw_dependencies: string list;
+}
+
+type namespace_map = {
+  key: string;
+  compiler_name: string;
+  namespace: string;
+  package_name: string;
+  package_root: string;
+  members: string list;
+}
+
+let namespace_map_key package_root = "\000namespace:" ^ package_root
+
+let dependency_head dependency =
+  match String.split_on_char '.' dependency with
+  | head :: _ -> head
+  | [] -> dependency
+
+let compiler_namespace (config : Config.t) =
+  Config.namespace_compiler_name config.namespace
+
+let validate_visible_namespaces ~(root_config : Config.t)
+    (package_plans : Package_plan.t list) =
+  let by_root = Hashtbl.create (List.length package_plans) in
+  List.iter
+    (fun (package : Package_plan.t) ->
+      Hashtbl.replace by_root package.root package)
+    package_plans;
+  package_plans
+  |> List.sort (fun first second ->
+      String.compare first.Package_plan.root second.root)
+  |> List.iter (fun consumer ->
+      let visible =
+        consumer
+        :: (consumer.Package_plan.dependencies
+           |> List.filter_map (fun dependency ->
+               Hashtbl.find_opt by_root dependency.Package_plan.directory))
+      in
+      let namespaces = Hashtbl.create (List.length visible) in
+      visible
+      |> List.sort (fun first second ->
+          String.compare first.Package_plan.root second.root)
+      |> List.iter (fun package ->
+          compiler_namespace package.Package_plan.compile_config
+          |> Option.iter (fun namespace ->
+              match Hashtbl.find_opt namespaces namespace with
+              | None -> Hashtbl.add namespaces namespace package
+              | Some previous when previous.Package_plan.root = package.root ->
+                ()
+              | Some previous ->
+                let display package =
+                  Printf.sprintf "%s (%s)" package.Package_plan.config.name
+                    (Project_context.display_path ~root:root_config.root
+                       package.root)
+                in
+                raise
+                  (Project_context.Error
+                     (Printf.sprintf
+                        "Could not initialize build: Namespace %s is provided \
+                         by both %s and %s while building %s. Give the \
+                         packages distinct namespaces."
+                        namespace (display previous) (display package)
+                        (display consumer))))))
+
+let resolve_dependency ~find_module ~find_namespace_maps (node : module_node)
+    dependency =
+  let raw_name = dependency_head dependency in
+  let local_name =
+    match
+      (Config.namespace_name node.namespace, String.split_on_char '.' dependency)
+    with
+    | Some namespace, first :: second :: _ when first = namespace -> second
+    | _ -> raw_name
+  in
+  let local_key = Config.namespaced_module_name node.namespace local_name in
+  let is_visible (dependency_node : module_node) =
+    Hashtbl.mem node.visible_packages dependency_node.package_name
+  in
+  match find_module local_key with
+  | Some dependency_node when is_visible dependency_node -> [local_key]
+  | _ when Config.namespace_name node.namespace = Some raw_name -> []
+  | _ -> (
+    match find_module raw_name with
+    | Some dependency_node when is_visible dependency_node -> [raw_name]
+    | _ -> (
+      let explicit_namespaced_module =
+        match String.split_on_char '.' dependency with
+        | namespace :: module_name :: _ ->
+          [module_name ^ "-" ^ namespace; module_name ^ "-@" ^ namespace]
+          |> List.find_opt (fun key ->
+              match find_module key with
+              | Some dependency_node
+                when Config.namespace_name dependency_node.namespace
+                     = Some namespace
+                     && is_visible dependency_node ->
+                true
+              | Some _ | None -> false)
+        | _ -> None
+      in
+      match explicit_namespaced_module with
+      | Some key -> [key]
+      | None ->
+        find_namespace_maps raw_name
+        |> Option.value ~default:[]
+        |> List.filter_map (fun (namespace_map : namespace_map) ->
+            if Hashtbl.mem node.visible_packages namespace_map.package_name then
+              Some namespace_map.key
+            else None)))
+
+let resolved_dependencies ~find_module ~find_namespace_maps (node : module_node)
+    =
+  let parsed =
+    node.raw_dependencies
+    |> List.concat_map
+         (resolve_dependency ~find_module ~find_namespace_maps node)
+  in
+  let implicit_namespace_entry =
+    match node.namespace with
+    | Config.Namespace_with_entry {name = namespace; entry}
+      when Source.module_name node.source_path = entry ->
+      find_namespace_maps namespace
+      |> Option.value ~default:[]
+      |> List.find_map (fun (namespace_map : namespace_map) ->
+          if namespace_map.package_root = node.package_root then
+            Some namespace_map.key
+          else None)
+      |> Option.to_list
+    | Config.Namespace_with_entry _ | Config.Namespace _ | Config.No_namespace
+      ->
+      []
+  in
+  parsed @ implicit_namespace_entry
+  |> List.filter (fun dependency -> dependency <> node.key)
+  |> List.sort_uniq String.compare
+
+type initialized = {
+  nodes: module_node list;
+  namespace_maps: namespace_map list;
+  build_state: Build_state.t;
+  use_existing_ast_paths: string list;
+}
+
+let initialize ~(root_config : Config.t) ~package_plans ~compile_assets
+    ~failed_parse_paths =
+  let nodes = ref [] in
+  let use_existing_ast_paths = ref [] in
+  List.iter
+    (fun (package : Package_plan.t) ->
+      let visible_packages =
+        Hashtbl.create (List.length package.dependencies + 1)
+      in
+      Hashtbl.replace visible_packages package.name ();
+      List.iter
+        (fun (dependency : Package_plan.dependency) ->
+          Hashtbl.replace visible_packages dependency.declaration.name ())
+        package.dependencies;
+      List.iter
+        (fun module_ ->
+          let dependencies path =
+            if
+              Hashtbl.mem failed_parse_paths (Filename.concat package.root path)
+            then []
+            else
+              Compiler_process.ast_dependencies ~build_dir:package.build_dir
+                (Source.ast_path path)
+          in
+          let raw_dependencies =
+            List.sort_uniq String.compare
+              (dependencies module_.Source.implementation
+              @
+              match module_.Source.interface with
+              | None -> []
+              | Some path -> dependencies path)
+          in
+          let compiler_base =
+            Source.compiler_basename package.compile_config module_.Source.name
+          in
+          if Option.is_none (Compile_assets.cmt compile_assets compiler_base)
+          then
+            use_existing_ast_paths :=
+              Filename.concat package.root module_.Source.implementation
+              :: !use_existing_ast_paths;
+          nodes :=
+            {
+              key = compiler_base;
+              package_name = package.name;
+              package_root = package.root;
+              source_path = module_.Source.implementation;
+              namespace = package.compile_config.namespace;
+              visible_packages;
+              raw_dependencies;
+            }
+            :: !nodes)
+        package.modules)
+    package_plans;
+  let nodes =
+    List.sort
+      (fun (first : module_node) second -> String.compare first.key second.key)
+      !nodes
+  in
+  let by_key : (string, module_node) Hashtbl.t =
+    Hashtbl.create (List.length nodes)
+  in
+  List.iter
+    (fun (node : module_node) ->
+      match Hashtbl.find_opt by_key node.key with
+      | None -> Hashtbl.add by_key node.key node
+      | Some previous ->
+        raise
+          (Source.duplicate_error ~display_root:root_config.root "" node.key
+             (Filename.concat previous.package_root previous.source_path)
+             (Filename.concat node.package_root node.source_path)))
+    nodes;
+  let namespace_maps =
+    package_plans
+    |> List.filter_map (fun (package : Package_plan.t) ->
+        let namespace = package.Package_plan.compile_config.namespace in
+        let namespace_details =
+          match namespace with
+          | Config.No_namespace -> None
+          | Config.Namespace name -> Some (name, name, None)
+          | Config.Namespace_with_entry {name; entry} ->
+            Some ("@" ^ name, name, Some entry)
+        in
+        namespace_details
+        |> Option.map (fun (compiler_name, name, namespace_entry) ->
+            let members =
+              Source.namespace_members ~entry:namespace_entry package.modules
+              |> List.map (fun module_ ->
+                  Source.compiler_basename package.compile_config
+                    module_.Source.name)
+              |> List.sort_uniq String.compare
+            in
+            {
+              key = namespace_map_key package.root;
+              compiler_name;
+              namespace = name;
+              package_name = package.name;
+              package_root = package.root;
+              members;
+            }))
+  in
+  let namespace_maps_by_name = Hashtbl.create (List.length namespace_maps) in
+  List.iter
+    (fun (namespace_map : namespace_map) ->
+      let existing =
+        Hashtbl.find_opt namespace_maps_by_name namespace_map.namespace
+        |> Option.value ~default:[]
+      in
+      Hashtbl.replace namespace_maps_by_name namespace_map.namespace
+        (namespace_map :: existing))
+    namespace_maps;
+  let source_graph_nodes =
+    List.map
+      (fun (node : module_node) ->
+        ( node,
+          resolved_dependencies ~find_module:(Hashtbl.find_opt by_key)
+            ~find_namespace_maps:(Hashtbl.find_opt namespace_maps_by_name)
+            node ))
+      nodes
+  in
+  let build_state =
+    Build_state.create
+      (List.length source_graph_nodes + List.length namespace_maps)
+  in
+  let modified = Option.map (fun entry -> entry.Compile_assets.modified) in
+  List.iter
+    (fun ((node : module_node), _) ->
+      Build_state.add build_state ~key:node.key ~kind:Build_state.Source_module
+        ~last_compiled_cmi:
+          (Compile_assets.cmi compile_assets node.key |> modified)
+        ~last_compiled_cmt:
+          (Compile_assets.cmt compile_assets node.key |> modified))
+    source_graph_nodes;
+  List.iter
+    (fun (namespace_map : namespace_map) ->
+      Build_state.add build_state ~key:namespace_map.key
+        ~kind:Build_state.Namespace_map
+        ~last_compiled_cmi:
+          (Compile_assets.cmi compile_assets namespace_map.compiler_name
+          |> modified)
+        ~last_compiled_cmt:
+          (Compile_assets.cmt compile_assets namespace_map.compiler_name
+          |> modified))
+    namespace_maps;
+  List.iter
+    (fun ((node : module_node), dependencies) ->
+      Build_state.set_dependencies build_state ~key:node.key dependencies)
+    source_graph_nodes;
+  List.iter
+    (fun (namespace_map : namespace_map) ->
+      Build_state.set_dependencies build_state ~key:namespace_map.key
+        namespace_map.members)
+    namespace_maps;
+  {
+    nodes;
+    namespace_maps;
+    build_state;
+    use_existing_ast_paths = !use_existing_ast_paths;
+  }
+
+let find_cycle modules namespace_maps build_state =
+  let nodes_by_key =
+    Hashtbl.create (List.length modules + List.length namespace_maps)
+  in
+  List.iter
+    (fun (node : module_node) ->
+      let key = node.key in
+      let module_name = Source.module_name node.source_path in
+      let display_name =
+        match node.namespace with
+        | Config.Namespace_with_entry {name = namespace; entry}
+          when entry <> module_name ->
+          namespace ^ "." ^ module_name
+        | Config.Namespace namespace -> namespace ^ "." ^ module_name
+        | Config.Namespace_with_entry _ | Config.No_namespace -> module_name
+      in
+      Hashtbl.add nodes_by_key key
+        {
+          key;
+          package_root = node.package_root;
+          source_path = Some node.source_path;
+          display_name;
+        })
+    modules;
+  List.iter
+    (fun (namespace_map : namespace_map) ->
+      let key = namespace_map.key in
+      Hashtbl.add nodes_by_key key
+        {
+          key;
+          package_root = namespace_map.package_root;
+          source_path = None;
+          display_name = namespace_map.compiler_name;
+        })
+    namespace_maps;
+  let graph_nodes =
+    Hashtbl.to_seq_values nodes_by_key
+    |> List.of_seq
+    |> List.sort (fun (first : cycle_node) second ->
+        String.compare first.key second.key)
+    |> List.map (fun (node : cycle_node) ->
+        (node, (Build_state.find_exn build_state node.key).dependencies))
+  in
+  let name ((node : cycle_node), _) = node.key in
+  let blocked_nodes = Graph.cycle_blocked_nodes graph_nodes ~name ~deps:snd in
+  match blocked_nodes with
+  | [] -> None
+  | _ ->
+    let cycle =
+      match Graph.shortest_cycle blocked_nodes ~name ~deps:snd with
+      | Some cycle -> cycle
+      | None ->
+        raise
+          (Project_context.Error
+             "cycle-blocked dependency graph contains no detectable cycle")
+    in
+    Some {cycle; blocked = List.map name blocked_nodes; nodes_by_key}
diff --git a/rewatch-ocaml/module_graph.mli b/rewatch-ocaml/module_graph.mli
new file mode 100644
index 00000000000..e1e904029c0
--- /dev/null
+++ b/rewatch-ocaml/module_graph.mli
@@ -0,0 +1,64 @@
+(** Namespace maps are graph nodes because adding or removing a namespace
+    member can invalidate consumers even when no ordinary source dependency
+    name changes. The graph therefore contains both source modules and these
+    synthetic nodes. *)
+
+type cycle_info = {
+  cycle: string list;
+  blocked: string list;
+  nodes_by_key: (string, cycle_node) Hashtbl.t;
+}
+
+and cycle_node = {
+  key: string;
+  package_root: string;
+  source_path: string option;
+  display_name: string;
+}
+
+type module_node = {
+  key: string;
+  package_name: string;
+  package_root: string;
+  source_path: string;
+  namespace: Config.namespace;
+  visible_packages: (string, unit) Hashtbl.t;
+  mutable raw_dependencies: string list;
+}
+
+type namespace_map = {
+  key: string;
+  compiler_name: string;
+  namespace: string;
+  package_name: string;
+  package_root: string;
+  members: string list;
+}
+
+val namespace_map_key : string -> string
+
+val validate_visible_namespaces :
+  root_config:Config.t -> Package_plan.t list -> unit
+
+val resolved_dependencies :
+  find_module:(string -> module_node option) ->
+  find_namespace_maps:(string -> namespace_map list option) ->
+  module_node ->
+  string list
+
+type initialized = {
+  nodes: module_node list;
+  namespace_maps: namespace_map list;
+  build_state: Build_state.t;
+  use_existing_ast_paths: string list;
+}
+
+val initialize :
+  root_config:Config.t ->
+  package_plans:Package_plan.t list ->
+  compile_assets:Compile_assets.t ->
+  failed_parse_paths:(string, unit) Hashtbl.t ->
+  initialized
+
+val find_cycle :
+  module_node list -> namespace_map list -> Build_state.t -> cycle_info option
diff --git a/rewatch-ocaml/native_watcher.ml b/rewatch-ocaml/native_watcher.ml
new file mode 100644
index 00000000000..ee8f670b75d
--- /dev/null
+++ b/rewatch-ocaml/native_watcher.ml
@@ -0,0 +1,304 @@
+type change_kind = Content | Structural
+type change = {path: string option; kind: change_kind}
+
+type wait_result = Changed of change list | Stopped | Failed of string
+
+type watch_path = {directory: string; recursive: bool}
+
+type watched_directory = {
+  directory: string;
+  identity: string;
+  handle: Luv.FS_event.t;
+}
+
+type t = {
+  loop: Luv.Loop.t;
+  timer: Luv.Timer.t;
+  mutable handles: watched_directory list;
+  mutable changes: change list;
+  mutable stopped: bool;
+  mutable error: string option;
+}
+
+let error_message error =
+  Printf.sprintf "%s: %s" (Luv.Error.err_name error) (Luv.Error.strerror error)
+
+let is_compiler_artifact_directory path =
+  let name = Filename.basename path in
+  (name = "bs" || name = "ocaml")
+  && Filename.basename (Filename.dirname path) = "lib"
+
+let directories_under paths =
+  let visited = Hashtbl.create 64 in
+  let rec walk acc directory =
+    match Platform.canonicalize_path directory with
+    | canonical -> (
+      if Hashtbl.mem visited canonical then acc
+      else
+        let entries =
+          try Some (File_util.directory_entries canonical)
+          with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> None
+        in
+        match entries with
+        | None -> acc
+        | Some entries ->
+          Hashtbl.add visited canonical ();
+          entries
+          |> List.fold_left
+               (fun acc name ->
+                 let path = Filename.concat canonical name in
+                 if is_compiler_artifact_directory path then acc
+                 else
+                   match Unix.stat path with
+                   | stat ->
+                     if stat.Unix.st_kind = Unix.S_DIR then walk acc path
+                     else acc
+                   | exception
+                       Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+                     acc)
+               (canonical :: acc))
+    | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> acc
+  in
+  paths
+  |> List.fold_left
+       (fun directories path ->
+         if path.recursive then walk directories path.directory
+         else
+           match Platform.canonicalize_path path.directory with
+           | canonical -> canonical :: directories
+           | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+             directories)
+       []
+  |> List.sort_uniq String.compare
+
+let close_fs_handles loop handles =
+  let pending = ref 0 in
+  List.iter
+    (fun handle ->
+      ignore (Luv.FS_event.stop handle);
+      if not (Luv.Handle.is_closing handle) then (
+        incr pending;
+        Luv.Handle.close handle (fun () -> decr pending)))
+    handles;
+  while !pending > 0 do
+    ignore (Luv.Loop.run ~loop ~mode:`NOWAIT ())
+  done
+
+let close_timer loop timer =
+  if not (Luv.Handle.is_closing timer) then (
+    let closed = ref false in
+    Luv.Handle.close timer (fun () -> closed := true);
+    while not !closed do
+      ignore (Luv.Loop.run ~loop ~mode:`NOWAIT ())
+    done)
+
+let remove_handles watcher =
+  close_fs_handles watcher.loop
+    (List.map (fun watched -> watched.handle) watcher.handles);
+  watcher.handles <- []
+
+let close watcher =
+  remove_handles watcher;
+  ignore (Luv.Timer.stop watcher.timer);
+  close_timer watcher.loop watcher.timer;
+  ignore (Luv.Loop.close watcher.loop)
+
+let directory_identity directory =
+  try
+    let metadata = Unix.stat directory in
+    Ok (Platform.directory_identity ~path:directory metadata)
+  with (Sys_error _ | Unix.Unix_error _) as error ->
+    Error
+      (Printf.sprintf "could not identify watched directory %s: %s" directory
+         (Printexc.to_string error))
+
+let identify_directories ~directory_identity directories =
+  let rec identify identified = function
+    | [] -> Ok (List.rev identified)
+    | directory :: rest -> (
+      match directory_identity directory with
+      | Ok identity -> identify ((directory, identity) :: identified) rest
+      | Error _ as error -> error)
+  in
+  identify [] directories
+
+let install_handles watcher identified_directories =
+  let existing = Hashtbl.create (List.length watcher.handles) in
+  List.iter
+    (fun watched -> Hashtbl.add existing watched.directory ())
+    watcher.handles;
+  let install (directory, identity) =
+    if not (Hashtbl.mem existing directory) then
+      match Luv.FS_event.init ~loop:watcher.loop () with
+      | Error error -> watcher.error <- Some (error_message error)
+      | Ok handle ->
+        watcher.handles <- {directory; identity; handle} :: watcher.handles;
+        Luv.FS_event.start handle directory (function
+          | Ok (filename, events) ->
+            let path =
+              match filename with
+              | None | Some "" -> None
+              | Some filename ->
+                Some
+                  (if Filename.is_relative filename then
+                     Filename.concat directory filename
+                   else filename)
+            in
+            let kind =
+              if List.mem `CHANGE events && not (List.mem `RENAME events) then
+                Content
+              else Structural
+            in
+            watcher.changes <- {path; kind} :: watcher.changes;
+            Luv.Loop.stop watcher.loop
+          | Error error ->
+            watcher.error <- Some (error_message error);
+            Luv.Loop.stop watcher.loop)
+  in
+  List.iter install identified_directories;
+  match watcher.error with
+  | None -> Ok ()
+  | Some message -> Error message
+
+let create_with_directory_identity ~directory_identity ~paths =
+  match Luv.Loop.init () with
+  | Error error -> Error (error_message error)
+  | Ok loop -> (
+    match Luv.Timer.init ~loop () with
+    | Error error ->
+      ignore (Luv.Loop.close loop);
+      Error (error_message error)
+    | Ok timer -> (
+      let watcher =
+        {loop; timer; handles = []; changes = []; stopped = false; error = None}
+      in
+      let fail message =
+        try
+          close watcher;
+          Error message
+        with cleanup_error ->
+          Error
+            (Printf.sprintf "%s (native watcher cleanup failed: %s)" message
+               (Printexc.to_string cleanup_error))
+      in
+      match
+        try
+          match
+            identify_directories ~directory_identity (directories_under paths)
+          with
+          | Error _ as error -> error
+          | Ok directories -> install_handles watcher directories
+        with error -> Error (Printexc.to_string error)
+      with
+      | Ok () -> Ok watcher
+      | Error message -> fail message))
+
+let create = create_with_directory_identity ~directory_identity
+
+let wait watcher ~keep_running =
+  watcher.stopped <- not (keep_running ());
+  (* Events can arrive while refresh closes handles and pumps the libuv loop.
+     Consume the event that wakes this call, but never erase one already queued
+     by reconciliation before the next wait begins. A failed handle or an
+     external stop must take precedence because another reconciliation could
+     otherwise erase the condition that requires native watch to end. *)
+  let ready_result () =
+    match watcher.error with
+    | Some message -> Some (Failed message)
+    | None when watcher.stopped -> Some Stopped
+    | None when watcher.changes <> [] ->
+      let changes = List.rev watcher.changes in
+      watcher.changes <- [];
+      Some (Changed changes)
+    | None -> None
+  in
+  match ready_result () with
+  | Some result -> result
+  | None -> (
+    let check_running () =
+      try
+        if not (keep_running ()) then (
+          watcher.stopped <- true;
+          Luv.Loop.stop watcher.loop)
+      with error ->
+        watcher.error <- Some (Printexc.to_string error);
+        Luv.Loop.stop watcher.loop
+    in
+    (match Luv.Timer.start ~repeat:100 watcher.timer 100 check_running with
+    | Ok () -> ()
+    | Error error -> watcher.error <- Some (error_message error));
+    while
+      watcher.changes = [] && (not watcher.stopped)
+      && Option.is_none watcher.error
+    do
+      ignore (Luv.Loop.run ~loop:watcher.loop ~mode:`ONCE ())
+    done;
+    ignore (Luv.Timer.stop watcher.timer);
+    match ready_result () with
+    | Some result -> result
+    | None -> Failed "native watcher loop stopped without a wakeup condition")
+
+let drain watcher =
+  (* The first callback stops the loop so the build can start promptly. Pump
+     already-ready callbacks after the debounce window to keep one editor save
+     together without waiting for another build cycle. *)
+  let rec pump remaining =
+    if remaining > 0 then (
+      let changes = watcher.changes in
+      ignore (Luv.Loop.run ~loop:watcher.loop ~mode:`NOWAIT ());
+      if watcher.changes != changes then pump (remaining - 1))
+  in
+  pump 1024;
+  let changes = List.rev watcher.changes in
+  watcher.changes <- [];
+  changes
+
+let watches_directory watcher path =
+  List.exists (fun watched -> watched.directory = path) watcher.handles
+
+let refresh_with_directory_identity ~directory_identity watcher ~paths =
+  (* Only failures from work completed before this refresh are stale. Clear
+     them before pumping close callbacks so a newly reported handle failure is
+     preserved and makes the caller fall back to polling. *)
+  watcher.error <- None;
+  match
+    try Ok (directories_under paths)
+    with (Unix.Unix_error _ | Sys_error _) as error ->
+      Error (Printexc.to_string error)
+  with
+  | Error _ as error -> error
+  | Ok directories -> (
+    match identify_directories ~directory_identity directories with
+    | Error _ as error -> error
+    | Ok identified_directories ->
+      let desired = Hashtbl.create (List.length directories) in
+      List.iter
+        (fun (directory, identity) -> Hashtbl.add desired directory identity)
+        identified_directories;
+      let kept, removed =
+        List.partition
+          (fun watched ->
+            Hashtbl.find_opt desired watched.directory = Some watched.identity)
+          watcher.handles
+      in
+      close_fs_handles watcher.loop
+        (List.map (fun watched -> watched.handle) removed);
+      watcher.handles <- kept;
+      install_handles watcher identified_directories)
+
+let refresh = refresh_with_directory_identity ~directory_identity
+
+module For_test = struct
+  let create_with_directory_identity = create_with_directory_identity
+  let refresh_with_directory_identity = refresh_with_directory_identity
+  let handle_count watcher = List.length watcher.handles
+  let directory_identity watcher directory =
+    watcher.handles
+    |> List.find_opt (fun watched -> watched.directory = directory)
+    |> Option.map (fun watched -> watched.identity)
+
+  let queue_change watcher =
+    watcher.changes <- {path = None; kind = Structural} :: watcher.changes
+
+  let queue_error watcher message = watcher.error <- Some message
+end
diff --git a/rewatch-ocaml/native_watcher.mli b/rewatch-ocaml/native_watcher.mli
new file mode 100644
index 00000000000..089a30fcb02
--- /dev/null
+++ b/rewatch-ocaml/native_watcher.mli
@@ -0,0 +1,37 @@
+type t
+(** A native watcher is only a low-latency notification source. Callers must
+    reconcile notifications against {!Watch_snapshot}; operating systems may
+    merge, duplicate, reorder, or omit the pathname attached to an event. *)
+
+type change_kind = Content | Structural
+type change = {path: string option; kind: change_kind}
+
+type watch_path = {directory: string; recursive: bool}
+
+type wait_result = Changed of change list | Stopped | Failed of string
+
+val create : paths:watch_path list -> (t, string) result
+val is_compiler_artifact_directory : string -> bool
+val wait : t -> keep_running:(unit -> bool) -> wait_result
+val drain : t -> change list
+val watches_directory : t -> string -> bool
+val refresh : t -> paths:watch_path list -> (unit, string) result
+val close : t -> unit
+
+module For_test : sig
+  val create_with_directory_identity :
+    directory_identity:(string -> (string, string) result) ->
+    paths:watch_path list ->
+    (t, string) result
+
+  val refresh_with_directory_identity :
+    directory_identity:(string -> (string, string) result) ->
+    t ->
+    paths:watch_path list ->
+    (unit, string) result
+
+  val handle_count : t -> int
+  val directory_identity : t -> string -> string option
+  val queue_change : t -> unit
+  val queue_error : t -> string -> unit
+end
diff --git a/rewatch-ocaml/output.ml b/rewatch-ocaml/output.ml
new file mode 100644
index 00000000000..2f6e5c923f5
--- /dev/null
+++ b/rewatch-ocaml/output.ml
@@ -0,0 +1,169 @@
+let line_clear = "\027[2K\r"
+
+let log ~minimum ~verbosity ~label message =
+  if verbosity >= minimum then Printf.printf "%s:\n%s\n%!" label message
+
+let debug ~verbosity message = log ~minimum:1 ~verbosity ~label:"DEBUG" message
+let trace ~verbosity message = log ~minimum:2 ~verbosity ~label:"TRACE" message
+let trace_enabled verbosity = verbosity >= 2
+
+let format_step ~color step =
+  if color then Printf.sprintf "\027[1m\027[2m[%s]\027[0m" step
+  else Printf.sprintf "[%s]" step
+
+module Progress = struct
+  type phase = {
+    step: string;
+    symbol: string;
+    label: string;
+    total: int;
+    mutable position: int;
+  }
+
+  type t = {
+    enabled: bool;
+    color: bool;
+    mutable phase: phase option;
+    mutable frame: int;
+    mutable next_draw: float;
+  }
+
+  let frames = [|"⠁"; "⠂"; "⠄"; "⡀"; "⢀"; "⠠"; "⠐"; "⠈"|]
+  let create ~enabled ~color =
+    {enabled; color; phase = None; frame = 0; next_draw = 0.}
+
+  let draw ?(force = false) progress =
+    if progress.enabled then
+      match progress.phase with
+      | None -> ()
+      | Some phase ->
+        let now = Unix.gettimeofday () in
+        if force || now >= progress.next_draw then (
+          let spinner = frames.(progress.frame mod Array.length frames) in
+          progress.frame <- progress.frame + 1;
+          progress.next_draw <- now +. 0.08;
+          Printf.printf "%s%s %s%s... %s %d/%d %!" line_clear
+            (format_step ~color:progress.color phase.step)
+            phase.symbol phase.label spinner phase.position phase.total)
+
+  let start progress ~step ~symbol ~label ~total =
+    progress.phase <- Some {step; symbol; label; total; position = 0};
+    progress.frame <- 0;
+    progress.next_draw <- 0.;
+    draw ~force:true progress
+
+  let advance progress =
+    Option.iter
+      (fun phase ->
+        if phase.position < phase.total then
+          phase.position <- phase.position + 1)
+      progress.phase;
+    draw progress
+
+  let tick progress = draw progress
+  let finish progress = progress.phase <- None
+
+  let debug progress ~verbosity message =
+    if verbosity >= 1 then (
+      let redraw = progress.enabled && Option.is_some progress.phase in
+      if redraw then Printf.printf "%s%!" line_clear;
+      log ~minimum:1 ~verbosity ~label:"DEBUG" message;
+      if redraw then draw ~force:true progress)
+
+  let start_grouped progress ~step ~symbol ~label groups =
+    let remaining = Hashtbl.create (List.length groups) in
+    List.iter
+      (fun group ->
+        let count =
+          Hashtbl.find_opt remaining group |> Option.value ~default:0
+        in
+        Hashtbl.replace remaining group (count + 1))
+      groups;
+    start progress ~step ~symbol ~label ~total:(Hashtbl.length remaining);
+    let groups = Array.of_list groups in
+    fun index ->
+      let group = groups.(index) in
+      let count = Hashtbl.find remaining group - 1 in
+      Hashtbl.replace remaining group count;
+      if count = 0 then advance progress
+end
+
+let yellow text =
+  if String.starts_with ~prefix:"\n" text then
+    "\n\027[33m" ^ String.sub text 1 (String.length text - 1) ^ "\027[0m"
+  else "\027[33m" ^ text ^ "\027[0m"
+
+let colors_enabled_with ~getenv ~win32 ~interactive =
+  let nonzero name default =
+    match getenv name with
+    | None -> default
+    | Some value -> value <> "0"
+  in
+  let terminal_supports_color =
+    interactive
+    &&
+    if win32 then true
+    else
+      Option.is_none (getenv "NO_COLOR")
+      &&
+      match getenv "TERM" with
+      | Some term -> term <> "dumb"
+      | None -> false
+  in
+  (terminal_supports_color && nonzero "CLICOLOR" true)
+  || nonzero "CLICOLOR_FORCE" false
+
+let colors_enabled ~interactive =
+  colors_enabled_with ~getenv:Sys.getenv_opt
+    ~win32:Platform.terminal_supports_color_without_term ~interactive
+
+let cleanup_message ~color ~step ~cleaned ~total ~seconds =
+  Printf.sprintf "%s%s %sCleaned %d/%d in %.2fs" line_clear
+    (format_step ~color step) Platform.clean_symbol cleaned total seconds
+
+let compiler_cleanup_message ~color ~step =
+  Printf.sprintf "%s%s %sCleaned previous build due to compiler update"
+    line_clear (format_step ~color step) Platform.clean_symbol
+
+let cleaning_command_message ~color ~step target =
+  Printf.sprintf "%s%s %sCleaning %s..." line_clear (format_step ~color step)
+    Platform.clean_symbol target
+
+let cleaned_command_message ~color ~step ~target ~seconds =
+  Printf.sprintf "%s%s %sCleaned %s in %.2fs" line_clear
+    (format_step ~color step) Platform.clean_symbol target seconds
+
+let parsing_message ~color ~step ~count ~seconds =
+  Printf.sprintf "%s%s %sParsed %d source files in %.2fs" line_clear
+    (format_step ~color step) Platform.parse_symbol count seconds
+
+let parsing_failed_message ~color ~step ~seconds =
+  Printf.sprintf "%s%s %sError parsing source files in %.2fs" line_clear
+    (format_step ~color step) Platform.error_symbol seconds
+
+let compiling_message ~color ~step ~count ~seconds =
+  Printf.sprintf "%s%s %sCompiled %d modules in %.2fs" line_clear
+    (format_step ~color step) Platform.build_symbol count seconds
+
+let compilation_failed_message ~color ~step ~count ~seconds =
+  Printf.sprintf "%s%s %sCompiled %d modules in %.2fs" line_clear
+    (format_step ~color step) Platform.error_symbol count seconds
+
+type compilation_label = Standard | Initial | Incremental
+
+let finished_compilation_message ~label ~warnings ~seconds =
+  let status =
+    if warnings then Platform.warning_symbol else Platform.success_symbol
+  in
+  let kind =
+    match label with
+    | Standard -> ""
+    | Initial -> "initial "
+    | Incremental -> "incremental "
+  in
+  let warning_suffix = if warnings then " with warnings" else "" in
+  Printf.sprintf "%s%sFinished %scompilation%s in %.2fs" line_clear status kind
+    warning_suffix seconds
+
+let should_clear_screen ~clear_screen ~show_progress ~interactive =
+  clear_screen && show_progress && interactive
diff --git a/rewatch-ocaml/output.mli b/rewatch-ocaml/output.mli
new file mode 100644
index 00000000000..78b22430bd3
--- /dev/null
+++ b/rewatch-ocaml/output.mli
@@ -0,0 +1,61 @@
+val debug : verbosity:int -> string -> unit
+val trace : verbosity:int -> string -> unit
+val trace_enabled : int -> bool
+
+module Progress : sig
+  type t
+
+  val create : enabled:bool -> color:bool -> t
+
+  val start :
+    t -> step:string -> symbol:string -> label:string -> total:int -> unit
+
+  val advance : t -> unit
+  val tick : t -> unit
+  val finish : t -> unit
+  val debug : t -> verbosity:int -> string -> unit
+
+  val start_grouped :
+    t -> step:string -> symbol:string -> label:string -> 'a list -> int -> unit
+end
+
+val yellow : string -> string
+
+val colors_enabled_with :
+  getenv:(string -> string option) -> win32:bool -> interactive:bool -> bool
+
+val colors_enabled : interactive:bool -> bool
+
+val cleanup_message :
+  color:bool ->
+  step:string ->
+  cleaned:int ->
+  total:int ->
+  seconds:float ->
+  string
+
+val compiler_cleanup_message : color:bool -> step:string -> string
+val cleaning_command_message : color:bool -> step:string -> string -> string
+
+val cleaned_command_message :
+  color:bool -> step:string -> target:string -> seconds:float -> string
+
+val parsing_message :
+  color:bool -> step:string -> count:int -> seconds:float -> string
+
+val parsing_failed_message :
+  color:bool -> step:string -> seconds:float -> string
+
+val compiling_message :
+  color:bool -> step:string -> count:int -> seconds:float -> string
+
+val compilation_failed_message :
+  color:bool -> step:string -> count:int -> seconds:float -> string
+
+type compilation_label = Standard | Initial | Incremental
+
+val finished_compilation_message :
+  label:compilation_label -> warnings:bool -> seconds:float -> string
+
+val should_clear_screen :
+  clear_screen:bool -> show_progress:bool -> interactive:bool -> bool
diff --git a/rewatch-ocaml/package_build.ml b/rewatch-ocaml/package_build.ml
new file mode 100644
index 00000000000..3dc61d22b15
--- /dev/null
+++ b/rewatch-ocaml/package_build.ml
@@ -0,0 +1,53 @@
+exception Error = Project_context.Error
+
+let prepare_removed_modules ~(package : Package_plan.t)
+    (attempt : Build_attempt.t) =
+  let cleanup =
+    match Build_attempt.find_cleanup_result attempt package.root with
+    | Some result -> result
+    | None ->
+      raise (Error ("Package cleanup was not prepared for " ^ package.root))
+  in
+  let removed = Hashtbl.create (List.length cleanup.removed_modules) in
+  List.iter
+    (fun module_name ->
+      Hashtbl.replace removed module_name ();
+      Hashtbl.replace attempt.removed_modules module_name ())
+    cleanup.removed_modules;
+  removed
+
+let rec prepare_tree ~seen ~(package : Package_plan.t) ~prepared ~watch
+    ~(attempt : Build_attempt.t) =
+  let root = package.root in
+  Hashtbl.replace seen root ();
+  attempt.diagnostics <-
+    List.rev_append
+      (Package_diagnostics.for_package ~is_local:package.is_local package.config)
+      attempt.diagnostics;
+  List.iter
+    (fun (dependency : Package_plan.dependency) ->
+      let root = dependency.directory in
+      if not (Hashtbl.mem seen root) then
+        match Build_session.find_package_plan attempt.session root with
+        | None -> ()
+        | Some dependency_package ->
+          prepare_tree ~seen ~package:dependency_package ~prepared ~watch
+            ~attempt)
+    package.dependencies;
+  File_util.ensure_dir package.build_dir;
+  File_util.ensure_dir package.ocaml_dir;
+  Compiler_log.initialize root;
+  Build_attempt.mark_log_initialized attempt root;
+  let prepared_package =
+    match Hashtbl.find_opt prepared.Build_session.packages root with
+    | Some package -> package
+    | None ->
+      raise (Error ("Package build was not prepared for " ^ package.root))
+  in
+  let removed_module_names = prepare_removed_modules ~package attempt in
+  let parse_dirty_modules =
+    Package_parse.run ~package ~prepared ~prepared_package ~attempt
+      ~removed_module_names
+  in
+  Package_compilation.prepare ~package ~prepared ~prepared_package ~attempt
+    ~watch ~removed_module_names ~parse_dirty_modules
diff --git a/rewatch-ocaml/package_build.mli b/rewatch-ocaml/package_build.mli
new file mode 100644
index 00000000000..bcdbc8f8635
--- /dev/null
+++ b/rewatch-ocaml/package_build.mli
@@ -0,0 +1,11 @@
+val prepare_tree :
+  seen:(string, unit) Hashtbl.t ->
+  package:Package_plan.t ->
+  prepared:Build_session.prepared ->
+  watch:bool ->
+  attempt:Build_attempt.t ->
+  unit
+(** Package preparation remains recursive because dependencies must register
+    their compiler work before a consumer can be scheduled. [seen] prevents a
+    package graph cycle or duplicate dependency edge from preparing a package
+    twice within the attempt. *)
diff --git a/rewatch-ocaml/package_compilation.ml b/rewatch-ocaml/package_compilation.ml
new file mode 100644
index 00000000000..c48cbf6c23a
--- /dev/null
+++ b/rewatch-ocaml/package_compilation.ml
@@ -0,0 +1,205 @@
+let prepare ~(package : Package_plan.t) ~(prepared : Build_session.prepared)
+    ~(prepared_package : Package_plan.compilation) ~(attempt : Build_attempt.t)
+    ~watch ~removed_module_names ~parse_dirty_modules =
+  let root = package.root in
+  let is_local = package.is_local in
+  let config = package.compile_config in
+  let build_state = prepared.build_state in
+  let compile_assets = prepared.compile_assets in
+  let build_dir = package.build_dir in
+  let ocaml_dir = package.ocaml_dir in
+  let modules = package.modules in
+  let cleanup =
+    match Build_attempt.find_cleanup_result attempt root with
+    | Some result -> result
+    | None ->
+      raise
+        (Project_context.Error ("Package cleanup was not prepared for " ^ root))
+  in
+  let module_is_dirty module_ (state : Build_state.module_) =
+    let global_key = Source.compiler_basename config module_.Source.name in
+    let module_name = Source.module_name module_.Source.implementation in
+    let source_artifact_is_pending path =
+      let source = Filename.concat root path in
+      match
+        (Compile_assets.ast compile_assets source, state.last_compiled_cmt)
+      with
+      | Some ast, Some cmt_time -> ast.modified >= cmt_time
+      | Some _, None -> true
+      | None, _ -> false
+    in
+    let outputs_exist =
+      List.for_all
+        (fun spec ->
+          Hashtbl.mem cleanup.present_public_outputs
+            (Build_artifacts.generated_js_path config
+               module_.Source.implementation spec))
+        config.package_specs
+    in
+    let raw_dependencies =
+      match Build_session.find_global_module attempt.session global_key with
+      | Some node -> node.raw_dependencies
+      | None ->
+        raise
+          (Project_context.Error
+             ("Build module was not prepared for " ^ global_key))
+    in
+    let dependency_is_newer dependency =
+      let dependency_state = Build_state.find_exn build_state dependency in
+      Build_state.dependency_tree_compiled_after
+        ~namespace_freshness:attempt.namespace_freshness build_state state
+        dependency_state
+    in
+    Hashtbl.mem parse_dirty_modules module_.Source.name
+    || Hashtbl.mem removed_module_names module_name
+    || List.exists source_artifact_is_pending
+         (module_.Source.implementation
+         :: Option.to_list module_.Source.interface)
+    || (not (Build_state.has_complete_compile_assets state))
+    || (not outputs_exist)
+    || List.exists (Hashtbl.mem removed_module_names) raw_dependencies
+    || List.exists
+         (fun dependency -> Hashtbl.mem attempt.removed_modules dependency)
+         raw_dependencies
+    || List.exists dependency_is_newer state.dependencies
+  in
+  if attempt.freshness_mode = Build_attempt.Initialize_freshness then
+    List.iter
+      (fun module_ ->
+        let key = Source.compiler_basename config module_.Source.name in
+        let state = Build_state.find_exn build_state key in
+        state.compile_dirty <-
+          state.compile_dirty || module_is_dirty module_ state)
+      modules;
+  if not (Build_attempt.has_parse_error attempt.parse_messages) then (
+    attempt.parsed <- attempt.parsed + Hashtbl.length parse_dirty_modules;
+    let compile_warning_paths = Hashtbl.create 8 in
+    let prepare_outputs module_ =
+      let path = module_.Source.implementation in
+      List.iter
+        (fun spec ->
+          let output = Build_artifacts.generated_js_path config path spec in
+          File_util.ensure_dir (Filename.dirname output))
+        config.package_specs
+    in
+    let compile_process module_ ~source_kind path =
+      Compiler_process.compile_job ~bsc:prepared.compiler_context.bsc_path
+        ~build_dir ~config
+        ~common_args:
+          (if module_.Source.is_dev then
+             prepared_package.development_common_args
+           else prepared_package.regular_common_args)
+        module_ ~source_kind path
+    in
+    let record_published_outputs ~source_kind path =
+      match source_kind with
+      | Source.Interface -> ()
+      | Source.Implementation ->
+        List.iter
+          (fun spec ->
+            let output = Build_artifacts.generated_js_path config path spec in
+            [output; output ^ ".map"]
+            |> List.iter (fun path ->
+                if File_util.is_regular_file path then
+                  Hashtbl.replace cleanup.present_public_outputs path ()))
+          config.package_specs
+    in
+    let candidates =
+      List.filter_map
+        (fun module_ ->
+          let key = Source.compiler_basename config module_.Source.name in
+          let state = Build_state.find_exn build_state key in
+          if Hashtbl.mem attempt.blocked_modules key then None
+          else
+            let cmi_path =
+              Filename.concat ocaml_dir
+                (Source.compiler_asset_basename config
+                   module_.Source.implementation
+                ^ ".cmi")
+            in
+            let warning_paths =
+              module_.Source.implementation
+              :: Option.to_list module_.Source.interface
+              |> List.map (Filename.concat config.root)
+            in
+            let make () =
+              Compiler_scheduler.create ~key ~dependencies:state.dependencies
+                ~source:module_ ~state ~cmi_path
+                ~prepare:(fun () -> prepare_outputs module_)
+                ~compile:(fun ~source_kind path ->
+                  compile_process module_ ~source_kind path)
+                ~publish:(fun ~source_kind path result ->
+                  Compiler_process.publish ~build_dir ~ocaml_dir ~is_local
+                    ~config ~source_kind path result)
+                ~record_published_outputs
+                ~post_build:(Compiler_process.post_build_tasks config)
+                ~package_root:config.root ~is_local
+                ~mark_warning:(fun _path ->
+                  module_.Source.implementation
+                  :: Option.to_list module_.Source.interface
+                  |> List.iter (fun path ->
+                      Hashtbl.replace compile_warning_paths
+                        (Build_artifacts.published_ast_path ~ocaml_dir path)
+                        ()))
+            in
+            Some (Compiler_scheduler.candidate ~key ~state ~warning_paths ~make))
+        modules
+    in
+    Config.namespace_compiler_name config.namespace
+    |> Option.iter (fun compiler_name ->
+        let namespace_map =
+          Build_session.find_namespace_map attempt.session
+            (Module_graph.namespace_map_key root)
+        in
+        let namespace_state =
+          Build_state.find_exn build_state namespace_map.key
+        in
+        let package_dirty =
+          List.exists Compiler_scheduler.candidate_requires_compile candidates
+        in
+        if
+          package_dirty || namespace_state.compile_dirty
+          || attempt.freshness_mode = Build_attempt.Initialize_freshness
+        then
+          Compiler_process.namespace_task
+            ~bsc:prepared.compiler_context.bsc_path
+            ~runtime:prepared.compiler_context.runtime_path ~build_dir
+            ~ocaml_dir
+            ~entry:(Config.namespace_entry config.namespace)
+            ~package_dirty ~force:namespace_state.compile_dirty compiler_name
+            modules
+          |> Option.iter (fun namespace_task ->
+              namespace_state.compile_dirty <- true;
+              let cmi_path =
+                Filename.concat ocaml_dir (compiler_name ^ ".cmi")
+              in
+              let finish result =
+                if not (Process.succeeded result) then
+                  ignore (namespace_task.Compiler_scheduler.publish result)
+                else
+                  match
+                    Compiler_scheduler.capture_publication (fun () ->
+                        namespace_task.Compiler_scheduler.publish result)
+                  with
+                  | Compiler_scheduler.Published {cmi_change; _} ->
+                    Build_state.record_published_cmi build_state ~compile_assets
+                      namespace_state ~path:cmi_path cmi_change;
+                    let cmt_path =
+                      Filename.concat ocaml_dir (compiler_name ^ ".cmt")
+                    in
+                    Build_state.record_successful_compile ~compile_assets
+                      namespace_state ~cmt_path
+                  | Compiler_scheduler.Failed_after_cmi_publication
+                      {error; cmi_change} ->
+                    Build_state.record_published_cmi build_state ~compile_assets
+                      namespace_state ~path:cmi_path cmi_change;
+                    raise error
+              in
+              Build_attempt.add_namespace_job attempt
+                Build_attempt.{job = namespace_task.job; finish}));
+    Build_attempt.add_compile_candidates attempt candidates;
+    Build_attempt.register_cleanup attempt (fun () ->
+        if not watch then
+          Hashtbl.iter
+            (fun path () -> File_util.remove_file path)
+            compile_warning_paths))
diff --git a/rewatch-ocaml/package_compilation.mli b/rewatch-ocaml/package_compilation.mli
new file mode 100644
index 00000000000..0e7efeebcc7
--- /dev/null
+++ b/rewatch-ocaml/package_compilation.mli
@@ -0,0 +1,13 @@
+val prepare :
+  package:Package_plan.t ->
+  prepared:Build_session.prepared ->
+  prepared_package:Package_plan.compilation ->
+  attempt:Build_attempt.t ->
+  watch:bool ->
+  removed_module_names:(string, unit) Hashtbl.t ->
+  parse_dirty_modules:(string, unit) Hashtbl.t ->
+  unit
+(** Compilation preparation turns one stable package plan into the parse,
+    namespace, and compiler work owned by the current attempt. It records
+    pending work separately from scheduler eligibility so cycle-blocked modules
+    remain dirty for a later recovery build. *)
diff --git a/rewatch-ocaml/package_diagnostics.ml b/rewatch-ocaml/package_diagnostics.ml
new file mode 100644
index 00000000000..ffb71567d47
--- /dev/null
+++ b/rewatch-ocaml/package_diagnostics.ml
@@ -0,0 +1,125 @@
+let member name = function
+  | `Assoc fields -> List.assoc_opt name fields
+  | _ -> None
+
+let last_member name = function
+  | `Assoc fields -> List.assoc_opt name (List.rev fields)
+  | _ -> None
+
+let package_name package_root =
+  let path = Filename.concat package_root "package.json" in
+  try
+    if not (File_util.exists path) then Ok None
+    else
+      let json = Yojson.Safe.from_file path in
+      match last_member "name" json with
+      | Some (`String name) -> Ok (Some name)
+      | Some _ | None -> Ok None
+  with
+  | Sys_error message -> Error ("Could not read package.json: " ^ message)
+  | Unix.Unix_error (error, _, _) ->
+    Error ("Could not read package.json: " ^ Unix.error_message error)
+  | Yojson.Json_error message ->
+    Error ("Could not parse package.json: " ^ message)
+
+let url_value = function
+  | `String value -> Some value
+  | `Assoc fields -> (
+    match List.assoc_opt "url" fields with
+    | Some (`String value) -> Some value
+    | _ -> None)
+  | _ -> None
+
+let remove_prefix prefix value =
+  if String.starts_with ~prefix value then
+    String.sub value (String.length prefix)
+      (String.length value - String.length prefix)
+  else value
+
+let remove_suffix suffix value =
+  if String.ends_with ~suffix value then
+    String.sub value 0 (String.length value - String.length suffix)
+  else value
+
+let issues_url_from_repository repository =
+  let cleaned = repository |> remove_prefix "git+" |> remove_suffix ".git" in
+  if
+    (not (String.contains cleaned '@'))
+    && not (String_util.contains cleaned "://")
+  then
+    let path = remove_prefix "github:" cleaned in
+    "https://github.com/" ^ path ^ "/issues"
+  else cleaned ^ "/issues"
+
+let issue_tracker_url package_root =
+  let path = Filename.concat package_root "package.json" in
+  try
+    let json = Yojson.Safe.from_file path in
+    match Option.bind (member "bugs" json) url_value with
+    | Some url -> Some url
+    | None ->
+      Option.bind (member "repository" json) url_value
+      |> Option.map issues_url_from_repository
+  with Sys_error _ | Yojson.Json_error _ -> None
+
+let for_package ~is_local (config : Config.t) =
+  if is_local then config.diagnostics
+  else if config.deprecation_diagnostics = [] then []
+  else
+    let report_suffix =
+      issue_tracker_url config.root
+      |> Option.map (fun url ->
+          "\nPlease report this to the package maintainer: " ^ url)
+      |> Option.value ~default:""
+    in
+    List.map
+      (fun diagnostic -> diagnostic ^ report_suffix)
+      config.deprecation_diagnostics
+
+let report_missing_source_folder (config : Config.t) path =
+  let prefix = Filename.concat config.root "" in
+  let relative =
+    if String.starts_with ~prefix path then
+      String.sub path (String.length prefix)
+        (String.length path - String.length prefix)
+    else path
+  in
+  Printf.eprintf
+    "ERROR:\n\
+     Could not read folder: %S. Specified in dependency: %s, located %S...\n\
+     %!"
+    relative config.name config.root
+
+let report_missing_sources ~is_root (config : Config.t) =
+  if (not is_root) && not config.sources_defined then
+    Printf.eprintf
+      "WARN:\n\
+       Package '%s' has not defined any sources, but is not the root package. \
+       This is likely a mistake. It is located: %s\n\
+       %!"
+      config.name config.root
+
+let package_identity ~report_diagnostics (config : Config.t) =
+  match package_name config.root with
+  | Error message ->
+    raise (Project_context.Error ("Could not initialize build: " ^ message))
+  | Ok (Some package_name) when package_name <> config.name ->
+    if report_diagnostics then
+      Printf.eprintf
+        "WARN:\n\n\
+         Package name mismatch for %s:\n\
+         The package.json name is %S, while the rescript.json name is %S\n\
+         This inconsistency will cause issues with package resolution.\n\n\
+         %!"
+        config.root package_name config.name;
+    package_name
+  | Ok (Some package_name) -> package_name
+  | Ok None -> config.name
+
+let validate_metadata config =
+  ignore (package_identity ~report_diagnostics:true config)
+
+module For_test = struct
+  let package_name = package_name
+  let issue_tracker_url = issue_tracker_url
+end
diff --git a/rewatch-ocaml/package_diagnostics.mli b/rewatch-ocaml/package_diagnostics.mli
new file mode 100644
index 00000000000..8bcccf0057f
--- /dev/null
+++ b/rewatch-ocaml/package_diagnostics.mli
@@ -0,0 +1,10 @@
+val for_package : is_local:bool -> Config.t -> string list
+val report_missing_source_folder : Config.t -> string -> unit
+val report_missing_sources : is_root:bool -> Config.t -> unit
+val package_identity : report_diagnostics:bool -> Config.t -> string
+val validate_metadata : Config.t -> unit
+
+module For_test : sig
+  val package_name : string -> (string option, string) result
+  val issue_tracker_url : string -> string option
+end
diff --git a/rewatch-ocaml/package_graph.ml b/rewatch-ocaml/package_graph.ml
new file mode 100644
index 00000000000..9e866cba602
--- /dev/null
+++ b/rewatch-ocaml/package_graph.ml
@@ -0,0 +1,183 @@
+exception Error = Project_context.Error
+
+let with_gentype_source_dirs directories (config : Config.t) =
+  if config.gentype_args = [] then config
+  else
+    {
+      config with
+      gentype_args =
+        config.gentype_args
+        @ List.concat_map
+            (fun directory -> ["-bs-gentype-source-dir"; directory])
+            directories;
+    }
+
+let dependent_is_allowed allowed_dependents dependent =
+  Option.fold ~none:true
+    ~some:(fun allowed -> List.mem dependent allowed)
+    allowed_dependents
+
+let discover ~(root_config : Config.t) ~prod ~features ~warn_error ~filter
+    ~(attempt : Build_attempt.t) =
+  let resolution = Package_resolution.create root_config in
+  let unallowed_dependencies = ref [] in
+  let discovered =
+    Package_traversal.discover ~root_config ~prod ~features ~resolution
+  in
+  List.iter
+    (fun (package : Package_traversal.package) ->
+      Output.debug ~verbosity:attempt.verbosity
+        ("Parsing package: " ^ package.config.name);
+      List.iter
+        (fun (resolved : Package_traversal.resolved) ->
+          let dependency = resolved.dependency in
+          if
+            not
+              (dependent_is_allowed dependency.config.allowed_dependents
+                 package.config.name)
+          then
+            unallowed_dependencies :=
+              ( package.config.name,
+                Package_traversal.dependency_kind_name resolved.request.kind,
+                dependency.config.name )
+              :: !unallowed_dependencies)
+        package.dependencies)
+    discovered.packages;
+  (if !unallowed_dependencies <> [] then
+     let details =
+       !unallowed_dependencies |> List.sort_uniq compare
+       |> List.map (fun (dependent, kind, dependency) ->
+           Printf.sprintf "%s %s: %s" dependent kind dependency)
+       |> String.concat "\n"
+     in
+     raise
+       (Error
+          ("The following packages use dependencies that do not allow them:\n"
+         ^ details
+         ^ "\nUpdate allowed-dependents in the dependency rescript.json files."
+          )));
+  let packages_by_root = Hashtbl.create (List.length discovered.packages) in
+  List.iter
+    (fun (package : Package_traversal.package) ->
+      Hashtbl.add packages_by_root package.config.root package)
+    discovered.packages;
+  let visited = Hashtbl.create 32 in
+  let package_plans = ref [] in
+  let rec visit root =
+    if not (Hashtbl.mem visited root) then (
+      Hashtbl.add visited root ();
+      let discovered_package = Hashtbl.find packages_by_root root in
+      let is_local = discovered_package.Package_traversal.is_local in
+      let features =
+        match Package_traversal.find_feature_selection discovered root with
+        | Some features ->
+          Package_traversal.feature_selection_to_option features
+        | None -> None
+      in
+      let config = discovered_package.config in
+      Package_diagnostics.report_missing_sources
+        ~is_root:(root = root_config.root) config;
+      let config =
+        match warn_error with
+        | None -> config
+        | Some value -> {config with warning_flags = ["-warn-error"; value]}
+      in
+      let dependencies =
+        List.map
+          (fun (resolved : Package_traversal.resolved) ->
+            let request = resolved.request in
+            Package_plan.
+              {
+                declaration = request.declaration;
+                directory = resolved.dependency.directory;
+                kind = request.kind;
+              })
+          discovered_package.dependencies
+      in
+      List.iter
+        (fun (dependency : Package_plan.dependency) ->
+          visit dependency.directory)
+        dependencies;
+      let discovery =
+        Output.debug ~verbosity:attempt.verbosity
+          ("Building source file-tree for package: " ^ config.name);
+        Source.discover_with_inventory config
+          ~prod:(Package_traversal.source_discovery_prod ~prod ~is_local)
+          ~features
+          ~filter:(if root = root_config.root then filter else None)
+          ~on_missing:(Package_diagnostics.report_missing_source_folder config)
+          ~on_orphan:(fun path ->
+            Printf.eprintf
+              "\027[2K\r No implementation file found for interface file \
+               (skipping): %s\n\
+               %!"
+              path)
+          ~display_root:root_config.root
+      in
+      let modules = discovery.modules in
+      let owns_outputs =
+        root <> root_config.root && Compiler_info.owns_outputs config
+      in
+      let compile_config =
+        let config = with_gentype_source_dirs discovery.gentype_dirs config in
+        let inherited = Config.with_root_options config root_config in
+        let output_config =
+          if owns_outputs then
+            {
+              inherited with
+              package_specs = config.package_specs;
+              suffix = config.suffix;
+            }
+          else inherited
+        in
+        output_config |> Compiler_args.with_local_warning_policy ~is_local
+      in
+      let build_dir = Build_artifacts.lib_path root "bs" in
+      let ocaml_dir = Build_artifacts.lib_path root "ocaml" in
+      File_util.ensure_dir build_dir;
+      let source_mtimes =
+        Hashtbl.create (List.length discovery.source_mtimes)
+      in
+      List.iter
+        (fun (path, modified) -> Hashtbl.replace source_mtimes path modified)
+        discovery.source_mtimes;
+      let package : Package_plan.t =
+        Package_plan.
+          {
+            name = discovered_package.name;
+            root;
+            build_owner = (if owns_outputs then root else root_config.root);
+            is_local;
+            config;
+            compile_config;
+            build_dir;
+            ocaml_dir;
+            dependencies;
+            gentype_dependency_args =
+              Compiler_args.gentype_dependency_args_from_paths compile_config
+                (List.map
+                   (fun (dependency : Package_plan.dependency) ->
+                     (dependency.declaration, dependency.directory))
+                   dependencies);
+            modules;
+            source_mtimes;
+            source_files = discovery.inventory_files;
+            present_source_files = discovery.present_files;
+          }
+      in
+      Build_session.add_package_plan attempt.session package;
+      List.iter
+        (fun module_ ->
+          module_.Source.implementation
+          :: Option.to_list module_.Source.interface
+          |> List.iter (fun relative_path ->
+              let absolute_path = Filename.concat root relative_path in
+              Build_session.add_source_reference attempt.session
+                (Platform.normalize_path_for_comparison absolute_path)
+                Build_session.
+                  {package_root = root; module_; relative_path; absolute_path}))
+        modules;
+      package_plans := package :: !package_plans)
+  in
+  visit root_config.root;
+  !package_plans
diff --git a/rewatch-ocaml/package_graph.mli b/rewatch-ocaml/package_graph.mli
new file mode 100644
index 00000000000..20647927aa8
--- /dev/null
+++ b/rewatch-ocaml/package_graph.mli
@@ -0,0 +1,12 @@
+val discover :
+  root_config:Config.t ->
+  prod:bool ->
+  features:string list option ->
+  warn_error:string option ->
+  filter:Source_filter.t option ->
+  attempt:Build_attempt.t ->
+  Package_plan.t list
+(** Package discovery resolves each dependency edge once, aggregates feature
+    requests across all incoming edges, and then constructs stable package
+    plans. Commands that need a different file projection reuse the lower-level
+    {!Package_traversal} graph. *)
diff --git a/rewatch-ocaml/package_parse.ml b/rewatch-ocaml/package_parse.ml
new file mode 100644
index 00000000000..ac7e31f0821
--- /dev/null
+++ b/rewatch-ocaml/package_parse.ml
@@ -0,0 +1,113 @@
+let run ~(package : Package_plan.t) ~(prepared : Build_session.prepared)
+    ~(prepared_package : Package_plan.compilation) ~(attempt : Build_attempt.t)
+    ~removed_module_names =
+  let root = package.root in
+  let is_local = package.is_local in
+  let config = package.compile_config in
+  let build_state = prepared.build_state in
+  let compile_assets = prepared.compile_assets in
+  let build_dir = package.build_dir in
+  let ocaml_dir = package.ocaml_dir in
+  let dirty_parse_paths =
+    prepared_package.parse_paths
+    |> List.filter (fun path ->
+        let forced =
+          Hashtbl.mem attempt.preliminary_parses (Filename.concat root path)
+        in
+        match attempt.freshness_mode with
+        | Build_attempt.Reuse_freshness -> forced
+        | Build_attempt.Initialize_freshness ->
+          Hashtbl.mem removed_module_names (Source.module_name path)
+          || forced
+          || Build_freshness.source_is_not_older_than_ast compile_assets ~root
+               ~source_mtimes:package.source_mtimes path)
+  in
+  let dirty_parse_path_set = Hashtbl.create (List.length dirty_parse_paths) in
+  List.iter
+    (fun path -> Hashtbl.replace dirty_parse_path_set path ())
+    dirty_parse_paths;
+  List.iter
+    (fun path ->
+      Filename.concat root path |> Platform.normalize_path_for_comparison
+      |> Build_session.mark_parse_pending attempt.session)
+    dirty_parse_paths;
+  let dirty_modules = Hashtbl.create (List.length package.modules) in
+  List.iter
+    (fun module_ ->
+      let paths =
+        module_.Source.implementation :: Option.to_list module_.Source.interface
+      in
+      if List.exists (Hashtbl.mem dirty_parse_path_set) paths then (
+        Hashtbl.replace dirty_modules module_.Source.name ();
+        let key = Source.compiler_basename config module_.Source.name in
+        (Build_state.find_exn build_state key).compile_dirty <- true))
+    package.modules;
+  let parse_paths_to_run =
+    dirty_parse_paths
+    |> List.filter (fun path ->
+        not (Hashtbl.mem attempt.preliminary_parses (Filename.concat root path)))
+  in
+  let parsed =
+    List.map2
+      (fun path result -> (path, Build_attempt.preliminary_parse result))
+      parse_paths_to_run
+      (Process.run_parallel_map ?poll:attempt.process_poll parse_paths_to_run
+         ~job:
+           (Compiler_process.parse_job ~bsc:prepared.compiler_context.bsc_path
+              ~build_dir ~config))
+    @ (dirty_parse_paths
+      |> List.filter_map (fun path ->
+          Hashtbl.find_opt attempt.preliminary_parses
+            (Filename.concat root path)
+          |> Option.map (fun result -> (path, result))))
+  in
+  List.iter
+    (fun (path, result) ->
+      let absolute_path = Filename.concat root path in
+      let pending_path = Platform.normalize_path_for_comparison absolute_path in
+      let publish_successful_parse stderr =
+        let stderr =
+          if is_local then stderr
+          else Compiler_process.retain_critical_external_warnings stderr
+        in
+        if stderr <> "" then (
+          attempt.had_warnings <- true;
+          Compiler_log.append root stderr;
+          attempt.parse_messages <-
+            Build_attempt.Parse_warning stderr :: attempt.parse_messages);
+        let ast = Source.ast_path path in
+        if is_local && stderr <> "" then
+          Build_attempt.register_cleanup attempt (fun () ->
+              let path = Filename.concat ocaml_dir (Filename.basename ast) in
+              File_util.remove_file path;
+              Compile_assets.refresh_ast compile_assets ~source:absolute_path
+                ~path);
+        let published_ast =
+          Build_artifacts.published_ast_path ~ocaml_dir path
+        in
+        File_util.copy_existing_file ~ensure_parent:false
+          (Filename.concat build_dir ast)
+          published_ast;
+        Compile_assets.refresh_ast compile_assets ~source:absolute_path
+          ~path:published_ast;
+        File_util.copy_existing_file ~ensure_parent:false
+          (Filename.concat config.root path)
+          (Filename.concat ocaml_dir (Filename.basename path));
+        if is_local && stderr <> "" then
+          Build_session.mark_parse_pending attempt.session pending_path
+        else Build_session.clear_parse_pending attempt.session pending_path
+      in
+      match result with
+      | Build_attempt.Parse_failed {stdout; stderr} ->
+        Build_session.mark_parse_pending attempt.session pending_path;
+        let output =
+          Printf.sprintf "Error in %s:\n%s%s" config.name stderr stdout
+        in
+        Compiler_log.append root output;
+        attempt.parse_messages <-
+          Build_attempt.Parse_error output :: attempt.parse_messages
+      | Build_attempt.Parsed_successfully {stderr} ->
+        publish_successful_parse stderr
+      | Build_attempt.Use_existing_ast -> publish_successful_parse "")
+    parsed;
+  dirty_modules
diff --git a/rewatch-ocaml/package_parse.mli b/rewatch-ocaml/package_parse.mli
new file mode 100644
index 00000000000..8b625e96374
--- /dev/null
+++ b/rewatch-ocaml/package_parse.mli
@@ -0,0 +1,11 @@
+val run :
+  package:Package_plan.t ->
+  prepared:Build_session.prepared ->
+  prepared_package:Package_plan.compilation ->
+  attempt:Build_attempt.t ->
+  removed_module_names:(string, unit) Hashtbl.t ->
+  (string, unit) Hashtbl.t
+(** Parsing returns the modules whose dependency information changed or whose
+    compilation is pending. Failed and warning-bearing source paths are also
+    retained in the session so later watch attempts cannot silently reuse an
+    older AST. *)
diff --git a/rewatch-ocaml/package_plan.ml b/rewatch-ocaml/package_plan.ml
new file mode 100644
index 00000000000..01b6ab66c10
--- /dev/null
+++ b/rewatch-ocaml/package_plan.ml
@@ -0,0 +1,28 @@
+type dependency = {
+  declaration: Config.dependency;
+  directory: string;
+  kind: Package_traversal.dependency_kind;
+}
+
+type t = {
+  name: string;
+  root: string;
+  build_owner: string;
+  is_local: bool;
+  config: Config.t;
+  compile_config: Config.t;
+  build_dir: string;
+  ocaml_dir: string;
+  dependencies: dependency list;
+  gentype_dependency_args: string list;
+  modules: Source.module_ list;
+  source_mtimes: (string, float) Hashtbl.t;
+  source_files: string list;
+  present_source_files: string list;
+}
+
+type compilation = {
+  regular_common_args: string list;
+  development_common_args: string list;
+  parse_paths: string list;
+}
diff --git a/rewatch-ocaml/package_plan.mli b/rewatch-ocaml/package_plan.mli
new file mode 100644
index 00000000000..163f79212ac
--- /dev/null
+++ b/rewatch-ocaml/package_plan.mli
@@ -0,0 +1,32 @@
+(** A package plan freezes discovery and configuration decisions that are
+    stable across retained watch attempts. Compilation-specific arguments are
+    stored separately so per-attempt dirtiness does not require rediscovery. *)
+
+type dependency = {
+  declaration: Config.dependency;
+  directory: string;
+  kind: Package_traversal.dependency_kind;
+}
+
+type t = {
+  name: string;
+  root: string;
+  build_owner: string;
+  is_local: bool;
+  config: Config.t;
+  compile_config: Config.t;
+  build_dir: string;
+  ocaml_dir: string;
+  dependencies: dependency list;
+  gentype_dependency_args: string list;
+  modules: Source.module_ list;
+  source_mtimes: (string, float) Hashtbl.t;
+  source_files: string list;
+  present_source_files: string list;
+}
+
+type compilation = {
+  regular_common_args: string list;
+  development_common_args: string list;
+  parse_paths: string list;
+}
diff --git a/rewatch-ocaml/package_resolution.ml b/rewatch-ocaml/package_resolution.ml
new file mode 100644
index 00000000000..d61a0d9c957
--- /dev/null
+++ b/rewatch-ocaml/package_resolution.ml
@@ -0,0 +1,121 @@
+type dependency = {
+  name: string;
+  directory: string;
+  config: Config.t;
+  is_local: bool;
+}
+
+type diagnostic_mode = Report_diagnostics | Suppress_diagnostics
+
+type t = {
+  root_config: Config.t;
+  context: Project_context.dependency_context;
+  loaded: (string, Config.t) Hashtbl.t;
+  edges: (string * string, dependency) Hashtbl.t;
+  selected: (string, dependency) Hashtbl.t;
+  reported_duplicates: (string * string, unit) Hashtbl.t;
+  diagnostic_mode: diagnostic_mode;
+  root_package_name: string;
+}
+
+let create ?(diagnostic_mode = Report_diagnostics) root_config =
+  let loaded = Hashtbl.create 32 in
+  Hashtbl.add loaded root_config.Config.root root_config;
+  {
+    root_config;
+    context = Project_context.dependency_context root_config;
+    loaded;
+    edges = Hashtbl.create 32;
+    selected = Hashtbl.create 32;
+    reported_duplicates = Hashtbl.create 8;
+    diagnostic_mode;
+    root_package_name =
+      Package_diagnostics.package_identity
+        ~report_diagnostics:(diagnostic_mode = Report_diagnostics)
+        root_config;
+  }
+
+let load_config resolution root =
+  match Hashtbl.find_opt resolution.loaded root with
+  | Some config -> config
+  | None ->
+    let config = Config.load_root root in
+    Hashtbl.add resolution.loaded root config;
+    config
+
+let is_local resolution directory =
+  Project_context.dependency_is_local_canonical resolution.context directory
+
+let dependency_path resolution ~package_root name =
+  Project_context.dependency_path_in resolution.context package_root name
+
+let dependency_candidates resolution ~package_root name =
+  Project_context.dependency_candidates_in resolution.context package_root name
+
+let root_package_name resolution = resolution.root_package_name
+
+let resolve resolution ~package_root (declaration : Config.dependency) =
+  let edge_key = (package_root, declaration.name) in
+  match Hashtbl.find_opt resolution.edges edge_key with
+  | Some identity -> identity
+  | None ->
+    let candidate =
+      Project_context.require_dependency_directory ~context:resolution.context
+        package_root declaration
+    in
+    let dependency =
+      match Hashtbl.find_opt resolution.selected declaration.name with
+      | Some selected ->
+        (if selected.directory <> candidate then
+           let key = (declaration.name, candidate) in
+           if
+             resolution.diagnostic_mode = Report_diagnostics
+             && not (Hashtbl.mem resolution.reported_duplicates key)
+           then (
+             Hashtbl.add resolution.reported_duplicates key ();
+             Printf.eprintf "Duplicated package: %s %s (chosen) vs %s in %s\n%!"
+               declaration.name
+               (Project_context.display_path ~root:resolution.root_config.root
+                  selected.directory)
+               (Project_context.display_path ~root:resolution.root_config.root
+                  candidate)
+               (Project_context.display_path ~root:resolution.root_config.root
+                  package_root)));
+        selected
+      | None ->
+        let config =
+          try load_config resolution candidate
+          with Config.Error message ->
+            raise
+              (Project_context.Package_error
+                 (Printf.sprintf
+                    "Could not build package tree for '%s' at path '%s'. \
+                     Error: %s"
+                    declaration.name resolution.root_config.root message))
+        in
+        let name =
+          Package_diagnostics.package_identity
+            ~report_diagnostics:(resolution.diagnostic_mode = Report_diagnostics)
+            config
+        in
+        if name <> declaration.name then
+          raise
+            (Project_context.Package_error
+               (Printf.sprintf
+                  "Could not build package tree reading dependency '%s' at \
+                   path '%s'. Error: resolved package identity '%s' does not \
+                   match the requested dependency name"
+                  declaration.name candidate name));
+        let identity =
+          {
+            name;
+            directory = candidate;
+            config;
+            is_local = is_local resolution candidate;
+          }
+        in
+        Hashtbl.add resolution.selected declaration.name identity;
+        identity
+    in
+    Hashtbl.add resolution.edges edge_key dependency;
+    dependency
diff --git a/rewatch-ocaml/package_resolution.mli b/rewatch-ocaml/package_resolution.mli
new file mode 100644
index 00000000000..71f0d30bc04
--- /dev/null
+++ b/rewatch-ocaml/package_resolution.mli
@@ -0,0 +1,20 @@
+type t
+(** Resolution caches package identity, not declaration-specific feature
+    requests. The same installed package may be reached through multiple edges
+    whose requested features must remain distinct for later aggregation. *)
+
+type dependency = {
+  name: string;
+  directory: string;
+  config: Config.t;
+  is_local: bool;
+}
+
+type diagnostic_mode = Report_diagnostics | Suppress_diagnostics
+
+val create : ?diagnostic_mode:diagnostic_mode -> Config.t -> t
+val resolve : t -> package_root:string -> Config.dependency -> dependency
+val dependency_path : t -> package_root:string -> string -> string option
+val dependency_candidates : t -> package_root:string -> string -> string list
+val root_package_name : t -> string
+val is_local : t -> string -> bool
diff --git a/rewatch-ocaml/package_traversal.ml b/rewatch-ocaml/package_traversal.ml
new file mode 100644
index 00000000000..46c96042ac9
--- /dev/null
+++ b/rewatch-ocaml/package_traversal.ml
@@ -0,0 +1,111 @@
+type dependency_kind = Regular | Development
+
+let source_discovery_prod ~prod ~is_local = prod || not is_local
+
+type request = {kind: dependency_kind; declaration: Config.dependency}
+
+type resolved = {request: request; dependency: Package_resolution.dependency}
+
+type package = {
+  name: string;
+  config: Config.t;
+  is_local: bool;
+  dependencies: resolved list;
+}
+
+type feature_selection = All_features | Selected_features of string list
+module String_set = Set.Make (String)
+
+type accumulated_features = All_requested | Selected_requested of String_set.t
+
+type feature_requests = (string, accumulated_features) Hashtbl.t
+type graph = {packages: package list; feature_requests: feature_requests}
+
+let add_feature_request requests root request =
+  match (Hashtbl.find_opt requests root, request) with
+  | None, None -> Hashtbl.add requests root All_requested
+  | None, Some requested ->
+    Hashtbl.add requests root
+      (Selected_requested (String_set.of_list requested))
+  | Some All_requested, _ | Some _, None ->
+    Hashtbl.replace requests root All_requested
+  | Some (Selected_requested current), Some requested ->
+    Hashtbl.replace requests root
+      (Selected_requested
+         (List.fold_left
+            (fun features feature -> String_set.add feature features)
+            current requested))
+
+let find_feature_selection graph root =
+  Hashtbl.find_opt graph.feature_requests root
+  |> Option.map (function
+    | All_requested -> All_features
+    | Selected_requested features ->
+      Selected_features (String_set.elements features))
+
+let feature_selection_to_option = function
+  | All_features -> None
+  | Selected_features features -> Some features
+
+let dependency_kind_name = function
+  | Regular -> "dependencies"
+  | Development -> "dev-dependencies"
+
+let requests ~prod ~is_local (config : Config.t) =
+  List.map
+    (fun declaration -> {kind = Regular; declaration})
+    config.dependencies
+  @
+  if prod || not is_local then []
+  else
+    List.map
+      (fun declaration -> {kind = Development; declaration})
+      config.dev_dependencies
+
+let resolve resolution ~package_root request =
+  {
+    request;
+    dependency =
+      Package_resolution.resolve resolution ~package_root request.declaration;
+  }
+
+let traverse ~root_config ~root_name ~prod ~features ~resolve =
+  let visited = Hashtbl.create 32 in
+  let feature_requests = Hashtbl.create 32 in
+  let packages = ref [] in
+  let rec visit ~name ~is_local ~features (config : Config.t) =
+    add_feature_request feature_requests config.root features;
+    if not (Hashtbl.mem visited config.root) then (
+      Hashtbl.add visited config.root ();
+      let dependencies =
+        requests ~prod ~is_local config
+        |> List.filter_map (fun request -> resolve config request)
+      in
+      List.iter
+        (fun resolved ->
+          visit ~is_local:resolved.dependency.is_local
+            ~name:resolved.dependency.name
+            ~features:resolved.request.declaration.features
+            resolved.dependency.config)
+        dependencies;
+      packages := {name; config; is_local; dependencies} :: !packages)
+  in
+  visit ~name:root_name ~is_local:true ~features root_config;
+  {packages = !packages; feature_requests}
+
+let discover ~root_config ~prod ~features ~resolution =
+  traverse ~root_config
+    ~root_name:(Package_resolution.root_package_name resolution) ~prod ~features
+    ~resolve:(fun config request ->
+      Some (resolve resolution ~package_root:config.root request))
+
+module For_test = struct
+  let create_feature_requests () = Hashtbl.create 4
+  let add_feature_request = add_feature_request
+  let find_feature_selection requests root =
+    Hashtbl.find_opt requests root
+    |> Option.map (function
+      | All_requested -> All_features
+      | Selected_requested features ->
+        Selected_features (String_set.elements features))
+end
diff --git a/rewatch-ocaml/package_traversal.mli b/rewatch-ocaml/package_traversal.mli
new file mode 100644
index 00000000000..bf8bb49ded5
--- /dev/null
+++ b/rewatch-ocaml/package_traversal.mli
@@ -0,0 +1,53 @@
+(** Traversal centralizes dependency admission and feature aggregation while
+    leaving command-specific failure policy to its [resolve] callback. This is
+    why build, watch, and format can share graph semantics without sharing all
+    recovery behavior. *)
+type dependency_kind = Regular | Development
+
+val source_discovery_prod : prod:bool -> is_local:bool -> bool
+
+type request = {kind: dependency_kind; declaration: Config.dependency}
+
+type resolved = {request: request; dependency: Package_resolution.dependency}
+
+type package = {
+  name: string;
+  config: Config.t;
+  is_local: bool;
+  dependencies: resolved list;
+}
+
+type feature_selection = All_features | Selected_features of string list
+type feature_requests
+type graph = {packages: package list; feature_requests: feature_requests}
+
+val find_feature_selection : graph -> string -> feature_selection option
+val feature_selection_to_option : feature_selection -> string list option
+
+val dependency_kind_name : dependency_kind -> string
+val requests : prod:bool -> is_local:bool -> Config.t -> request list
+
+val resolve : Package_resolution.t -> package_root:string -> request -> resolved
+
+val traverse :
+  root_config:Config.t ->
+  root_name:string ->
+  prod:bool ->
+  features:string list option ->
+  resolve:(Config.t -> request -> resolved option) ->
+  graph
+
+val discover :
+  root_config:Config.t ->
+  prod:bool ->
+  features:string list option ->
+  resolution:Package_resolution.t ->
+  graph
+
+module For_test : sig
+  val create_feature_requests : unit -> feature_requests
+  val add_feature_request :
+    feature_requests -> string -> string list option -> unit
+  val find_feature_selection :
+    feature_requests -> string -> feature_selection option
+end
diff --git a/rewatch-ocaml/platform.mli b/rewatch-ocaml/platform.mli
new file mode 100644
index 00000000000..d6711f9f2ea
--- /dev/null
+++ b/rewatch-ocaml/platform.mli
@@ -0,0 +1,55 @@
+(** Platform-specific filesystem and process operations live behind this
+    interface because path identity, command-line quoting, handle inheritance,
+    and process-tree termination have materially different Unix and Windows
+    contracts. Higher layers should not infer those rules from path strings. *)
+
+val normalize_path_for_comparison : string -> string
+val directory_identity : path:string -> Unix.stats -> string
+val canonicalize_path : string -> string
+val resolve_program : cwd:string -> string -> string
+val terminal_supports_color_without_term : bool
+val inherit_streaming_terminal_stdin : bool
+val configure_standard_streams : unit -> unit
+val clean_symbol : string
+val parse_symbol : string
+val build_symbol : string
+val success_symbol : string
+val warning_symbol : string
+val error_symbol : string
+
+type command = {env: Spawn.Env.t option; program: string; args: string list}
+
+val post_build_command : command:string -> output:string -> command
+
+type process
+
+val spawn :
+  env:Spawn.Env.t option ->
+  cwd:string ->
+  program:string ->
+  args:string list ->
+  stdin:Unix.file_descr ->
+  stdout:Unix.file_descr ->
+  stderr:Unix.file_descr ->
+  process
+
+val null_device : string
+
+val current_process_id : unit -> int
+val process_id : process -> int
+val release_process : process -> unit
+
+val create_capture_pipes :
+  unit ->
+  (Unix.file_descr * Unix.file_descr) * (Unix.file_descr * Unix.file_descr)
+
+val signal_process_tree :
+  root_reaped:bool -> process -> int -> (unit, string) result
+val defer_termination_signals : unit -> unit -> unit
+val graceful_termination_signal : int
+val escalate_process_groups : bool
+
+val process_is_active :
+  run:(string -> string list -> (Unix.process_status * string) option) ->
+  string ->
+  bool
diff --git a/rewatch-ocaml/platform_common.ml b/rewatch-ocaml/platform_common.ml
new file mode 100644
index 00000000000..0ad629386fe
--- /dev/null
+++ b/rewatch-ocaml/platform_common.ml
@@ -0,0 +1,41 @@
+let resolve_program ~path_separator ~executable_extensions ~search_directories
+    ~normalize_directory ~executable_is_usable ~cwd program =
+  if (not (Filename.is_implicit program)) || Filename.dirname program <> "."
+  then program
+  else
+    let path_directories =
+      Sys.getenv_opt "PATH" |> Option.value ~default:""
+      |> String.split_on_char path_separator
+    in
+    search_directories ~cwd path_directories
+    |> List.find_map (fun directory ->
+        let directory = normalize_directory directory in
+        let directory =
+          if directory = "" then cwd
+          else if Filename.is_relative directory then
+            Filename.concat cwd directory
+          else directory
+        in
+        executable_extensions ~program
+        |> List.find_map (fun extension ->
+            let candidate = Filename.concat directory (program ^ extension) in
+            if executable_is_usable candidate then Some candidate else None))
+    |> Option.value ~default:program
+
+let process_is_active ~probe value =
+  try
+    let pid = int_of_string value in
+    (* PID zero names a process group on Unix and is not a usable child-process
+       identity on Windows, so it can never prove ownership of a build lock. *)
+    if pid = 0 then false else probe pid
+  with
+  | Failure _ | Unix.Unix_error (Unix.ESRCH, _, _) -> false
+  | Unix.Unix_error (Unix.EPERM, _, _) -> true
+
+let create_capture_pipes () =
+  let stdout = Spawn.safe_pipe () in
+  try (stdout, Spawn.safe_pipe ())
+  with exn ->
+    Unix.close (fst stdout);
+    Unix.close (snd stdout);
+    raise exn
diff --git a/rewatch-ocaml/platform_common.mli b/rewatch-ocaml/platform_common.mli
new file mode 100644
index 00000000000..49a6e78659c
--- /dev/null
+++ b/rewatch-ocaml/platform_common.mli
@@ -0,0 +1,14 @@
+val resolve_program :
+  path_separator:char ->
+  executable_extensions:(program:string -> string list) ->
+  search_directories:(cwd:string -> string list -> string list) ->
+  normalize_directory:(string -> string) ->
+  executable_is_usable:(string -> bool) ->
+  cwd:string ->
+  string ->
+  string
+
+val process_is_active : probe:(int -> bool) -> string -> bool
+val create_capture_pipes :
+  unit ->
+  (Unix.file_descr * Unix.file_descr) * (Unix.file_descr * Unix.file_descr)
diff --git a/rewatch-ocaml/platform_unix.ml b/rewatch-ocaml/platform_unix.ml
new file mode 100644
index 00000000000..b3ab49f354e
--- /dev/null
+++ b/rewatch-ocaml/platform_unix.ml
@@ -0,0 +1,105 @@
+let path_separator = ':'
+let terminal_supports_color_without_term = false
+let inherit_streaming_terminal_stdin = false
+let configure_standard_streams () = ()
+let clean_symbol = "🧹 "
+let parse_symbol = "🧱 "
+let build_symbol = "🤺 "
+let success_symbol = "✅ "
+let warning_symbol = "⚠️ "
+let error_symbol = "❌ "
+let normalize_path_for_comparison value = value
+let directory_identity ~path:_ metadata =
+  Printf.sprintf "%d:%d" metadata.Unix.st_dev metadata.Unix.st_ino
+
+let canonicalize_path = Unix.realpath
+let executable_extensions ~program:_ = [""]
+let search_directories ~cwd:_ directories = directories
+
+let executable_is_usable candidate =
+  try
+    (Unix.stat candidate).Unix.st_kind = Unix.S_REG
+    &&
+      try
+        Unix.access candidate [Unix.X_OK];
+        true
+      with Unix.Unix_error _ -> false
+  with Unix.Unix_error _ -> false
+
+let resolve_program =
+  Platform_common.resolve_program ~path_separator ~executable_extensions
+    ~search_directories ~normalize_directory:Fun.id ~executable_is_usable
+
+type command = {env: Spawn.Env.t option; program: string; args: string list}
+
+let post_build_command ~command ~output =
+  {
+    env = None;
+    program = "/bin/sh";
+    args = ["-c"; command ^ " " ^ Filename.quote output];
+  }
+
+type process = int
+
+let spawn ~env ~cwd ~program ~args ~stdin ~stdout ~stderr =
+  let program = resolve_program ~cwd program in
+  Spawn.spawn ?env ~cwd:(Spawn.Working_dir.Path cwd) ~prog:program
+    ~argv:(program :: args) ~stdin ~stdout ~stderr
+    ~setpgid:Spawn.Pgid.new_process_group ()
+
+let null_device = "/dev/null"
+
+let current_process_id = Unix.getpid
+let process_id process = process
+let release_process _process = ()
+
+let create_capture_pipes = Platform_common.create_capture_pipes
+
+let rec signal_process_tree ~root_reaped process signal =
+  try
+    Unix.kill (-process) signal;
+    Ok ()
+  with
+  (* A missing group is already in the requested terminal state. Other errors
+     must prevent the caller from waiting indefinitely for an undelivered
+     signal. *)
+  | Unix.Unix_error (Unix.ESRCH, _, _) -> Ok ()
+  | Unix.Unix_error (Unix.EINTR, _, _) ->
+    signal_process_tree ~root_reaped process signal
+  | Unix.Unix_error (error, operation, _) ->
+    Error
+      (Printf.sprintf "%s while sending signal %d to process group %d: %s"
+         operation signal process (Unix.error_message error))
+
+let defer_termination_signals () =
+  let previous = Unix.sigprocmask Unix.SIG_BLOCK [Sys.sigint; Sys.sigterm] in
+  fun () -> ignore (Unix.sigprocmask Unix.SIG_SETMASK previous)
+
+let graceful_termination_signal = Sys.sigterm
+let escalate_process_groups = true
+
+let process_name_from_ps ~run pid =
+  match run "/bin/ps" ["-p"; string_of_int pid; "-o"; "comm="] with
+  | Some (Unix.WEXITED 0, output) ->
+    output |> String.trim |> Filename.basename
+    |> String.starts_with ~prefix:"rescript"
+  | Some (Unix.WEXITED _, _) -> false
+  | Some (Unix.WSIGNALED _, _) | Some (Unix.WSTOPPED _, _) | None -> true
+
+let probe_process ~run pid =
+  Unix.kill pid 0;
+  let executable = Printf.sprintf "/proc/%d/exe" pid in
+  if Sys.file_exists executable then
+    try
+      let basename = Unix.realpath executable |> Filename.basename in
+      String.starts_with ~prefix:"rescript" basename
+    with Unix.Unix_error _ -> true
+  else
+    (* macOS has no procfs. `ps` supplies the same executable-name check so a
+       reused PID from an abandoned lock is not mistaken for this tool. A
+       failed probe remains conservative because stealing a live build's lock
+       is worse than asking the user to remove an inconclusive stale lock. *)
+    process_name_from_ps ~run pid
+
+let process_is_active ~run value =
+  Platform_common.process_is_active ~probe:(probe_process ~run) value
diff --git a/rewatch-ocaml/platform_windows.ml b/rewatch-ocaml/platform_windows.ml
new file mode 100644
index 00000000000..de7716ddce5
--- /dev/null
+++ b/rewatch-ocaml/platform_windows.ml
@@ -0,0 +1,291 @@
+let path_separator = ';'
+let terminal_supports_color_without_term = true
+let inherit_streaming_terminal_stdin = true
+
+external enable_virtual_terminal_processing : unit -> unit
+  = "rewatch_windows_enable_virtual_terminal_processing"
+
+let configure_standard_streams () =
+  set_binary_mode_in stdin true;
+  set_binary_mode_out stdout true;
+  set_binary_mode_out stderr true;
+  enable_virtual_terminal_processing ()
+let clean_symbol = "[clean] "
+let parse_symbol = "[parse] "
+let build_symbol = "[build] "
+let success_symbol = "[ok] "
+let warning_symbol = "[warn] "
+let error_symbol = "[error] "
+let normalize_path_for_comparison path =
+  path |> String.lowercase_ascii
+  |> String.map (function
+    | '/' -> '\\'
+    | character -> character)
+
+let strip_verbatim_prefix path =
+  if String.starts_with ~prefix:"\\\\?\\UNC\\" path then
+    "\\\\" ^ String.sub path 8 (String.length path - 8)
+  else if String.starts_with ~prefix:"\\\\?\\" path then
+    String.sub path 4 (String.length path - 4)
+  else path
+
+let canonicalize_path path = Unix.realpath path |> strip_verbatim_prefix
+
+external directory_file_identity : string -> string
+  = "rewatch_windows_directory_identity"
+
+let directory_identity ~path _metadata = directory_file_identity path
+
+let executable_extensions ~program =
+  if Filename.extension program <> "" then [""]
+  else
+    Sys.getenv_opt "PATHEXT"
+    |> Option.value ~default:".COM;.EXE;.BAT;.CMD"
+    |> String.split_on_char ';'
+
+let search_directories ~cwd directories = cwd :: directories
+
+let executable_is_usable candidate =
+  try (Unix.stat candidate).Unix.st_kind = Unix.S_REG
+  with Unix.Unix_error _ -> false
+
+let normalize_path_directory directory =
+  let directory = String.trim directory in
+  let length = String.length directory in
+  if length >= 2 && directory.[0] = '"' && directory.[length - 1] = '"' then
+    String.sub directory 1 (length - 2)
+  else directory
+
+let resolve_program =
+  Platform_common.resolve_program ~path_separator ~executable_extensions
+    ~search_directories ~normalize_directory:normalize_path_directory
+    ~executable_is_usable
+
+type command = {env: Spawn.Env.t option; program: string; args: string list}
+
+let post_build_command ~command ~output =
+  let variable = "REWATCH_JS_POST_BUILD_FILE" in
+  let prefix = String.lowercase_ascii (variable ^ "=") in
+  let environment =
+    Unix.environment () |> Array.to_list
+    |> List.filter (fun entry ->
+        not (String.starts_with ~prefix (String.lowercase_ascii entry)))
+    |> List.cons (variable ^ "=" ^ output)
+    |> Spawn.Env.of_list
+  in
+  {
+    env = Some environment;
+    program = "cmd.exe";
+    args = ["/D"; "/V:OFF"; "/S"; "/C"; command ^ " \"%" ^ variable ^ "%\""];
+  }
+
+let is_batch_file program =
+  List.mem
+    (Filename.extension program |> String.lowercase_ascii)
+    [".bat"; ".cmd"]
+
+type process_job
+
+type process = {wait_id: int; job: process_job}
+
+external spawn_owned :
+  env:Spawn.Env.t option ->
+  cwd:string ->
+  program:string ->
+  command_line:string ->
+  stdin:Unix.file_descr ->
+  stdout:Unix.file_descr ->
+  stderr:Unix.file_descr ->
+  process = "rewatch_windows_spawn_owned_byte" "rewatch_windows_spawn_owned"
+
+external terminate_process_job : process_job -> bool
+  = "rewatch_windows_terminate_process_job"
+
+external close_process_job : process_job -> unit
+  = "rewatch_windows_close_process_job"
+
+let quote_argument argument =
+  if
+    argument = ""
+    || String.contains argument ' '
+    || String.contains argument '\t'
+    || String.contains argument '"'
+  then Filename.quote argument
+  else argument
+
+let ensure_no_null label value =
+  if String.contains value '\x00' then
+    invalid_arg (Printf.sprintf "%s contains a NUL byte" label)
+
+let program_for_working_directory ~cwd program =
+  if Filename.is_relative program then Filename.concat cwd program else program
+
+let serialize_command_line ~shell_command_is_quoted ~program ~args =
+  match args with
+  | ["/D"; "/V:OFF"; "/S"; "/C"; command] ->
+    let command =
+      if shell_command_is_quoted then command else "\"" ^ command ^ "\""
+    in
+    String.concat " " [quote_argument program; "/D"; "/V:OFF"; "/S"; "/C"]
+    ^ " " ^ command
+  | _ -> program :: args |> List.map quote_argument |> String.concat " "
+
+let spawn ~env ~cwd ~program ~args ~stdin ~stdout ~stderr =
+  let program = resolve_program ~cwd program in
+  ensure_no_null "working directory" cwd;
+  ensure_no_null "program" program;
+  List.iter (ensure_no_null "argument") args;
+  let program, args, shell_command_is_quoted =
+    if is_batch_file program then
+      let command = Filename.quote_command program args in
+      ( resolve_program ~cwd "cmd.exe",
+        ["/D"; "/V:OFF"; "/S"; "/C"; command],
+        true )
+    else (program, args, false)
+  in
+  let command_line =
+    serialize_command_line ~shell_command_is_quoted ~program ~args
+  in
+  let program = program_for_working_directory ~cwd program in
+  (* Starting suspended closes the only interval in which a child could create
+     descendants before the job owns its process tree. *)
+  spawn_owned ~env ~cwd ~program ~command_line ~stdin ~stdout ~stderr
+
+let null_device = "NUL"
+
+external current_process_id : unit -> int = "rewatch_windows_current_process_id"
+
+let process_id process = process.wait_id
+let release_process process = close_process_job process.job
+
+let create_capture_pipes = Platform_common.create_capture_pipes
+
+let signal_process_tree ~root_reaped:_ process _signal =
+  if terminate_process_job process.job then Ok ()
+  else Error "TerminateJobObject failed"
+
+let termination_signal_mutex = Mutex.create ()
+
+let defer_termination_signals () =
+  (* Windows has no per-thread signal mask, so temporary handlers affect every
+     domain. Serializing installation keeps concurrently published artifacts
+     from restoring one another's handlers out of order. *)
+  Mutex.lock termination_signal_mutex;
+  let pending = ref [] in
+  let defer signal =
+    if not (List.mem signal !pending) then pending := signal :: !pending
+  in
+  let previous_int =
+    try Sys.signal Sys.sigint (Sys.Signal_handle defer)
+    with exn ->
+      Mutex.unlock termination_signal_mutex;
+      raise exn
+  in
+  let previous_term =
+    try Sys.signal Sys.sigterm (Sys.Signal_handle defer)
+    with exn ->
+      let exn =
+        try
+          ignore (Sys.signal Sys.sigint previous_int);
+          exn
+        with restore_exn -> restore_exn
+      in
+      Mutex.unlock termination_signal_mutex;
+      raise exn
+  in
+  let restored = ref false in
+  let dispatch signal behavior =
+    match behavior with
+    | Sys.Signal_ignore -> ()
+    | Sys.Signal_handle handler -> handler signal
+    | Sys.Signal_default -> raise Sys.Break
+  in
+  fun () ->
+    if not !restored then (
+      restored := true;
+      let restore_error = ref None in
+      let restore signal behavior =
+        try ignore (Sys.signal signal behavior)
+        with exn ->
+          if Option.is_none !restore_error then restore_error := Some exn
+      in
+      restore Sys.sigint previous_int;
+      restore Sys.sigterm previous_term;
+      Mutex.unlock termination_signal_mutex;
+      Option.iter raise !restore_error;
+      List.rev !pending
+      |> List.iter (fun signal ->
+          dispatch signal
+            (if signal = Sys.sigint then previous_int else previous_term)))
+
+let graceful_termination_signal = Sys.sigkill
+let escalate_process_groups = false
+
+let parse_tasklist_csv_line line =
+  let length = String.length line in
+  let rec parse_field fields index =
+    if index >= length || line.[index] <> '"' then None
+    else
+      let buffer = Buffer.create 32 in
+      let rec parse_char index =
+        if index >= length then None
+        else
+          match line.[index] with
+          | '"' when index + 1 < length && line.[index + 1] = '"' ->
+            Buffer.add_char buffer '"';
+            parse_char (index + 2)
+          | '"' ->
+            let fields = Buffer.contents buffer :: fields in
+            let next = index + 1 in
+            if next = length then Some (List.rev fields)
+            else if line.[next] = ',' then parse_field fields (next + 1)
+            else None
+          | character ->
+            Buffer.add_char buffer character;
+            parse_char (index + 1)
+      in
+      parse_char (index + 1)
+  in
+  if length = 0 then None else parse_field [] 0
+
+type tasklist_probe = Process_found | Process_absent | Malformed_output
+
+let tasklist_probe ~pid output =
+  let lines =
+    output |> String.trim |> String.split_on_char '\n' |> List.map String.trim
+    |> List.filter (( <> ) "")
+  in
+  let rows = List.map parse_tasklist_csv_line lines in
+  let valid_row = function
+    | Some [_image; row_pid; _session; _session_number; _memory] ->
+      Option.is_some (int_of_string_opt row_pid)
+    | Some _ | None -> false
+  in
+  if lines = [] || not (List.for_all valid_row rows) then Malformed_output
+  else if
+    List.exists
+      (function
+        | Some [image; row_pid; _session; _session_number; _memory] ->
+          String.starts_with ~prefix:"rescript" (String.lowercase_ascii image)
+          && row_pid = string_of_int pid
+        | Some _ | None -> false)
+      rows
+  then Process_found
+  else Process_absent
+
+let probe_process ~run pid =
+  let tasklist =
+    match Sys.getenv_opt "SystemRoot" with
+    | Some root ->
+      Filename.concat (Filename.concat root "System32") "tasklist.exe"
+    | None -> "tasklist.exe"
+  in
+  match run tasklist ["/FO"; "CSV"; "/NH"] with
+  | Some (Unix.WEXITED 0, stdout) -> (
+    match tasklist_probe ~pid stdout with
+    | Process_found | Malformed_output -> true
+    | Process_absent -> false)
+  | Some _ | None -> true
+
+let process_is_active ~run value =
+  Platform_common.process_is_active ~probe:(probe_process ~run) value
diff --git a/rewatch-ocaml/process.ml b/rewatch-ocaml/process.ml
new file mode 100644
index 00000000000..81a883768eb
--- /dev/null
+++ b/rewatch-ocaml/process.ml
@@ -0,0 +1,549 @@
+module Child = Process_child
+
+type result = Child.result = {
+  status: Unix.process_status;
+  stdout: string;
+  stderr: string;
+}
+
+type job = Child.job = {program: string; args: string list; cwd: string}
+type task = {job: job; env: Spawn.Env.t option; on_result: result -> result}
+
+exception Error = Child.Error
+exception Interrupted of int
+
+let decode_utf8_lossy = Child.decode_utf8_lossy
+let succeeded = Child.succeeded
+let status_string = Child.status_string
+
+(* Reader threads are needed because a child can block when either output pipe
+   fills, and Windows cannot use select to drain anonymous pipes concurrently.
+   The threads perform only blocking I/O; dependency scheduling stays on the
+   calling thread. *)
+let default_max_jobs = min 32 (max 1 (Domain.recommended_domain_count ()))
+
+let task ?env ?(on_result = fun result -> result) job = {job; env; on_result}
+
+let run_parallel_map_with_notifier ~max_jobs ~poll ~on_complete notifier values
+    ~job =
+  let indexed = List.mapi (fun index value -> (index, value)) values in
+  let results = Array.make (List.length values) None in
+  let active = ref [] in
+  let launch_indexed (index, value) =
+    active := Child.launch ~notifier index (job value) :: !active
+  in
+  let rec fill slots queued =
+    if slots = 0 then queued
+    else
+      match queued with
+      | [] -> []
+      | job :: rest ->
+        launch_indexed job;
+        fill (slots - 1) rest
+  in
+  let rec schedule queued =
+    let queued = fill (max_jobs - List.length !active) queued in
+    match !active with
+    | [] -> ()
+    | _ ->
+      let (child, result), deferred_signals =
+        Child.wait_for_running ~poll notifier !active
+      in
+      Signal_restore.protect deferred_signals (fun () ->
+          active := List.filter (fun running -> running != child) !active;
+          Child.release_running child;
+          results.(Child.payload child) <- Some result;
+          on_complete (Child.payload child));
+      schedule queued
+  in
+  try
+    schedule indexed;
+    Array.to_list results
+    |> List.map (function
+      | Some result -> result
+      | None -> raise (Error "subprocess result was not collected"))
+  with exn ->
+    Child.terminate_running !active;
+    raise exn
+
+let run_parallel_map ?(max_jobs = default_max_jobs) ?poll
+    ?(on_complete = fun _ -> ()) values ~job =
+  if max_jobs < 1 then raise (Error "max_jobs must be at least one");
+  match values with
+  | [] -> []
+  | _ ->
+    let poll, ticker_enabled =
+      match poll with
+      | Some poll -> (poll, true)
+      | None -> ((fun () -> ()), false)
+    in
+    Child.with_completion_notifier ~ticker_enabled (fun notifier ->
+        run_parallel_map_with_notifier ~max_jobs ~poll ~on_complete notifier
+          values ~job)
+
+let run_parallel ?max_jobs ?poll ?on_complete jobs =
+  run_parallel_map ?max_jobs ?poll ?on_complete jobs ~job:Fun.id
+
+type 'a work = {key: string; dependencies: string list; value: 'a}
+type failure_action =
+  | Abort_immediately
+  | Stop_new_work
+  | Continue_independent_work
+
+module Work_ready = Set.Make (struct
+  type t = int * string
+
+  let compare (first_priority, first_key) (second_priority, second_key) =
+    let by_priority = compare second_priority first_priority in
+    if by_priority <> 0 then by_priority
+    else String.compare first_key second_key
+end)
+
+type cancellation_state =
+  | Cancellation_not_requested
+  | Cancellation_requested
+  | Cancellation_failed of exn
+
+type termination_state =
+  | Termination_confirmed
+  | Termination_unconfirmed of exn
+
+type pool_active = {
+  id: int;
+  child: unit Child.running;
+  mutable cancellation: cancellation_state;
+}
+
+let cancellation_was_requested = function
+  | Cancellation_not_requested -> false
+  | Cancellation_requested | Cancellation_failed _ -> true
+
+type 'a pool_completion =
+  | Task_completed of 'a * result
+  | Task_failed of 'a * exn
+
+type 'a worker_pool = {
+  notifier: Child.completion_notifier;
+  mutex: Mutex.t;
+  work_available: Condition.t;
+  cancellation_finished: Condition.t;
+  queued: ('a * task) Queue.t;
+  completed_tasks: 'a pool_completion Queue.t;
+  mutable active_children: pool_active list;
+  mutable next_active_id: int;
+  mutable stopping: bool;
+  mutable signalling_cancellation: bool;
+  mutable workers: unit Domain.t list;
+}
+
+let launch_worker_task notifier task =
+  (* Signal handlers are process-wide and may execute on a worker domain. A
+     worker therefore keeps ownership across launch without temporarily
+     replacing those handlers: an interruption unwinds through [launch] or the
+     completion queue, and the scheduler then cancels the other process trees. *)
+  Child.launch ?env:task.env ~defer_signals:false ~notifier () task.job
+
+let remove_active pool active =
+  Child.with_lock pool.mutex (fun () ->
+      while
+        pool.signalling_cancellation
+        && cancellation_was_requested active.cancellation
+      do
+        Condition.wait pool.cancellation_finished pool.mutex
+      done;
+      pool.active_children <-
+        List.filter
+          (fun current -> current.id <> active.id)
+          pool.active_children)
+
+let release_active active = Child.release active.child
+
+let release_active_after_completion active =
+  Child.release_after_completion active.child
+
+let complete_pool_task pool completion =
+  Child.with_lock pool.mutex (fun () ->
+      Queue.add completion pool.completed_tasks);
+  Child.notify_completion pool.notifier
+
+let run_pool_task pool payload task =
+  match
+    try Ok (launch_worker_task pool.notifier task) with exn -> Error exn
+  with
+  | Error exn -> complete_pool_task pool (Task_failed (payload, exn))
+  | Ok child ->
+    let active, cancel_after_launch =
+      Child.with_lock pool.mutex (fun () ->
+          let active =
+            {
+              id = pool.next_active_id;
+              child;
+              cancellation =
+                (if pool.stopping then Cancellation_requested
+                 else Cancellation_not_requested);
+            }
+          in
+          pool.next_active_id <- pool.next_active_id + 1;
+          pool.active_children <- active :: pool.active_children;
+          (active, cancellation_was_requested active.cancellation))
+    in
+    let wait_result =
+      try
+        if cancel_after_launch then Child.signal_running [child];
+        Ok
+          (Child.wait_for_running ~defer_signals:false
+             ~poll:(fun () ->
+               match
+                 Child.with_lock pool.mutex (fun () -> active.cancellation)
+               with
+               | Cancellation_failed exn -> raise exn
+               | Cancellation_not_requested | Cancellation_requested -> ())
+             pool.notifier [child])
+      with exn -> Error exn
+    in
+    let completion =
+      match wait_result with
+      | Ok ((_, result), deferred_signals) -> (
+        remove_active pool active;
+        try
+          Signal_restore.protect deferred_signals (fun () ->
+              Child.await_termination child;
+              release_active active;
+              Task_completed (payload, task.on_result result))
+        with exn -> Task_failed (payload, exn))
+      | Error exn -> (
+        let termination =
+          try
+            Child.signal_running [child];
+            Termination_confirmed
+          with cancellation_exn -> Termination_unconfirmed cancellation_exn
+        in
+        match termination with
+        | Termination_confirmed -> (
+          remove_active pool active;
+          try
+            Child.await_termination child;
+            release_active active;
+            Task_failed (payload, exn)
+          with release_exn -> Task_failed (payload, release_exn))
+        | Termination_unconfirmed cancellation_exn ->
+          remove_active pool active;
+          release_active_after_completion active;
+          Task_failed (payload, cancellation_exn))
+    in
+    complete_pool_task pool completion
+
+let rec worker_loop pool =
+  let queued =
+    Child.with_lock pool.mutex (fun () ->
+        while Queue.is_empty pool.queued && not pool.stopping do
+          Condition.wait pool.work_available pool.mutex
+        done;
+        if Queue.is_empty pool.queued then None
+        else Some (Queue.take pool.queued))
+  in
+  match queued with
+  | None -> ()
+  | Some (payload, task) ->
+    run_pool_task pool payload task;
+    worker_loop pool
+
+let create_worker_pool ~max_jobs notifier =
+  let pool =
+    {
+      notifier;
+      mutex = Mutex.create ();
+      work_available = Condition.create ();
+      cancellation_finished = Condition.create ();
+      queued = Queue.create ();
+      completed_tasks = Queue.create ();
+      active_children = [];
+      next_active_id = 0;
+      stopping = false;
+      signalling_cancellation = false;
+      workers = [];
+    }
+  in
+  let rec start_workers remaining =
+    if remaining > 0 then (
+      let worker = Domain.spawn (fun () -> worker_loop pool) in
+      pool.workers <- worker :: pool.workers;
+      start_workers (remaining - 1))
+  in
+  try
+    start_workers max_jobs;
+    pool
+  with exn ->
+    Child.with_lock pool.mutex (fun () ->
+        pool.stopping <- true;
+        Condition.broadcast pool.work_available);
+    List.iter Domain.join pool.workers;
+    raise exn
+
+let submit_pool_task pool payload task =
+  Child.with_lock pool.mutex (fun () ->
+      if pool.stopping then raise (Error "subprocess worker pool is stopping");
+      Queue.add (payload, task) pool.queued;
+      Condition.signal pool.work_available)
+
+let await_pool_completion ~poll pool =
+  let rec wait generation =
+    match
+      Child.with_lock pool.mutex (fun () ->
+          if Queue.is_empty pool.completed_tasks then None
+          else Some (Queue.take pool.completed_tasks))
+    with
+    | Some completion -> completion
+    | None ->
+      poll ();
+      Child.await_notification pool.notifier generation |> wait
+  in
+  Child.notifier_generation pool.notifier |> wait
+
+let stop_worker_pool ~cancel pool =
+  let active =
+    Child.with_lock pool.mutex (fun () ->
+        pool.stopping <- true;
+        Queue.clear pool.queued;
+        Condition.broadcast pool.work_available;
+        if cancel then (
+          pool.signalling_cancellation <- true;
+          pool.active_children
+          |> List.filter_map (fun active ->
+              match active.cancellation with
+              | Cancellation_requested | Cancellation_failed _ -> None
+              | Cancellation_not_requested ->
+                active.cancellation <- Cancellation_requested;
+                Some active.child))
+        else [])
+  in
+  let signal_error =
+    try
+      Child.signal_running active;
+      None
+    with exn -> Some exn
+  in
+  Child.with_lock pool.mutex (fun () ->
+      Option.iter
+        (fun exn ->
+          List.iter
+            (fun active ->
+              match active.cancellation with
+              | Cancellation_requested ->
+                active.cancellation <- Cancellation_failed exn
+              | Cancellation_not_requested | Cancellation_failed _ -> ())
+            pool.active_children)
+        signal_error;
+      pool.signalling_cancellation <- false;
+      Condition.broadcast pool.cancellation_finished);
+  Option.iter (fun _ -> Child.notify_completion pool.notifier) signal_error;
+  List.iter Domain.join pool.workers;
+  Option.iter raise signal_error
+
+let with_worker_pool ~max_jobs notifier action =
+  let pool = create_worker_pool ~max_jobs notifier in
+  match action pool with
+  | result ->
+    stop_worker_pool ~cancel:false pool;
+    result
+  | exception exn ->
+    let exn =
+      try
+        stop_worker_pool ~cancel:true pool;
+        exn
+      with cancellation_exn -> cancellation_exn
+    in
+    raise exn
+
+let run_dependency_graph_with_notifier ~max_jobs ~on_failure ~poll notifier
+    works ~next =
+  let graph =
+    Graph.create_index works
+      ~name:(fun work -> work.key)
+      ~deps:(fun work -> work.dependencies)
+      ~validation:
+        (Graph.Reject_invalid
+           {
+             duplicate_node =
+               (fun key -> Error ("duplicate subprocess work key: " ^ key));
+             unknown_dependency =
+               (fun ~node ~dependency ->
+                 Error
+                   (Printf.sprintf
+                      "unknown dependency %s for subprocess work %s" dependency
+                      node));
+           })
+  in
+  let count = Graph.node_count graph in
+  let pending = Hashtbl.create count in
+  List.iter
+    (fun work ->
+      Hashtbl.add pending work.key (Graph.dependency_count graph work.key))
+    works;
+  let priorities = Hashtbl.create count in
+  let remaining_dependents = Hashtbl.create count in
+  let leaves = Queue.create () in
+  List.iter
+    (fun work ->
+      let dependent_count = List.length (Graph.dependents graph work.key) in
+      Hashtbl.add remaining_dependents work.key dependent_count;
+      if dependent_count = 0 then (
+        Hashtbl.add priorities work.key 1;
+        Queue.add work.key leaves))
+    works;
+  let prioritized = ref 0 in
+  while not (Queue.is_empty leaves) do
+    let key = Queue.take leaves in
+    incr prioritized;
+    let key_priority = Hashtbl.find priorities key in
+    Graph.dependencies graph key
+    |> List.iter (fun dependency ->
+        let candidate = key_priority + 1 in
+        let current =
+          Hashtbl.find_opt priorities dependency |> Option.value ~default:1
+        in
+        if candidate > current then
+          Hashtbl.replace priorities dependency candidate;
+        let remaining = Hashtbl.find remaining_dependents dependency - 1 in
+        Hashtbl.replace remaining_dependents dependency remaining;
+        if remaining = 0 then Queue.add dependency leaves)
+  done;
+  let () =
+    if !prioritized <> count then
+      let cycle =
+        Graph.shortest_cycle_in_index graph |> Option.value ~default:[]
+      in
+      let details =
+        match cycle with
+        | [] -> ""
+        | cycle -> ": " ^ String.concat " -> " cycle
+      in
+      raise (Error ("subprocess dependency graph contains a cycle" ^ details))
+  in
+  let ready = ref Work_ready.empty in
+  let add_ready work =
+    ready := Work_ready.add (Hashtbl.find priorities work.key, work.key) !ready
+  in
+  List.iter
+    (fun work -> if Hashtbl.find pending work.key = 0 then add_ready work)
+    works;
+  let in_flight = ref 0 in
+  let completed = ref 0 in
+  let stopped = ref false in
+  let errors = ref [] in
+  let record_error work exn =
+    match on_failure exn with
+    | Abort_immediately -> raise exn
+    | Stop_new_work ->
+      stopped := true;
+      errors := (work.key, exn) :: !errors
+    | Continue_independent_work -> errors := (work.key, exn) :: !errors
+  in
+  let complete work =
+    incr completed;
+    Graph.dependents graph work.key
+    |> List.iter (fun dependent_key ->
+        let remaining = Hashtbl.find pending dependent_key - 1 in
+        Hashtbl.replace pending dependent_key remaining;
+        if remaining = 0 then add_ready (Graph.find_node graph dependent_key))
+  in
+  let rec fill pool =
+    if (not !stopped) && !in_flight < max_jobs then
+      match Work_ready.min_elt_opt !ready with
+      | None -> ()
+      | Some ((_, key) as ready_key) ->
+        ready := Work_ready.remove ready_key !ready;
+        let work = Graph.find_node graph key in
+        (try
+           match next work.value None with
+           | None -> complete work
+           | Some task ->
+             submit_pool_task pool work task;
+             incr in_flight
+         with exn -> record_error work exn);
+        fill pool
+  in
+  let rec schedule pool =
+    fill pool;
+    if !in_flight = 0 then
+      match
+        !errors
+        |> List.sort (fun (first, _) (second, _) -> String.compare first second)
+      with
+      | (_, exn) :: _ -> raise exn
+      | [] when !completed <> count ->
+        raise (Error "subprocess dependency graph stalled")
+      | [] -> ()
+    else
+      let completion = await_pool_completion ~poll pool in
+      decr in_flight;
+      (match completion with
+      | Task_failed (work, exn) -> record_error work exn
+      | Task_completed (work, result) -> (
+        try
+          match next work.value (Some result) with
+          | Some task ->
+            submit_pool_task pool work task;
+            incr in_flight
+          | None -> complete work
+        with exn -> record_error work exn));
+      schedule pool
+  in
+  with_worker_pool ~max_jobs:(min max_jobs count) notifier schedule
+
+let run_dependency_graph ?(max_jobs = default_max_jobs)
+    ?(on_failure =
+      function
+      | Sys.Break | Interrupted _ -> Abort_immediately
+      | _ -> Stop_new_work) ?poll works ~next =
+  if max_jobs < 1 then raise (Error "max_jobs must be at least one");
+  match works with
+  | [] -> ()
+  | _ ->
+    let poll, ticker_enabled =
+      match poll with
+      | Some poll -> (poll, true)
+      | None -> ((fun () -> ()), false)
+    in
+    Child.with_completion_notifier ~ticker_enabled (fun notifier ->
+        run_dependency_graph_with_notifier ~max_jobs ~on_failure ~poll notifier
+          works ~next)
+
+let run_one ?poll ?stdout_chunk ?stderr_chunk ?stdin ~cwd program args =
+  let poll, ticker_enabled =
+    match poll with
+    | Some poll -> (poll, true)
+    | None -> ((fun () -> ()), false)
+  in
+  Child.with_completion_notifier ~ticker_enabled (fun notifier ->
+      let child =
+        Child.launch ?stdout_chunk ?stderr_chunk ?stdin ~notifier ()
+          {program; args; cwd}
+      in
+      let completion_received = ref false in
+      try
+        let (_, result), deferred_signals =
+          Child.wait_for_running ~poll notifier [child]
+        in
+        completion_received := true;
+        Signal_restore.protect deferred_signals (fun () ->
+            Child.release_running child;
+            result)
+      with exn ->
+        if not !completion_received then Child.terminate_running [child];
+        raise exn)
+
+let run ?poll ~cwd program args = run_one ?poll ~cwd program args
+
+let run_streaming ?poll ~cwd program args =
+  let write channel bytes count =
+    output channel bytes 0 count;
+    flush channel
+  in
+  let stdin =
+    if Platform.inherit_streaming_terminal_stdin || not (Unix.isatty Unix.stdin)
+    then Child.Inherit_stdin
+    else Child.Null_stdin
+  in
+  run_one ?poll ~stdout_chunk:(write stdout) ~stderr_chunk:(write stderr) ~stdin
+    ~cwd program args
diff --git a/rewatch-ocaml/process.mli b/rewatch-ocaml/process.mli
new file mode 100644
index 00000000000..efaf3b79ffe
--- /dev/null
+++ b/rewatch-ocaml/process.mli
@@ -0,0 +1,57 @@
+(** Process scheduling has two policies but one child-lifecycle owner. Parallel
+    lists stop immediately on interruption; dependency graphs can stop admitting
+    new work while already-started children drain. {!Process_child} owns the
+    operating-system resources in both cases. *)
+
+type result = {status: Unix.process_status; stdout: string; stderr: string}
+type job = {program: string; args: string list; cwd: string}
+type task
+
+exception Error of string
+exception Interrupted of int
+
+val decode_utf8_lossy : string -> string
+val succeeded : result -> bool
+val status_string : Unix.process_status -> string
+val default_max_jobs : int
+
+val task : ?env:Spawn.Env.t -> ?on_result:(result -> result) -> job -> task
+
+val run_parallel :
+  ?max_jobs:int ->
+  ?poll:(unit -> unit) ->
+  ?on_complete:(int -> unit) ->
+  job list ->
+  result list
+
+val run_parallel_map :
+  ?max_jobs:int ->
+  ?poll:(unit -> unit) ->
+  ?on_complete:(int -> unit) ->
+  'a list ->
+  job:('a -> job) ->
+  result list
+
+type 'a work = {key: string; dependencies: string list; value: 'a}
+
+(* [Stop_new_work] preserves results from children that already started while
+    ensuring no newly ready dependency is launched after a build failure.
+    [Continue_independent_work] also drains work that was already ready, while
+    failed prerequisites continue to block their dependents. *)
+type failure_action =
+  | Abort_immediately
+  | Stop_new_work
+  | Continue_independent_work
+
+val run_dependency_graph :
+  ?max_jobs:int ->
+  ?on_failure:(exn -> failure_action) ->
+  ?poll:(unit -> unit) ->
+  'a work list ->
+  next:('a -> result option -> task option) ->
+  unit
+
+val run : ?poll:(unit -> unit) -> cwd:string -> string -> string list -> result
+
+val run_streaming :
+  ?poll:(unit -> unit) -> cwd:string -> string -> string list -> result
diff --git a/rewatch-ocaml/process_child.ml b/rewatch-ocaml/process_child.ml
new file mode 100644
index 00000000000..99341f134b3
--- /dev/null
+++ b/rewatch-ocaml/process_child.ml
@@ -0,0 +1,452 @@
+type result = {status: Unix.process_status; stdout: string; stderr: string}
+type job = {program: string; args: string list; cwd: string}
+type stdin_policy = Inherit_stdin | Null_stdin
+
+exception Error of string
+
+let decode_utf8_lossy value =
+  if String.is_valid_utf_8 value then value
+  else
+    let output = Buffer.create (String.length value) in
+    let rec loop index =
+      if index < String.length value then (
+        let decoded = String.get_utf_8_uchar value index in
+        let length = max 1 (Uchar.utf_decode_length decoded) in
+        if Uchar.utf_decode_is_valid decoded then
+          Buffer.add_substring output value index length
+        else Buffer.add_utf_8_uchar output Uchar.rep;
+        loop (index + length))
+    in
+    loop 0;
+    Buffer.contents output
+
+let succeeded result = result.status = Unix.WEXITED 0
+
+let status_string = function
+  | Unix.WEXITED code -> Printf.sprintf "exit code %d" code
+  | Unix.WSIGNALED signal -> Printf.sprintf "signal %d" signal
+  | Unix.WSTOPPED signal -> Printf.sprintf "stopped by signal %d" signal
+
+type capture = {
+  thread: Thread.t;
+  outcome: (string, exn) Stdlib.result option ref;
+}
+
+type child_wait = {
+  thread: Thread.t;
+  direct_outcome: (Unix.process_status, exn) Stdlib.result option Atomic.t;
+  outcome: (result, exn) Stdlib.result option Atomic.t;
+}
+
+type 'a running = {
+  payload: 'a;
+  process: Platform.process;
+  pid: int;
+  child_wait: child_wait;
+}
+
+type launch_ownership = {
+  mutable stdin: Unix.file_descr option;
+  mutable stdout_read: Unix.file_descr option;
+  mutable stdout_write: Unix.file_descr option;
+  mutable stderr_read: Unix.file_descr option;
+  mutable stderr_write: Unix.file_descr option;
+  mutable stdout_capture: capture option;
+  mutable stderr_capture: capture option;
+  mutable process: Platform.process option;
+  mutable child_wait: child_wait option;
+  mutable termination_error: string option;
+}
+
+let empty_launch_ownership () =
+  {
+    stdin = None;
+    stdout_read = None;
+    stdout_write = None;
+    stderr_read = None;
+    stderr_write = None;
+    stdout_capture = None;
+    stderr_capture = None;
+    process = None;
+    child_wait = None;
+    termination_error = None;
+  }
+
+type completion_notifier = {
+  mutex: Mutex.t;
+  condition: Condition.t;
+  mutable generation: int;
+  mutable stopped: bool;
+}
+
+let with_lock mutex action =
+  Mutex.lock mutex;
+  (* Unlocking in the exception path prevents one failed callback from
+     permanently blocking every waiter that shares this notifier. *)
+  Fun.protect ~finally:(fun () -> Mutex.unlock mutex) action
+
+let create_completion_notifier () =
+  {
+    mutex = Mutex.create ();
+    condition = Condition.create ();
+    generation = 0;
+    stopped = false;
+  }
+
+let notify_completion notifier =
+  with_lock notifier.mutex (fun () ->
+      if not notifier.stopped then (
+        notifier.generation <- notifier.generation + 1;
+        Condition.broadcast notifier.condition))
+
+let notifier_generation notifier =
+  with_lock notifier.mutex (fun () -> notifier.generation)
+
+let await_notification notifier generation =
+  with_lock notifier.mutex (fun () ->
+      while notifier.generation = generation && not notifier.stopped do
+        Condition.wait notifier.condition notifier.mutex
+      done;
+      notifier.generation)
+
+let with_completion_notifier ~ticker_enabled action =
+  (* The scheduler needs immediate child completion without repeatedly asking
+     the operating system about every running PID. A condition variable wakes
+     it when status and captured output are both ready; one ticker also wakes a
+     supplied watch poll callback every five milliseconds while children are
+     busy. *)
+  let notifier = create_completion_notifier () in
+  let rec send_tick () =
+    Thread.delay 0.005;
+    let continue =
+      with_lock notifier.mutex (fun () ->
+          if notifier.stopped then false
+          else (
+            notifier.generation <- notifier.generation + 1;
+            Condition.broadcast notifier.condition;
+            true))
+    in
+    if continue then send_tick ()
+  in
+  let deferred_signals = Signal_restore.create ~defer:true in
+  let ticker = ref None in
+  let stopped = ref false in
+  let stop () =
+    if not !stopped then (
+      with_lock notifier.mutex (fun () ->
+          notifier.stopped <- true;
+          Condition.broadcast notifier.condition);
+      Option.iter Thread.join !ticker;
+      stopped := true)
+  in
+  try
+    if ticker_enabled then ticker := Some (Thread.create send_tick ());
+    Fun.protect ~finally:stop (fun () ->
+        Signal_restore.restore deferred_signals;
+        action notifier)
+  with exn ->
+    stop ();
+    raise (Signal_restore.exception_after_restore deferred_signals exn)
+
+let close_noerr descriptor =
+  try Unix.close descriptor with Unix.Unix_error _ -> ()
+
+let start_capture ?on_chunk descriptor : capture =
+  let outcome = ref None in
+  let thread =
+    Thread.create
+      (fun () ->
+        outcome :=
+          Some
+            (try
+               Fun.protect
+                 ~finally:(fun () -> close_noerr descriptor)
+                 (fun () ->
+                   let output = Buffer.create 4096 in
+                   let bytes = Bytes.create 65536 in
+                   let rec read () =
+                     try
+                       match
+                         Unix.read descriptor bytes 0 (Bytes.length bytes)
+                       with
+                       | 0 -> ()
+                       | count ->
+                         (match on_chunk with
+                         | Some on_chunk -> on_chunk bytes count
+                         | None -> Buffer.add_subbytes output bytes 0 count);
+                         read ()
+                     with Unix.Unix_error (Unix.EINTR, _, _) -> read ()
+                   in
+                   read ();
+                   Ok
+                     (match on_chunk with
+                     | Some _ -> ""
+                     | None -> Buffer.contents output |> decode_utf8_lossy))
+             with exn -> Error exn))
+      ()
+  in
+  {thread; outcome}
+
+let capture_outcome (capture : capture) =
+  Thread.join capture.thread;
+  match !(capture.outcome) with
+  | Some outcome -> outcome
+  | None -> Error (Failure "subprocess output reader did not finish")
+
+let capture_error exn =
+  Error ("failed to capture subprocess output: " ^ Printexc.to_string exn)
+
+let start_child_wait pid notifier stdout_capture stderr_capture : child_wait =
+  let direct_outcome = Atomic.make None in
+  let outcome = Atomic.make None in
+  let rec wait () =
+    try
+      let _, status = Unix.waitpid [] pid in
+      status
+    with Unix.Unix_error (Unix.EINTR, _, _) -> wait ()
+  in
+  let thread =
+    Thread.create
+      (fun () ->
+        let status = try Ok (wait ()) with exn -> Error exn in
+        Atomic.set direct_outcome (Some status);
+        let stdout = capture_outcome stdout_capture in
+        let stderr = capture_outcome stderr_capture in
+        let result =
+          match (status, stdout, stderr) with
+          | Ok status, Ok stdout, Ok stderr -> Ok {status; stdout; stderr}
+          | Error exn, _, _ -> Error exn
+          | _, Error exn, _ | _, _, Error exn -> Error (capture_error exn)
+        in
+        Atomic.set outcome (Some result);
+        notify_completion notifier)
+      ()
+  in
+  {thread; direct_outcome; outcome}
+
+let fail_launch ownership deferred_signals launch_error =
+  Option.iter
+    (fun process ->
+      let pid = Platform.process_id process in
+      let root_reaped =
+        match ownership.child_wait with
+        | Some wait -> Option.is_some (Atomic.get wait.direct_outcome)
+        | None -> false
+      in
+      (match Platform.signal_process_tree ~root_reaped process Sys.sigkill with
+      | Ok () -> ()
+      | Error message -> ownership.termination_error <- Some message);
+      if
+        Option.is_none ownership.termination_error
+        && Option.is_none ownership.child_wait
+      then try ignore (Unix.waitpid [] pid) with Unix.Unix_error _ -> ())
+    ownership.process;
+  List.iter
+    (fun descriptor -> Option.iter close_noerr descriptor)
+    [
+      ownership.stdout_write;
+      ownership.stderr_write;
+      ownership.stdout_read;
+      ownership.stderr_read;
+      ownership.stdin;
+    ];
+  (match ownership.child_wait with
+  | Some wait when Option.is_none ownership.termination_error ->
+    Thread.join wait.thread
+  | Some _ -> ()
+  | None
+    when Option.is_none ownership.termination_error
+         && Option.is_some ownership.process ->
+    Option.iter
+      (fun (capture : capture) -> Thread.join capture.thread)
+      ownership.stdout_capture;
+    Option.iter
+      (fun (capture : capture) -> Thread.join capture.thread)
+      ownership.stderr_capture
+  | None -> ());
+  let release_error =
+    try
+      Option.iter Platform.release_process ownership.process;
+      None
+    with release_exn -> Some release_exn
+  in
+  let restore_error =
+    try
+      Signal_restore.restore deferred_signals;
+      None
+    with signal_exn -> Some signal_exn
+  in
+  let error =
+    match ownership.termination_error with
+    | Some message ->
+      Error
+        ("Could not terminate a partially launched subprocess tree: " ^ message)
+    | None -> (
+      match (restore_error, release_error) with
+      | Some signal_exn, _ -> signal_exn
+      | None, Some release_exn -> release_exn
+      | None, None -> launch_error)
+  in
+  raise error
+
+let launch ?env ?stdout_chunk ?stderr_chunk ?(stdin = Null_stdin)
+    ?(defer_signals = true) ~notifier payload job =
+  (* Capture descriptors need a cleanup owner before asynchronous watch
+     termination can raise. Signals are therefore deferred across pipe
+     acquisition and restored only after every descriptor has an owner. *)
+  let deferred_signals = Signal_restore.create ~defer:defer_signals in
+  let ownership = empty_launch_ownership () in
+  try
+    let pipes = Platform.create_capture_pipes () in
+    let (stdout_read, stdout_write), (stderr_read, stderr_write) = pipes in
+    ownership.stdout_read <- Some stdout_read;
+    ownership.stdout_write <- Some stdout_write;
+    ownership.stderr_read <- Some stderr_read;
+    ownership.stderr_write <- Some stderr_write;
+    let stdout = start_capture ?on_chunk:stdout_chunk stdout_read in
+    ownership.stdout_read <- None;
+    ownership.stdout_capture <- Some stdout;
+    let stderr = start_capture ?on_chunk:stderr_chunk stderr_read in
+    ownership.stderr_read <- None;
+    ownership.stderr_capture <- Some stderr;
+    let stdin =
+      match stdin with
+      | Inherit_stdin -> Unix.stdin
+      | Null_stdin ->
+        let descriptor =
+          Unix.openfile Platform.null_device [Unix.O_RDONLY; Unix.O_CLOEXEC] 0
+        in
+        ownership.stdin <- Some descriptor;
+        descriptor
+    in
+    let process =
+      Platform.spawn ~env ~cwd:job.cwd ~program:job.program ~args:job.args
+        ~stdin ~stdout:stdout_write ~stderr:stderr_write
+    in
+    ownership.process <- Some process;
+    let pid = Platform.process_id process in
+    Option.iter close_noerr ownership.stdin;
+    ownership.stdin <- None;
+    ownership.stdout_write <- None;
+    close_noerr stdout_write;
+    ownership.stderr_write <- None;
+    close_noerr stderr_write;
+    let wait = start_child_wait pid notifier stdout stderr in
+    ownership.child_wait <- Some wait;
+    Signal_restore.restore deferred_signals;
+    {payload; process; pid; child_wait = wait}
+  with exn -> fail_launch ownership deferred_signals exn
+
+let wait_for_running ~poll ?(defer_signals = true) notifier active =
+  let rec find_completed = function
+    | [] -> None
+    | (child : _ running) :: rest -> (
+      match Atomic.get child.child_wait.outcome with
+      | Some outcome -> Some (child, outcome)
+      | None -> find_completed rest)
+  in
+  let rec wait generation =
+    match find_completed active with
+    | Some (child, Ok result) ->
+      let deferred_signals = Signal_restore.create ~defer:defer_signals in
+      ((child, result), deferred_signals)
+    | Some (_, Error exn) -> raise exn
+    | None ->
+      poll ();
+      await_notification notifier generation |> wait
+  in
+  notifier_generation notifier |> wait
+
+let signal_running (children : _ running list) =
+  if children <> [] then (
+    let completion_ready (child : _ running) =
+      Option.is_some (Atomic.get child.child_wait.outcome)
+    in
+    let root_identity_lost (child : _ running) =
+      Option.is_some (Atomic.get child.child_wait.direct_outcome)
+    in
+    let signal_group signal (child : _ running) =
+      let root_reaped = root_identity_lost child in
+      Platform.signal_process_tree ~root_reaped child.process signal
+    in
+    let signal_all signal =
+      List.fold_left
+        (fun errors child ->
+          if completion_ready child then errors
+          else
+            match signal_group signal child with
+            | Ok () -> errors
+            | Error message -> (child, message) :: errors)
+        [] children
+    in
+    let graceful_signal = Platform.graceful_termination_signal in
+    let graceful_errors = signal_all graceful_signal in
+    let deadline = Unix.gettimeofday () +. 0.25 in
+    let rec wait_until_deadline children =
+      let remaining =
+        List.filter (fun child -> not (root_identity_lost child)) children
+      in
+      if remaining <> [] && Unix.gettimeofday () < deadline then (
+        ignore (Unix.select [] [] [] 0.01);
+        wait_until_deadline remaining)
+      else remaining
+    in
+    ignore (wait_until_deadline children);
+    (* Every original process group needs escalation because a direct child can
+       exit while a PPX or helper in its group remains alive. *)
+    let termination_errors =
+      if Platform.escalate_process_groups then signal_all Sys.sigkill
+      else graceful_errors
+    in
+    (* On Darwin, signalling a group containing only zombies returns EPERM.
+       Pipe closure and waiter publication happen on separate threads, so the
+       process result can become ready just after that harmless failure. A
+       bounded wait distinguishes this race from a live tree that the platform
+       genuinely could not terminate. *)
+    let completion_deadline = Unix.gettimeofday () +. 0.25 in
+    let rec unresolved errors =
+      let errors =
+        List.filter (fun (child, _) -> not (completion_ready child)) errors
+      in
+      if errors <> [] && Unix.gettimeofday () < completion_deadline then (
+        ignore (Unix.select [] [] [] 0.01);
+        unresolved errors)
+      else errors
+    in
+    match unresolved termination_errors |> List.rev with
+    | [] -> ()
+    | errors ->
+      raise
+        (Error
+           ("Could not terminate a subprocess tree: "
+           ^ String.concat "; " (List.map snd errors))))
+
+let release_after_completion (child : _ running) =
+  ignore
+    (Thread.create
+       (fun () ->
+         Thread.join child.child_wait.thread;
+         Platform.release_process child.process)
+       ())
+
+let payload (child : _ running) = child.payload
+let pid (child : _ running) = child.pid
+let await_termination (child : _ running) = Thread.join child.child_wait.thread
+let release (child : _ running) = Platform.release_process child.process
+
+let terminate_running (children : _ running list) =
+  if children <> [] then (
+    try
+      signal_running children;
+      List.iter
+        (fun (child : _ running) ->
+          Thread.join child.child_wait.thread;
+          Platform.release_process child.process)
+        children
+    with exn ->
+      List.iter release_after_completion children;
+      raise exn)
+
+let release_running (child : _ running) =
+  await_termination child;
+  release child
diff --git a/rewatch-ocaml/process_child.mli b/rewatch-ocaml/process_child.mli
new file mode 100644
index 00000000000..cf824b43c76
--- /dev/null
+++ b/rewatch-ocaml/process_child.mli
@@ -0,0 +1,52 @@
+(** A running child is abstract so descriptors, reader threads, the waiter, and
+    the platform process handle always have one cleanup owner. Callers may wait,
+    cancel, or release it, but cannot reconstruct a partially owned child. *)
+
+type result = {status: Unix.process_status; stdout: string; stderr: string}
+type job = {program: string; args: string list; cwd: string}
+type stdin_policy = Inherit_stdin | Null_stdin
+
+exception Error of string
+
+val decode_utf8_lossy : string -> string
+val succeeded : result -> bool
+val status_string : Unix.process_status -> string
+
+type completion_notifier
+type 'a running
+
+val with_lock : Mutex.t -> (unit -> 'a) -> 'a
+
+val with_completion_notifier :
+  ticker_enabled:bool -> (completion_notifier -> 'a) -> 'a
+
+val notify_completion : completion_notifier -> unit
+val notifier_generation : completion_notifier -> int
+val await_notification : completion_notifier -> int -> int
+
+val launch :
+  ?env:Spawn.Env.t ->
+  ?stdout_chunk:(bytes -> int -> unit) ->
+  ?stderr_chunk:(bytes -> int -> unit) ->
+  ?stdin:stdin_policy ->
+  ?defer_signals:bool ->
+  notifier:completion_notifier ->
+  'a ->
+  job ->
+  'a running
+
+val wait_for_running :
+  poll:(unit -> unit) ->
+  ?defer_signals:bool ->
+  completion_notifier ->
+  'a running list ->
+  ('a running * result) * Signal_restore.t
+
+val payload : 'a running -> 'a
+val pid : 'a running -> int
+val signal_running : 'a running list -> unit
+val await_termination : 'a running -> unit
+val release : 'a running -> unit
+val release_after_completion : 'a running -> unit
+val terminate_running : 'a running list -> unit
+val release_running : 'a running -> unit
diff --git a/rewatch-ocaml/project_context.ml b/rewatch-ocaml/project_context.ml
new file mode 100644
index 00000000000..a72fc4745ba
--- /dev/null
+++ b/rewatch-ocaml/project_context.ml
@@ -0,0 +1,189 @@
+exception Error of string
+exception Package_error of string
+
+let canonical_project_root folder =
+  if not (File_util.exists folder) then
+    raise
+      (Error
+         ("Could not start Rescript build: Could not write lockfile because \
+           the specified project folder does not exist: " ^ folder));
+  Platform.canonicalize_path folder
+
+type dependency_context = {
+  current_root: string;
+  workspace_root: string;
+  allow_upward_search: bool;
+  include_workspace_dependencies: bool;
+}
+
+let path_is_within_canonical ~root path =
+  let normalize = Platform.normalize_path_for_comparison in
+  let root = normalize root in
+  let path = normalize path in
+  path = root || String.starts_with ~prefix:(Filename.concat root "") path
+
+let rec nearest_config_path directory =
+  if Config.exists_in_root directory then Some (Config.path_in_root directory)
+  else
+    let parent = Filename.dirname directory in
+    if parent = directory then None else nearest_config_path parent
+
+(* Graph roots and resolved dependency paths are already canonical. Keeping this
+   predicate pure avoids repeating realpath calls throughout package traversal. *)
+let is_local_dependency_canonical ~workspace path =
+  let equal_component left right =
+    Platform.normalize_path_for_comparison left
+    = Platform.normalize_path_for_comparison right
+  in
+  let rec contains_component path component =
+    if equal_component (Filename.basename path) component then true
+    else
+      let parent = Filename.dirname path in
+      parent <> path && contains_component parent component
+  in
+  path_is_within_canonical ~root:workspace path
+  && not (contains_component path "node_modules")
+
+let workspace_lock_root_for (current : Config.t) =
+  match nearest_config_path (Filename.dirname current.root) with
+  | Some path -> (
+    match Config.load path with
+    | parent
+      when List.exists
+             (fun (dependency : Config.dependency) ->
+               dependency.name = current.name)
+             (parent.dependencies @ parent.dev_dependencies) ->
+      parent.root
+    | _ -> current.root)
+  | None -> current.root
+
+let workspace_lock_root folder =
+  workspace_lock_root_for (Config.load_root folder)
+
+let dependency_context (current : Config.t) =
+  let workspace_root = workspace_lock_root_for current in
+  let has_local_dependency (dependency : Config.dependency) =
+    let candidate =
+      Filename.concat
+        (Filename.concat current.root "node_modules")
+        dependency.name
+    in
+    File_util.exists candidate
+    && is_local_dependency_canonical ~workspace:current.root
+         (Platform.canonicalize_path candidate)
+  in
+  let is_monorepo_root =
+    List.exists has_local_dependency
+      (current.dependencies @ current.dev_dependencies)
+  in
+  {
+    current_root = current.root;
+    workspace_root;
+    allow_upward_search = workspace_root = current.root && not is_monorepo_root;
+    include_workspace_dependencies = workspace_root = current.root;
+  }
+
+let dependency_is_local_canonical context path =
+  (* A command run from one package listed by a parent workspace owns only that
+     package. Sibling packages still resolve through the workspace, but treating
+     them as local would also include their development sources, clean their
+     outputs, and watch their source trees. *)
+  context.include_workspace_dependencies
+  && is_local_dependency_canonical ~workspace:context.workspace_root path
+
+let dependency_candidates_in context package_root name =
+  let candidate root =
+    Filename.concat (Filename.concat root "node_modules") name
+  in
+  let rec in_ancestors directory acc =
+    let candidate =
+      Filename.concat (Filename.concat directory "node_modules") name
+    in
+    let parent = Filename.dirname directory in
+    if parent = directory then List.rev (candidate :: acc)
+    else in_ancestors parent (candidate :: acc)
+  in
+  let deduplicate values =
+    let seen = Hashtbl.create (List.length values) in
+    List.fold_left
+      (fun unique value ->
+        if Hashtbl.mem seen value then unique
+        else (
+          Hashtbl.add seen value ();
+          value :: unique))
+      [] values
+    |> List.rev
+  in
+  let direct =
+    [
+      candidate package_root;
+      candidate context.current_root;
+      candidate context.workspace_root;
+    ]
+    |> deduplicate
+  in
+  if context.allow_upward_search then
+    deduplicate (direct @ in_ancestors (Filename.dirname package_root) [])
+  else direct
+
+let dependency_path_in context package_root name =
+  let existing_realpath path =
+    if File_util.exists path then Some (Platform.canonicalize_path path)
+    else None
+  in
+  dependency_candidates_in context package_root name
+  |> List.find_map existing_realpath
+
+let standalone_dependency_context root =
+  {
+    current_root = root;
+    workspace_root = root;
+    allow_upward_search = true;
+    include_workspace_dependencies = true;
+  }
+
+let dependency_path root name =
+  dependency_path_in (standalone_dependency_context root) root name
+
+let require_dependency_directory ~context package_root
+    (dependency : Config.dependency) =
+  match dependency_path_in context package_root dependency.name with
+  | None ->
+    raise
+      (Package_error
+         (Printf.sprintf
+            "Could not build package tree reading dependency '%s' at path \
+             '%s'. Error: Could not resolve dependency %s"
+            dependency.name context.current_root dependency.name))
+  | Some directory when not (Config.exists_in_root directory) ->
+    raise
+      (Package_error
+         (Printf.sprintf
+            "Could not build package tree for '%s' at path '%s'. Error: no \
+             rescript.json or bsconfig.json in %s"
+            dependency.name context.current_root directory))
+  | Some directory -> directory
+
+let relative_to_opt root path =
+  let prefix = Filename.concat root "" in
+  let comparable = Platform.normalize_path_for_comparison in
+  if comparable path = comparable root then Some "."
+  else if String.starts_with ~prefix:(comparable prefix) (comparable path) then
+    Some
+      (String.sub path (String.length prefix)
+         (String.length path - String.length prefix))
+  else None
+
+let relative_to root path =
+  match relative_to_opt root path with
+  | Some relative -> relative
+  | None -> raise (Error (path ^ " is not inside " ^ root))
+
+let relative_or_absolute ~root path =
+  Option.value (relative_to_opt root path) ~default:path
+
+let display_path ~root path =
+  match relative_or_absolute ~root path with
+  | "." -> "."
+  | relative when path_is_within_canonical ~root path -> "./" ^ relative
+  | absolute -> absolute
diff --git a/rewatch-ocaml/project_context.mli b/rewatch-ocaml/project_context.mli
new file mode 100644
index 00000000000..4e29d95f05a
--- /dev/null
+++ b/rewatch-ocaml/project_context.mli
@@ -0,0 +1,29 @@
+exception Error of string
+exception Package_error of string
+
+type dependency_context
+
+val canonical_project_root : string -> string
+
+val nearest_config_path : string -> string option
+
+val workspace_lock_root_for : Config.t -> string
+val workspace_lock_root : string -> string
+val dependency_context : Config.t -> dependency_context
+val dependency_is_local_canonical : dependency_context -> string -> bool
+
+val dependency_candidates_in :
+  dependency_context -> string -> string -> string list
+
+val dependency_path_in : dependency_context -> string -> string -> string option
+
+val dependency_path : string -> string -> string option
+
+val require_dependency_directory :
+  context:dependency_context -> string -> Config.dependency -> string
+
+val relative_to : string -> string -> string
+val relative_to_opt : string -> string -> string option
+val relative_or_absolute : root:string -> string -> string
+val display_path : root:string -> string -> string
+val path_is_within_canonical : root:string -> string -> bool
diff --git a/rewatch-ocaml/rescript_ocaml.ml b/rewatch-ocaml/rescript_ocaml.ml
new file mode 100644
index 00000000000..9dacdeed9f2
--- /dev/null
+++ b/rewatch-ocaml/rescript_ocaml.ml
@@ -0,0 +1,99 @@
+(* Signal handlers can run on any domain, including between spawning a child
+   and registering its cleanup owner. Recording the request and raising from
+   the command's poll point keeps unwinding on the domain that owns scheduler,
+   lock, and temporary-output cleanup. *)
+let with_termination_handlers action =
+  let requested_exit = Atomic.make 0 in
+  let interrupt signal =
+    let exit_code =
+      if signal = Sys.sigint then 130
+      else if signal = Sys.sigterm then 143
+      else 1
+    in
+    ignore (Atomic.compare_and_set requested_exit 0 exit_code)
+  in
+  let poll () =
+    let exit_code = Atomic.get requested_exit in
+    if exit_code <> 0 then raise (Process.Interrupted exit_code)
+  in
+  let previous_sigint = Sys.signal Sys.sigint (Sys.Signal_handle interrupt) in
+  let result =
+    Fun.protect
+      (fun () ->
+        let previous_sigterm =
+          Sys.signal Sys.sigterm (Sys.Signal_handle interrupt)
+        in
+        Fun.protect
+          (fun () -> action ~poll)
+          ~finally:(fun () -> ignore (Sys.signal Sys.sigterm previous_sigterm)))
+      ~finally:(fun () -> ignore (Sys.signal Sys.sigint previous_sigint))
+  in
+  poll ();
+  result
+
+let run_command ~poll = function
+  | Cli.Build
+      {
+        verbosity;
+        folder;
+        prod;
+        features;
+        warn_error;
+        after_build;
+        filter;
+        clear_screen;
+        no_timing;
+      } ->
+    ignore clear_screen;
+    Build.run ~poll ~verbosity ~folder ~prod ~features ~warn_error ~after_build
+      ~filter ~no_timing
+  | Cli.Watch
+      {
+        verbosity;
+        folder;
+        prod;
+        features;
+        warn_error;
+        after_build;
+        filter;
+        clear_screen;
+        no_timing;
+      } ->
+    ignore no_timing;
+    Build.watch ~verbosity ~folder ~prod ~features ~warn_error ~after_build
+      ~filter ~clear_screen
+  | Cli.Format (Cli.Format_stdin extension) ->
+    Format.format_stdin ~poll extension
+  | Cli.Format (Cli.Format_files {check; paths}) ->
+    Format.run_files ~poll ~check paths
+  | Cli.Compiler_args path -> print_endline (Compiler_args_command.run path)
+  | Cli.Clean {verbosity; folder; prod} ->
+    Clean.run ~poll ~verbosity ~folder ~prod
+
+let run = function
+  | Cli.Watch _ as command -> run_command ~poll:(fun () -> ()) command
+  | command ->
+    with_termination_handlers (fun ~poll -> run_command ~poll command)
+
+let () =
+  Platform.configure_standard_streams ();
+  try
+    match Cli.eval Sys.argv with
+    | Cli.Run command -> run command
+    | Cli.Exit code -> exit code
+  with
+  | Project_context.Package_error message ->
+    prerr_endline message;
+    exit 2
+  | Config.Error message
+  | Source.Error message
+  | Project_context.Error message
+  | Process.Error message
+  | Format.Error message ->
+    prerr_endline message;
+    exit 1
+  | Watcher.Stop -> exit 0
+  | Process.Interrupted exit_code -> exit exit_code
+  | (Sys_error _ as exn) | (Unix.Unix_error _ as exn) ->
+    prerr_endline (Printexc.to_string exn);
+    exit 1
diff --git a/rewatch-ocaml/rewatch_version.ml b/rewatch-ocaml/rewatch_version.ml
new file mode 100644
index 00000000000..f57117ff29e
--- /dev/null
+++ b/rewatch-ocaml/rewatch_version.ml
@@ -0,0 +1 @@
+let version = "13.0.0-alpha.6"
diff --git a/rewatch-ocaml/rewatch_version.mli b/rewatch-ocaml/rewatch_version.mli
new file mode 100644
index 00000000000..8579193c227
--- /dev/null
+++ b/rewatch-ocaml/rewatch_version.mli
@@ -0,0 +1 @@
+val version : string
diff --git a/rewatch-ocaml/signal_restore.ml b/rewatch-ocaml/signal_restore.ml
new file mode 100644
index 00000000000..16fc46fd20d
--- /dev/null
+++ b/rewatch-ocaml/signal_restore.ml
@@ -0,0 +1,26 @@
+type t = {restore_once: unit -> unit; mutable restored: bool}
+
+let create ~defer =
+  {
+    restore_once =
+      (if defer then Platform.defer_termination_signals () else Fun.id);
+    restored = false;
+  }
+
+let restore state =
+  if not state.restored then (
+    state.restored <- true;
+    state.restore_once ())
+
+let exception_after_restore state original =
+  try
+    restore state;
+    original
+  with restoration_error -> restoration_error
+
+let protect state action =
+  try
+    let result = action () in
+    restore state;
+    result
+  with error -> raise (exception_after_restore state error)
diff --git a/rewatch-ocaml/signal_restore.mli b/rewatch-ocaml/signal_restore.mli
new file mode 100644
index 00000000000..ded3c89bcaa
--- /dev/null
+++ b/rewatch-ocaml/signal_restore.mli
@@ -0,0 +1,9 @@
+type t
+(** Signal restoration is explicit because termination is deferred across
+    short ownership-transfer windows. [protect] restores exactly once and
+    preserves the original exception if restoration also fails. *)
+
+val create : defer:bool -> t
+val restore : t -> unit
+val exception_after_restore : t -> exn -> exn
+val protect : t -> (unit -> 'a) -> 'a
diff --git a/rewatch-ocaml/source.ml b/rewatch-ocaml/source.ml
new file mode 100644
index 00000000000..08819878ddb
--- /dev/null
+++ b/rewatch-ocaml/source.ml
@@ -0,0 +1,447 @@
+type module_ = {
+  name: string;
+  implementation: string;
+  interface: string option;
+  is_dev: bool;
+}
+
+type discovery = {
+  modules: module_ list;
+  source_mtimes: (string * float) list;
+  inventory_files: string list;
+  present_files: string list;
+  gentype_dirs: string list;
+}
+
+type discovered_file = {path: string; modified: float}
+type source_kind = Implementation | Interface
+
+type scanned_file = {file: discovered_file; kind: source_kind; is_dev: bool}
+
+type scanned_sources = {
+  files: scanned_file list;
+  inventory_files: string list;
+  present_files: string list;
+  gentype_dirs: string list;
+}
+
+exception Error of string
+
+let source_kind path =
+  match Filename.extension path with
+  | ".res" -> Some Implementation
+  | ".resi" -> Some Interface
+  | _ -> None
+
+let module_name path =
+  path |> Filename.basename |> Filename.remove_extension
+  |> String.capitalize_ascii
+
+let is_non_exotic_module_name name =
+  let is_ascii_uppercase = function
+    | 'A' .. 'Z' -> true
+    | _ -> false
+  in
+  let is_ascii_alphanumeric = function
+    | 'A' .. 'Z' | 'a' .. 'z' | '0' .. '9' -> true
+    | _ -> false
+  in
+  let rec valid_tail index =
+    if index = String.length name then true
+    else
+      let character = name.[index] in
+      (is_ascii_alphanumeric character || character = '_')
+      && valid_tail (index + 1)
+  in
+  String.length name > 0 && is_ascii_uppercase name.[0] && valid_tail 1
+
+let display_path ~display_root root path =
+  let absolute =
+    if Filename.is_relative path then Filename.concat root path else path
+  in
+  let display_root = Platform.canonicalize_path display_root in
+  Project_context.relative_or_absolute ~root:display_root absolute
+
+let duplicate_error ~display_root root name first second =
+  let first, second =
+    let first = display_path ~display_root root first in
+    let second = display_path ~display_root root second in
+    if String.compare first second <= 0 then (first, second) else (second, first)
+  in
+  Error
+    (Printf.sprintf
+       "Could not initialize build: Duplicate module name: %s. Found in %s and \
+        %s. Rename one of these files."
+       name first second)
+
+let interface_mismatch_error implementation interface =
+  Error
+    (Printf.sprintf
+       "Could not initialize build: Implementation and interface have \
+        different path names or different cases: `%s` vs `%s`"
+       implementation interface)
+
+(* A package source tree has three consumers with deliberately different
+   recursion rules. Compilation follows directory links and honors source
+   activation/subdirs; cleanup inventories every real descendant but treats
+   links as leaves; GenType records every configured directory that its
+   subdirs setting reaches. Keeping the views in one walk gives every consumer
+   the same filesystem snapshot without weakening stale-output cleanup. *)
+let scan_source ~root (source : Config.source) ~discover_modules ~on_missing
+    ~visited_dirs ~visited_inventory_dirs ~collect_inventory ~collect_gentype
+    ~visited_gentype_dirs candidates inventory_files present_files gentype_dirs
+    =
+  let rec scan_directory ~relative ~collect_inventory ~discover_requested
+      ~collect_gentype ~identity =
+    let absolute = Filename.concat root relative in
+    let coverage table requested =
+      if requested then
+        Traversal_coverage.admit table identity ~recursive:source.recurse
+      else Traversal_coverage.Skip
+    in
+    let discovery = coverage visited_dirs discover_requested in
+    let gentype = coverage visited_gentype_dirs collect_gentype in
+    let inventory =
+      if collect_inventory then
+        Traversal_coverage.admit visited_inventory_dirs identity ~recursive:true
+      else Traversal_coverage.Skip
+    in
+    let discover_here = Traversal_coverage.visits_current discovery in
+    let discover_children = Traversal_coverage.visits_descendants discovery in
+    let inventory_here = Traversal_coverage.visits_current inventory in
+    let inventory_children = Traversal_coverage.visits_descendants inventory in
+    let gentype_here = Traversal_coverage.visits_current gentype in
+    let gentype_children = Traversal_coverage.visits_descendants gentype in
+    if gentype_here then gentype_dirs := relative :: !gentype_dirs;
+    if
+      inventory <> Traversal_coverage.Skip
+      || discovery <> Traversal_coverage.Skip
+      || gentype <> Traversal_coverage.Skip
+    then
+      let entries =
+        try File_util.directory_entries absolute |> List.sort String.compare
+        with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+          if discover_here || discover_children then on_missing absolute;
+          []
+      in
+      let record_candidate relative_path absolute_path metadata =
+        present_files := absolute_path :: !present_files;
+        if inventory_here then
+          inventory_files := absolute_path :: !inventory_files;
+        if discover_here && metadata.Unix.st_kind = Unix.S_REG then
+          match source_kind relative_path with
+          | None -> ()
+          | Some kind ->
+            candidates :=
+              {
+                file = {path = relative_path; modified = metadata.Unix.st_mtime};
+                kind;
+                is_dev = source.is_dev;
+              }
+              :: !candidates
+      in
+      List.iter
+        (fun name ->
+          let relative_path = Filename.concat relative name in
+          let absolute_path = Filename.concat root relative_path in
+          match Unix.lstat absolute_path with
+          | metadata -> (
+            match metadata.Unix.st_kind with
+            | Unix.S_DIR ->
+              if inventory_children || discover_children || gentype_children
+              then
+                let identity =
+                  Platform.directory_identity ~path:absolute_path metadata
+                in
+                scan_directory ~relative:relative_path
+                  ~collect_inventory:inventory_children
+                  ~discover_requested:discover_children
+                  ~collect_gentype:gentype_children ~identity
+            | Unix.S_LNK -> (
+              match Unix.stat absolute_path with
+              | target_metadata -> (
+                match target_metadata.Unix.st_kind with
+                | Unix.S_DIR ->
+                  if inventory_here then
+                    inventory_files := absolute_path :: !inventory_files;
+                  if discover_children || gentype_children then
+                    let identity =
+                      Platform.directory_identity ~path:absolute_path
+                        target_metadata
+                    in
+                    scan_directory ~relative:relative_path
+                      ~collect_inventory:false
+                      ~discover_requested:discover_children
+                      ~collect_gentype:gentype_children ~identity
+                | _ ->
+                  record_candidate relative_path absolute_path target_metadata)
+              | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _)
+                ->
+                ())
+            | _ -> record_candidate relative_path absolute_path metadata)
+          | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> ())
+        entries
+  in
+  let relative = source.dir in
+  let absolute = Filename.concat root relative in
+  match Unix.lstat absolute with
+  | metadata -> (
+    match metadata.Unix.st_kind with
+    | Unix.S_DIR ->
+      let identity = Platform.directory_identity ~path:absolute metadata in
+      scan_directory ~relative ~collect_inventory
+        ~discover_requested:discover_modules ~collect_gentype ~identity
+    | Unix.S_LNK -> (
+      match Unix.stat absolute with
+      | target_metadata -> (
+        match target_metadata.Unix.st_kind with
+        | Unix.S_DIR ->
+          inventory_files := absolute :: !inventory_files;
+          let identity =
+            Platform.directory_identity ~path:absolute target_metadata
+          in
+          scan_directory ~relative ~collect_inventory:false
+            ~discover_requested:discover_modules ~collect_gentype ~identity
+        | _ ->
+          present_files := absolute :: !present_files;
+          inventory_files := absolute :: !inventory_files;
+          if discover_modules then on_missing absolute)
+      | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+        if discover_modules then on_missing absolute)
+    | _ ->
+      present_files := absolute :: !present_files;
+      inventory_files := absolute :: !inventory_files;
+      if discover_modules then on_missing absolute)
+  | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+    if discover_modules then on_missing absolute
+
+let resolve_active_features (config : Config.t) requested =
+  let active_features = Hashtbl.create 16 in
+  let visiting_features = Hashtbl.create 16 in
+  let features = Hashtbl.create (List.length config.features) in
+  List.iter
+    (fun (name, implied) -> Hashtbl.replace features name implied)
+    config.features;
+  let raise_feature_cycle feature visiting =
+    let chain = List.rev (feature :: visiting) |> String.concat " -> " in
+    raise (Error ("Cycle detected in `features` map: " ^ chain))
+  in
+  let rec activate feature visiting =
+    if Hashtbl.mem visiting_features feature then
+      raise_feature_cycle feature visiting;
+    if not (Hashtbl.mem active_features feature) then (
+      Hashtbl.add visiting_features feature ();
+      match Hashtbl.find_opt features feature with
+      | None ->
+        Hashtbl.remove visiting_features feature;
+        Hashtbl.add active_features feature ()
+      | Some implied ->
+        List.iter (fun name -> activate name (feature :: visiting)) implied;
+        Hashtbl.remove visiting_features feature;
+        Hashtbl.add active_features feature ())
+  in
+  List.iter (fun feature -> activate feature []) requested;
+  active_features
+
+let source_is_active ~prod ~all_features ~active_features
+    (source : Config.source) =
+  let feature_enabled =
+    all_features
+    || Option.fold ~none:true
+         ~some:(fun feature -> Hashtbl.mem active_features feature)
+         source.feature
+  in
+  (not (prod && source.is_dev)) && feature_enabled
+
+let active_sources (config : Config.t) ~prod ~features =
+  let active_features =
+    resolve_active_features config (Option.value features ~default:[])
+  in
+  let all_features = Option.is_none features in
+  List.filter
+    (source_is_active ~prod ~all_features ~active_features)
+    config.sources
+
+let namespace_members ~entry modules =
+  modules
+  |> List.filter (fun module_ -> Some module_.name <> entry)
+  |> List.filter (fun module_ -> is_non_exotic_module_name module_.name)
+
+let scan_sources ~on_missing (config : Config.t) ~prod ~features
+    ~collect_inventory ~collect_gentype =
+  let active_features =
+    resolve_active_features config (Option.value features ~default:[])
+  in
+  let all_features = features = None in
+  let visited_dirs = Hashtbl.create 32 in
+  let visited_inventory_dirs = Hashtbl.create 32 in
+  let visited_gentype_dirs = Hashtbl.create 32 in
+  let files = ref [] in
+  let inventory_files = ref [] in
+  let present_files = ref [] in
+  let gentype_dirs = ref [] in
+  config.sources
+  |> List.iter (fun (source : Config.source) ->
+      let feature_enabled =
+        all_features
+        || Option.fold ~none:true
+             ~some:(fun feature -> Hashtbl.mem active_features feature)
+             source.feature
+      in
+      let discover_modules =
+        source_is_active ~prod ~all_features ~active_features source
+      in
+      scan_source ~root:config.root source ~discover_modules ~on_missing
+        ~visited_dirs ~visited_inventory_dirs ~collect_inventory
+        ~collect_gentype:(collect_gentype && feature_enabled)
+        ~visited_gentype_dirs files inventory_files present_files gentype_dirs);
+  {
+    files = !files;
+    inventory_files = List.sort_uniq String.compare !inventory_files;
+    present_files = List.sort_uniq String.compare !present_files;
+    gentype_dirs = List.sort_uniq String.compare !gentype_dirs;
+  }
+
+let discover_for_cleanup
+    ?(on_missing =
+      fun path -> Printf.eprintf "Could not read folder %s\n%!" path)
+    (config : Config.t) ~prod =
+  let scanned =
+    scan_sources ~on_missing config ~prod ~features:None
+      ~collect_inventory:false ~collect_gentype:false
+  in
+  let implementations =
+    scanned.files
+    |> List.filter_map (fun source ->
+        match source.kind with
+        | Interface -> None
+        | Implementation -> Some source.file.path)
+    |> List.sort_uniq String.compare
+  in
+  implementations
+
+let discover_files
+    ?(on_missing =
+      fun path -> Printf.eprintf "Could not read folder %s\n%!" path)
+    (config : Config.t) ~prod ~features ~filter =
+  let matches_filter =
+    match filter with
+    | None -> fun _ -> true
+    | Some filter -> Source_filter.matches_basename filter
+  in
+  let scanned =
+    scan_sources ~on_missing config ~prod ~features ~collect_inventory:false
+      ~collect_gentype:false
+  in
+  scanned.files
+  |> List.filter_map (fun source ->
+      if matches_filter source.file.path then Some source.file.path else None)
+  |> List.sort_uniq String.compare
+
+let discover_with_inventory ?(on_orphan = fun _ -> ())
+    ?(on_missing =
+      fun path -> Printf.eprintf "Could not read folder %s\n%!" path)
+    ?(display_root = Sys.getcwd ()) (config : Config.t) ~prod ~features ~filter
+    =
+  let matches_filter =
+    match filter with
+    | None -> fun _ -> true
+    | Some filter -> Source_filter.matches_basename filter
+  in
+  let scanned =
+    scan_sources ~on_missing config ~prod ~features ~collect_inventory:true
+      ~collect_gentype:(config.gentype_args <> [])
+  in
+  let files = scanned.files in
+  let table = Hashtbl.create (List.length files) in
+  List.iter
+    (fun source ->
+      let file = source.file in
+      let is_dev = source.is_dev in
+      let name = module_name file.path in
+      let implementation, interface, old_dev =
+        match Hashtbl.find_opt table name with
+        | None -> (None, None, is_dev)
+        | Some values -> values
+      in
+      match source.kind with
+      | Interface -> (
+        match interface with
+        | Some previous ->
+          raise
+            (duplicate_error ~display_root config.root name previous.path
+               file.path)
+        | None ->
+          Hashtbl.replace table name
+            (implementation, Some file, old_dev || is_dev))
+      | Implementation -> (
+        match implementation with
+        | Some previous ->
+          raise
+            (duplicate_error ~display_root config.root name previous.path
+               file.path)
+        | None ->
+          Hashtbl.replace table name (Some file, interface, old_dev || is_dev)))
+    (List.filter (fun source -> matches_filter source.file.path) files);
+  Hashtbl.iter
+    (fun _ (implementation, interface, _) ->
+      match (implementation, interface) with
+      | Some implementation, Some interface
+        when Filename.remove_extension implementation.path
+             <> Filename.remove_extension interface.path ->
+        raise (interface_mismatch_error implementation.path interface.path)
+      | _ -> ())
+    table;
+  Hashtbl.to_seq table
+  |> Seq.filter_map (fun (_, (implementation, interface, _)) ->
+      match (implementation, interface) with
+      | None, Some interface -> Some interface.path
+      | _ -> None)
+  |> List.of_seq |> List.sort String.compare |> List.iter on_orphan;
+  let modules =
+    Hashtbl.to_seq table
+    |> Seq.filter_map (fun (name, (implementation, interface, is_dev)) ->
+        match implementation with
+        | None -> None
+        | Some implementation ->
+          Some
+            {
+              name;
+              implementation = implementation.path;
+              interface = Option.map (fun file -> file.path) interface;
+              is_dev;
+            })
+    |> List.of_seq
+    |> List.sort (fun a b -> String.compare a.name b.name)
+  in
+  let source_mtimes =
+    Hashtbl.to_seq_values table
+    |> Seq.flat_map (fun (implementation, interface, _) ->
+        List.to_seq (Option.to_list implementation @ Option.to_list interface))
+    |> Seq.map (fun file -> (file.path, file.modified))
+    |> List.of_seq
+  in
+  {
+    modules;
+    source_mtimes;
+    inventory_files = scanned.inventory_files;
+    present_files = scanned.present_files;
+    gentype_dirs = scanned.gentype_dirs;
+  }
+
+let ast_path path =
+  Filename.remove_extension path
+  ^ if Filename.extension path = ".resi" then ".iast" else ".ast"
+
+let compiler_basename config module_name =
+  Config.namespaced_module_name config.Config.namespace module_name
+
+(* Compiler artifacts preserve the source filename's case, while dependency
+   graph module names are capitalized. Keep those two names distinct. *)
+let compiler_asset_basename config path =
+  let basename = path |> Filename.basename |> Filename.remove_extension in
+  match config.Config.namespace with
+  | Config.No_namespace -> basename
+  | Config.Namespace namespace -> basename ^ "-" ^ namespace
+  | Config.Namespace_with_entry {name; entry} ->
+    if entry = module_name path then basename else basename ^ "-@" ^ name
diff --git a/rewatch-ocaml/source.mli b/rewatch-ocaml/source.mli
new file mode 100644
index 00000000000..6809f8d0337
--- /dev/null
+++ b/rewatch-ocaml/source.mli
@@ -0,0 +1,60 @@
+(** Discovery keeps filesystem files separate from compilable modules. Commands
+    such as formatting can operate on orphan interfaces and duplicate basenames,
+    while compilation uses [module_] after pairing and duplicate validation. *)
+
+type module_ = {
+  name: string;
+  implementation: string;
+  interface: string option;
+  is_dev: bool;
+}
+
+type discovery = {
+  modules: module_ list;
+  source_mtimes: (string * float) list;
+  inventory_files: string list;
+  present_files: string list;
+  gentype_dirs: string list;
+}
+
+exception Error of string
+
+type source_kind = Implementation | Interface
+
+val source_kind : string -> source_kind option
+val module_name : string -> string
+val namespace_members : entry:string option -> module_ list -> module_ list
+
+val duplicate_error :
+  display_root:string -> string -> string -> string -> string -> exn
+
+val resolve_active_features :
+  Config.t -> string list -> (string, unit) Hashtbl.t
+
+val active_sources :
+  Config.t -> prod:bool -> features:string list option -> Config.source list
+
+val discover_for_cleanup :
+  ?on_missing:(string -> unit) -> Config.t -> prod:bool -> string list
+
+val discover_files :
+  ?on_missing:(string -> unit) ->
+  Config.t ->
+  prod:bool ->
+  features:string list option ->
+  filter:Source_filter.t option ->
+  string list
+
+val discover_with_inventory :
+  ?on_orphan:(string -> unit) ->
+  ?on_missing:(string -> unit) ->
+  ?display_root:string ->
+  Config.t ->
+  prod:bool ->
+  features:string list option ->
+  filter:Source_filter.t option ->
+  discovery
+
+val ast_path : string -> string
+val compiler_basename : Config.t -> string -> string
+val compiler_asset_basename : Config.t -> string -> string
diff --git a/rewatch-ocaml/source_dirs.ml b/rewatch-ocaml/source_dirs.ml
new file mode 100644
index 00000000000..5bb0c46bd98
--- /dev/null
+++ b/rewatch-ocaml/source_dirs.ml
@@ -0,0 +1,98 @@
+type scan = {
+  build_root: string;
+  scan_dirs: string list;
+  also_scan_build_root: bool;
+}
+
+let scan_json scan =
+  `Assoc
+    [
+      ("also_scan_build_root", `Bool scan.also_scan_build_root);
+      ("build_root", `String scan.build_root);
+      ("scan_dirs", `List (List.map (fun path -> `String path) scan.scan_dirs));
+    ]
+
+let write ~root ~dirs ~packages ~scans =
+  let path = File_util.path_of_parts root ["lib"; "bs"; ".sourcedirs.json"] in
+  File_util.ensure_dir (Filename.dirname path);
+  let json =
+    `Assoc
+      [
+        ("cmt_scan", `List (List.map scan_json scans));
+        ("dirs", `List (List.map (fun path -> `String path) dirs));
+        ("generated", `List []);
+        ( "pkgs",
+          `List
+            (List.map
+               (fun (name, path) -> `List [`String name; `String path])
+               packages) );
+        ("version", `Int 2);
+      ]
+  in
+  File_util.write_file_atomic ~ensure_parent:false ~perm:0o644 path
+    (Yojson.Safe.to_string json)
+
+let write_build ~(root_config : Config.t) session =
+  let packages =
+    Build_session.package_plan_values session
+    |> List.of_seq
+    |> List.sort (fun (left : Package_plan.t) right ->
+        String.compare left.root right.root)
+  in
+  packages
+  |> List.iter (fun package ->
+      if package.Package_plan.root <> root_config.root then
+        File_util.remove_file
+          (File_util.path_of_parts package.root
+             ["lib"; "bs"; ".sourcedirs.json"]));
+  let local_packages =
+    List.filter (fun package -> package.Package_plan.is_local) packages
+  in
+  let source_directories package =
+    package.Package_plan.modules
+    |> List.map (fun module_ -> Filename.dirname module_.Source.implementation)
+    |> List.sort_uniq String.compare
+  in
+  let relative_package_root package =
+    if package.Package_plan.root = root_config.root then ""
+    else Project_context.relative_to root_config.root package.root
+  in
+  let dirs =
+    local_packages
+    |> List.concat_map (fun package ->
+        let relative_root = relative_package_root package in
+        source_directories package
+        |> List.map (fun directory ->
+            if relative_root = "" then directory
+            else Filename.concat relative_root directory))
+    |> List.sort_uniq String.compare
+  in
+  let package_roots = Hashtbl.create 16 in
+  local_packages
+  |> List.iter (fun package ->
+      package.Package_plan.dependencies
+      |> List.iter (fun dependency ->
+          Hashtbl.replace package_roots dependency.Package_plan.declaration.name
+            dependency.directory));
+  let package_roots =
+    Hashtbl.to_seq package_roots
+    |> List.of_seq
+    |> List.sort (fun (left, _) (right, _) -> String.compare left right)
+  in
+  let scans =
+    local_packages
+    |> List.map (fun package ->
+        let relative_root = relative_package_root package in
+        let build_root =
+          if relative_root = "" then File_util.path_of_parts "" ["lib"; "bs"]
+          else File_util.path_of_parts relative_root ["lib"; "bs"]
+        in
+        {
+          build_root;
+          scan_dirs = source_directories package;
+          also_scan_build_root = true;
+        })
+    |> List.sort (fun (left : scan) right ->
+        String.compare left.build_root right.build_root)
+  in
+  write ~root:root_config.root ~dirs ~packages:package_roots ~scans
diff --git a/rewatch-ocaml/source_dirs.mli b/rewatch-ocaml/source_dirs.mli
new file mode 100644
index 00000000000..4a0af8b3043
--- /dev/null
+++ b/rewatch-ocaml/source_dirs.mli
@@ -0,0 +1,7 @@
+type scan = {
+  build_root: string;
+  scan_dirs: string list;
+  also_scan_build_root: bool;
+}
+
+val write_build : root_config:Config.t -> Build_session.t -> unit
diff --git a/rewatch-ocaml/source_filter.ml b/rewatch-ocaml/source_filter.ml
new file mode 100644
index 00000000000..bf864d8db6d
--- /dev/null
+++ b/rewatch-ocaml/source_filter.ml
@@ -0,0 +1,60 @@
+type t = {pattern: string; regex: Re.re}
+
+let unsupported = Error "unsupported regular expression"
+
+(* A filter that compiles with different semantics can silently select the
+   wrong source set. Keep the accepted grammar to the CLI's shared safe subset;
+   the regex parser below rejects unsupported syntax outside this check. *)
+let validate_compatibility pattern =
+  let length = String.length pattern in
+  let rec loop ~in_class ~class_start index =
+    if index >= length then Ok ()
+    else
+      match pattern.[index] with
+      | '\\' when index + 1 < length -> (
+        let escaped = pattern.[index + 1] in
+        match escaped with
+        | '0' .. '9' | 'Q' | 'E' | 'G' | 'Z' | 'e' | 'o' -> unsupported
+        | ('b' | 'W') when in_class -> unsupported
+        | _ -> loop ~in_class ~class_start (index + 2))
+      | '('
+        when (not in_class)
+             && index + 2 < length
+             && pattern.[index + 1] = '?'
+             && pattern.[index + 2] = '#' ->
+        unsupported
+      | '[' when not in_class ->
+        loop ~in_class:true ~class_start:(index + 1) (index + 1)
+      | '[' when index + 1 >= length || pattern.[index + 1] <> ':' ->
+        unsupported
+      | ']' when in_class && index > class_start ->
+        loop ~in_class:false ~class_start:0 (index + 1)
+      | ('&' | '-' | '~') as operator
+        when in_class && index + 1 < length && pattern.[index + 1] = operator ->
+        unsupported
+      | '-'
+        when in_class && index > class_start
+             && index + 1 < length
+             && pattern.[index + 1] <> ']' ->
+        let left = pattern.[index - 1] in
+        let right = pattern.[index + 1] in
+        let escaped_left = index >= 2 && pattern.[index - 2] = '\\' in
+        if escaped_left || left = '\\' || right = '\\' || left > right then
+          unsupported
+        else loop ~in_class ~class_start (index + 1)
+      | _ -> loop ~in_class ~class_start (index + 1)
+  in
+  loop ~in_class:false ~class_start:0 0
+
+let compile pattern =
+  match validate_compatibility pattern with
+  | Error _ as error -> error
+  | Ok () -> (
+    match Re.Perl.re_result pattern with
+    | Ok regex -> Ok {pattern; regex = Re.compile regex}
+    | Error `Parse_error -> Error "invalid regular expression"
+    | Error `Not_supported -> unsupported)
+
+let pattern filter = filter.pattern
+let matches_basename filter path =
+  Re.execp filter.regex (Filename.basename path)
diff --git a/rewatch-ocaml/source_filter.mli b/rewatch-ocaml/source_filter.mli
new file mode 100644
index 00000000000..2b533c9dc5a
--- /dev/null
+++ b/rewatch-ocaml/source_filter.mli
@@ -0,0 +1,5 @@
+type t
+
+val compile : string -> (t, string) result
+val pattern : t -> string
+val matches_basename : t -> string -> bool
diff --git a/rewatch-ocaml/string_util.ml b/rewatch-ocaml/string_util.ml
new file mode 100644
index 00000000000..6b26af1ad27
--- /dev/null
+++ b/rewatch-ocaml/string_util.ml
@@ -0,0 +1,22 @@
+(* OCaml 5.5 provides [String.includes], but Rewatch also supports OCaml 5.0.
+   The affixes checked here are short markers, so this allocation-free scan
+   keeps the compatibility implementation simpler than a general-purpose
+   substring-search algorithm. *)
+let contains value substring =
+  let substring_length = String.length substring in
+  let last_start = String.length value - substring_length in
+  let rec matches_at start offset =
+    offset = substring_length
+    || String.get value (start + offset) = String.get substring offset
+       && matches_at start (offset + 1)
+  in
+  let rec search start =
+    start <= last_start && (matches_at start 0 || search (start + 1))
+  in
+  search 0
+
+let strip_prefix ~prefix value =
+  if String.starts_with ~prefix value then
+    String.sub value (String.length prefix)
+      (String.length value - String.length prefix)
+  else value
diff --git a/rewatch-ocaml/string_util.mli b/rewatch-ocaml/string_util.mli
new file mode 100644
index 00000000000..cda9b6907ff
--- /dev/null
+++ b/rewatch-ocaml/string_util.mli
@@ -0,0 +1,2 @@
+val contains : string -> string -> bool
+val strip_prefix : prefix:string -> string -> string
diff --git a/rewatch-ocaml/tests/basic/rescript.json b/rewatch-ocaml/tests/basic/rescript.json
new file mode 100644
index 00000000000..6abd6654a34
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/rescript.json
@@ -0,0 +1,6 @@
+{
+  "name": "rewatch-ocaml-basic",
+  "sources": {"dir": "src", "subdirs": true},
+  "package-specs": {"module": "esmodule", "in-source": true},
+  "suffix": ".mjs"
+}
diff --git a/rewatch-ocaml/tests/basic/src/A.res b/rewatch-ocaml/tests/basic/src/A.res
new file mode 100644
index 00000000000..5f5aa16a1ae
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/src/A.res
@@ -0,0 +1 @@
+let value = 41
diff --git a/rewatch-ocaml/tests/basic/src/Authored.js b/rewatch-ocaml/tests/basic/src/Authored.js
new file mode 100644
index 00000000000..e65eaf2a3a6
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/src/Authored.js
@@ -0,0 +1 @@
+authored same-stem JavaScript
diff --git a/rewatch-ocaml/tests/basic/src/Authored.res b/rewatch-ocaml/tests/basic/src/Authored.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/src/Authored.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/basic/src/B.res b/rewatch-ocaml/tests/basic/src/B.res
new file mode 100644
index 00000000000..f12af2ffc7d
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/src/B.res
@@ -0,0 +1 @@
+let answer = A.value + 1
diff --git a/rewatch-ocaml/tests/basic/src/WithInterface.res b/rewatch-ocaml/tests/basic/src/WithInterface.res
new file mode 100644
index 00000000000..ec69b5dd4a5
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/src/WithInterface.res
@@ -0,0 +1 @@
+let value = B.answer
diff --git a/rewatch-ocaml/tests/basic/src/WithInterface.resi b/rewatch-ocaml/tests/basic/src/WithInterface.resi
new file mode 100644
index 00000000000..14829e3b698
--- /dev/null
+++ b/rewatch-ocaml/tests/basic/src/WithInterface.resi
@@ -0,0 +1 @@
+let value: int
diff --git a/rewatch-ocaml/tests/check_canonical_test_coverage.sh b/rewatch-ocaml/tests/check_canonical_test_coverage.sh
new file mode 100755
index 00000000000..db93f31a404
--- /dev/null
+++ b/rewatch-ocaml/tests/check_canonical_test_coverage.sh
@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root=$(cd "$(dirname "$0")/../.." && pwd)
+tests="$root/rewatch/tests"
+inventory=$(mktemp)
+referenced=$(mktemp)
+cleanup() {
+  rm -f "$inventory" "$referenced"
+}
+trap cleanup EXIT
+
+for file in "$tests"/*/*.sh; do
+  relative=${file#"$tests/"}
+  printf './%s\n' "$relative"
+done | sort >"$inventory"
+
+sed -n 's/^\(\.\/[^ ]*\.sh\).*$/\1/p' "$tests/suite.sh" | sort \
+  >"$referenced"
+
+duplicates=$(uniq -d "$referenced")
+missing=$(comm -23 "$inventory" "$referenced")
+stale=$(comm -13 "$inventory" "$referenced")
+if [[ -n "$duplicates" || -n "$missing" || -n "$stale" ]]; then
+  [[ -z "$duplicates" ]] \
+    || printf 'Canonical tests referenced more than once:\n%s\n' "$duplicates" >&2
+  [[ -z "$missing" ]] \
+    || printf 'Canonical tests omitted from suite.sh:\n%s\n' "$missing" >&2
+  [[ -z "$stale" ]] \
+    || printf 'Stale canonical test references in suite.sh:\n%s\n' "$stale" >&2
+  exit 1
+fi
+
+total=$(wc -l <"$inventory" | tr -d ' ')
+printf 'Canonical integration tests: %s; all referenced exactly once\n' "$total"
diff --git a/rewatch-ocaml/tests/check_command_validation.sh b/rewatch-ocaml/tests/check_command_validation.sh
new file mode 100755
index 00000000000..0b5a32cadf7
--- /dev/null
+++ b/rewatch-ocaml/tests/check_command_validation.sh
@@ -0,0 +1,2029 @@
+#!/bin/bash
+set -eu
+
+root=$(CDPATH= cd -- "$(dirname "$0")/../.." && pwd)
+rust=${1:-$root/rewatch/target/debug/rescript}
+ocaml=${2:-$root/_build/default/rewatch-ocaml/rescript_ocaml.exe}
+rust=$(realpath "$rust")
+ocaml=$(realpath "$ocaml")
+bsc_test_proxy="$root/_build/default/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.exe"
+work=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-command-validation-XXXXXX")
+# Compiler diagnostics contain canonical paths. Resolve platform aliases such
+# as macOS's /var -> /private/var before deriving paths used for comparison.
+work=$(realpath "$work")
+native_work=$work
+native_short_work=$work
+windows_posix_shell=false
+case $(uname -s) in
+  MINGW*|MSYS*)
+    windows_posix_shell=true
+    native_work=$(cd "$work" && pwd -W)
+    native_user=$(cygpath -w "$USERPROFILE" | tr '\\' '/')
+    native_short_user=$(cygpath -w -s "$USERPROFILE" | tr '\\' '/')
+    native_short_work=${native_work/"$native_user"/"$native_short_user"}
+    bsc_test_proxy=$(cygpath -aw \
+      "$root/_build/default/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.exe")
+    ;;
+esac
+delete_source_bsc=$bsc_test_proxy
+delete_parse_sources_bsc=$bsc_test_proxy
+delete_ast_bsc=$bsc_test_proxy
+lock_owner_pid() {
+  local shell_pid=$1
+  if $windows_posix_shell; then
+    # MSYS assigns a synthetic PID to native Windows children. Lock files are
+    # consumed by native executables and therefore must contain the WINPID.
+    ps -p "$shell_pid" -l | awk 'NR == 2 { print $4 }'
+  else
+    printf '%s\n' "$shell_pid"
+  fi
+}
+directory_link() {
+  local target=$1
+  local link=$2
+  if $windows_posix_shell; then
+    LINK_PATH=$(cygpath -aw "$link") TARGET_PATH=$(cygpath -aw "$target") \
+      powershell.exe -NoProfile -NonInteractive -Command \
+        '$ErrorActionPreference = "Stop"; $null = New-Item -ItemType Junction -Path $env:LINK_PATH -Target $env:TARGET_PATH'
+  else
+    ln -s "$target" "$link"
+  fi
+}
+file_link_if_supported() {
+  local target=$1
+  local link=$2
+  if $windows_posix_shell; then
+    LINK_PATH=$(cygpath -aw "$link") TARGET_PATH=$(cygpath -aw "$target") \
+      powershell.exe -NoProfile -NonInteractive -Command \
+        '$ErrorActionPreference = "Stop"; $null = New-Item -ItemType SymbolicLink -Path $env:LINK_PATH -Target $env:TARGET_PATH' \
+        >/dev/null 2>&1
+  else
+    ln -s "$target" "$link"
+  fi
+}
+command_work=$native_work
+command_path() {
+  case $1 in
+    "$work"*) printf '%s\n' "$command_work${1#"$work"}" ;;
+    *) printf '%s\n' "$1" ;;
+  esac
+}
+terminate_and_wait() {
+  local pid=$1
+  local label=$2
+  kill -TERM "$pid"
+  set +e
+  wait "$pid"
+  local status=$?
+  set -e
+  if [ "$status" -ne 0 ] && \
+    { ! $windows_posix_shell || [ "$status" -ne 143 ]; }; then
+    printf '%s exited with status %s after shutdown\n' "$label" "$status" >&2
+    return 1
+  fi
+}
+background_pids=""
+cleanup() {
+  for pid in $background_pids; do
+    kill -TERM "$pid" 2>/dev/null || true
+    wait "$pid" 2>/dev/null || true
+  done
+  rm -rf "$work"
+}
+trap cleanup EXIT
+
+project="$work/project"
+mkdir -p "$project/src" "$work/orphan" "$work/empty" "$work/malformed"
+mkdir -p "$work/malformed-parent/child/src" "$work/config-directory/rescript.json"
+mkdir -p "$work/missing-dependency/src"
+mkdir -p "$work/malformed-lock/src" "$work/malformed-lock/lib"
+mkdir -p "$work/watch-lock-order/src" "$work/watch-lock-order/lib"
+mkdir -p "$work/signal-lock/src" "$work/signal-lock/lib"
+mkdir -p "$work/signal-lock-owner/src"
+mkdir -p "$work/interface-mismatch/src"
+mkdir -p "$work/exotic-module-rust/src" "$work/exotic-module-ocaml/src"
+mkdir -p "$work/filter-basename-rust/src/nested" \
+  "$work/filter-basename-ocaml/src/nested"
+mkdir -p "$work/external-dev-source/src" \
+  "$work/external-dev-source/node_modules/dep/src" \
+  "$work/external-dev-source/node_modules/dep/test"
+mkdir -p "$work/external-dev-permission/src" \
+  "$work/external-dev-permission/node_modules/a" \
+  "$work/external-dev-permission/node_modules/b"
+mkdir -p "$work/active-permission/node_modules/a" \
+  "$work/active-permission/node_modules/b"
+mkdir -p "$work/source-path-file"
+mkdir -p "$work/missing-runtime-package"
+mkdir -p "$work/clean-missing-bsc/src" "$work/clean-missing-bsc/lib/bs"
+mkdir -p "$work/clean-missing-runtime/src" \
+  "$work/clean-missing-runtime/lib/bs"
+for implementation in rust ocaml; do
+  mkdir -p "$work/clean-duplicate-$implementation/src/one" \
+    "$work/clean-duplicate-$implementation/src/two" \
+    "$work/clean-duplicate-$implementation/lib/bs"
+done
+mkdir -p "$work/missing-source-folder/src" \
+  "$work/missing-source-folder/node_modules/dep"
+mkdir -p "$work/dependency-without-sources/src" \
+  "$work/dependency-without-sources/node_modules/dep"
+mkdir -p "$work/default-feature-cycle/src"
+mkdir -p "$work/format-feature-cycle/src" \
+  "$work/format-feature-cycle/node_modules/dep/src"
+mkdir -p "$work/format-source-selection/src" \
+  "$work/format-source-selection/node_modules/installed" \
+  "$work/format-source-selection/packages/local/base" \
+  "$work/format-source-selection/packages/local/native" \
+  "$work/format-source-selection/packages/local/other"
+mkdir -p "$work/package-name-mismatch/src" "$work/malformed-package-json/src"
+mkdir -p "$work/failed-js-post-build/src"
+mkdir -p "$work/mismatched-dependency/src" \
+  "$work/mismatched-dependency/node_modules/dep/src"
+mkdir -p "$work/configless-dependency/src" \
+  "$work/configless-dependency/node_modules/no-config"
+mkdir -p "$work/malformed-dependency/src" \
+  "$work/malformed-dependency/node_modules/bad-config"
+mkdir -p "$work/duplicate-dependency/src" \
+  "$work/duplicate-dependency/node_modules/a/src" \
+  "$work/duplicate-dependency/node_modules/shared/src" \
+  "$work/duplicate-dependency/node_modules/a/node_modules/shared/src"
+mkdir -p "$work/publication-race-rust/src" \
+  "$work/publication-race-ocaml/src"
+mkdir -p "$work/ast-race-rust/src" "$work/ast-race-ocaml/src"
+mkdir -p "$work/parse-source-race-rust/src" \
+  "$work/parse-source-race-ocaml/src"
+mkdir -p "$work/watch-config-rust/src" "$work/watch-config-ocaml/src"
+mkdir -p "$work/watch-retained-graph/src"
+mkdir -p "$work/watch-dependency-recovery/src" \
+  "$work/watch-dependency-recovery/node_modules" \
+  "$work/watch-dependency-recovery/packages/dep/src"
+mkdir -p "$work/watch-dependency-install/src" \
+  "$work/watch-dependency-install/node_modules"
+mkdir -p "$work/watch-dependency-fallback/src" \
+  "$work/watch-dependency-fallback/node_modules/dep" \
+  "$work/watch-dependency-fallback/packages/dep/src" \
+  "$work/node_modules"
+mkdir -p "$work/watch-symlink-target/src" "$work/watch-symlink-external/sub"
+mkdir -p "$work/watch-feature-scope/src" "$work/watch-feature-scope/inactive"
+mkdir -p "$work/watch-filter-rust/src" "$work/watch-filter-rust/inactive" \
+  "$work/watch-filter-ocaml/src" "$work/watch-filter-ocaml/inactive"
+mkdir -p "$work/quiet-watch-rust/src" "$work/quiet-watch-ocaml/src"
+printf '{"name":"command-validation","sources":["src"]}\n' \
+  >"$project/rescript.json"
+printf 'let value = 1\n' >"$project/src/A.res"
+printf 'not a ReScript source\n' >"$project/src/A.txt"
+printf '{"name":"signal-lock","sources":["src"]}\n' \
+  >"$work/signal-lock/rescript.json"
+printf 'let value = 1\n' >"$work/signal-lock/src/A.res"
+printf '{"name":"signal-lock-owner","sources":["src"]}\n' \
+  >"$work/signal-lock-owner/rescript.json"
+printf 'let value = 1\n' >"$work/signal-lock-owner/src/A.res"
+printf '{"name":"watch-lock-order","sources":["src"]}\n' \
+  >"$work/watch-lock-order/rescript.json"
+printf 'let value = 1\n' >"$work/watch-lock-order/src/A.res"
+mkdir -p "$work/redirected-parse-fixture/src"
+mkdir -p "$work/multiple-parse-errors/src"
+mkdir -p "$work/redirected-config-diagnostics/src"
+printf '{"name":"parse-output","sources":["src"]}\n' \
+  >"$work/redirected-parse-fixture/rescript.json"
+printf 'let value =\n' >"$work/redirected-parse-fixture/src/A.res"
+printf '{"name":"multiple-parse-errors","sources":["src"]}\n' \
+  >"$work/multiple-parse-errors/rescript.json"
+printf 'let value =\n' >"$work/multiple-parse-errors/src/A.res"
+printf 'let other =\n' >"$work/multiple-parse-errors/src/B.res"
+printf '%s\n' \
+  '{"name":"config-diagnostics","sources":["src"],"bsc-flags":[],"ignored-dirs":[],"future-field":true}' \
+  >"$work/redirected-config-diagnostics/rescript.json"
+printf 'let value = 1\n' >"$work/redirected-config-diagnostics/src/A.res"
+printf 'process.stderr.write("hook failed\\n"); process.exit(7)\n' \
+  >"$work/failing-after-build.js"
+printf 'let value = 1\n' >"$work/orphan/A.res"
+printf '{ invalid json\n' >"$work/malformed/rescript.json"
+printf '{ invalid json\n' >"$work/malformed-parent/rescript.json"
+printf '{"name":"child","sources":["src"]}\n' \
+  >"$work/malformed-parent/child/rescript.json"
+printf 'let value = 1\n' >"$work/malformed-parent/child/src/A.res"
+printf '{"name":"missing-dependency","sources":["src"],"dependencies":["absent"]}\n' \
+  >"$work/missing-dependency/rescript.json"
+printf 'let value = 1\n' >"$work/missing-dependency/src/A.res"
+printf '{"name":"malformed-lock","sources":["src"]}\n' \
+  >"$work/malformed-lock/rescript.json"
+printf 'let value = 1\n' >"$work/malformed-lock/src/A.res"
+printf '{"name":"interface-mismatch","sources":["src"]}\n' \
+  >"$work/interface-mismatch/rescript.json"
+printf 'let value = 1\n' >"$work/interface-mismatch/src/lower.res"
+printf 'let value: int\n' >"$work/interface-mismatch/src/Lower.resi"
+for implementation in rust ocaml; do
+  printf '{"name":"exotic-module","namespace":"Ns","sources":["src"]}\n' \
+    >"$work/exotic-module-$implementation/rescript.json"
+  printf 'let value = 1\n' \
+    >"$work/exotic-module-$implementation/src/Main.res"
+  printf 'let value = 2\n' \
+    >"$work/exotic-module-$implementation/src/foo-bar.res"
+  printf '{"name":"filter-basename","sources":[{"dir":"src","subdirs":true}]}\n' \
+    >"$work/filter-basename-$implementation/rescript.json"
+  printf 'let value = 1\n' \
+    >"$work/filter-basename-$implementation/src/nested/A.res"
+  printf 'let value = 2\n' \
+    >"$work/filter-basename-$implementation/src/nested/B2.res"
+done
+printf '{"name":"external-dev-source","sources":["src"],"dependencies":["dep"]}\n' \
+  >"$work/external-dev-source/rescript.json"
+printf 'let value = DepPublic.value\n' \
+  >"$work/external-dev-source/src/App.res"
+printf '{"name":"dep","sources":["src",{"dir":"test","type":"dev"}]}\n' \
+  >"$work/external-dev-source/node_modules/dep/rescript.json"
+printf 'let value = 1\n' \
+  >"$work/external-dev-source/node_modules/dep/src/DepPublic.res"
+printf 'this is deliberately invalid ReScript\n' \
+  >"$work/external-dev-source/node_modules/dep/test/DevOnly.res"
+printf '{"name":"root","sources":["src"],"dependencies":["a","b"]}\n' \
+  >"$work/external-dev-permission/rescript.json"
+printf '{"name":"a","sources":[],"dev-dependencies":["b"]}\n' \
+  >"$work/external-dev-permission/node_modules/a/rescript.json"
+printf '{"name":"b","sources":[],"allowed-dependents":["root"]}\n' \
+  >"$work/external-dev-permission/node_modules/b/rescript.json"
+printf '{"name":"root","sources":[],"dependencies":["a","b"]}\n' \
+  >"$work/active-permission/rescript.json"
+printf '{"name":"a","sources":[],"allowed-dependents":["someone-else"]}\n' \
+  >"$work/active-permission/node_modules/a/rescript.json"
+printf '{"name":"b","sources":[],"allowed-dependents":["someone-else"]}\n' \
+  >"$work/active-permission/node_modules/b/rescript.json"
+printf '{"name":"source-path-file","sources":["src"]}\n' \
+  >"$work/source-path-file/rescript.json"
+printf 'not a directory\n' >"$work/source-path-file/src"
+printf '{"name":"missing-runtime-package","sources":[]}\n' \
+  >"$work/missing-runtime-package/rescript.json"
+for clean_project in clean-missing-bsc clean-missing-runtime; do
+  printf '{"name":"%s","sources":["src"]}\n' "$clean_project" \
+    >"$work/$clean_project/rescript.json"
+  printf 'let value = 1\n' >"$work/$clean_project/src/A.res"
+  printf 'owned compiler artifact\n' >"$work/$clean_project/lib/bs/marker"
+done
+for implementation in rust ocaml; do
+  duplicate_clean="$work/clean-duplicate-$implementation"
+  printf '%s\n' \
+    '{"name":"duplicate-clean","sources":{"dir":"src","subdirs":true},"package-specs":{"module":"esmodule","in-source":true}}' \
+    >"$duplicate_clean/rescript.json"
+  printf 'let value = 1\n' >"$duplicate_clean/src/one/A.res"
+  printf 'let value = 2\n' >"$duplicate_clean/src/two/A.res"
+  printf 'generated\n' >"$duplicate_clean/src/one/A.js"
+  printf 'generated\n' >"$duplicate_clean/src/two/A.js"
+  printf 'owned compiler artifact\n' >"$duplicate_clean/lib/bs/marker"
+done
+printf '{"name":"missing-source-folder","sources":["src"],"dependencies":["dep"]}\n' \
+  >"$work/missing-source-folder/rescript.json"
+printf 'let value = 1\n' >"$work/missing-source-folder/src/App.res"
+printf '{"name":"dep","sources":["missing"]}\n' \
+  >"$work/missing-source-folder/node_modules/dep/rescript.json"
+printf '{"name":"dependency-without-sources","sources":["src"],"dependencies":["dep"]}\n' \
+  >"$work/dependency-without-sources/rescript.json"
+printf 'let value = 1\n' >"$work/dependency-without-sources/src/App.res"
+printf '{"name":"dep"}\n' \
+  >"$work/dependency-without-sources/node_modules/dep/rescript.json"
+printf '{"name":"dep"}\n' \
+  >"$work/dependency-without-sources/node_modules/dep/package.json"
+printf '{"name":"default-feature-cycle","sources":["src"],"features":{"a":["b"],"b":["a"]}}\n' \
+  >"$work/default-feature-cycle/rescript.json"
+printf 'let value = 1\n' >"$work/default-feature-cycle/src/App.res"
+printf '{"name":"format-feature-cycle","sources":["src"],"dependencies":[{"name":"dep","features":["a"]}]}\n' \
+  >"$work/format-feature-cycle/rescript.json"
+printf 'let value = 1\n' >"$work/format-feature-cycle/src/App.res"
+printf '{"name":"dep","sources":["src"],"features":{"a":["b"],"b":["a"]}}\n' \
+  >"$work/format-feature-cycle/node_modules/dep/rescript.json"
+printf '{"name":"dep"}\n' \
+  >"$work/format-feature-cycle/node_modules/dep/package.json"
+printf 'let value = 1\n' \
+  >"$work/format-feature-cycle/node_modules/dep/src/Dep.res"
+printf '{"name":"format-source-selection","sources":["src"],"dependencies":["installed",{"name":"local","features":["native"]}]}\n' \
+  >"$work/format-source-selection/rescript.json"
+printf '{"name":"installed","sources":["missing"]}\n' \
+  >"$work/format-source-selection/node_modules/installed/rescript.json"
+printf '{"name":"local","sources":["base",{"dir":"native","feature":"native"},{"dir":"other","feature":"other"}]}\n' \
+  >"$work/format-source-selection/packages/local/rescript.json"
+printf 'let value=1\n' \
+  >"$work/format-source-selection/packages/local/base/Base.res"
+printf 'let value=2\n' \
+  >"$work/format-source-selection/packages/local/native/Native.res"
+printf 'let value=3\n' \
+  >"$work/format-source-selection/packages/local/other/Other.res"
+directory_link "$work/format-source-selection/packages/local" \
+  "$work/format-source-selection/node_modules/local"
+printf '{"name":"config-name","sources":["src"]}\n' \
+  >"$work/package-name-mismatch/rescript.json"
+printf '{"name":"package-name"}\n' >"$work/package-name-mismatch/package.json"
+printf 'let value = 1\n' >"$work/package-name-mismatch/src/A.res"
+printf '{"name":"malformed-package-json","sources":["src"]}\n' \
+  >"$work/malformed-package-json/rescript.json"
+printf '{invalid\n' >"$work/malformed-package-json/package.json"
+printf 'let value = 1\n' >"$work/malformed-package-json/src/A.res"
+printf '{"name":"failed-js-post-build","sources":["src"],"js-post-build":{"cmd":"exit 7"}}\n' \
+  >"$work/failed-js-post-build/rescript.json"
+printf 'let value = 1\n' >"$work/failed-js-post-build/src/A.res"
+printf '{"name":"mismatched-dependency","sources":["src"],"dependencies":["dep"]}\n' \
+  >"$work/mismatched-dependency/rescript.json"
+printf 'let value = Dep.value\n' >"$work/mismatched-dependency/src/A.res"
+printf '{"name":"dep","sources":["src"]}\n' \
+  >"$work/mismatched-dependency/node_modules/dep/rescript.json"
+printf '{"name":"different-name"}\n' \
+  >"$work/mismatched-dependency/node_modules/dep/package.json"
+printf 'let value = 1\n' \
+  >"$work/mismatched-dependency/node_modules/dep/src/Dep.res"
+printf '{"name":"configless-dependency","sources":["src"],"dependencies":["no-config"]}\n' \
+  >"$work/configless-dependency/rescript.json"
+printf 'let value = 1\n' >"$work/configless-dependency/src/A.res"
+printf '{"name":"malformed-dependency","sources":["src"],"dependencies":["bad-config"]}\n' \
+  >"$work/malformed-dependency/rescript.json"
+printf 'let value = 1\n' >"$work/malformed-dependency/src/A.res"
+printf '{ invalid json\n' \
+  >"$work/malformed-dependency/node_modules/bad-config/rescript.json"
+printf '{"name":"duplicate-dependency","sources":["src"],"dependencies":["a","shared"]}\n' \
+  >"$work/duplicate-dependency/rescript.json"
+printf 'let value = Shared.value + A.value\n' \
+  >"$work/duplicate-dependency/src/Main.res"
+printf '{"name":"a","sources":["src"],"dependencies":["shared"]}\n' \
+  >"$work/duplicate-dependency/node_modules/a/rescript.json"
+printf 'let value = Shared.value\n' \
+  >"$work/duplicate-dependency/node_modules/a/src/A.res"
+printf '{"name":"shared","sources":["src"]}\n' \
+  >"$work/duplicate-dependency/node_modules/shared/rescript.json"
+printf 'let value = 1\n' \
+  >"$work/duplicate-dependency/node_modules/shared/src/Shared.res"
+printf '{"name":"shared","sources":["src"]}\n' \
+  >"$work/duplicate-dependency/node_modules/a/node_modules/shared/rescript.json"
+printf 'let value = 2\n' \
+  >"$work/duplicate-dependency/node_modules/a/node_modules/shared/src/Shared.res"
+printf '{"name":"publication-race","sources":["src"]}\n' \
+  >"$work/publication-race-rust/rescript.json"
+cp "$work/publication-race-rust/rescript.json" \
+  "$work/publication-race-ocaml/rescript.json"
+printf 'let value = 1\n' >"$work/publication-race-rust/src/A.res"
+cp "$work/publication-race-rust/src/A.res" \
+  "$work/publication-race-ocaml/src/A.res"
+printf '{"name":"ast-race","sources":["src"]}\n' \
+  >"$work/ast-race-rust/rescript.json"
+cp "$work/ast-race-rust/rescript.json" "$work/ast-race-ocaml/rescript.json"
+printf 'let value = 1\n' >"$work/ast-race-rust/src/A.res"
+cp "$work/ast-race-rust/src/A.res" "$work/ast-race-ocaml/src/A.res"
+printf '{"name":"parse-source-race","sources":["src"]}\n' \
+  >"$work/parse-source-race-rust/rescript.json"
+cp "$work/parse-source-race-rust/rescript.json" \
+  "$work/parse-source-race-ocaml/rescript.json"
+for implementation in rust ocaml; do
+  printf 'let value = 1\n' \
+    >"$work/parse-source-race-$implementation/src/A.res"
+  printf 'let value = 2\n' \
+    >"$work/parse-source-race-$implementation/src/B.res"
+done
+printf '{"name":"watch-config","sources":["src"]}\n' \
+  >"$work/watch-config-rust/rescript.json"
+cp "$work/watch-config-rust/rescript.json" \
+  "$work/watch-config-ocaml/rescript.json"
+printf 'let value = 1\n' >"$work/watch-config-rust/src/A.res"
+cp "$work/watch-config-rust/src/A.res" "$work/watch-config-ocaml/src/A.res"
+printf '{"name":"watch-retained-graph","sources":["src"]}\n' \
+  >"$work/watch-retained-graph/rescript.json"
+printf 'let value = 1\n' >"$work/watch-retained-graph/src/A.res"
+printf 'let oldValue = 1\n' >"$work/watch-retained-graph/src/C.res"
+printf 'let value = A.value\n' >"$work/watch-retained-graph/src/B.res"
+printf '{"name":"watch-dependency-recovery","sources":["src"],"dependencies":["dep"]}\n' \
+  >"$work/watch-dependency-recovery/rescript.json"
+printf 'let value = 1\n' >"$work/watch-dependency-recovery/src/A.res"
+printf '{ invalid json\n' \
+  >"$work/watch-dependency-recovery/packages/dep/rescript.json"
+printf 'let dependency = 1\n' \
+  >"$work/watch-dependency-recovery/packages/dep/src/Dep.res"
+directory_link "$work/watch-dependency-recovery/packages/dep" \
+  "$work/watch-dependency-recovery/node_modules/dep"
+printf '{"name":"watch-dependency-install","sources":["src"],"dependencies":["@scope/dep"]}\n' \
+  >"$work/watch-dependency-install/rescript.json"
+printf 'let value = 1\n' >"$work/watch-dependency-install/src/A.res"
+printf '{"name":"watch-feature-scope","sources":["src",{"dir":"inactive","feature":"inactive"}]}\n' \
+  >"$work/watch-feature-scope/rescript.json"
+printf 'let value = 1\n' >"$work/watch-feature-scope/src/A.res"
+printf 'let inactive = 1\n' >"$work/watch-feature-scope/inactive/Inactive.res"
+printf '{"name":"watch-dependency-fallback","sources":["src"],"dependencies":["dep"]}\n' \
+  >"$work/watch-dependency-fallback/rescript.json"
+printf 'let value = 1\n' >"$work/watch-dependency-fallback/src/A.res"
+printf '{"name":"dep","sources":["src"]}\n' \
+  >"$work/watch-dependency-fallback/packages/dep/rescript.json"
+printf 'let dependency = 1\n' \
+  >"$work/watch-dependency-fallback/packages/dep/src/Dep.res"
+printf '{"name":"watch-symlink-target","sources":["src"]}\n' \
+  >"$work/watch-symlink-target/rescript.json"
+printf 'let linked = 1\n' >"$work/watch-symlink-external/sub/Linked.res"
+file_symlinks_supported=true
+if ! file_link_if_supported "$work/watch-symlink-external/sub/Linked.res" \
+    "$work/watch-symlink-target/src/Linked.res"; then
+  file_symlinks_supported=false
+fi
+for implementation in rust ocaml; do
+  printf '{"name":"watch-filter","sources":["src",{"dir":"inactive","feature":"inactive"}]}\n' \
+    >"$work/watch-filter-$implementation/rescript.json"
+  printf 'let value = 1\n' \
+    >"$work/watch-filter-$implementation/src/Include.res"
+  printf 'let value = 10\n' \
+    >"$work/watch-filter-$implementation/src/Exclude.res"
+  printf 'let value = 20\n' \
+    >"$work/watch-filter-$implementation/inactive/Inactive.res"
+  printf '{"name":"quiet-watch","sources":["src"]}\n' \
+    >"$work/quiet-watch-$implementation/rescript.json"
+  printf 'let value = 1\n' \
+    >"$work/quiet-watch-$implementation/src/A.res"
+done
+printf 'require("fs").appendFileSync(process.env.REWATCH_WATCH_FILTER_MARKER, "done\\n")\n' \
+  >"$work/watch-filter-marker.js"
+printf '%s\n' \
+  '#!/bin/sh' \
+  'if [ -f "$REWATCH_SCOPE_BLOCK_REQUEST" ] && [ ! -f "$REWATCH_SCOPE_BLOCK_STARTED" ]; then' \
+  '  : >"$REWATCH_SCOPE_BLOCK_STARTED"' \
+  '  attempts=0' \
+  '  while [ ! -f "$REWATCH_SCOPE_BLOCK_RELEASE" ] && [ "$attempts" -lt 200 ]; do' \
+  '    attempts=$((attempts + 1))' \
+  '    sleep 0.05' \
+  '  done' \
+  'fi' \
+  'exec "$REWATCH_SCOPE_REAL_BSC" "$@"' \
+  >"$work/watch-scope-bsc.sh"
+chmod +x "$work/watch-scope-bsc.sh"
+watch_scope_bsc="$work/watch-scope-bsc.sh"
+if $windows_posix_shell; then
+  watch_scope_bsc=$bsc_test_proxy
+fi
+
+default_bsc=$root/_build/default/compiler/bsc/rescript_compiler_main.exe
+default_runtime=$root/packages/@rescript/runtime
+case $(uname -s) in
+  MINGW*|MSYS*)
+    default_bsc=$(cygpath -w "$default_bsc")
+    default_runtime=$(cygpath -w "$default_runtime")
+    ;;
+esac
+export RESCRIPT_BSC_EXE=${RESCRIPT_BSC_EXE:-$default_bsc}
+export RESCRIPT_RUNTIME=${RESCRIPT_RUNTIME:-$default_runtime}
+
+classify() {
+  case "$1" in
+    0) printf accept ;;
+    101) printf panic ;;
+    2) printf exit2 ;;
+    *) printf reject ;;
+  esac
+}
+
+strip_ansi() {
+  LC_ALL=C sed $'s/\033\\[[0-9;]*m//g' "$1"
+}
+
+normalize_project_path() {
+  local input=$1
+  local output=$2
+  local project=$3
+  local native_project=$project
+  local native_short_project=$project
+  case $(uname -s) in
+    MINGW*|MSYS*)
+      native_project=$(cd "$project" && pwd -W)
+      native_short_project=${native_project/"$native_user"/"$native_short_user"}
+      ;;
+  esac
+  PROJECT_PATH=$project PROJECT_NATIVE_PATH=$native_project \
+    PROJECT_SHORT_PATH=$native_short_project node -e '
+    const fs = require("fs");
+    let text = fs.readFileSync(process.argv[1], "utf8");
+    for (const project of [process.env.PROJECT_PATH, process.env.PROJECT_NATIVE_PATH, process.env.PROJECT_SHORT_PATH]) {
+      for (const spelling of [project, project.replaceAll("/", "\\")]) {
+        text = text.split(spelling).join("<ROOT>");
+      }
+    }
+    text = text.replaceAll("\\", "/");
+    text = text.replaceAll("./<ROOT>/", "./");
+    fs.writeFileSync(process.argv[2], text);
+  ' "$input" "$output"
+}
+
+checked=0
+run_case() {
+  name=$1
+  rust_expected=$2
+  ocaml_expected=$3
+  shift 3
+  set +e
+  "$rust" "$@" >"$work/rust.out" 2>"$work/rust.err"
+  rust_status=$?
+  "$ocaml" "$@" >"$work/ocaml.out" 2>"$work/ocaml.err"
+  ocaml_status=$?
+  set -e
+  rust_actual=$(classify "$rust_status")
+  ocaml_actual=$(classify "$ocaml_status")
+  if [ "$rust_actual" != "$rust_expected" ] || \
+    [ "$ocaml_actual" != "$ocaml_expected" ]; then
+    printf '%s: expected Rust=%s/OCaml=%s, got Rust=%s/OCaml=%s\n' \
+      "$name" "$rust_expected" "$ocaml_expected" \
+      "$rust_status" "$ocaml_status" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+  checked=$((checked + 1))
+}
+
+run_cwd_case() {
+  name=$1
+  rust_expected=$2
+  ocaml_expected=$3
+  cwd=$4
+  shift 4
+  set +e
+  (cd "$cwd" && "$rust" "$@") >"$work/rust.out" 2>"$work/rust.err"
+  rust_status=$?
+  (cd "$cwd" && "$ocaml" "$@") >"$work/ocaml.out" 2>"$work/ocaml.err"
+  ocaml_status=$?
+  set -e
+  rust_actual=$(classify "$rust_status")
+  ocaml_actual=$(classify "$ocaml_status")
+  if [ "$rust_actual" != "$rust_expected" ] || \
+    [ "$ocaml_actual" != "$ocaml_expected" ]; then
+    printf '%s: expected Rust=%s/OCaml=%s, got Rust=%s/OCaml=%s\n' \
+      "$name" "$rust_expected" "$ocaml_expected" \
+      "$rust_status" "$ocaml_status" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+  checked=$((checked + 1))
+}
+
+require_same_output() {
+  name=$1
+  if ! cmp -s "$work/rust.out" "$work/ocaml.out" || \
+    ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+    echo "$name: Rust and OCaml output differ" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+}
+
+require_both_errors_contain() {
+  local name=$1
+  local fragment=$2
+  local native_fragment=${fragment/"$work"/"$native_work"}
+  local native_short_fragment=${fragment/"$work"/"$native_short_work"}
+  if ! { grep -F "$fragment" "$work/rust.err" >/dev/null || \
+      tr '\\' '/' <"$work/rust.err" | grep -F "$native_fragment" >/dev/null || \
+      tr '\\' '/' <"$work/rust.err" | grep -F "$native_short_fragment" >/dev/null; } || \
+    ! { grep -F "$fragment" "$work/ocaml.err" >/dev/null || \
+      tr '\\' '/' <"$work/ocaml.err" | grep -F "$native_fragment" >/dev/null || \
+      tr '\\' '/' <"$work/ocaml.err" | grep -F "$native_short_fragment" >/dev/null; }; then
+    printf '%s: expected both errors to contain %s\n' "$name" "$fragment" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+}
+
+run_missing_bsc_case() {
+  name=$1
+  shift
+  set +e
+  RESCRIPT_BSC_EXE="$work/missing-bsc" "$rust" "$@" \
+    >"$work/rust.out" 2>"$work/rust.err"
+  rust_status=$?
+  RESCRIPT_BSC_EXE="$work/missing-bsc" "$ocaml" "$@" \
+    >"$work/ocaml.out" 2>"$work/ocaml.err"
+  ocaml_status=$?
+  set -e
+  if [ "$(classify "$rust_status")" != panic ] || \
+    [ "$(classify "$ocaml_status")" != reject ]; then
+    printf '%s: expected Rust=panic/OCaml=reject, got Rust=%s/OCaml=%s\n' \
+      "$name" "$rust_status" "$ocaml_status" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+  if ! grep -F 'RESCRIPT_BSC_EXE points to missing path' \
+    "$work/ocaml.err" >/dev/null; then
+    echo "$name: OCaml did not report the stale compiler path" >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+  checked=$((checked + 1))
+}
+
+run_build_output_case() {
+  name=$1
+  expected_status=$2
+  fixture=$3
+  mode=$4
+  rust_project="$work/$name-rust"
+  ocaml_project="$work/$name-ocaml"
+  cp -R "$fixture" "$rust_project"
+  cp -R "$fixture" "$ocaml_project"
+  set +e
+  if [ "$mode" = quiet ]; then
+    "$rust" -q build "$rust_project" >"$work/rust.out" 2>"$work/rust.err"
+    rust_status=$?
+    "$ocaml" -q build "$ocaml_project" >"$work/ocaml.out" 2>"$work/ocaml.err"
+    ocaml_status=$?
+  elif [ "$mode" = forced-color ]; then
+    CLICOLOR_FORCE=1 "$rust" build "$rust_project" \
+      >"$work/rust.out" 2>"$work/rust.err"
+    rust_status=$?
+    CLICOLOR_FORCE=1 "$ocaml" build "$ocaml_project" \
+      >"$work/ocaml.out" 2>"$work/ocaml.err"
+    ocaml_status=$?
+  else
+    "$rust" build "$rust_project" >"$work/rust.out" 2>"$work/rust.err"
+    rust_status=$?
+    "$ocaml" build "$ocaml_project" >"$work/ocaml.out" 2>"$work/ocaml.err"
+    ocaml_status=$?
+  fi
+  set -e
+  normalize_project_path "$work/rust.out" "$work/rust.out.norm" "$rust_project"
+  normalize_project_path "$work/rust.err" "$work/rust.err.norm" "$rust_project"
+  normalize_project_path "$work/ocaml.out" "$work/ocaml.out.norm" "$ocaml_project"
+  normalize_project_path "$work/ocaml.err" "$work/ocaml.err.norm" "$ocaml_project"
+  if [ "$rust_status" -ne "$expected_status" ] || \
+    [ "$ocaml_status" -ne "$expected_status" ] || \
+    ! cmp -s "$work/rust.out.norm" "$work/ocaml.out.norm" || \
+    ! cmp -s "$work/rust.err.norm" "$work/ocaml.err.norm"; then
+    echo "$name: $mode build output differs" >&2
+    printf '%s\n' \
+      "statuses: expected=$expected_status Rust=$rust_status OCaml=$ocaml_status" \
+      >&2
+    printf '%s\n' '--- Rust stdout ---' >&2
+    cat "$work/rust.out.norm" >&2
+    printf '%s\n' '--- OCaml stdout ---' >&2
+    cat "$work/ocaml.out.norm" >&2
+    printf '%s\n' '--- Rust stderr ---' >&2
+    cat "$work/rust.err.norm" >&2
+    printf '%s\n' '--- OCaml stderr ---' >&2
+    cat "$work/ocaml.err.norm" >&2
+    for stream in out err; do
+      if ! cmp -s "$work/rust.$stream.norm" "$work/ocaml.$stream.norm"; then
+        printf '%s\n' "--- $stream byte counts ---" >&2
+        wc -c "$work/rust.$stream.norm" "$work/ocaml.$stream.norm" >&2
+        printf '%s\n' "--- first differing $stream bytes ---" >&2
+        cmp -l "$work/rust.$stream.norm" "$work/ocaml.$stream.norm" \
+          | head -n 20 >&2 || true
+        printf '%s\n' "--- Rust $stream tail bytes ---" >&2
+        tail -c 64 "$work/rust.$stream.norm" | od -An -tx1c >&2
+        printf '%s\n' "--- OCaml $stream tail bytes ---" >&2
+        tail -c 64 "$work/ocaml.$stream.norm" | od -An -tx1c >&2
+      fi
+    done
+    exit 1
+  fi
+  checked=$((checked + 1))
+}
+
+run_multiple_parse_errors_case() {
+  rust_project="$work/multiple-parse-errors-rust"
+  ocaml_project="$work/multiple-parse-errors-ocaml"
+  cp -R "$work/multiple-parse-errors" "$rust_project"
+  cp -R "$work/multiple-parse-errors" "$ocaml_project"
+  set +e
+  "$rust" build "$rust_project" >"$work/rust.out" 2>"$work/rust.err"
+  rust_status=$?
+  "$ocaml" build "$ocaml_project" >"$work/ocaml.out" 2>"$work/ocaml.err"
+  ocaml_status=$?
+  set -e
+  if [ "$rust_status" -ne 1 ] || [ "$ocaml_status" -ne 1 ]; then
+    printf 'multiple-parse-errors: expected status 1, got Rust=%s OCaml=%s\n' \
+      "$rust_status" "$ocaml_status" >&2
+    exit 1
+  fi
+  for implementation in rust ocaml; do
+    error="$work/$implementation.err"
+    if [ "$(grep -cF 'Error in multiple-parse-errors:' "$error")" -ne 2 ] || \
+      ! grep -E '[/\\]src[/\\]A\.res' "$error" >/dev/null || \
+      ! grep -E '[/\\]src[/\\]B\.res' "$error" >/dev/null; then
+      echo "$implementation did not retain every independent parse error" >&2
+      cat "$error" >&2
+      exit 1
+    fi
+  done
+  checked=$((checked + 1))
+}
+
+wait_for_file() {
+  path=$1
+  attempts=0
+  while [ "$attempts" -lt 150 ] && [ ! -f "$path" ]; do
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  if [ -f "$path" ]; then
+    return 0
+  fi
+  printf 'Timed out waiting for file %s (requested at line %s)\n' \
+    "$path" "${BASH_LINENO[0]:-unknown}" >&2
+  return 1
+}
+
+wait_for_text() {
+  path=$1
+  pattern=$2
+  attempts=0
+  while [ "$attempts" -lt 150 ] && \
+    ! grep -F "$pattern" "$path" >/dev/null 2>&1; do
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  if grep -F "$pattern" "$path" >/dev/null 2>&1; then
+    return 0
+  fi
+  printf 'Timed out waiting for %s in %s\n' "$pattern" "$path" >&2
+  if [ -f "$path" ]; then
+    printf '%s\n' '--- observed contents ---' >&2
+    cat "$path" >&2
+  else
+    printf '%s\n' '--- file does not exist ---' >&2
+  fi
+  return 1
+}
+
+wait_for_exit() {
+  pid=$1
+  attempts=0
+  while [ "$attempts" -lt 150 ] && kill -0 "$pid" 2>/dev/null; do
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  ! kill -0 "$pid" 2>/dev/null
+}
+
+wait_for_line_count() {
+  path=$1
+  expected=$2
+  attempts=0
+  while [ "$attempts" -lt 150 ]; do
+    actual=0
+    if [ -f "$path" ]; then
+      actual=$(wc -l <"$path" | tr -d ' ')
+    fi
+    if [ "$actual" -ge "$expected" ]; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+line_count_stays() {
+  path=$1
+  expected=$2
+  attempts=0
+  while [ "$attempts" -lt 20 ]; do
+    actual=0
+    if [ -f "$path" ]; then
+      actual=$(wc -l <"$path" | tr -d ' ')
+    fi
+    if [ "$actual" -ne "$expected" ]; then
+      return 1
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+}
+
+run_build_output_case redirected-success 0 \
+  "$root/rewatch-ocaml/tests/basic" redirected
+run_build_output_case redirected-compile-error 1 \
+  "$root/rewatch-ocaml/tests/failure" redirected
+run_build_output_case redirected-parse-error 1 \
+  "$work/redirected-parse-fixture" redirected
+run_multiple_parse_errors_case
+run_build_output_case redirected-warning 0 \
+  "$root/rewatch-ocaml/tests/warning-replay" redirected
+run_build_output_case redirected-config-diagnostics 0 \
+  "$work/redirected-config-diagnostics" redirected
+run_build_output_case forced-color-config-diagnostics 0 \
+  "$work/redirected-config-diagnostics" forced-color
+
+run_build_output_case quiet-success 0 "$project" quiet
+if [ -s "$work/rust.out" ] || [ -s "$work/rust.err" ] || \
+  [ -s "$work/ocaml.out" ] || [ -s "$work/ocaml.err" ]; then
+  echo "quiet-success: a clean build emitted output" >&2
+  for implementation in rust ocaml; do
+    printf '%s\n' "--- $implementation stdout ---" >&2
+    cat "$work/$implementation.out" >&2
+    printf '%s\n' "--- $implementation stderr ---" >&2
+    cat "$work/$implementation.err" >&2
+  done
+  exit 1
+fi
+run_build_output_case quiet-compile-error 1 \
+  "$root/rewatch-ocaml/tests/failure" quiet
+run_build_output_case quiet-parse-error 1 "$work/redirected-parse-fixture" quiet
+run_build_output_case quiet-warning 0 \
+  "$root/rewatch-ocaml/tests/warning-replay" quiet
+
+run_case build-subcommand-version exit2 accept build --version
+run_case clustered-global-version accept accept -vV build
+run_case clustered-global-verbosity accept accept -vvvvv build "$project"
+run_case clustered-global-version-before-help accept accept -Vh build
+run_case clustered-global-help-before-version accept accept -hV build
+run_case clustered-subcommand-help-before-version accept accept build -hV
+run_case clustered-subcommand-version-before-help exit2 accept build -Vh
+"$ocaml" build --help=groff >"$work/ocaml-help-groff.out"
+if ! grep -F '.\" Pipe this output to groff' \
+  "$work/ocaml-help-groff.out" >/dev/null; then
+  echo "formatted-help: OCaml did not preserve the requested groff format" >&2
+  cat "$work/ocaml-help-groff.out" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+run_case implicit-conflicting-verbosity exit2 exit2 -v -q
+run_case build-conflicting-verbosity exit2 exit2 build --verbose --quiet
+run_case filter-perl-quoting exit2 exit2 build --filter '\QFoo.res\E' "$project"
+run_case filter-comment-group exit2 exit2 build --filter '(?#note)Foo' "$project"
+run_case filter-octal-escape exit2 exit2 build --filter '\123' "$project"
+run_case filter-braced-octal-escape exit2 exit2 build --filter '\o{123}' "$project"
+run_case filter-perl-control-escape-e exit2 exit2 build --filter '\e' "$project"
+run_case filter-perl-anchor-g exit2 exit2 build --filter '\G' "$project"
+run_case filter-perl-anchor-z exit2 exit2 build --filter '\Z' "$project"
+run_case filter-descending-range exit2 exit2 build --filter '[z-a]' "$project"
+run_case filter-escaped-range-start exit2 exit2 build --filter '[\d-z]' "$project"
+run_case filter-in-class-nonword accept exit2 build --filter \
+  '^[\W]+\.res$' "$project"
+run_case filter-collating-element accept exit2 build --filter '[[.a.]]' "$project"
+run_case filter-class-algebra accept exit2 build --filter \
+  '[a-z&&[^aeiou]]' "$project"
+run_case build-no-timing-preserves-folder exit2 accept build --no-timing "$project"
+run_case compiler-args-source accept accept compiler-args "$project/src/A.res"
+run_case compiler-args-extension accept reject compiler-args "$project/src/A.txt"
+run_case compiler-args-missing panic reject compiler-args "$project/src/Missing.res"
+run_case compiler-args-no-project panic reject compiler-args "$work/orphan/A.res"
+
+run_missing_bsc_case build-missing-bsc build "$project"
+run_missing_bsc_case format-missing-bsc format "$project/src/A.res"
+set +e
+RESCRIPT_BSC_EXE="$work/missing-bsc" \
+  "$rust" clean "$work/clean-missing-bsc" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+RESCRIPT_BSC_EXE="$work/missing-bsc" \
+  "$ocaml" clean "$work/clean-missing-bsc" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != panic ] || [ "$ocaml_status" -ne 0 ] || \
+  [ -e "$work/clean-missing-bsc/lib/bs/marker" ]; then
+  echo "clean-missing-bsc: expected Rust panic and successful OCaml cleanup" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+env -u RESCRIPT_RUNTIME "$rust" clean "$work/clean-missing-runtime" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+env -u RESCRIPT_RUNTIME "$ocaml" clean "$work/clean-missing-runtime" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != reject ] || [ "$ocaml_status" -ne 0 ] || \
+  [ -e "$work/clean-missing-runtime/lib/bs/marker" ]; then
+  echo "clean-missing-runtime: expected Rust rejection and successful OCaml cleanup" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+env -u RESCRIPT_RUNTIME "$rust" build "$work/missing-runtime-package" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+env -u RESCRIPT_RUNTIME "$ocaml" build "$work/missing-runtime-package" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -eq 0 ] || [ "$ocaml_status" -eq 0 ]; then
+  echo "build-missing-runtime-package: expected both builds to reject" >&2
+  exit 1
+fi
+require_both_errors_contain build-missing-runtime-package \
+  'The rescript runtime package could not be found.'
+require_both_errors_contain build-missing-runtime-package \
+  'Please set RESCRIPT_RUNTIME environment variable'
+checked=$((checked + 1))
+set +e
+RESCRIPT_RUNTIME="$work/missing-runtime" "$rust" build "$project" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+RESCRIPT_RUNTIME="$work/missing-runtime" "$ocaml" build "$project" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -eq 0 ] || [ "$ocaml_status" -eq 0 ] || \
+  ! grep -F "RESCRIPT_RUNTIME points to missing path" \
+    "$work/ocaml.err" >/dev/null || \
+  ! grep -F "missing-runtime" "$work/ocaml.err" >/dev/null || \
+  ! grep -F "The module or file Pervasives can't be found." \
+    "$work/rust.err" >/dev/null; then
+  echo "build-stale-runtime: expected contextual OCaml preflight rejection" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+run_case format-missing-file reject reject format "$project/src/Missing.res"
+require_same_output format-missing-file
+run_case format-directory reject reject format "$project/src"
+require_same_output format-directory
+run_case format-unsupported-extension reject reject format "$project/src/A.txt"
+require_same_output format-unsupported-extension
+run_cwd_case format-no-config reject reject "$work/empty" format
+require_both_errors_contain format-no-config \
+  "Could not read rescript.json at $work/empty:"
+require_both_errors_contain format-no-config "$work/empty/bsconfig.json"
+run_cwd_case format-malformed-config reject reject "$work/malformed" format
+require_both_errors_contain format-malformed-config \
+  "Could not read rescript.json at $work/malformed:"
+if ! grep -F 'Failed to parse rescript.json' "$work/rust.err" >/dev/null || \
+  ! grep -F 'invalid JSON' "$work/ocaml.err" >/dev/null; then
+  echo "format-malformed-config: JSON parser context was lost" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_cwd_case format-config-directory reject reject "$work/config-directory" format
+require_both_errors_contain format-config-directory \
+  "$work/config-directory/rescript.json"
+if ! grep -E 'Is a directory|Access is denied' "$work/rust.err" >/dev/null || \
+  ! grep -F 'Is a directory' "$work/ocaml.err" >/dev/null; then
+  echo "format-config-directory: file-kind diagnostic was lost" >&2
+  exit 1
+fi
+set +e
+printf 'let value =\n' | "$rust" format --stdin .res \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+printf 'let value =\n' | "$ocaml" format --stdin .res \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -eq 0 ] || [ "$ocaml_status" -eq 0 ] || \
+  [ -s "$work/rust.out" ] || [ -s "$work/ocaml.out" ]; then
+  echo "format-invalid-stdin: expected both formatters to reject" >&2
+  exit 1
+fi
+require_both_errors_contain format-invalid-stdin 'Error formatting stdin:'
+require_both_errors_contain format-invalid-stdin \
+  'This let-binding misses an expression'
+checked=$((checked + 1))
+mkdir -p "$work/format-write-rust" "$work/format-write-ocaml"
+printf 'let value=1\n' >"$work/format-write-rust/A.res"
+printf 'let value=1\n' >"$work/format-write-ocaml/A.res"
+chmod 0444 "$work/format-write-rust/A.res" "$work/format-write-ocaml/A.res"
+if [ ! -w "$work/format-write-rust/A.res" ] && \
+  [ ! -w "$work/format-write-ocaml/A.res" ]; then
+  set +e
+  "$rust" format "$work/format-write-rust/A.res" \
+    >"$work/rust.out" 2>"$work/rust.err"
+  rust_status=$?
+  "$ocaml" format "$work/format-write-ocaml/A.res" \
+    >"$work/ocaml.out" 2>"$work/ocaml.err"
+  ocaml_status=$?
+  set -e
+  if [ "$rust_status" -eq 0 ] || [ "$ocaml_status" -eq 0 ] || \
+    ! grep -E 'Permission denied|Access is denied' "$work/rust.err" >/dev/null || \
+    ! grep -F "Could not write formatted file" "$work/ocaml.err" >/dev/null || \
+    ! grep -F "format-write-ocaml/A.res" \
+      < <(tr '\\' '/' <"$work/ocaml.err") >/dev/null; then
+    echo "format-write-failure: formatter write failures lost context" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" "$work/ocaml.err" >&2
+    exit 1
+  fi
+  checked=$((checked + 1))
+fi
+chmod 0644 "$work/format-write-rust/A.res" "$work/format-write-ocaml/A.res"
+
+run_case build-missing-folder reject reject build "$work/missing"
+run_case build-existing-folder-without-config reject reject build "$work/empty"
+require_both_errors_contain build-existing-folder-without-config "$work/empty"
+run_case build-malformed-config reject reject build "$work/malformed"
+require_both_errors_contain build-malformed-config \
+  "$work/malformed"
+run_case build-malformed-parent reject reject build "$work/malformed-parent/child"
+require_both_errors_contain build-malformed-parent \
+  "$work/malformed-parent"
+run_case build-config-path-is-directory reject reject build "$work/config-directory"
+require_both_errors_contain build-config-path-is-directory \
+  "$work/config-directory"
+run_case after-build-nonzero-is-not-ignored accept reject build --after-build \
+  "node $command_work/failing-after-build.js" "$project"
+if ! grep -F 'hook failed' "$work/rust.err" >/dev/null || \
+  ! grep -F -- '--after-build command failed with exit code 7' \
+    "$work/ocaml.err" >/dev/null || \
+  ! grep -F 'hook failed' "$work/ocaml.err" >/dev/null; then
+  echo "Nonzero --after-build handling differs from its recorded outcomes" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case after-build-empty panic reject build --after-build '' "$project"
+if ! grep -F -- '--after-build command cannot be empty' \
+  "$work/ocaml.err" >/dev/null; then
+  echo "Empty --after-build did not produce a contextual OCaml error" >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case after-build-missing-program panic reject build --after-build \
+  rewatch-command-that-does-not-exist "$project"
+if ! grep -F 'Could not run --after-build command' \
+    "$work/ocaml.err" >/dev/null || \
+  ! grep -F 'rewatch-command-that-does-not-exist' \
+    "$work/ocaml.err" >/dev/null; then
+  echo "Missing --after-build program did not produce a contextual OCaml error" >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case js-post-build-nonzero reject reject build "$work/failed-js-post-build"
+js_post_build_error="js-post-build command failed for $work/failed-js-post-build/src/A.js"
+require_both_errors_contain js-post-build-nonzero "$js_post_build_error"
+printf 'not-a-pid' >"$work/malformed-lock/lib/build.lock"
+run_case build-malformed-lock reject reject build "$work/malformed-lock"
+if [ "$(cat "$work/malformed-lock/lib/build.lock")" != not-a-pid ]; then
+  echo "OCaml replaced a malformed build lock with unknown ownership" >&2
+  exit 1
+fi
+if [ -n "$(find "$work/malformed-lock/lib" -maxdepth 1 \
+  -name '.build-lock-*.tmp' -print -quit)" ]; then
+  echo "OCaml left a build-lock candidate after acquisition failed" >&2
+  exit 1
+fi
+# Use the real executable as the lock owner. Copying an MSYS utility to a
+# rescript-prefixed name does not change the image name reported by tasklist,
+# while both implementations deliberately reject locks owned by unrelated
+# executables to protect against PID reuse.
+"$ocaml" watch "$work/signal-lock-owner" \
+  >"$work/signal-lock-owner.out" 2>"$work/signal-lock-owner.err" &
+signal_lock_owner_pid=$!
+background_pids="$background_pids $signal_lock_owner_pid"
+wait_for_text "$work/signal-lock-owner.out" "Finished initial compilation"
+printf '%s' "$(lock_owner_pid "$signal_lock_owner_pid")" \
+  >"$work/signal-lock/lib/build.lock"
+"$ocaml" watch "$work/signal-lock" \
+  >"$work/signal-lock.out" 2>"$work/signal-lock.err" &
+signal_lock_watch_pid=$!
+background_pids="$background_pids $signal_lock_watch_pid"
+wait_for_text "$work/signal-lock.out" "Waiting for other build to finish"
+kill -TERM "$signal_lock_watch_pid"
+set +e
+wait "$signal_lock_watch_pid"
+signal_lock_status=$?
+set -e
+if ! $windows_posix_shell && [ "$signal_lock_status" -ne 0 ]; then
+  echo "Signal during build-lock wait exited with $signal_lock_status" >&2
+  exit 1
+fi
+# MSYS implements kill for native processes by terminating them externally,
+# so no Windows console control event reaches OCaml and the status is nonzero.
+# The native Windows unit tests cover deferred signal dispatch; this integration
+# case still proves that terminating a waiter cannot emit a spurious diagnostic.
+if [ -s "$work/signal-lock.err" ]; then
+  echo "Signal during build-lock wait emitted a diagnostic" >&2
+  cat "$work/signal-lock.err" >&2
+  exit 1
+fi
+kill -TERM "$signal_lock_owner_pid"
+wait "$signal_lock_owner_pid" 2>/dev/null || true
+checked=$((checked + 1))
+printf 'not-a-pid' >"$work/malformed-lock/lib/watch.lock"
+run_case watch-malformed-lock reject reject watch "$work/malformed-lock"
+if [ "$(cat "$work/malformed-lock/lib/watch.lock")" != not-a-pid ]; then
+  echo "OCaml replaced a malformed watch lock with unknown ownership" >&2
+  exit 1
+fi
+if [ -n "$(find "$work/malformed-lock/lib" -maxdepth 1 \
+  -name '.watch-lock-*.tmp' -print -quit)" ]; then
+  echo "OCaml left a watch-lock candidate after acquisition failed" >&2
+  exit 1
+fi
+printf '{ invalid json\n' >"$work/watch-lock-order/rescript.json"
+# A copied MSYS utility keeps its original Windows image name and is therefore
+# rejected as an unrelated lock owner. Run the implementation under test as a
+# real watcher so both process identity and the native PID match production.
+for implementation in rust ocaml; do
+  case "$implementation" in
+    rust) executable=$rust ;;
+    ocaml) executable=$ocaml ;;
+  esac
+  owner_dir="$work/watch-lock-owner-$implementation"
+  mkdir -p "$owner_dir/src"
+  printf '{"name":"watch-lock-owner-%s","sources":["src"]}\n' \
+    "$implementation" >"$owner_dir/rescript.json"
+  printf 'let value = 1\n' >"$owner_dir/src/A.res"
+  "$executable" watch "$owner_dir" \
+    >"$owner_dir/watch.out" 2>"$owner_dir/watch.err" &
+  watch_lock_owner_pid=$!
+  background_pids="$background_pids $watch_lock_owner_pid"
+  wait_for_text "$owner_dir/watch.out" "Finished initial compilation"
+  native_owner_pid=$(lock_owner_pid "$watch_lock_owner_pid")
+  if [ -z "$native_owner_pid" ]; then
+    echo "watch-lock-before-config: could not resolve $implementation owner PID" >&2
+    exit 1
+  fi
+  printf '%s' "$native_owner_pid" \
+    >"$work/watch-lock-order/lib/watch.lock"
+  set +e
+  "$executable" watch "$work/watch-lock-order" \
+    >"$work/$implementation.out" 2>"$work/$implementation.err"
+  status=$?
+  set -e
+  if [ "$(classify "$status")" != reject ] || \
+    ! grep -F 'A ReScript build is already running' \
+      "$work/$implementation.err" >/dev/null || \
+    grep -F 'invalid JSON' "$work/$implementation.err" >/dev/null; then
+    echo "watch-lock-before-config: $implementation lock acquisition did not precede config parsing" >&2
+    cat "$work/$implementation.out" "$work/$implementation.err" >&2
+    exit 1
+  fi
+  kill -TERM "$watch_lock_owner_pid"
+  wait "$watch_lock_owner_pid" 2>/dev/null || true
+done
+checked=$((checked + 1))
+run_case build-interface-path-mismatch reject reject build \
+  "$work/interface-mismatch"
+if ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+  echo "Implementation/interface mismatch diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.err" >&2
+  exit 1
+fi
+set +e
+"$rust" build "$work/exotic-module-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" build "$work/exotic-module-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+rust_mlmap="$work/exotic-module-rust/lib/bs/Ns.mlmap"
+ocaml_mlmap="$work/exotic-module-ocaml/lib/bs/Ns.mlmap"
+if [ "$(classify "$rust_status")" != accept ] || \
+  [ "$(classify "$ocaml_status")" != accept ] || \
+  ! cmp -s "$rust_mlmap" "$ocaml_mlmap" || \
+  ! grep -Fx Main "$ocaml_mlmap" >/dev/null || \
+  grep -F 'Foo-bar' "$ocaml_mlmap" >/dev/null; then
+  printf 'namespace-exotic-module: expected matching successful builds, got Rust=%s/OCaml=%s\n' \
+    "$rust_status" "$ocaml_status" >&2
+  printf '%s\n' '--- Rust output / namespace map ---' >&2
+  cat "$work/rust.out" "$work/rust.err" "$rust_mlmap" >&2
+  printf '%s\n' '--- OCaml output / namespace map ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" "$ocaml_mlmap" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+"$rust" build --filter nested "$work/filter-basename-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" build --filter nested "$work/filter-basename-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != accept ] || \
+  [ "$(classify "$ocaml_status")" != accept ] || \
+  [ -e "$work/filter-basename-rust/src/nested/A.js" ] || \
+  [ -e "$work/filter-basename-ocaml/src/nested/A.js" ]; then
+  echo "Source filters did not consistently ignore directory-only matches" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+"$rust" build --filter 'A\.res$' "$work/filter-basename-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" build --filter 'A\.res$' "$work/filter-basename-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != accept ] || \
+  [ "$(classify "$ocaml_status")" != accept ] || \
+  [ ! -e "$work/filter-basename-rust/src/nested/A.js" ] || \
+  [ ! -e "$work/filter-basename-ocaml/src/nested/A.js" ]; then
+  echo "Source filters did not consistently include a basename match" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+"$rust" build --filter '(?:A|B\d)\.res$' "$work/filter-basename-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" build --filter '(?:A|B\d)\.res$' "$work/filter-basename-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != accept ] || \
+  [ "$(classify "$ocaml_status")" != accept ] || \
+  [ ! -e "$work/filter-basename-rust/src/nested/B2.js" ] || \
+  [ ! -e "$work/filter-basename-ocaml/src/nested/B2.js" ]; then
+  echo "Source filters did not consistently support Rust-style regex syntax" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+run_case build-excludes-external-dev-source accept accept build \
+  "$work/external-dev-source"
+run_case build-ignores-dormant-external-dev-permission reject accept build \
+  "$work/external-dev-permission"
+strip_ansi "$work/rust.out" >"$work/rust.out.plain"
+strip_ansi "$work/rust.err" >"$work/rust.err.plain"
+if ! grep -F 'a has the following unallowed dependencies' \
+    "$work/rust.err.plain" >/dev/null; then
+  echo "Rust dormant external dev-dependency rejection was not reproduced" >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  exit 1
+fi
+run_case build-reports-all-active-permission-failures reject reject build \
+  "$work/active-permission"
+strip_ansi "$work/rust.out" >"$work/rust.out.plain"
+strip_ansi "$work/rust.err" >"$work/rust.err.plain"
+strip_ansi "$work/ocaml.out" >"$work/ocaml.out.plain"
+strip_ansi "$work/ocaml.err" >"$work/ocaml.err.plain"
+rust_permission_details=$(grep -Ec '^dependencies dependencies: (a|b)$' \
+  "$work/rust.out.plain" || true)
+if [ "$rust_permission_details" -ne 1 ] || \
+  ! grep -Fx 'root dependencies: a' "$work/ocaml.err.plain" >/dev/null || \
+  ! grep -Fx 'root dependencies: b' "$work/ocaml.err.plain" >/dev/null || \
+  ! grep -F 'unallowed_dependents' "$work/rust.err.plain" >/dev/null || \
+  ! grep -F 'config.json' "$work/rust.err.plain" >/dev/null || \
+  ! grep -F 'Update allowed-dependents in the dependency rescript.json files.' \
+    "$work/ocaml.err.plain" >/dev/null || [ -s "$work/ocaml.out.plain" ]; then
+  echo "Active dependency-permission diagnostics changed unexpectedly" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-source-path-is-file accept accept build "$work/source-path-file"
+if ! cmp -s "$work/rust.err" "$work/ocaml.err" || \
+  ! grep -F 'Could not read folder: "src"' "$work/ocaml.err" >/dev/null; then
+  echo "Non-directory source diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-missing-source-folder accept accept build \
+  "$work/missing-source-folder"
+if ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+  echo "Missing source-folder diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-dependency-without-sources accept accept build \
+  "$work/dependency-without-sources"
+if ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+  echo "Dependency-without-sources diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-default-feature-cycle accept accept build \
+  "$work/default-feature-cycle"
+run_cwd_case format-requested-feature-cycle reject reject \
+  "$work/format-feature-cycle" format
+if ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+  echo "Requested format feature-cycle diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_cwd_case format-scans-graph-with-effective-features \
+  reject reject \
+  "$work/format-source-selection" format --check
+normalize_project_path "$work/rust.err" "$work/rust.err.norm" \
+  "$work/format-source-selection"
+normalize_project_path "$work/ocaml.err" "$work/ocaml.err.norm" \
+  "$work/format-source-selection"
+missing_folder='Could not read folder: "missing". Specified in dependency: installed'
+base_file='[format check] <ROOT>/packages/local/base/Base.res'
+native_file='[format check] <ROOT>/packages/local/native/Native.res'
+other_file='<ROOT>/packages/local/other/Other.res'
+if ! grep -F "$missing_folder" "$work/rust.err.norm" >/dev/null || \
+  ! grep -F "$missing_folder" "$work/ocaml.err.norm" >/dev/null || \
+  ! grep -F "$base_file" "$work/rust.err.norm" >/dev/null || \
+  ! grep -F "$base_file" "$work/ocaml.err.norm" >/dev/null || \
+  ! grep -F "$native_file" "$work/rust.err.norm" >/dev/null || \
+  ! grep -F "$native_file" "$work/ocaml.err.norm" >/dev/null || \
+  grep -F "$other_file" "$work/rust.err.norm" >/dev/null || \
+  grep -F "$other_file" "$work/ocaml.err.norm" >/dev/null || \
+  ! grep -F 'The 2 files listed above need formatting' "$work/rust.err.norm" >/dev/null || \
+  ! grep -F 'The 2 files listed above need formatting' "$work/ocaml.err.norm" >/dev/null; then
+  echo "Implicit format package scanning or feature selection differs" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case clean-dependency-without-sources accept accept clean \
+  "$work/dependency-without-sources"
+if ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+  echo "Clean dependency-without-sources diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.err" >&2
+  exit 1
+fi
+set +e
+"$rust" clean "$project" >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" clean "$project" >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -ne 0 ] || [ "$ocaml_status" -ne 0 ] || \
+  ! cmp -s "$work/rust.out" "$work/ocaml.out" || \
+  ! cmp -s "$work/rust.err" "$work/ocaml.err" || \
+  ! grep -Fx 'Cleaning command-validation' "$work/ocaml.out" >/dev/null; then
+  echo "Redirected clean progress differs" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+"$rust" -q clean "$project" >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" -q clean "$project" >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -ne 0 ] || [ "$ocaml_status" -ne 0 ] || \
+  [ -s "$work/rust.out" ] || [ -s "$work/rust.err" ] || \
+  [ -s "$work/ocaml.out" ] || [ -s "$work/ocaml.err" ]; then
+  echo "Quiet redirected clean emitted output" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+run_cwd_case format-dependency-without-sources accept accept \
+  "$work/dependency-without-sources" format
+if ! cmp -s "$work/rust.err" "$work/ocaml.err"; then
+  echo "Format dependency-without-sources diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-package-name-mismatch accept accept build \
+  "$work/package-name-mismatch"
+normalize_project_path "$work/rust.err" "$work/rust.err.norm" \
+  "$work/package-name-mismatch"
+normalize_project_path "$work/ocaml.err" "$work/ocaml.err.norm" \
+  "$work/package-name-mismatch"
+if ! cmp -s "$work/rust.err.norm" "$work/ocaml.err.norm"; then
+  echo "Package-name mismatch diagnostics differ" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-malformed-package-json reject reject build \
+  "$work/malformed-package-json"
+require_both_errors_contain build-malformed-package-json \
+  'Could not initialize build: Could not parse package.json:'
+run_case build-mismatched-dependency-name panic exit2 build \
+  "$work/mismatched-dependency"
+strip_ansi "$work/ocaml.err" >"$work/ocaml.err.plain"
+if ! grep -F \
+    "resolved package identity 'different-name' does not match the requested dependency name" \
+    "$work/ocaml.err.plain" >/dev/null; then
+  echo "build-mismatched-dependency-name: OCaml did not reject the conflicting identity" >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_case build-missing-dependency exit2 exit2 build "$work/missing-dependency"
+require_both_errors_contain build-missing-dependency \
+  "Could not build package tree reading dependency 'absent' at path '$work/missing-dependency'. Error:"
+run_case build-configless-dependency exit2 exit2 build "$work/configless-dependency"
+require_both_errors_contain build-configless-dependency \
+  "Could not build package tree for 'no-config' at path '$work/configless-dependency'. Error:"
+run_case build-malformed-dependency exit2 exit2 build "$work/malformed-dependency"
+require_both_errors_contain build-malformed-dependency \
+  "Could not build package tree for 'bad-config' at path '$work/malformed-dependency'. Error:"
+run_case build-duplicate-dependency accept accept build "$work/duplicate-dependency"
+duplicate_warning='Duplicated package: shared ./node_modules/shared (chosen) vs ./node_modules/a/node_modules/shared in ./node_modules/a'
+normalize_project_path "$work/rust.err" "$work/rust.err.norm" \
+  "$work/duplicate-dependency"
+normalize_project_path "$work/ocaml.err" "$work/ocaml.err.norm" \
+  "$work/duplicate-dependency"
+if ! grep -F "$duplicate_warning" "$work/rust.err.norm" >/dev/null || \
+  ! grep -F "$duplicate_warning" "$work/ocaml.err.norm" >/dev/null; then
+  echo "Duplicate dependency warning was not emitted by both implementations" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+run_cwd_case format-duplicate-dependency accept accept \
+  "$work/duplicate-dependency" format
+normalize_project_path "$work/rust.err" "$work/rust.err.norm" \
+  "$work/duplicate-dependency"
+normalize_project_path "$work/ocaml.err" "$work/ocaml.err.norm" \
+  "$work/duplicate-dependency"
+if ! grep -F "$duplicate_warning" "$work/rust.err.norm" >/dev/null || \
+  ! grep -F "$duplicate_warning" "$work/ocaml.err.norm" >/dev/null; then
+  echo "Format duplicate dependency warning was not emitted by both implementations" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+
+set +e
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=delete-source \
+REWATCH_SOURCE_TO_DELETE="$(command_path "$work/publication-race-rust/src/A.res")" \
+REWATCH_SOURCE_DELETED="$(command_path "$work/publication-race-rust/source-deleted")" \
+RESCRIPT_BSC_EXE="$delete_source_bsc" \
+  "$rust" build "$work/publication-race-rust" \
+  >"$work/rust.out" 2>"$work/rust.err" &
+rust_pid=$!
+attempts=0
+while kill -0 "$rust_pid" 2>/dev/null && [ "$attempts" -lt 150 ]; do
+  attempts=$((attempts + 1))
+  sleep 0.1
+done
+if kill -0 "$rust_pid" 2>/dev/null; then
+  kill -TERM "$rust_pid" 2>/dev/null
+  wait "$rust_pid" 2>/dev/null
+  rust_status=124
+else
+  wait "$rust_pid"
+  rust_status=$?
+fi
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=delete-source \
+REWATCH_SOURCE_TO_DELETE="$(command_path "$work/publication-race-ocaml/src/A.res")" \
+REWATCH_SOURCE_DELETED="$(command_path "$work/publication-race-ocaml/source-deleted")" \
+RESCRIPT_BSC_EXE="$delete_source_bsc" \
+  "$ocaml" build "$work/publication-race-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -ne 124 ] || \
+  ! grep -F "copying source file failed" "$work/rust.err" >/dev/null || \
+  [ "$(classify "$ocaml_status")" != reject ] || \
+  ! grep -F "A.res" "$work/ocaml.err" >/dev/null; then
+  printf 'build-source-disappears-during-publication: expected Rust=worker-panic/timeout and OCaml=path-bearing rejection, got Rust=%s/OCaml=%s\n' \
+    "$rust_status" "$ocaml_status" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+
+set +e
+RAYON_NUM_THREADS=1 \
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=delete-parse-sources \
+REWATCH_SOURCE_A="$(command_path "$work/parse-source-race-rust/src/A.res")" \
+REWATCH_SOURCE_B="$(command_path "$work/parse-source-race-rust/src/B.res")" \
+REWATCH_SOURCES_DELETED="$(command_path "$work/parse-source-race-rust/sources-deleted")" \
+RESCRIPT_BSC_EXE="$delete_parse_sources_bsc" \
+  "$rust" build "$work/parse-source-race-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=delete-parse-sources \
+REWATCH_SOURCE_A="$(command_path "$work/parse-source-race-ocaml/src/A.res")" \
+REWATCH_SOURCE_B="$(command_path "$work/parse-source-race-ocaml/src/B.res")" \
+REWATCH_SOURCES_DELETED="$(command_path "$work/parse-source-race-ocaml/sources-deleted")" \
+RESCRIPT_BSC_EXE="$delete_parse_sources_bsc" \
+  "$ocaml" build "$work/parse-source-race-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -ne 101 ] || \
+  ! grep -F "file not found" "$work/rust.err" >/dev/null || \
+  [ "$(classify "$ocaml_status")" != reject ] || \
+  ! grep -F "parse-source-race" "$work/ocaml.err" >/dev/null || \
+  ! grep -E 'A\.(res|ast)' "$work/ocaml.err" >/dev/null; then
+  printf 'build-source-disappears-before-parse-read: expected Rust=panic and OCaml=path-bearing rejection, got Rust=%s/OCaml=%s\n' \
+    "$rust_status" "$ocaml_status" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+
+set +e
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=delete-ast \
+REWATCH_AST_DELETED="$(command_path "$work/ast-race-rust/ast-deleted")" \
+RESCRIPT_BSC_EXE="$delete_ast_bsc" \
+  "$rust" build "$work/ast-race-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=delete-ast \
+REWATCH_AST_DELETED="$(command_path "$work/ast-race-ocaml/ast-deleted")" \
+RESCRIPT_BSC_EXE="$delete_ast_bsc" \
+  "$ocaml" build "$work/ast-race-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$rust_status" -ne 101 ] || \
+  ! grep -F "Could not read file" "$work/rust.err" >/dev/null || \
+  [ "$(classify "$ocaml_status")" != reject ] || \
+  ! grep -F "A.ast" "$work/ocaml.err" >/dev/null; then
+  printf 'build-ast-disappears-before-dependency-read: expected Rust=panic and OCaml=path-bearing rejection, got Rust=%s/OCaml=%s\n' \
+    "$rust_status" "$ocaml_status" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+
+"$rust" watch "$work/watch-config-rust" \
+  >"$work/watch-rust.out" 2>"$work/watch-rust.err" &
+rust_watch_pid=$!
+background_pids="$background_pids $rust_watch_pid"
+wait_for_file "$work/watch-config-rust/src/A.js"
+wait_for_text "$work/watch-config-rust/lib/ocaml/.compiler.log" "#Done("
+printf '{ invalid json\n' >"$work/watch-config-rust/rescript.json"
+wait_for_text "$work/watch-rust.err" "Could not initialize build"
+wait_for_exit "$rust_watch_pid"
+set +e
+wait "$rust_watch_pid"
+rust_watch_status=$?
+set -e
+if [ "$rust_watch_status" -ne 101 ]; then
+  printf 'watch-invalid-config-rebuild: expected Rust panic exit 101, got %s\n' \
+    "$rust_watch_status" >&2
+  cat "$work/watch-rust.out" "$work/watch-rust.err" >&2
+  exit 1
+fi
+
+"$ocaml" watch "$work/watch-config-ocaml" \
+  >"$work/watch-ocaml.out" 2>"$work/watch-ocaml.err" &
+ocaml_watch_pid=$!
+background_pids="$background_pids $ocaml_watch_pid"
+wait_for_file "$work/watch-config-ocaml/src/A.js"
+wait_for_text "$work/watch-config-ocaml/lib/ocaml/.compiler.log" "#Done("
+printf '{ invalid json\n' >"$work/watch-config-ocaml/rescript.json"
+wait_for_text "$work/watch-ocaml.err" "invalid JSON"
+if ! kill -0 "$ocaml_watch_pid" 2>/dev/null; then
+  echo "OCaml watcher exited after a recoverable config error" >&2
+  cat "$work/watch-ocaml.out" "$work/watch-ocaml.err" >&2
+  exit 1
+fi
+printf '{"name":"watch-config","sources":["src"],"dependencies":["definitely-missing-dep"]}\n' \
+  >"$work/watch-config-ocaml/rescript.json"
+wait_for_text "$work/watch-ocaml.err" "definitely-missing-dep"
+if ! kill -0 "$ocaml_watch_pid" 2>/dev/null; then
+  echo "OCaml watcher exited after a recoverable dependency error" >&2
+  cat "$work/watch-ocaml.out" "$work/watch-ocaml.err" >&2
+  exit 1
+fi
+printf '{"name":"watch-config","sources":["src"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}\n' \
+  >"$work/watch-config-ocaml/rescript.json"
+wait_for_file "$work/watch-config-ocaml/src/A.mjs"
+wait_for_file "$work/watch-config-ocaml/lib/bs/build.ninja"
+if ! terminate_and_wait "$ocaml_watch_pid" "recoverable-config watcher"; then
+  cat "$work/watch-ocaml.out" "$work/watch-ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+
+retained_graph_marker="$work/watch-retained-graph/after-build.log"
+REWATCH_WATCH_FILTER_MARKER="$retained_graph_marker" \
+  "$ocaml" watch --after-build "node $command_work/watch-filter-marker.js" \
+    "$work/watch-retained-graph" \
+    >"$work/watch-retained-graph.out" \
+    2>"$work/watch-retained-graph.err" &
+retained_graph_pid=$!
+background_pids="$background_pids $retained_graph_pid"
+wait_for_line_count "$retained_graph_marker" 1
+printf 'let newValue = 2\n' >"$work/watch-retained-graph/src/C.res"
+printf 'let value = C.newValue\n' >"$work/watch-retained-graph/src/B.res"
+wait_for_line_count "$retained_graph_marker" 2
+wait_for_text "$work/watch-retained-graph/src/B.js" "C.newValue"
+printf 'let value = C.newValue\n' >"$work/watch-retained-graph/src/B.res"
+printf 'let newValue = B.value\n' >"$work/watch-retained-graph/src/C.res"
+wait_for_text "$work/watch-retained-graph.err" \
+  "Found a circular dependency in your code"
+if ! kill -0 "$retained_graph_pid" 2>/dev/null; then
+  echo "OCaml watcher exited after an incremental dependency cycle" >&2
+  exit 1
+fi
+printf 'let newValue = 3\n' >"$work/watch-retained-graph/src/C.res"
+wait_for_line_count "$retained_graph_marker" 3
+terminate_and_wait "$retained_graph_pid" "retained-graph watcher"
+checked=$((checked + 1))
+
+"$ocaml" watch "$work/watch-dependency-recovery" \
+  >"$work/watch-dependency-recovery.out" \
+  2>"$work/watch-dependency-recovery.err" &
+dependency_recovery_pid=$!
+background_pids="$background_pids $dependency_recovery_pid"
+wait_for_text "$work/watch-dependency-recovery.err" \
+  "Could not build package tree for 'dep'"
+if ! kill -0 "$dependency_recovery_pid" 2>/dev/null; then
+  echo "OCaml watcher exited after a malformed dependency config" >&2
+  exit 1
+fi
+printf '{"name":"dep","sources":["src"]}\n' \
+  >"$work/watch-dependency-recovery/packages/dep/rescript.json"
+wait_for_file "$work/watch-dependency-recovery/src/A.js"
+terminate_and_wait "$dependency_recovery_pid" "dependency-recovery watcher"
+checked=$((checked + 1))
+
+"$ocaml" watch "$work/watch-dependency-install" \
+  >"$work/watch-dependency-install.out" \
+  2>"$work/watch-dependency-install.err" &
+dependency_install_pid=$!
+background_pids="$background_pids $dependency_install_pid"
+if ! wait_for_text "$work/watch-dependency-install.err" \
+    "Could not resolve dependency @scope/dep"; then
+  printf '%s\n' '--- watcher stdout ---' >&2
+  cat "$work/watch-dependency-install.out" >&2
+  exit 1
+fi
+if ! kill -0 "$dependency_install_pid" 2>/dev/null; then
+  echo "OCaml watcher exited while waiting for a missing dependency" >&2
+  exit 1
+fi
+mkdir "$work/watch-dependency-install/node_modules/@scope"
+sleep 1
+mkdir -p "$work/watch-dependency-install/node_modules/@scope/dep/src"
+printf '{"name":"@scope/dep","sources":["src"]}\n' \
+  >"$work/watch-dependency-install/node_modules/@scope/dep/rescript.json"
+printf 'let dependency = 1\n' \
+  >"$work/watch-dependency-install/node_modules/@scope/dep/src/Dep.res"
+wait_for_file "$work/watch-dependency-install/src/A.js"
+terminate_and_wait "$dependency_install_pid" "dependency-install watcher"
+checked=$((checked + 1))
+
+directory_link "$work/watch-dependency-fallback/packages/dep" \
+  "$work/node_modules/dep"
+"$ocaml" watch "$work/watch-dependency-fallback" \
+  >"$work/watch-dependency-fallback.out" \
+  2>"$work/watch-dependency-fallback.err" &
+dependency_fallback_pid=$!
+background_pids="$background_pids $dependency_fallback_pid"
+wait_for_text "$work/watch-dependency-fallback.err" \
+  "no rescript.json or bsconfig.json"
+mv "$work/watch-dependency-fallback/node_modules/dep" \
+  "$work/watch-dependency-fallback/node_modules/dep-disabled"
+wait_for_file "$work/watch-dependency-fallback/src/A.js"
+terminate_and_wait "$dependency_fallback_pid" "dependency-fallback watcher"
+checked=$((checked + 1))
+
+if $file_symlinks_supported; then
+  symlink_target_marker="$work/watch-symlink-target/after-build.log"
+  REWATCH_WATCH_FILTER_MARKER="$symlink_target_marker" \
+    "$ocaml" watch --after-build "node $command_work/watch-filter-marker.js" \
+    "$work/watch-symlink-target" \
+    >"$work/watch-symlink-target.out" 2>"$work/watch-symlink-target.err" &
+  symlink_target_pid=$!
+  background_pids="$background_pids $symlink_target_pid"
+  wait_for_file "$work/watch-symlink-target/src/Linked.js"
+  wait_for_line_count "$symlink_target_marker" 1
+  printf 'let linked = 9876\n' \
+    >"$work/watch-symlink-external/sub/Linked.res.next"
+  mv "$work/watch-symlink-external/sub/Linked.res.next" \
+    "$work/watch-symlink-external/sub/Linked.res"
+  wait_for_text "$work/watch-symlink-target/src/Linked.js" "9876"
+  wait_for_line_count "$symlink_target_marker" 2
+  mv "$work/watch-symlink-external/sub/Linked.res" \
+    "$work/watch-symlink-external/sub/Linked.res.removed"
+  wait_for_line_count "$symlink_target_marker" 3
+  if [ -e "$work/watch-symlink-target/src/Linked.js" ]; then
+    echo "OCaml watcher retained output for a dangling source symlink" >&2
+    exit 1
+  fi
+  mv "$work/watch-symlink-external/sub/Linked.res.removed" \
+    "$work/watch-symlink-external/sub/Linked.res"
+  wait_for_file "$work/watch-symlink-target/src/Linked.js"
+  wait_for_line_count "$symlink_target_marker" 4
+  mv "$work/watch-symlink-external/sub" \
+    "$work/watch-symlink-external/sub.removed"
+  wait_for_line_count "$symlink_target_marker" 5
+  if [ -e "$work/watch-symlink-target/src/Linked.js" ]; then
+    echo "OCaml watcher retained output after a symlink target parent moved" >&2
+    exit 1
+  fi
+  mv "$work/watch-symlink-external/sub.removed" \
+    "$work/watch-symlink-external/sub"
+  wait_for_file "$work/watch-symlink-target/src/Linked.js"
+  wait_for_line_count "$symlink_target_marker" 6
+  terminate_and_wait "$symlink_target_pid" "symlink-target watcher"
+  checked=$((checked + 1))
+fi
+
+feature_scope_marker="$work/watch-feature-scope/after-build.log"
+scope_block_request="$work/watch-feature-scope/block-request"
+scope_block_started="$work/watch-feature-scope/block-started"
+scope_block_release="$work/watch-feature-scope/block-release"
+REWATCH_SCOPE_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=scope-block \
+REWATCH_SCOPE_BLOCK_REQUEST="$(command_path "$scope_block_request")" \
+REWATCH_SCOPE_BLOCK_STARTED="$(command_path "$scope_block_started")" \
+REWATCH_SCOPE_BLOCK_RELEASE="$(command_path "$scope_block_release")" \
+RESCRIPT_BSC_EXE="$watch_scope_bsc" \
+REWATCH_WATCH_FILTER_MARKER="$feature_scope_marker" \
+  "$ocaml" watch --features other \
+    --after-build "node $command_work/watch-filter-marker.js" \
+    "$work/watch-feature-scope" \
+    >"$work/watch-feature-scope.out" 2>"$work/watch-feature-scope.err" &
+feature_scope_pid=$!
+background_pids="$background_pids $feature_scope_pid"
+if ! wait_for_line_count "$feature_scope_marker" 1; then
+  echo "OCaml feature-scope watcher did not finish its initial build" >&2
+  cat "$work/watch-feature-scope.out" "$work/watch-feature-scope.err" >&2
+  exit 1
+fi
+: >"$scope_block_request"
+printf '{"name":"watch-feature-scope","sources":["src",{"dir":"inactive","feature":"inactive"}],"features":{"other":["inactive"]}}\n' \
+  >"$work/watch-feature-scope/rescript.json"
+wait_for_file "$scope_block_started"
+printf 'let createdDuringBuild = 2\n' \
+  >"$work/watch-feature-scope/inactive/CreatedDuringBuild.res"
+: >"$scope_block_release"
+wait_for_file "$work/watch-feature-scope/inactive/Inactive.js"
+wait_for_file "$work/watch-feature-scope/inactive/CreatedDuringBuild.js"
+wait_for_line_count "$feature_scope_marker" 3
+if ! line_count_stays "$feature_scope_marker" 3; then
+  echo "OCaml watcher lost or duplicated an edit during a source-scope transition" >&2
+  cat "$work/watch-feature-scope.out" "$work/watch-feature-scope.err" >&2
+  exit 1
+fi
+terminate_and_wait "$feature_scope_pid" "feature-scope watcher"
+checked=$((checked + 1))
+
+rust_filter_marker="$work/watch-filter-rust/after-build.log"
+REWATCH_WATCH_FILTER_MARKER="$rust_filter_marker" \
+  "$rust" watch --features other --filter 'Include\.res$' \
+    --after-build "node $command_work/watch-filter-marker.js" \
+    "$work/watch-filter-rust" \
+    >"$work/watch-filter-rust.out" 2>"$work/watch-filter-rust.err" &
+rust_filter_pid=$!
+background_pids="$background_pids $rust_filter_pid"
+wait_for_file "$work/watch-filter-rust/src/Include.js"
+wait_for_line_count "$rust_filter_marker" 1
+if [ -e "$work/watch-filter-rust/src/Exclude.js" ]; then
+  echo "Rust filter unexpectedly compiled the excluded source initially" >&2
+  exit 1
+fi
+cp "$work/watch-filter-rust/src/Include.js" "$work/watch-filter-rust-initial.js"
+printf 'let value = 2\n' >"$work/watch-filter-rust/src/Include.res"
+printf 'let value = 11\n' >"$work/watch-filter-rust/src/Exclude.res"
+wait_for_line_count "$rust_filter_marker" 2
+if ! cmp -s "$work/watch-filter-rust-initial.js" \
+    "$work/watch-filter-rust/src/Include.js"; then
+  echo "Rust no longer reproduces the inverted watch-filter event behavior" >&2
+  cat "$work/watch-filter-rust.out" "$work/watch-filter-rust.err" >&2
+  exit 1
+fi
+terminate_and_wait "$rust_filter_pid" "Rust filter watcher"
+
+ocaml_filter_marker="$work/watch-filter-ocaml/after-build.log"
+REWATCH_WATCH_FILTER_MARKER="$ocaml_filter_marker" \
+  "$ocaml" watch --features other --filter 'Include\.res$' \
+    --after-build "node $command_work/watch-filter-marker.js" \
+    "$work/watch-filter-ocaml" \
+    >"$work/watch-filter-ocaml.out" 2>"$work/watch-filter-ocaml.err" &
+ocaml_filter_pid=$!
+background_pids="$background_pids $ocaml_filter_pid"
+wait_for_file "$work/watch-filter-ocaml/src/Include.js"
+wait_for_line_count "$ocaml_filter_marker" 1
+if [ -e "$work/watch-filter-ocaml/src/Exclude.js" ]; then
+  echo "OCaml filter unexpectedly compiled the excluded source initially" >&2
+  exit 1
+fi
+cp "$work/watch-filter-ocaml/src/Include.js" "$work/watch-filter-ocaml-initial.js"
+printf 'let value = 2\n' >"$work/watch-filter-ocaml/src/Include.res"
+wait_for_line_count "$ocaml_filter_marker" 2
+if cmp -s "$work/watch-filter-ocaml-initial.js" \
+    "$work/watch-filter-ocaml/src/Include.js" || \
+  ! grep -F '2' "$work/watch-filter-ocaml/src/Include.js" >/dev/null; then
+  echo "OCaml watch filter did not rebuild its included source" >&2
+  cat "$work/watch-filter-ocaml.out" "$work/watch-filter-ocaml.err" >&2
+  exit 1
+fi
+printf 'let value = 11\n' >"$work/watch-filter-ocaml/src/Exclude.res"
+if ! line_count_stays "$ocaml_filter_marker" 2; then
+  echo "OCaml watch filter rebuilt for an excluded-only edit" >&2
+  cat "$work/watch-filter-ocaml.out" "$work/watch-filter-ocaml.err" >&2
+  exit 1
+fi
+printf 'let value = 21\n' \
+  >"$work/watch-filter-ocaml/inactive/Inactive.res"
+if ! line_count_stays "$ocaml_filter_marker" 2; then
+  echo "OCaml watcher rebuilt for a feature-disabled source edit" >&2
+  cat "$work/watch-filter-ocaml.out" "$work/watch-filter-ocaml.err" >&2
+  exit 1
+fi
+terminate_and_wait "$ocaml_filter_pid" "OCaml filter watcher"
+checked=$((checked + 1))
+
+for implementation in rust ocaml; do
+  if [ "$implementation" = rust ]; then
+    executable=$rust
+  else
+    executable=$ocaml
+  fi
+  quiet_watch_project="$work/quiet-watch-$implementation"
+  quiet_watch_marker="$quiet_watch_project/after-build.log"
+  REWATCH_WATCH_FILTER_MARKER="$quiet_watch_marker" \
+    "$executable" -q watch \
+      --after-build "node $command_work/watch-filter-marker.js" \
+      "$quiet_watch_project" \
+      >"$work/quiet-watch-$implementation.out" \
+      2>"$work/quiet-watch-$implementation.err" &
+  quiet_watch_pid=$!
+  background_pids="$background_pids $quiet_watch_pid"
+  wait_for_line_count "$quiet_watch_marker" 1
+  printf 'let value = 2\n' >"$quiet_watch_project/src/A.res"
+  wait_for_line_count "$quiet_watch_marker" 2
+  printf 'let value =\n' >"$quiet_watch_project/src/A.res"
+  wait_for_text "$work/quiet-watch-$implementation.err" \
+    "This let-binding misses an expression"
+  if ! kill -0 "$quiet_watch_pid" 2>/dev/null; then
+    echo "quiet-watch-$implementation: watcher exited after a parse error" >&2
+    exit 1
+  fi
+  printf 'let value = 3\n' >"$quiet_watch_project/src/A.res"
+  wait_for_line_count "$quiet_watch_marker" 3
+  rm "$quiet_watch_project/lib/watch.lock"
+  wait_for_exit "$quiet_watch_pid"
+  wait "$quiet_watch_pid"
+  if [ -s "$work/quiet-watch-$implementation.out" ] || \
+    grep -F "Incremental build failed" \
+      "$work/quiet-watch-$implementation.err" >/dev/null; then
+    echo "quiet-watch-$implementation: quiet watch emitted progress or a duplicate failure summary" >&2
+    cat "$work/quiet-watch-$implementation.out" \
+      "$work/quiet-watch-$implementation.err" >&2
+    exit 1
+  fi
+  normalize_project_path "$work/quiet-watch-$implementation.err" \
+    "$work/quiet-watch-$implementation.err.norm" "$quiet_watch_project"
+done
+if ! cmp -s "$work/quiet-watch-rust.err.norm" \
+  "$work/quiet-watch-ocaml.err.norm"; then
+  echo "Quiet watch failure diagnostics differ" >&2
+  printf '%s\n' '--- Rust stderr ---' >&2
+  cat "$work/quiet-watch-rust.err.norm" >&2
+  printf '%s\n' '--- OCaml stderr ---' >&2
+  cat "$work/quiet-watch-ocaml.err.norm" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+
+run_case clean-missing-dependency exit2 exit2 clean "$work/missing-dependency"
+run_case clean-configless-dependency exit2 exit2 clean "$work/configless-dependency"
+run_case clean-malformed-dependency exit2 exit2 clean "$work/malformed-dependency"
+set +e
+"$rust" clean "$work/clean-duplicate-rust" \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+"$ocaml" clean "$work/clean-duplicate-ocaml" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != reject ] || [ "$ocaml_status" -ne 0 ] || \
+  [ ! -e "$work/clean-duplicate-rust/src/one/A.js" ] || \
+  [ ! -e "$work/clean-duplicate-rust/src/two/A.js" ] || \
+  [ -e "$work/clean-duplicate-rust/lib/bs/marker" ] || \
+  [ -e "$work/clean-duplicate-ocaml/src/one/A.js" ] || \
+  [ -e "$work/clean-duplicate-ocaml/src/two/A.js" ] || \
+  [ -e "$work/clean-duplicate-ocaml/lib/bs/marker" ]; then
+  echo "clean-duplicate-modules: cleanup behavior differs unexpectedly" >&2
+  printf '%s\n' '--- Rust output ---' >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  printf '%s\n' '--- OCaml output ---' >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+set +e
+"$rust" watch "$work/missing-dependency" >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+set -e
+if [ "$rust_status" -ne 2 ]; then
+  echo "watch-missing-dependency: expected Rust exit 2, got $rust_status" >&2
+  cat "$work/rust.out" "$work/rust.err" >&2
+  exit 1
+fi
+"$ocaml" watch "$work/missing-dependency" \
+  >"$work/ocaml.out" 2>"$work/ocaml.err" &
+missing_dependency_watch_pid=$!
+background_pids="$background_pids $missing_dependency_watch_pid"
+wait_for_text "$work/ocaml.err" "Could not resolve dependency absent"
+if ! kill -0 "$missing_dependency_watch_pid" 2>/dev/null; then
+  echo "OCaml watcher exited after its initial dependency error" >&2
+  cat "$work/ocaml.out" "$work/ocaml.err" >&2
+  exit 1
+fi
+printf '{"name":"missing-dependency","sources":["src"]}\n' \
+  >"$work/missing-dependency/rescript.json"
+wait_for_file "$work/missing-dependency/src/A.js"
+terminate_and_wait "$missing_dependency_watch_pid" \
+  "missing-dependency watcher"
+if $windows_posix_shell; then
+  # MSYS cannot deliver the graceful console event that lets a native watcher
+  # remove its lock. Its external termination intentionally leaves a stale
+  # watch lock, so remove that harness artifact before checking failure paths.
+  rm -f "$work/missing-dependency/lib/watch.lock"
+fi
+printf '{"name":"missing-dependency","sources":["src"],"dependencies":["absent"]}\n' \
+  >"$work/missing-dependency/rescript.json"
+checked=$((checked + 1))
+run_cwd_case format-missing-dependency exit2 exit2 \
+  "$work/missing-dependency" format
+run_cwd_case format-configless-dependency exit2 exit2 \
+  "$work/configless-dependency" format
+run_cwd_case format-malformed-dependency exit2 exit2 \
+  "$work/malformed-dependency" format
+if [ -e "$work/missing-dependency/lib/build.lock" ] || \
+  [ -e "$work/missing-dependency/lib/watch.lock" ]; then
+  echo "OCaml dependency failures left a build or watch lock behind" >&2
+  exit 1
+fi
+
+set +e
+(cd "$project/src" && "$rust" format --check) \
+  >"$work/rust.out" 2>"$work/rust.err"
+rust_status=$?
+(cd "$project/src" && "$ocaml" format --check) \
+  >"$work/ocaml.out" 2>"$work/ocaml.err"
+ocaml_status=$?
+set -e
+if [ "$(classify "$rust_status")" != reject ] || \
+  [ "$(classify "$ocaml_status")" != reject ]; then
+  printf 'format-nested: expected both implementations to reject, got Rust=%s/OCaml=%s\n' \
+    "$rust_status" "$ocaml_status" >&2
+  exit 1
+fi
+checked=$((checked + 1))
+
+printf 'Command validation cases: %d; expected outcomes matched\n' "$checked"
diff --git a/rewatch-ocaml/tests/check_config_acceptance.sh b/rewatch-ocaml/tests/check_config_acceptance.sh
new file mode 100755
index 00000000000..c414fcb1083
--- /dev/null
+++ b/rewatch-ocaml/tests/check_config_acceptance.sh
@@ -0,0 +1,112 @@
+#!/bin/bash
+set -eu
+
+root=$(CDPATH= cd -- "$(dirname "$0")/../.." && pwd)
+rust=${1:-$root/rewatch/target/debug/rescript}
+ocaml=${2:-$root/_build/default/rewatch-ocaml/rescript_ocaml.exe}
+cases=$root/rewatch-ocaml/tests/config_acceptance_cases.tsv
+
+rust=$(realpath "$rust")
+ocaml=$(realpath "$ocaml")
+mkdir -p "$root/tmp"
+work=$(mktemp -d "$root/tmp/rewatch-config-acceptance-XXXXXX")
+trap 'rm -rf "$work"' EXIT
+mkdir -p "$work/src" "$work/node_modules/dep/lib/ocaml" "$work/node_modules/ppx"
+printf 'let value = 1\n' >"$work/src/A.res"
+
+default_bsc=$root/_build/default/compiler/bsc/rescript_compiler_main.exe
+default_runtime=$root/packages/@rescript/runtime
+case $(uname -s) in
+  MINGW*|MSYS*)
+    default_bsc=$(cygpath -w "$default_bsc")
+    default_runtime=$(cygpath -w "$default_runtime")
+    ;;
+esac
+export RESCRIPT_BSC_EXE=${RESCRIPT_BSC_EXE:-$default_bsc}
+export RESCRIPT_RUNTIME=${RESCRIPT_RUNTIME:-$default_runtime}
+
+checked=0
+divergences=0
+diagnostic_checks=0
+while IFS=$'\t' read -r area name expected json; do
+  if [[ -z "$area" || "$area" == \#* ]]; then
+    continue
+  fi
+  printf '%s\n' "$json" >"$work/rescript.json"
+  set +e
+  "$rust" compiler-args "$work/src/A.res" >"$work/rust.out" 2>"$work/rust.err"
+  rust_status=$?
+  "$ocaml" compiler-args "$work/src/A.res" >"$work/ocaml.out" 2>"$work/ocaml.err"
+  ocaml_status=$?
+  set -e
+
+  if [[ "$rust_status" -eq 0 ]]; then
+    rust_actual=accept
+  elif [[ "$rust_status" -eq 101 ]]; then
+    rust_actual=panic
+  else
+    rust_actual=reject
+  fi
+  if [[ "$ocaml_status" -eq 0 ]]; then
+    ocaml_actual=accept
+  else
+    ocaml_actual=reject
+  fi
+  rust_expected=${expected%%/*}
+  if [[ "$expected" == */* ]]; then
+    ocaml_expected=${expected#*/}
+    compare_arguments=false
+    divergences=$((divergences + 1))
+  else
+    ocaml_expected=$expected
+    compare_arguments=true
+  fi
+  if [[ "$rust_actual" != "$rust_expected" || "$ocaml_actual" != "$ocaml_expected" ]]; then
+    printf 'Config case %s/%s: expected Rust=%s/OCaml=%s, got Rust=%s/OCaml=%s\n' \
+      "$area" "$name" "$rust_expected" "$ocaml_expected" \
+      "$rust_status" "$ocaml_status" >&2
+    printf '%s\n' '--- Rust output ---' >&2
+    cat "$work/rust.out" >&2
+    cat "$work/rust.err" >&2
+    printf '%s\n' '--- OCaml output ---' >&2
+    cat "$work/ocaml.out" >&2
+    cat "$work/ocaml.err" >&2
+    exit 1
+  fi
+  diagnostic_fragment=
+  case "$area" in
+    jsx) diagnostic_fragment=jsx ;;
+    source-map) diagnostic_fragment=sourceMap ;;
+  esac
+  if [[ -n "$diagnostic_fragment" && "$rust_expected" != accept ]]; then
+    if ! grep -Fi "$diagnostic_fragment" "$work/rust.err" >/dev/null ||
+      ! grep -Fi "$diagnostic_fragment" "$work/ocaml.err" >/dev/null; then
+      printf 'Config case %s/%s lost the %s diagnostic context\n' \
+        "$area" "$name" "$diagnostic_fragment" >&2
+      printf '%s\n' '--- Rust output ---' >&2
+      cat "$work/rust.out" "$work/rust.err" >&2
+      printf '%s\n' '--- OCaml output ---' >&2
+      cat "$work/ocaml.out" "$work/ocaml.err" >&2
+      exit 1
+    fi
+    diagnostic_checks=$((diagnostic_checks + 1))
+  fi
+  if [[ "$compare_arguments" == true && "$rust_expected" == accept ]] &&
+    ! node -e '
+      const fs = require("fs");
+      const assert = require("assert");
+      assert.deepStrictEqual(
+        JSON.parse(fs.readFileSync(process.argv[1], "utf8")),
+        JSON.parse(fs.readFileSync(process.argv[2], "utf8")),
+      );
+    ' "$work/rust.out" "$work/ocaml.out"; then
+    printf 'Config case %s/%s produced different compiler arguments\n' \
+      "$area" "$name" >&2
+    diff -u "$work/rust.out" "$work/ocaml.out" >&2 || true
+    exit 1
+  fi
+  checked=$((checked + 1))
+done <"$cases"
+
+printf 'Configuration cases: %d (%d documented divergences, %d JSX/source-map diagnostic checks); Rust/OCaml expectations and parity arguments matched\n' \
+  "$checked" "$divergences" "$diagnostic_checks"
diff --git a/rewatch-ocaml/tests/check_interactive_output.sh b/rewatch-ocaml/tests/check_interactive_output.sh
new file mode 100755
index 00000000000..1f599c3e87b
--- /dev/null
+++ b/rewatch-ocaml/tests/check_interactive_output.sh
@@ -0,0 +1,790 @@
+#!/bin/bash
+set -eu
+
+root=$(CDPATH= cd -- "$(dirname "$0")/../.." && pwd)
+rust=${1:-$root/rewatch/target/debug/rescript}
+ocaml=${2:-$root/_build/default/rewatch-ocaml/rescript_ocaml.exe}
+rust=$(realpath "$rust")
+ocaml=$(realpath "$ocaml")
+work=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-interactive-output-XXXXXX")
+windows_posix_shell=false
+compare_rust_watch_output=true
+case $(uname -s) in
+  CYGWIN*|MINGW*|MSYS*) windows_posix_shell=true ;;
+  # Rust's macOS watcher can classify an ordinary source write as one or more
+  # structural events and legitimately perform full rebuilds. Linux provides
+  # the stable event classification needed for exact incremental comparison;
+  # macOS still exercises the OCaml watch presentation and recovery paths.
+  Darwin) compare_rust_watch_output=false ;;
+esac
+
+if $windows_posix_shell; then
+  echo "Skipping interactive output gate: a POSIX pseudo-terminal does not exercise the native Windows console"
+  exit 0
+fi
+
+command_path() {
+  if $windows_posix_shell; then
+    cygpath -am "$1"
+  else
+    printf '%s\n' "$1"
+  fi
+}
+active_script_pid=""
+active_watch_project=""
+active_watch_transcript=""
+wait_for_pid_gone() {
+  pid=$1
+  attempts=${2:-200}
+  while [ "$attempts" -gt 0 ]; do
+    if ! kill -0 "$pid" 2>/dev/null; then return 0; fi
+    attempts=$((attempts - 1))
+    sleep 0.1
+  done
+  return 1
+}
+cleanup() {
+  if [ -n "$active_script_pid" ]; then
+    if [ -n "$active_watch_project" ]; then
+      rm -f "$active_watch_project/lib/watch.lock"
+    fi
+    if ! wait_for_pid_gone "$active_script_pid" 50; then
+      kill -TERM "$active_script_pid" 2>/dev/null || true
+    fi
+    if ! wait_for_pid_gone "$active_script_pid" 50; then
+      kill -KILL "$active_script_pid" 2>/dev/null || true
+    fi
+    wait "$active_script_pid" 2>/dev/null || true
+  fi
+  rm -rf "$work"
+}
+trap cleanup EXIT
+
+if ! command -v script >/dev/null 2>&1; then
+  echo "Interactive output gate requires the util-linux script command" >&2
+  exit 1
+fi
+
+# macOS buffers typescript files for up to 30 seconds unless -F is used. The
+# watcher checks below must observe each message as soon as it is written.
+
+normalize_output() {
+  sed -E $'s/\033\\[[0-9;]*[[:alpha:]]//g' \
+    | sed -e 's/\[clean\]/🧹/g' -e 's/\[parse\]/🧱/g' \
+        -e 's/\[build\]/🤺/g' -e 's/\[ok\]/✅/g' \
+        -e 's/\[warn\]/⚠️/g' -e 's/\[error\]/❌/g'
+}
+
+for implementation in rust ocaml; do
+  mkdir -p "$work/$implementation/src"
+  printf '{"name":"interactive-output","sources":["src"],"namespace":"Interactive"}\n' \
+    >"$work/$implementation/rescript.json"
+  printf 'let value = 1\n' >"$work/$implementation/src/A.res"
+  printf 'let value: int\n' >"$work/$implementation/src/A.resi"
+  cp -R "$work/$implementation" "$work/$implementation-after-build"
+  cp -R "$work/$implementation" "$work/$implementation-parse-warning"
+  cp -R "$work/$implementation" "$work/$implementation-watch"
+  cp -R "$work/$implementation" "$work/$implementation-initial-failure-watch"
+  printf 'let value = (\n' \
+    >"$work/$implementation-initial-failure-watch/src/A.res"
+  cp -R "$root/rewatch-ocaml/tests/basic" \
+    "$work/$implementation-partial-initial-failure-watch"
+  printf 'let answer: int = "not an int"\n' \
+    >"$work/$implementation-partial-initial-failure-watch/src/B.res"
+  cp -R "$work/$implementation" "$work/$implementation-warning-watch"
+  printf '%s\n' \
+    '{"name":"interactive-output","sources":["src"],"namespace":"Interactive","package-specs":{"module":"es6","in-source":true}}' \
+    >"$work/$implementation-warning-watch/rescript.json"
+done
+
+cat >"$work/after-build-marker.sh" <<'EOF'
+#!/bin/sh
+printf '%s\n' AFTER_BUILD_MARKER
+EOF
+chmod +x "$work/after-build-marker.sh"
+printf 'console.log("AFTER_BUILD_MARKER")\n' >"$work/after-build-marker.js"
+cat >"$work/after-build-input.js" <<'EOF'
+process.stdin.setEncoding("utf8");
+process.stdin.once("data", input => {
+  console.log(`AFTER_BUILD_INPUT:${input.trim()}`);
+});
+EOF
+
+export RESCRIPT_BSC_EXE=${RESCRIPT_BSC_EXE:-$root/_build/default/compiler/bsc/rescript_compiler_main.exe}
+export RESCRIPT_RUNTIME=${RESCRIPT_RUNTIME:-$root/packages/@rescript/runtime}
+parse_warning_bsc="$root/_build/default/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.exe"
+after_build_command="$work/after-build-marker.sh"
+if $windows_posix_shell; then
+  RESCRIPT_BSC_EXE=$(command_path "$RESCRIPT_BSC_EXE")
+  RESCRIPT_RUNTIME=$(command_path "$RESCRIPT_RUNTIME")
+  parse_warning_bsc=$(command_path "$parse_warning_bsc")
+  after_build_command="node $(command_path "$work/after-build-marker.js")"
+fi
+
+capture() {
+  implementation=$1
+  executable=$2
+  transcript="$work/$implementation.tty"
+  command_executable=$(command_path "$executable")
+  command_project=$(command_path "$work/$implementation")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" build "$work/$implementation" --no-timing >/dev/null
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable build $command_project --no-timing" \
+      "$transcript" >/dev/null
+  fi
+  tr '\r' '\n' <"$transcript" \
+    | normalize_output \
+    | grep -E '^\[[123]/3\] .* (Cleaned|Parsed|Compiled) |^✅ Finished compilation in ' \
+    >"$work/$implementation.phases"
+}
+
+echo "Checking interactive build output..."
+capture rust "$rust"
+capture ocaml "$ocaml"
+
+require_spinner_frames() {
+  implementation=$1
+  if ! grep -F $'\033[1m\033[2m[2/3]\033[0m' \
+      "$work/$implementation.tty" >/dev/null; then
+    echo "$implementation did not render the interactive step style" >&2
+    cat "$work/$implementation.tty" >&2
+    exit 1
+  fi
+  tr '\r' '\n' <"$work/$implementation.tty" \
+    | normalize_output \
+    >"$work/$implementation.frames"
+  if ! grep -E '^\[2/3\] 🧱 Parsing\.\.\. .+ [0-9]+/1' \
+      "$work/$implementation.frames" >/dev/null || \
+    ! grep -E '^\[3/3\] 🤺 Compiling\.\.\. .+ [0-9]+/2' \
+      "$work/$implementation.frames" >/dev/null; then
+    echo "$implementation did not render both live spinner phases" >&2
+    cat "$work/$implementation.frames" >&2
+    exit 1
+  fi
+}
+
+require_spinner_frames rust
+require_spinner_frames ocaml
+
+if ! cmp -s "$work/rust.phases" "$work/ocaml.phases"; then
+  echo "Interactive phase output differs" >&2
+  printf '%s\n' '--- Rust phases ---' >&2
+  cat "$work/rust.phases" >&2
+  printf '%s\n' '--- OCaml phases ---' >&2
+  cat "$work/ocaml.phases" >&2
+  exit 1
+fi
+
+cat >"$work/expected" <<'EOF'
+[1/3] 🧹 Cleaned 0/0 in 0.00s
+[2/3] 🧱 Parsed 1 source files in 0.00s
+[3/3] 🤺 Compiled 1 modules in 0.00s
+✅ Finished compilation in 0.00s
+EOF
+
+if ! cmp -s "$work/expected" "$work/ocaml.phases"; then
+  echo "Interactive phase output no longer has the expected stable shape" >&2
+  cat "$work/ocaml.phases" >&2
+  exit 1
+fi
+
+capture_parse_warning_order() {
+  implementation=$1
+  executable=$2
+  transcript="$work/$implementation-parse-warning.tty"
+  project="$work/$implementation-parse-warning"
+  command_executable=$(command_path "$executable")
+  command_project=$(command_path "$project")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "REWATCH_REAL_BSC=$RESCRIPT_BSC_EXE" \
+      "REWATCH_BSC_PROXY_MODE=parse-warning" \
+      "RESCRIPT_BSC_EXE=$parse_warning_bsc" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" build "$project" --no-timing >/dev/null
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 REAL_BSC_EXE=$RESCRIPT_BSC_EXE REWATCH_REAL_BSC=$RESCRIPT_BSC_EXE REWATCH_BSC_PROXY_MODE=parse-warning RESCRIPT_BSC_EXE=$parse_warning_bsc RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable build $command_project --no-timing" \
+      "$transcript" >/dev/null
+  fi
+  tr '\r' '\n' <"$transcript" \
+    | normalize_output \
+    | grep -E '(^\[[123]/3\] .* Parsed |PARSE_WARNING_MARKER)' \
+    >"$work/$implementation-parse-warning.order"
+}
+
+echo "Checking interactive parser warnings..."
+capture_parse_warning_order rust "$rust"
+capture_parse_warning_order ocaml "$ocaml"
+
+if ! cmp -s "$work/rust-parse-warning.order" \
+  "$work/ocaml-parse-warning.order"; then
+  echo "Parser warning phase order differs" >&2
+  printf '%s\n' '--- Rust order ---' >&2
+  cat "$work/rust-parse-warning.order" >&2
+  printf '%s\n' '--- OCaml order ---' >&2
+  cat "$work/ocaml-parse-warning.order" >&2
+  exit 1
+fi
+
+if ! sed -n '1p' "$work/ocaml-parse-warning.order" \
+  | grep -E '^\[2/3\] .* Parsed 1 source files in 0.00s$' >/dev/null; then
+  echo "Parser warnings were emitted before the completed parse phase" >&2
+  cat "$work/ocaml-parse-warning.order" >&2
+  exit 1
+fi
+
+capture_after_build_order() {
+  implementation=$1
+  executable=$2
+  transcript="$work/$implementation-after-build.tty"
+  project="$work/$implementation-after-build"
+  command_executable=$(command_path "$executable")
+  command_project=$(command_path "$project")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" build --after-build "$after_build_command" \
+      "$project" --no-timing >/dev/null
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable build --after-build '$after_build_command' $command_project --no-timing" \
+      "$transcript" >/dev/null
+  fi
+  tr '\r' '\n' <"$transcript" \
+    | normalize_output \
+    | grep -E '^(✅ Finished compilation in |AFTER_BUILD_MARKER$)' \
+    | sed -E 's/in [0-9]+\.[0-9]+s$/in 0.00s/' \
+    >"$work/$implementation-after-build.order"
+}
+
+echo "Checking interactive after-build output..."
+capture_after_build_order rust "$rust"
+capture_after_build_order ocaml "$ocaml"
+
+cat >"$work/expected-after-build-order" <<'EOF'
+✅ Finished compilation in 0.00s
+AFTER_BUILD_MARKER
+EOF
+
+for implementation in rust ocaml; do
+  if ! cmp -s "$work/expected-after-build-order" \
+    "$work/$implementation-after-build.order"; then
+    echo "$implementation ran one-shot --after-build in the wrong phase" >&2
+    cat "$work/$implementation-after-build.order" >&2
+    exit 1
+  fi
+done
+
+if $windows_posix_shell; then
+  for implementation in rust ocaml; do
+    if [ "$implementation" = rust ]; then executable=$rust; else executable=$ocaml; fi
+    command_executable=$(command_path "$executable")
+    command_project=$(command_path "$work/$implementation-after-build")
+    input_script=$(command_path "$work/after-build-input.js")
+    transcript="$work/$implementation-after-build-input.tty"
+    printf 'from-terminal\n' | script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable build --after-build 'node $input_script' $command_project --no-timing" \
+      "$transcript" >/dev/null
+    if ! tr '\r' '\n' <"$transcript" \
+      | grep -F 'AFTER_BUILD_INPUT:from-terminal' >/dev/null; then
+      echo "$implementation did not pass terminal input to --after-build" >&2
+      cat "$transcript" >&2
+      exit 1
+    fi
+  done
+fi
+
+capture_quiet_build() {
+  implementation=$1
+  executable=$2
+  transcript="$work/$implementation-quiet.tty"
+  command_executable=$(command_path "$executable")
+  command_project=$(command_path "$work/$implementation")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" -q build "$work/$implementation" >/dev/null
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable -q build $command_project" \
+      "$transcript" >/dev/null
+  fi
+  if tr '\r' '\n' <"$transcript" \
+    | grep -E '(Cleaned|Parsed|Parsing\.\.\.|Compiled|Compiling\.\.\.|Finished .*compilation)' >/dev/null; then
+    echo "$implementation quiet interactive build emitted progress" >&2
+    cat "$transcript" >&2
+    exit 1
+  fi
+}
+
+echo "Checking quiet interactive builds..."
+capture_quiet_build rust "$rust"
+capture_quiet_build ocaml "$ocaml"
+
+capture_clean() {
+  implementation=$1
+  executable=$2
+  transcript="$work/$implementation-clean.tty"
+  command_executable=$(command_path "$executable")
+  command_project=$(command_path "$work/$implementation")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "$executable" clean "$work/$implementation" >/dev/null
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 $command_executable clean $command_project" \
+      "$transcript" >/dev/null
+  fi
+  # The initial generic label is overwritten by the first package label on the
+  # same terminal line, so it is not part of the visible phase sequence.
+  tr '\r' '\n' <"$transcript" \
+    | normalize_output \
+    | grep -E '^\[[12]/2\] 🧹 (Cleaning|Cleaned)' \
+    | awk '$0 != "[1/2] 🧹 Cleaning compiler assets..."' \
+    | sed -E 's/in [0-9]+\.[0-9]+s$/in 0.00s/' \
+    >"$work/$implementation-clean.phases"
+}
+
+echo "Checking interactive clean output..."
+capture_clean rust "$rust"
+capture_clean ocaml "$ocaml"
+
+if ! cmp -s "$work/rust-clean.phases" "$work/ocaml-clean.phases"; then
+  echo "Interactive clean phase output differs" >&2
+  printf '%s\n' '--- Rust clean phases ---' >&2
+  cat "$work/rust-clean.phases" >&2
+  printf '%s\n' '--- OCaml clean phases ---' >&2
+  cat "$work/ocaml-clean.phases" >&2
+  exit 1
+fi
+
+cat >"$work/expected-clean" <<'EOF'
+[1/2] 🧹 Cleaning interactive-output...
+[1/2] 🧹 Cleaned compiler assets in 0.00s
+[2/2] 🧹 Cleaning .js files...
+[2/2] 🧹 Cleaned .js files in 0.00s
+EOF
+
+if ! cmp -s "$work/expected-clean" "$work/ocaml-clean.phases"; then
+  echo "Interactive clean output no longer has the expected stable shape" >&2
+  cat "$work/ocaml-clean.phases" >&2
+  exit 1
+fi
+
+capture_quiet_clean() {
+  implementation=$1
+  executable=$2
+  transcript="$work/$implementation-clean-quiet.tty"
+  command_executable=$(command_path "$executable")
+  command_project=$(command_path "$work/$implementation")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "$executable" -q clean "$work/$implementation" >/dev/null
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 $command_executable -q clean $command_project" \
+      "$transcript" >/dev/null
+  fi
+  if tr '\r' '\n' <"$transcript" | grep -E '(Cleaning|Cleaned)' >/dev/null; then
+    echo "$implementation quiet interactive clean emitted progress" >&2
+    cat "$transcript" >&2
+    exit 1
+  fi
+}
+
+echo "Checking quiet interactive cleans..."
+capture_quiet_clean rust "$rust"
+capture_quiet_clean ocaml "$ocaml"
+
+wait_for_text() {
+  path=$1
+  pattern=$2
+  count=$3
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    actual=$(grep -cF "$pattern" "$path" 2>/dev/null || true)
+    actual=${actual:-0}
+    if [ "$actual" -ge "$count" ]; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  printf 'Timed out waiting for occurrence %s of %s in %s\n' \
+    "$count" "$pattern" "$path" >&2
+  if [ -f "$path" ]; then cat "$path" >&2; fi
+  return 1
+}
+
+wait_for_file() {
+  path=$1
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    if [ -f "$path" ]; then return 0; fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  printf 'Timed out waiting for %s\n' "$path" >&2
+  return 1
+}
+
+# Opening an existing source with shell redirection truncates it first. Some
+# native watcher backends report that as a structural change, which correctly
+# requests a full rebuild. Equal-length writes without truncation make these
+# fixtures exercise the content-change/incremental path they are testing.
+overwrite_line_in_place() {
+  path=$1
+  line=$2
+  current_size=$(wc -c <"$path" | tr -d ' ')
+  replacement_size=$(printf '%s\n' "$line" | wc -c | tr -d ' ')
+  if [ "$current_size" -ne "$replacement_size" ]; then
+    echo "In-place test update changed size for $path" >&2
+    return 1
+  fi
+  printf '%s\n' "$line" | dd of="$path" conv=notrunc 2>/dev/null
+}
+
+stop_active_watch() {
+  rm -f "$active_watch_project/lib/watch.lock"
+  if ! wait_for_pid_gone "$active_script_pid"; then
+    echo "Timed out waiting for the interactive watcher to stop" >&2
+    if [ -f "$active_watch_transcript" ]; then
+      cat "$active_watch_transcript" >&2
+    fi
+    return 1
+  fi
+  status=0
+  wait "$active_script_pid" || status=$?
+  active_script_pid=""
+  active_watch_project=""
+  active_watch_transcript=""
+  return "$status"
+}
+
+capture_watch_rebuild() {
+  local implementation=$1
+  local executable=$2
+  local project="$work/$implementation-watch"
+  local transcript="$work/$implementation-watch.tty"
+  local command_executable=$(command_path "$executable")
+  local command_project=$(command_path "$project")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" watch --clear-screen "$project" >/dev/null &
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable watch --clear-screen $command_project" \
+      "$transcript" >/dev/null &
+  fi
+  active_script_pid=$!
+  active_watch_project=$project
+  active_watch_transcript=$transcript
+  if ! wait_for_text "$transcript" "Finished initial compilation" 1; then
+    return 1
+  fi
+  overwrite_line_in_place "$project/src/A.res" 'let value = 2'
+  if ! wait_for_text "$transcript" "Finished incremental compilation" 1; then
+    return 1
+  fi
+  cp "$transcript" "$work/$implementation-watch-phases.tty"
+  overwrite_line_in_place "$project/src/A.res" 'let value = ('
+  if ! wait_for_text "$transcript" "Build failed. Watching for changes..." 1; then
+    return 1
+  fi
+  overwrite_line_in_place "$project/src/A.res" 'let value = 3'
+  if ! wait_for_text "$transcript" "Finished incremental compilation" 2; then
+    return 1
+  fi
+  printf '%s\n' \
+    '{"name":"interactive-output","sources":["src"],"namespace":"Interactive","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+    >"$project/rescript.next"
+  mv "$project/rescript.next" "$project/rescript.json"
+  if ! wait_for_text "$transcript" "Change detected. Full rebuild..." 1 || \
+    ! wait_for_text "$transcript" "Finished compilation" 1; then
+    return 1
+  fi
+  stop_active_watch
+  tr '\r' '\n' <"$work/$implementation-watch-phases.tty" \
+    | normalize_output \
+    | sed -E 's/in [0-9]+\.[0-9]+s/in <TIME>/' \
+    >"$work/$implementation-watch.normalized"
+  awk '
+    /^\[[123]\/3\] .* (Cleaned|Parsed|Compiled) / ||
+      /^✅ Finished initial compilation in / {
+      print
+    }
+    /^✅ Finished initial compilation in / { exit }
+  ' "$work/$implementation-watch.normalized" \
+    >"$work/$implementation-watch-initial.phases"
+  grep -E '^\[[12]/2\] .* (Parsed|Compiled) |^✅ Finished incremental compilation in ' \
+    "$work/$implementation-watch.normalized" \
+    >"$work/$implementation-watch.phases"
+  tr '\r' '\n' <"$transcript" \
+    | normalize_output \
+    >"$work/$implementation-watch.presentation"
+  rebuilds=$(grep -cF 'Change detected. Rebuilding...' \
+    "$work/$implementation-watch.presentation" || true)
+  rebuilds=${rebuilds:-0}
+  full_rebuilds=$(grep -cF 'Change detected. Full rebuild...' \
+    "$work/$implementation-watch.presentation" || true)
+  full_rebuilds=${full_rebuilds:-0}
+  failures=$(grep -cF 'Build failed. Watching for changes...' \
+    "$work/$implementation-watch.presentation" || true)
+  failures=${failures:-0}
+  if [ "$rebuilds" -lt 3 ] || [ "$full_rebuilds" -ne 1 ] || \
+    [ "$failures" -ne 1 ]; then
+    echo "$implementation watch rebuild presentation changed" >&2
+    cat "$work/$implementation-watch.presentation" >&2
+    exit 1
+  fi
+}
+
+echo "Checking interactive watch rebuilds..."
+if $compare_rust_watch_output; then
+  capture_watch_rebuild rust "$rust"
+fi
+capture_watch_rebuild ocaml "$ocaml"
+
+if $compare_rust_watch_output && \
+  ! cmp -s "$work/rust-watch-initial.phases" \
+  "$work/ocaml-watch-initial.phases"; then
+  echo "Interactive initial-watch output differs" >&2
+  printf '%s\n' '--- Rust initial phases ---' >&2
+  cat "$work/rust-watch-initial.phases" >&2
+  printf '%s\n' '--- OCaml initial phases ---' >&2
+  cat "$work/ocaml-watch-initial.phases" >&2
+  exit 1
+fi
+
+cat >"$work/expected-watch-initial" <<'EOF'
+[1/3] 🧹 Cleaned 0/0 in <TIME>
+[2/3] 🧱 Parsed 1 source files in <TIME>
+[3/3] 🤺 Compiled 1 modules in <TIME>
+✅ Finished initial compilation in <TIME>
+EOF
+
+if ! cmp -s "$work/expected-watch-initial" \
+  "$work/ocaml-watch-initial.phases"; then
+  echo "Interactive initial-watch output lost its stable shape" >&2
+  cat "$work/ocaml-watch-initial.phases" >&2
+  exit 1
+fi
+
+if $compare_rust_watch_output && \
+  ! cmp -s "$work/rust-watch.phases" "$work/ocaml-watch.phases"; then
+  echo "Interactive watch rebuild output differs" >&2
+  printf '%s\n' '--- Rust rebuild phases ---' >&2
+  cat "$work/rust-watch.phases" >&2
+  printf '%s\n' '--- OCaml rebuild phases ---' >&2
+  cat "$work/ocaml-watch.phases" >&2
+  exit 1
+fi
+
+cat >"$work/expected-watch" <<'EOF'
+[1/2] 🧱 Parsed 1 source files in <TIME>
+[2/2] 🤺 Compiled 1 modules in <TIME>
+✅ Finished incremental compilation in <TIME>
+EOF
+
+if ! cmp -s "$work/expected-watch" "$work/ocaml-watch.phases"; then
+  echo "Interactive watch rebuild output lost its stable shape" >&2
+  cat "$work/ocaml-watch.phases" >&2
+  exit 1
+fi
+
+capture_initial_failure_recovery() {
+  local implementation=$1
+  local executable=$2
+  local project="$work/$implementation-initial-failure-watch"
+  local transcript="$work/$implementation-initial-failure-watch.tty"
+  local command_executable=$(command_path "$executable")
+  local command_project=$(command_path "$project")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" watch --clear-screen "$project" >/dev/null &
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable watch --clear-screen $command_project" \
+      "$transcript" >/dev/null &
+  fi
+  active_script_pid=$!
+  active_watch_project=$project
+  active_watch_transcript=$transcript
+  if ! wait_for_text "$transcript" "Error parsing source files" 1; then
+    return 1
+  fi
+  overwrite_line_in_place "$project/src/A.res" 'let value = 1'
+  if ! wait_for_text "$transcript" "Finished incremental compilation" 1; then
+    return 1
+  fi
+  stop_active_watch
+  tr '\r' '\n' <"$transcript" \
+    | normalize_output \
+    | sed -E 's/in [0-9]+\.[0-9]+s/in <TIME>/' \
+    | sed -n '/Change detected\. Rebuilding\.\.\./,$p' \
+    | grep -E '^(Change detected|\[[12]/2\] .* (Parsed|Compiled) |✅ Finished incremental compilation)' \
+    | awk '{ print } /✅ Finished incremental compilation/ { exit }' \
+    >"$work/$implementation-initial-failure-recovery.phases"
+}
+
+echo "Checking interactive initial-failure recovery..."
+if $compare_rust_watch_output; then
+  capture_initial_failure_recovery rust "$rust"
+fi
+capture_initial_failure_recovery ocaml "$ocaml"
+
+if $compare_rust_watch_output && \
+  ! cmp -s "$work/rust-initial-failure-recovery.phases" \
+  "$work/ocaml-initial-failure-recovery.phases"; then
+  echo "Interactive initial-failure recovery output differs" >&2
+  printf '%s\n' '--- Rust recovery phases ---' >&2
+  cat "$work/rust-initial-failure-recovery.phases" >&2
+  printf '%s\n' '--- OCaml recovery phases ---' >&2
+  cat "$work/ocaml-initial-failure-recovery.phases" >&2
+  exit 1
+fi
+
+cat >"$work/expected-initial-failure-recovery" <<'EOF'
+Change detected. Rebuilding...
+[1/2] 🧱 Parsed 1 source files in <TIME>
+[2/2] 🤺 Compiled 1 modules in <TIME>
+✅ Finished incremental compilation in <TIME>
+EOF
+
+if ! cmp -s "$work/expected-initial-failure-recovery" \
+  "$work/ocaml-initial-failure-recovery.phases"; then
+  echo "Initial-failure recovery did not retain incremental state" >&2
+  cat "$work/ocaml-initial-failure-recovery.phases" >&2
+  exit 1
+fi
+
+capture_partial_initial_failure_recovery() {
+  local implementation=$1
+  local executable=$2
+  local project="$work/$implementation-partial-initial-failure-watch"
+  local transcript="$work/$implementation-partial-initial-failure-watch.log"
+  local command_executable=$(command_path "$executable")
+  local command_project=$(command_path "$project")
+  "$command_executable" watch "$command_project" >"$transcript" 2>&1 &
+  active_script_pid=$!
+  active_watch_project=$project
+  active_watch_transcript=$transcript
+  if ! wait_for_text "$transcript" "expected to have type" 1; then return 1; fi
+  printf 'let answer = A.value + 1\n' >"$project/src/B.res"
+  if ! wait_for_text "$transcript" "Finished incremental compilation" 1; then
+    return 1
+  fi
+  wait_for_file "$project/src/A.mjs"
+  wait_for_file "$project/src/B.mjs"
+  if grep -F "I/O error: src/A.ast" "$transcript" >/dev/null; then
+    echo "$implementation retained build state without its parsed AST" >&2
+    cat "$transcript" >&2
+    return 1
+  fi
+  stop_active_watch
+}
+
+echo "Checking partial initial-failure recovery..."
+if $compare_rust_watch_output; then
+  capture_partial_initial_failure_recovery rust "$rust"
+fi
+capture_partial_initial_failure_recovery ocaml "$ocaml"
+
+capture_warning_watch() {
+  local implementation=$1
+  local executable=$2
+  local project="$work/$implementation-warning-watch"
+  local transcript="$work/$implementation-warning-watch.tty"
+  local command_executable=$(command_path "$executable")
+  local command_project=$(command_path "$project")
+  if [ "$(uname -s)" = Darwin ]; then
+    script -qF "$transcript" env -u NO_COLOR \
+      "TERM=xterm" \
+      "CLICOLOR=1" \
+      "CLICOLOR_FORCE=0" \
+      "RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE" \
+      "RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME" \
+      "$executable" watch "$project" >/dev/null &
+  else
+    script -qefc \
+      "env -u NO_COLOR TERM=xterm CLICOLOR=1 CLICOLOR_FORCE=0 RESCRIPT_BSC_EXE=$RESCRIPT_BSC_EXE RESCRIPT_RUNTIME=$RESCRIPT_RUNTIME $command_executable watch $command_project" \
+      "$transcript" >/dev/null &
+  fi
+  active_script_pid=$!
+  active_watch_project=$project
+  active_watch_transcript=$transcript
+  if ! wait_for_text "$transcript" "Finished initial compilation" 1; then
+    return 1
+  fi
+  overwrite_line_in_place "$project/src/A.res" 'let value = 2'
+  if ! wait_for_text "$transcript" "Finished incremental compilation" 1; then
+    return 1
+  fi
+  printf 'let added = 1\n' >"$project/src/B.res"
+  if ! wait_for_text "$transcript" "Finished compilation" 1; then
+    return 1
+  fi
+  stop_active_watch
+
+  if [ "$(grep -cF "uses deprecated config" "$transcript")" -ne 1 ]; then
+    echo "$implementation repeated a configuration warning during watch" >&2
+    cat "$transcript" >&2
+    exit 1
+  fi
+  if grep -F "Finished incremental compilation with warnings" \
+    "$transcript" >/dev/null; then
+    echo "$implementation carried a static configuration warning into the incremental footer" >&2
+    cat "$transcript" >&2
+    exit 1
+  fi
+  if [ "$implementation" = ocaml ] && \
+    ! grep -F $'\033[33mPackage' "$transcript" >/dev/null; then
+    echo "$implementation did not render interactive configuration warnings in yellow" >&2
+    cat "$transcript" >&2
+    exit 1
+  fi
+}
+
+echo "Checking interactive watch warnings..."
+if $compare_rust_watch_output; then
+  capture_warning_watch rust "$rust"
+fi
+capture_warning_watch ocaml "$ocaml"
+
+echo "Interactive output phases matched"
diff --git a/rewatch-ocaml/tests/check_rust_test_coverage.sh b/rewatch-ocaml/tests/check_rust_test_coverage.sh
new file mode 100755
index 00000000000..e2db0b0ba78
--- /dev/null
+++ b/rewatch-ocaml/tests/check_rust_test_coverage.sh
@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root=$(cd "$(dirname "$0")/../.." && pwd)
+mapping="$root/rewatch-ocaml/tests/rust_test_coverage.tsv"
+inventory=$(mktemp)
+mapped=$(mktemp)
+cleanup() {
+  rm -f "$inventory" "$mapped"
+}
+trap cleanup EXIT
+
+while IFS= read -r file; do
+  relative=${file#"$root/rewatch/src/"}
+  awk -v path="$relative" '
+    /#\[(tokio::)?test\]/ { pending = 1; next }
+    pending && /fn[[:space:]]+[[:alnum:]_]+/ {
+      line = $0
+      sub(/^.*fn[[:space:]]+/, "", line)
+      sub(/[^[:alnum:]_].*$/, "", line)
+      print path "::" line
+      pending = 0
+    }
+  ' "$file"
+done < <(find "$root/rewatch/src" -type f -name '*.rs' \
+  -exec grep -lE '#\[(tokio::)?test\]' {} + | sort) \
+  | sort >"$inventory"
+
+awk -F '\t' '
+  /^[[:space:]]*#/ || /^[[:space:]]*$/ { next }
+  NF != 3 { print "invalid mapping row " NR > "/dev/stderr"; invalid = 1; next }
+  $2 !~ /^(covered|shared|intentional|omitted|gap|unreviewed)$/ {
+    print "invalid status on mapping row " NR ": " $2 > "/dev/stderr"
+    invalid = 1
+  }
+  { print $1 }
+  END { if (invalid) exit 1 }
+' "$mapping" | sort >"$mapped"
+
+duplicates=$(uniq -d "$mapped")
+if [[ -n "$duplicates" ]]; then
+  printf 'Duplicate Rust test mappings:\n%s\n' "$duplicates" >&2
+  exit 1
+fi
+
+missing=$(comm -23 "$inventory" "$mapped")
+stale=$(comm -13 "$inventory" "$mapped")
+if [[ -n "$missing" || -n "$stale" ]]; then
+  [[ -z "$missing" ]] || printf 'Unmapped Rust tests:\n%s\n' "$missing" >&2
+  [[ -z "$stale" ]] || printf 'Stale Rust test mappings:\n%s\n' "$stale" >&2
+  exit 1
+fi
+
+total=$(wc -l <"$inventory" | tr -d ' ')
+reviewed=$(awk -F '\t' '!/^#/ && NF && $2 != "unreviewed" { count++ } END { print count + 0 }' "$mapping")
+gaps=$(awk -F '\t' '!/^#/ && $2 == "gap" { count++ } END { print count + 0 }' "$mapping")
+unreviewed=$(awk -F '\t' '!/^#/ && $2 == "unreviewed" { count++ } END { print count + 0 }' "$mapping")
+printf 'Rust unit tests: %s; reviewed: %s; gaps: %s; unreviewed: %s\n' \
+  "$total" "$reviewed" "$gaps" "$unreviewed"
+
+if [[ ${1:-} == "--require-complete" ]] && ((gaps > 0 || unreviewed > 0)); then
+  exit 1
+fi
diff --git a/rewatch-ocaml/tests/check_verbose_output.sh b/rewatch-ocaml/tests/check_verbose_output.sh
new file mode 100755
index 00000000000..2c877278ef6
--- /dev/null
+++ b/rewatch-ocaml/tests/check_verbose_output.sh
@@ -0,0 +1,134 @@
+#!/bin/bash
+set -eu
+
+root=$(CDPATH= cd -- "$(dirname "$0")/../.." && pwd)
+rust=${1:-$root/rewatch/target/debug/rescript}
+ocaml=${2:-$root/_build/default/rewatch-ocaml/rescript_ocaml.exe}
+rust=$(realpath "$rust")
+ocaml=$(realpath "$ocaml")
+work=$(mktemp -d "${TMPDIR:-/tmp}/rewatch-verbose-output-XXXXXX")
+cleanup() {
+  rm -rf "$work"
+}
+trap cleanup EXIT
+
+for implementation in rust-v ocaml-v rust-vv ocaml-vv; do
+  mkdir -p "$work/$implementation/src"
+  printf '{"name":"verbose-output","sources":["src"]}\n' \
+    >"$work/$implementation/rescript.json"
+  printf 'let value = 1\n' >"$work/$implementation/src/A.res"
+  printf 'let value = A.value\n' >"$work/$implementation/src/B.res"
+  printf 'let value = 2\n' >"$work/$implementation/src/Authored.res"
+  printf 'let value: int\n' >"$work/$implementation/src/WithInterface.resi"
+  printf 'let value = 3\n' >"$work/$implementation/src/WithInterface.res"
+done
+
+export RESCRIPT_BSC_EXE=${RESCRIPT_BSC_EXE:-$root/_build/default/compiler/bsc/rescript_compiler_main.exe}
+export RESCRIPT_RUNTIME=${RESCRIPT_RUNTIME:-$root/packages/@rescript/runtime}
+
+normalize_events() {
+  input=$1
+  output=$2
+  sed -E \
+    's# at ".*rescript\.json" for ".*"# at "<PROJECT>/rescript.json" for "<PROJECT>"#' \
+    "$input" \
+    | awk '
+        /^(DEBUG|TRACE):$/ {
+          label = $0
+          if ((getline message) > 0) print label "\t" message
+          next
+        }
+        /^compile dirty: / { print }
+      ' \
+    | sort >"$output"
+}
+
+capture() {
+  implementation=$1
+  executable=$2
+  level=$3
+  project="$work/$implementation"
+  (
+    cd "$project"
+    "$executable" "$level" build >"$work/$implementation.stdout" \
+      2>"$work/$implementation.stderr"
+  )
+  if [ -s "$work/$implementation.stderr" ]; then
+    echo "$implementation unexpectedly wrote verbose build output to stderr" >&2
+    cat "$work/$implementation.stderr" >&2
+    exit 1
+  fi
+  normalize_events "$work/$implementation.stdout" \
+    "$work/$implementation.events"
+}
+
+capture rust-v "$rust" -v
+capture ocaml-v "$ocaml" -v
+capture rust-vv "$rust" -vv
+capture ocaml-vv "$ocaml" -vv
+
+compare_events() {
+  rust_events=$1
+  ocaml_events=$2
+  if ! cmp -s "$rust_events" "$ocaml_events"; then
+    echo "Verbose semantic output differs" >&2
+    printf '%s\n' '--- Rust events ---' >&2
+    cat "$rust_events" >&2
+    printf '%s\n' '--- OCaml events ---' >&2
+    cat "$ocaml_events" >&2
+    exit 1
+  fi
+}
+
+compare_events "$work/rust-v.events" "$work/ocaml-v.events"
+compare_events "$work/rust-vv.events" "$work/ocaml-vv.events"
+
+if grep -E '^(TRACE:|compile dirty:)' "$work/ocaml-v.events" >/dev/null; then
+  echo "-v unexpectedly emitted trace-level events" >&2
+  cat "$work/ocaml-v.events" >&2
+  exit 1
+fi
+if [ "$(grep -c '^compile dirty: ' "$work/ocaml-vv.events")" -ne 4 ]; then
+  echo "-vv did not identify the four dirty modules" >&2
+  cat "$work/ocaml-vv.events" >&2
+  exit 1
+fi
+if ! grep -F $'TRACE:\tCompiled 4 out of 4 in the universe' \
+  "$work/ocaml-vv.events" >/dev/null; then
+  echo "-vv did not report the scheduler universe" >&2
+  cat "$work/ocaml-vv.events" >&2
+  exit 1
+fi
+
+capture_incremental() {
+  implementation=$1
+  executable=$2
+  project="$work/$implementation"
+  printf 'let value = 2\n' >"$project/src/A.res"
+  (
+    cd "$project"
+    "$executable" -vv build >"$work/$implementation-incremental.stdout" \
+      2>"$work/$implementation-incremental.stderr"
+  )
+  if [ -s "$work/$implementation-incremental.stderr" ]; then
+    echo "$implementation incremental verbose output reached stderr" >&2
+    cat "$work/$implementation-incremental.stderr" >&2
+    exit 1
+  fi
+  normalize_events "$work/$implementation-incremental.stdout" \
+    "$work/$implementation-incremental.events"
+}
+
+capture_incremental rust-vv "$rust"
+capture_incremental ocaml-vv "$ocaml"
+compare_events "$work/rust-vv-incremental.events" \
+  "$work/ocaml-vv-incremental.events"
+if [ "$(grep -c '^compile dirty: ' "$work/ocaml-vv-incremental.events")" -ne 1 ] \
+  || ! grep -F $'TRACE:\tCompiled 2 out of 2 in the universe' \
+    "$work/ocaml-vv-incremental.events" >/dev/null; then
+  echo "Incremental -vv output lost its dirty/universe boundary" >&2
+  cat "$work/ocaml-vv-incremental.events" >&2
+  exit 1
+fi
+
+echo "Verbose semantic output matched"
diff --git a/rewatch-ocaml/tests/check_windows_job_stub.sh b/rewatch-ocaml/tests/check_windows_job_stub.sh
new file mode 100755
index 00000000000..2abf63e14aa
--- /dev/null
+++ b/rewatch-ocaml/tests/check_windows_job_stub.sh
@@ -0,0 +1,22 @@
+#!/bin/sh
+set -eu
+
+root=$(CDPATH= cd -- "$(dirname "$0")/../.." && pwd)
+if [ "$#" -ne 2 ]; then
+  echo "usage: $0 WINDOWS_CC WINDOWS_OCAML_INCLUDE" >&2
+  exit 2
+fi
+
+compiler=$1
+ocaml_include=$2
+object=$(mktemp "${TMPDIR:-/tmp}/rewatch-windows-job-stub-XXXXXX.o")
+trap 'rm -f "$object"' EXIT
+
+if ! printf '#ifndef _WIN32\n#error compiler does not target Windows\n#endif\n' \
+  | "$compiler" -x c -E - >/dev/null 2>&1; then
+  echo "$compiler does not target Windows" >&2
+  exit 2
+fi
+
+"$compiler" -Wall -Wextra -Werror -I "$ocaml_include" -c \
+  "$root/rewatch-ocaml/windows_job_stubs.c" -o "$object"
diff --git a/rewatch-ocaml/tests/config_acceptance_cases.tsv b/rewatch-ocaml/tests/config_acceptance_cases.tsv
new file mode 100644
index 00000000000..0bd3a9ad8f9
--- /dev/null
+++ b/rewatch-ocaml/tests/config_acceptance_cases.tsv
@@ -0,0 +1,298 @@
+# area	case	expected (or rust/ocaml divergence)	rescript.json
+sources	omitted	accept	{"name":"config-shape"}
+sources	null	accept	{"name":"config-shape","sources":null}
+sources	shorthand	accept	{"name":"config-shape","sources":"src"}
+sources	empty-list	accept	{"name":"config-shape","sources":[]}
+sources	qualified	accept	{"name":"config-shape","sources":{"dir":"src"}}
+sources	empty-dir	accept	{"name":"config-shape","sources":{"dir":""}}
+sources	subdirs-null	accept	{"name":"config-shape","sources":{"dir":"src","subdirs":null}}
+sources	subdirs-false	accept	{"name":"config-shape","sources":{"dir":"src","subdirs":false}}
+sources	subdirs-true	accept	{"name":"config-shape","sources":{"dir":"src","subdirs":true}}
+sources	subdirs-empty	accept	{"name":"config-shape","sources":{"dir":"src","subdirs":[]}}
+sources	subdirs-shorthand	accept	{"name":"config-shape","sources":{"dir":"src","subdirs":["nested"]}}
+sources	subdirs-qualified	accept	{"name":"config-shape","sources":{"dir":"src","subdirs":[{"dir":"nested"}]}}
+sources	type-null	accept	{"name":"config-shape","sources":{"dir":"src","type":null}}
+sources	type-dev	accept	{"name":"config-shape","sources":{"dir":"src","type":"dev"}}
+sources	type-custom	accept	{"name":"config-shape","sources":{"dir":"src","type":"custom"}}
+sources	feature-null	accept	{"name":"config-shape","sources":{"dir":"src","feature":null}}
+sources	feature-string	accept	{"name":"config-shape","sources":{"dir":"src","feature":"native"}}
+sources	unknown-qualified-field	accept	{"name":"config-shape","sources":{"dir":"src","future":true}}
+sources	sources-boolean	reject	{"name":"config-shape","sources":true}
+sources	sources-number	reject	{"name":"config-shape","sources":1}
+sources	sources-invalid-list-item	reject	{"name":"config-shape","sources":[null]}
+sources	missing-dir	reject	{"name":"config-shape","sources":{"subdirs":true}}
+sources	null-dir	reject	{"name":"config-shape","sources":{"dir":null}}
+sources	numeric-dir	reject	{"name":"config-shape","sources":{"dir":1}}
+sources	string-subdirs	reject	{"name":"config-shape","sources":{"dir":"src","subdirs":"nested"}}
+sources	numeric-subdirs	reject	{"name":"config-shape","sources":{"dir":"src","subdirs":1}}
+sources	object-subdirs	reject	{"name":"config-shape","sources":{"dir":"src","subdirs":{"dir":"nested"}}}
+sources	invalid-subdir-item	reject	{"name":"config-shape","sources":{"dir":"src","subdirs":[false]}}
+sources	boolean-type	reject	{"name":"config-shape","sources":{"dir":"src","type":true}}
+sources	array-type	reject	{"name":"config-shape","sources":{"dir":"src","type":[]}}
+sources	boolean-feature	reject	{"name":"config-shape","sources":{"dir":"src","feature":true}}
+sources	array-feature	reject	{"name":"config-shape","sources":{"dir":"src","feature":[]}}
+sources	duplicate-dir	reject	{"name":"config-shape","sources":{"dir":"src","dir":"other"}}
+sources	duplicate-subdirs	reject	{"name":"config-shape","sources":{"dir":"src","subdirs":true,"subdirs":false}}
+sources	duplicate-type	reject	{"name":"config-shape","sources":{"dir":"src","type":"dev","type":"other"}}
+sources	duplicate-feature	reject	{"name":"config-shape","sources":{"dir":"src","feature":"a","feature":"b"}}
+dependencies	omitted	accept	{"name":"config-shape"}
+dependencies	null	accept	{"name":"config-shape","dependencies":null}
+dependencies	empty	accept	{"name":"config-shape","dependencies":[]}
+dependencies	shorthand	accept	{"name":"config-shape","dependencies":["dep"]}
+dependencies	qualified	accept	{"name":"config-shape","dependencies":[{"name":"dep"}]}
+dependencies	features-null	accept	{"name":"config-shape","dependencies":[{"name":"dep","features":null}]}
+dependencies	features-empty	accept	{"name":"config-shape","dependencies":[{"name":"dep","features":[]}]}
+dependencies	features-list	accept	{"name":"config-shape","dependencies":[{"name":"dep","features":["native"]}]}
+dependencies	empty-name	accept	{"name":"config-shape","dependencies":[""]}
+dependencies	unknown-qualified-field	accept	{"name":"config-shape","dependencies":[{"name":"dep","future":true}]}
+dependencies	legacy-alias	accept	{"name":"config-shape","bs-dependencies":["dep"]}
+dependencies	dev	accept	{"name":"config-shape","dev-dependencies":["dep"]}
+dependencies	legacy-dev-alias	accept	{"name":"config-shape","bs-dev-dependencies":["dep"]}
+dependencies	string-outer	reject	{"name":"config-shape","dependencies":"dep"}
+dependencies	object-outer	reject	{"name":"config-shape","dependencies":{"name":"dep"}}
+dependencies	invalid-entry	reject	{"name":"config-shape","dependencies":[false]}
+dependencies	missing-name	reject	{"name":"config-shape","dependencies":[{"features":[]}]}
+dependencies	null-name	reject	{"name":"config-shape","dependencies":[{"name":null}]}
+dependencies	boolean-name	reject	{"name":"config-shape","dependencies":[{"name":true}]}
+dependencies	boolean-features	reject	{"name":"config-shape","dependencies":[{"name":"dep","features":true}]}
+dependencies	mixed-features	reject	{"name":"config-shape","dependencies":[{"name":"dep","features":["native",false]}]}
+dependencies	duplicate-name	reject	{"name":"config-shape","dependencies":[{"name":"dep","name":"other"}]}
+dependencies	duplicate-features	reject	{"name":"config-shape","dependencies":[{"name":"dep","features":[],"features":["native"]}]}
+dependencies	modern-and-legacy	reject	{"name":"config-shape","dependencies":[],"bs-dependencies":[]}
+dependencies	modern-and-legacy-dev	reject	{"name":"config-shape","dev-dependencies":[],"bs-dev-dependencies":[]}
+features	omitted	accept	{"name":"config-shape"}
+features	null	accept	{"name":"config-shape","features":null}
+features	empty	accept	{"name":"config-shape","features":{}}
+features	leaf	accept	{"name":"config-shape","features":{"native":[]}}
+features	transitive	accept	{"name":"config-shape","features":{"all":["native"],"native":[]}}
+features	duplicate-last	accept/reject	{"name":"config-shape","features":{"all":["old"],"all":["new"]}}
+features	boolean-outer	reject	{"name":"config-shape","features":true}
+features	array-outer	reject	{"name":"config-shape","features":[]}
+features	string-value	reject	{"name":"config-shape","features":{"all":"native"}}
+features	null-value	reject	{"name":"config-shape","features":{"all":null}}
+features	mixed-value	reject	{"name":"config-shape","features":{"all":["native",false]}}
+allowed-dependents	omitted	accept	{"name":"config-shape"}
+allowed-dependents	null	accept	{"name":"config-shape","allowed-dependents":null}
+allowed-dependents	empty	accept	{"name":"config-shape","allowed-dependents":[]}
+allowed-dependents	list	accept	{"name":"config-shape","allowed-dependents":["consumer"]}
+allowed-dependents	string-outer	reject	{"name":"config-shape","allowed-dependents":"consumer"}
+allowed-dependents	mixed-list	reject	{"name":"config-shape","allowed-dependents":["consumer",false]}
+package-specs	omitted	accept	{"name":"config-shape"}
+package-specs	null	accept	{"name":"config-shape","package-specs":null}
+package-specs	esmodule	accept	{"name":"config-shape","package-specs":{"module":"esmodule"}}
+package-specs	commonjs	accept	{"name":"config-shape","package-specs":{"module":"commonjs"}}
+package-specs	es6-alias	accept	{"name":"config-shape","package-specs":{"module":"es6"}}
+package-specs	cjs-alias	accept	{"name":"config-shape","package-specs":{"module":"cjs"}}
+package-specs	empty-list	accept	{"name":"config-shape","package-specs":[]}
+package-specs	in-source-false	accept	{"name":"config-shape","package-specs":{"module":"esmodule","in-source":false}}
+package-specs	null-suffix	accept	{"name":"config-shape","package-specs":{"module":"esmodule","suffix":null}}
+package-specs	explicit-suffix	accept	{"name":"config-shape","package-specs":{"module":"esmodule","suffix":".mjs"}}
+package-specs	empty-suffix	accept	{"name":"config-shape","package-specs":{"module":"esmodule","suffix":""}}
+package-specs	top-level-suffix	accept	{"name":"config-shape","suffix":".mjs","package-specs":{"module":"esmodule"}}
+package-specs	same-suffix-different-location	accept	{"name":"config-shape","package-specs":[{"module":"esmodule","in-source":true,"suffix":".mjs"},{"module":"commonjs","in-source":false,"suffix":".mjs"}]}
+package-specs	unknown-field	accept	{"name":"config-shape","package-specs":{"module":"esmodule","future":true}}
+package-specs	boolean-outer	reject	{"name":"config-shape","package-specs":true}
+package-specs	string-outer	reject	{"name":"config-shape","package-specs":"esmodule"}
+package-specs	invalid-list-item	reject	{"name":"config-shape","package-specs":[false]}
+package-specs	missing-module	reject	{"name":"config-shape","package-specs":{}}
+package-specs	null-module	reject	{"name":"config-shape","package-specs":{"module":null}}
+package-specs	unknown-module	reject	{"name":"config-shape","package-specs":{"module":"global"}}
+package-specs	string-in-source	reject	{"name":"config-shape","package-specs":{"module":"esmodule","in-source":"true"}}
+package-specs	null-in-source	reject	{"name":"config-shape","package-specs":{"module":"esmodule","in-source":null}}
+package-specs	boolean-suffix	reject	{"name":"config-shape","package-specs":{"module":"esmodule","suffix":true}}
+package-specs	boolean-top-level-suffix	reject	{"name":"config-shape","suffix":true}
+package-specs	duplicate-module	reject	{"name":"config-shape","package-specs":{"module":"esmodule","module":"commonjs"}}
+package-specs	duplicate-in-source	reject	{"name":"config-shape","package-specs":{"module":"esmodule","in-source":true,"in-source":false}}
+package-specs	duplicate-spec-suffix	reject	{"name":"config-shape","package-specs":{"module":"esmodule","suffix":".js","suffix":".mjs"}}
+package-specs	duplicate-output	reject	{"name":"config-shape","package-specs":[{"module":"esmodule","suffix":".mjs"},{"module":"commonjs","suffix":".mjs"}]}
+jsx	omitted	accept	{"name":"config-shape"}
+jsx	null	accept	{"name":"config-shape","jsx":null}
+jsx	empty	accept	{"name":"config-shape","jsx":{}}
+jsx	version-null	accept	{"name":"config-shape","jsx":{"version":null}}
+jsx	version-four	accept	{"name":"config-shape","jsx":{"version":4}}
+jsx	module-null	accept	{"name":"config-shape","jsx":{"module":null}}
+jsx	module-react-constructor	accept	{"name":"config-shape","jsx":{"module":"React"}}
+jsx	module-react-lowercase	accept	{"name":"config-shape","jsx":{"module":"react"}}
+jsx	module-custom	accept	{"name":"config-shape","jsx":{"module":"Voby.JSX"}}
+jsx	mode-null	accept	{"name":"config-shape","jsx":{"mode":null}}
+jsx	mode-classic	accept	{"name":"config-shape","jsx":{"mode":"classic"}}
+jsx	mode-automatic	accept	{"name":"config-shape","jsx":{"mode":"automatic"}}
+jsx	v3-dependencies-null	accept	{"name":"config-shape","jsx":{"v3-dependencies":null}}
+jsx	v3-dependencies	accept	{"name":"config-shape","jsx":{"v3-dependencies":["react"]}}
+jsx	preserve-null	accept	{"name":"config-shape","jsx":{"preserve":null}}
+jsx	preserve-false	accept	{"name":"config-shape","jsx":{"preserve":false}}
+jsx	preserve-true	accept	{"name":"config-shape","jsx":{"preserve":true}}
+jsx	unknown-field	accept	{"name":"config-shape","jsx":{"future":true}}
+jsx	boolean-outer	reject	{"name":"config-shape","jsx":true}
+jsx	version-three	panic/reject	{"name":"config-shape","jsx":{"version":3}}
+jsx	string-version	reject	{"name":"config-shape","jsx":{"version":"4"}}
+jsx	boolean-module	reject	{"name":"config-shape","jsx":{"module":true}}
+jsx	unknown-mode	reject	{"name":"config-shape","jsx":{"mode":"modern"}}
+jsx	boolean-mode	reject	{"name":"config-shape","jsx":{"mode":true}}
+jsx	string-v3-dependencies	reject	{"name":"config-shape","jsx":{"v3-dependencies":"react"}}
+jsx	mixed-v3-dependencies	reject	{"name":"config-shape","jsx":{"v3-dependencies":["react",false]}}
+jsx	string-preserve	reject	{"name":"config-shape","jsx":{"preserve":"true"}}
+jsx	duplicate-version	reject	{"name":"config-shape","jsx":{"version":4,"version":null}}
+jsx	duplicate-module	reject	{"name":"config-shape","jsx":{"module":"react","module":"voby"}}
+jsx	duplicate-mode	reject	{"name":"config-shape","jsx":{"mode":"classic","mode":"automatic"}}
+jsx	duplicate-v3-dependencies	reject	{"name":"config-shape","jsx":{"v3-dependencies":[],"v3-dependencies":["react"]}}
+jsx	duplicate-preserve	reject	{"name":"config-shape","jsx":{"preserve":true,"preserve":false}}
+source-map	omitted	accept	{"name":"config-shape"}
+source-map	null	accept	{"name":"config-shape","sourceMap":null}
+source-map	disabled	accept	{"name":"config-shape","sourceMap":false}
+source-map	linked-always	accept	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"linked"}}
+source-map	inline-dev	accept	{"name":"config-shape","sourceMap":{"enabled":"dev","mode":"inline"}}
+source-map	hidden	accept	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"hidden"}}
+source-map	null-options	accept	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"linked","sourcesContent":null,"sourceRoot":null}}
+source-map	options	accept	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"linked","sourcesContent":true,"sourceRoot":"/source"}}
+source-map	unknown-field	accept	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"linked","future":true}}
+source-map	duplicate-fields-last	accept/reject	{"name":"config-shape","sourceMap":{"enabled":"dev","enabled":"always","mode":"inline","mode":"linked"}}
+source-map	true	reject	{"name":"config-shape","sourceMap":true}
+source-map	string-outer	reject	{"name":"config-shape","sourceMap":"linked"}
+source-map	empty-object	reject	{"name":"config-shape","sourceMap":{}}
+source-map	missing-enabled	reject	{"name":"config-shape","sourceMap":{"mode":"linked"}}
+source-map	missing-mode	reject	{"name":"config-shape","sourceMap":{"enabled":"always"}}
+source-map	null-enabled	reject	{"name":"config-shape","sourceMap":{"enabled":null,"mode":"linked"}}
+source-map	unknown-enabled	reject	{"name":"config-shape","sourceMap":{"enabled":"build","mode":"linked"}}
+source-map	null-mode	reject	{"name":"config-shape","sourceMap":{"enabled":"always","mode":null}}
+source-map	unknown-mode	reject	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"external"}}
+source-map	string-sources-content	reject	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"linked","sourcesContent":"true"}}
+source-map	boolean-source-root	reject	{"name":"config-shape","sourceMap":{"enabled":"always","mode":"linked","sourceRoot":true}}
+post-build	omitted	accept	{"name":"config-shape"}
+post-build	null	accept	{"name":"config-shape","js-post-build":null}
+post-build	command	accept	{"name":"config-shape","js-post-build":{"cmd":"echo"}}
+post-build	empty-command	accept	{"name":"config-shape","js-post-build":{"cmd":""}}
+post-build	unknown-field	accept	{"name":"config-shape","js-post-build":{"cmd":"echo","future":true}}
+post-build	boolean-outer	reject	{"name":"config-shape","js-post-build":true}
+post-build	missing-command	reject	{"name":"config-shape","js-post-build":{}}
+post-build	null-command	reject	{"name":"config-shape","js-post-build":{"cmd":null}}
+post-build	boolean-command	reject	{"name":"config-shape","js-post-build":{"cmd":true}}
+post-build	duplicate-command	reject	{"name":"config-shape","js-post-build":{"cmd":"a","cmd":"b"}}
+top-level	minimal	accept	{"name":"config-shape"}
+top-level	empty-object	reject	{}
+top-level	null-root	reject	null
+top-level	array-root	reject	[]
+top-level	string-root	reject	"config-shape"
+top-level	null-name	reject	{"name":null}
+top-level	boolean-name	reject	{"name":true}
+top-level	empty-name	accept	{"name":""}
+top-level	unknown-field	accept	{"name":"config-shape","future":{"anything":true}}
+top-level	editor-arbitrary	accept	{"name":"config-shape","editor":[true,null,1]}
+top-level	reanalyze-arbitrary	accept	{"name":"config-shape","reanalyze":{"anything":true}}
+top-level	internal-path-string	accept	{"name":"config-shape","path":"ignored"}
+top-level	internal-path-null	reject/accept	{"name":"config-shape","path":null}
+top-level	internal-path-boolean	reject/accept	{"name":"config-shape","path":false}
+top-level	duplicate-name	reject	{"name":"first","name":"second"}
+top-level	duplicate-editor	reject	{"name":"config-shape","editor":true,"editor":false}
+warnings	omitted	accept	{"name":"config-shape"}
+warnings	null	accept	{"name":"config-shape","warnings":null}
+warnings	empty	accept	{"name":"config-shape","warnings":{}}
+warnings	number	accept	{"name":"config-shape","warnings":{"number":"+A"}}
+warnings	error-true	accept	{"name":"config-shape","warnings":{"error":true}}
+warnings	error-false	accept	{"name":"config-shape","warnings":{"error":false}}
+warnings	error-string	accept	{"name":"config-shape","warnings":{"error":"+101"}}
+warnings	null-fields	accept	{"name":"config-shape","warnings":{"number":null,"error":null}}
+warnings	unknown-field	accept	{"name":"config-shape","warnings":{"future":true}}
+warnings	boolean-outer	reject	{"name":"config-shape","warnings":true}
+warnings	boolean-number	reject	{"name":"config-shape","warnings":{"number":true}}
+warnings	numeric-error	reject	{"name":"config-shape","warnings":{"error":1}}
+warnings	array-error	reject	{"name":"config-shape","warnings":{"error":[]}}
+warnings	duplicate-number	reject	{"name":"config-shape","warnings":{"number":"A","number":"B"}}
+warnings	duplicate-error	reject	{"name":"config-shape","warnings":{"error":true,"error":false}}
+compiler-flags	omitted	accept	{"name":"config-shape"}
+compiler-flags	null	accept	{"name":"config-shape","compiler-flags":null}
+compiler-flags	empty	accept	{"name":"config-shape","compiler-flags":[]}
+compiler-flags	string-entry	accept	{"name":"config-shape","compiler-flags":["-open Belt"]}
+compiler-flags	array-entry	accept	{"name":"config-shape","compiler-flags":[["-open","Belt"]]}
+compiler-flags	mixed-entries	accept	{"name":"config-shape","compiler-flags":["-w +A",["-open","Belt"]]}
+compiler-flags	empty-array-entry	accept	{"name":"config-shape","compiler-flags":[[]]}
+compiler-flags	legacy-alias	accept	{"name":"config-shape","bsc-flags":["-open Belt"]}
+compiler-flags	string-outer	reject	{"name":"config-shape","compiler-flags":"-open Belt"}
+compiler-flags	boolean-entry	reject	{"name":"config-shape","compiler-flags":[true]}
+compiler-flags	mixed-array-entry	reject	{"name":"config-shape","compiler-flags":[["-open",false]]}
+compiler-flags	modern-and-legacy	reject	{"name":"config-shape","compiler-flags":[],"bsc-flags":[]}
+ppx-flags	omitted	accept	{"name":"config-shape"}
+ppx-flags	null	accept	{"name":"config-shape","ppx-flags":null}
+ppx-flags	empty	accept	{"name":"config-shape","ppx-flags":[]}
+ppx-flags	package-entry	accept	{"name":"config-shape","ppx-flags":["ppx"]}
+ppx-flags	array-entry	accept	{"name":"config-shape","ppx-flags":[["ppx","--flag"]]}
+ppx-flags	empty-array-entry	panic/accept	{"name":"config-shape","ppx-flags":[[]]}
+ppx-flags	string-outer	reject	{"name":"config-shape","ppx-flags":"ppx"}
+ppx-flags	boolean-entry	reject	{"name":"config-shape","ppx-flags":[true]}
+ppx-flags	mixed-array-entry	reject	{"name":"config-shape","ppx-flags":[["ppx",false]]}
+namespace	omitted	accept	{"name":"config-shape"}
+namespace	null	accept	{"name":"config-shape","namespace":null}
+namespace	false	accept	{"name":"config-shape","namespace":false}
+namespace	true	accept	{"name":"scope/config-shape","namespace":true}
+namespace	string-true	accept	{"name":"scope/config-shape","namespace":"true"}
+namespace	custom	accept	{"name":"config-shape","namespace":"custom-name"}
+namespace	empty-string	accept	{"name":"config-shape","namespace":""}
+namespace	entry	accept	{"name":"config-shape","namespace":true,"namespace-entry":"Entry"}
+namespace	null-entry	accept	{"name":"config-shape","namespace":true,"namespace-entry":null}
+namespace	boolean-outer	reject	{"name":"config-shape","namespace":1}
+namespace	boolean-entry	reject	{"name":"config-shape","namespace":true,"namespace-entry":false}
+namespace	entry-without-namespace	accept/reject	{"name":"config-shape","namespace-entry":"Entry"}
+compiler-flags	whitespace-normalization	accept/accept	{"name":"config-shape","compiler-flags":["  -w  +A  "]}
+experimental	omitted	accept	{"name":"config-shape"}
+experimental	null	accept	{"name":"config-shape","experimental-features":null}
+experimental	empty	accept	{"name":"config-shape","experimental-features":{}}
+experimental	enabled	accept	{"name":"config-shape","experimental-features":{"LetUnwrap":true}}
+experimental	disabled	accept	{"name":"config-shape","experimental-features":{"LetUnwrap":false}}
+experimental	duplicate-last	accept/reject	{"name":"config-shape","experimental-features":{"LetUnwrap":true,"LetUnwrap":false}}
+experimental	boolean-outer	reject	{"name":"config-shape","experimental-features":true}
+experimental	unknown-feature	reject	{"name":"config-shape","experimental-features":{"Future":true}}
+experimental	null-value	reject	{"name":"config-shape","experimental-features":{"LetUnwrap":null}}
+experimental	string-value	reject	{"name":"config-shape","experimental-features":{"LetUnwrap":"true"}}
+gentype	omitted	accept	{"name":"config-shape"}
+gentype	null	accept	{"name":"config-shape","gentypeconfig":null}
+gentype	empty	accept	{"name":"config-shape","gentypeconfig":{}}
+gentype	module-null	accept	{"name":"config-shape","gentypeconfig":{"module":null}}
+gentype	module-esmodule	accept	{"name":"config-shape","gentypeconfig":{"module":"esmodule"}}
+gentype	module-commonjs	accept	{"name":"config-shape","gentypeconfig":{"module":"commonjs"}}
+gentype	package-module-fallback	accept	{"name":"config-shape","package-specs":{"module":"commonjs"},"gentypeconfig":{}}
+gentype	module-overrides-package	accept	{"name":"config-shape","package-specs":{"module":"commonjs"},"gentypeconfig":{"module":"esmodule"}}
+gentype	resolution-null	accept	{"name":"config-shape","gentypeconfig":{"moduleResolution":null}}
+gentype	resolution-node	accept	{"name":"config-shape","gentypeconfig":{"moduleResolution":"node"}}
+gentype	resolution-node16	accept	{"name":"config-shape","gentypeconfig":{"moduleResolution":"node16"}}
+gentype	resolution-bundler	accept	{"name":"config-shape","gentypeconfig":{"moduleResolution":"bundler"}}
+gentype	export-null	accept	{"name":"config-shape","gentypeconfig":{"exportInterfaces":null}}
+gentype	export-false	accept	{"name":"config-shape","gentypeconfig":{"exportInterfaces":false}}
+gentype	export-true	accept	{"name":"config-shape","gentypeconfig":{"exportInterfaces":true}}
+gentype	extension-null	accept	{"name":"config-shape","gentypeconfig":{"generatedFileExtension":null}}
+gentype	extension	accept	{"name":"config-shape","gentypeconfig":{"generatedFileExtension":".gen.tsx"}}
+gentype	explicit-suffix	accept	{"name":"config-shape","suffix":".mjs","gentypeconfig":{}}
+gentype	shims-empty-object	accept	{"name":"config-shape","gentypeconfig":{"shims":{}}}
+gentype	shims-object	accept	{"name":"config-shape","gentypeconfig":{"shims":{"B":"Two","A":"One"}}}
+gentype	shims-object-duplicate-last	accept/reject	{"name":"config-shape","gentypeconfig":{"shims":{"A":"One","A":"Two"}}}
+gentype	shims-empty-list	accept	{"name":"config-shape","gentypeconfig":{"shims":[]}}
+gentype	shims-list	accept	{"name":"config-shape","gentypeconfig":{"shims":["B=Two"," A = One "]}}
+gentype	shims-list-duplicate-last	accept/reject	{"name":"config-shape","gentypeconfig":{"shims":["A=One","A=Two"]}}
+gentype	shims-empty-source	accept	{"name":"config-shape","gentypeconfig":{"shims":["=Target"]}}
+gentype	shims-empty-target	accept	{"name":"config-shape","gentypeconfig":{"shims":["Source="]}}
+gentype	debug-empty	accept	{"name":"config-shape","gentypeconfig":{"debug":{}}}
+gentype	debug-values	accept	{"name":"config-shape","gentypeconfig":{"debug":{"z":true,"a":false,"m":true}}}
+gentype	debug-duplicate-last	accept/reject	{"name":"config-shape","gentypeconfig":{"debug":{"all":true,"all":false}}}
+gentype	dependency	accept	{"name":"config-shape","dependencies":["dep"],"gentypeconfig":{}}
+gentype	source	accept	{"name":"config-shape","sources":["src"],"gentypeconfig":{}}
+gentype	unknown-field	accept	{"name":"config-shape","gentypeconfig":{"future":true}}
+gentype	boolean-outer	reject	{"name":"config-shape","gentypeconfig":true}
+gentype	unknown-module	reject	{"name":"config-shape","gentypeconfig":{"module":"amd"}}
+gentype	boolean-module	reject	{"name":"config-shape","gentypeconfig":{"module":true}}
+gentype	unknown-resolution	reject	{"name":"config-shape","gentypeconfig":{"moduleResolution":"classic"}}
+gentype	boolean-resolution	reject	{"name":"config-shape","gentypeconfig":{"moduleResolution":true}}
+gentype	string-export	reject	{"name":"config-shape","gentypeconfig":{"exportInterfaces":"true"}}
+gentype	boolean-extension	reject	{"name":"config-shape","gentypeconfig":{"generatedFileExtension":true}}
+gentype	null-shims	reject	{"name":"config-shape","gentypeconfig":{"shims":null}}
+gentype	boolean-shims	reject	{"name":"config-shape","gentypeconfig":{"shims":true}}
+gentype	boolean-shim-target	reject	{"name":"config-shape","gentypeconfig":{"shims":{"A":true}}}
+gentype	boolean-shim-entry	reject	{"name":"config-shape","gentypeconfig":{"shims":[true]}}
+gentype	malformed-shim-entry	reject	{"name":"config-shape","gentypeconfig":{"shims":["A"]}}
+gentype	null-debug	reject	{"name":"config-shape","gentypeconfig":{"debug":null}}
+gentype	boolean-debug	reject	{"name":"config-shape","gentypeconfig":{"debug":true}}
+gentype	string-debug-value	reject	{"name":"config-shape","gentypeconfig":{"debug":{"all":"true"}}}
+gentype	duplicate-module	reject	{"name":"config-shape","gentypeconfig":{"module":"esmodule","module":"commonjs"}}
+gentype	duplicate-resolution	reject	{"name":"config-shape","gentypeconfig":{"moduleResolution":"node","moduleResolution":"bundler"}}
+gentype	duplicate-export	reject	{"name":"config-shape","gentypeconfig":{"exportInterfaces":true,"exportInterfaces":false}}
+gentype	duplicate-extension	reject	{"name":"config-shape","gentypeconfig":{"generatedFileExtension":".ts","generatedFileExtension":".tsx"}}
+gentype	duplicate-shims	reject	{"name":"config-shape","gentypeconfig":{"shims":{},"shims":[]}}
+gentype	duplicate-debug	reject	{"name":"config-shape","gentypeconfig":{"debug":{},"debug":{"all":true}}}
diff --git a/rewatch-ocaml/tests/cycle/rescript.json b/rewatch-ocaml/tests/cycle/rescript.json
new file mode 100644
index 00000000000..99e2747dd0c
--- /dev/null
+++ b/rewatch-ocaml/tests/cycle/rescript.json
@@ -0,0 +1 @@
+{"name": "rewatch-ocaml-cycle", "sources": "src"}
diff --git a/rewatch-ocaml/tests/cycle/src/A.res b/rewatch-ocaml/tests/cycle/src/A.res
new file mode 100644
index 00000000000..7677fcab7b3
--- /dev/null
+++ b/rewatch-ocaml/tests/cycle/src/A.res
@@ -0,0 +1 @@
+let value = B.value
diff --git a/rewatch-ocaml/tests/cycle/src/B.res b/rewatch-ocaml/tests/cycle/src/B.res
new file mode 100644
index 00000000000..5f412bb1633
--- /dev/null
+++ b/rewatch-ocaml/tests/cycle/src/B.res
@@ -0,0 +1 @@
+let value = A.value
diff --git a/rewatch-ocaml/tests/dependency/rescript.json b/rewatch-ocaml/tests/dependency/rescript.json
new file mode 100644
index 00000000000..b2ec8455ef0
--- /dev/null
+++ b/rewatch-ocaml/tests/dependency/rescript.json
@@ -0,0 +1 @@
+{"name":"consumer","sources":"src","dependencies":["dep"]}
diff --git a/rewatch-ocaml/tests/dependency/src/Main.res b/rewatch-ocaml/tests/dependency/src/Main.res
new file mode 100644
index 00000000000..2758312616d
--- /dev/null
+++ b/rewatch-ocaml/tests/dependency/src/Main.res
@@ -0,0 +1 @@
+let value = Dep.value
diff --git a/rewatch-ocaml/tests/external-boundary/external/package.json b/rewatch-ocaml/tests/external-boundary/external/package.json
new file mode 100644
index 00000000000..fceb3afcf97
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/external/package.json
@@ -0,0 +1,4 @@
+{
+  "name": "external",
+  "bugs": "https://example.com/external/issues"
+}
diff --git a/rewatch-ocaml/tests/external-boundary/external/rescript.json b/rewatch-ocaml/tests/external-boundary/external/rescript.json
new file mode 100644
index 00000000000..f82d7866fe6
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/external/rescript.json
@@ -0,0 +1 @@
+{"name":"external","sources":"src","warnings":{"error":true},"bsc-flags":[]}
diff --git a/rewatch-ocaml/tests/external-boundary/external/src/Foo.js b/rewatch-ocaml/tests/external-boundary/external/src/Foo.js
new file mode 100644
index 00000000000..e65eaf2a3a6
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/external/src/Foo.js
@@ -0,0 +1 @@
+authored same-stem JavaScript
diff --git a/rewatch-ocaml/tests/external-boundary/external/src/Foo.res b/rewatch-ocaml/tests/external-boundary/external/src/Foo.res
new file mode 100644
index 00000000000..45603114c23
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/external/src/Foo.res
@@ -0,0 +1,4 @@
+let value = {
+  let unused = 2
+  1
+}
diff --git a/rewatch-ocaml/tests/external-boundary/external/src/Foo.resi b/rewatch-ocaml/tests/external-boundary/external/src/Foo.resi
new file mode 100644
index 00000000000..14829e3b698
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/external/src/Foo.resi
@@ -0,0 +1 @@
+let value: int
diff --git a/rewatch-ocaml/tests/external-boundary/external/src/Sentinel.js b/rewatch-ocaml/tests/external-boundary/external/src/Sentinel.js
new file mode 100644
index 00000000000..343e68677ec
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/external/src/Sentinel.js
@@ -0,0 +1 @@
+external sentinel
diff --git a/rewatch-ocaml/tests/external-boundary/project/packages/main/rescript.json b/rewatch-ocaml/tests/external-boundary/project/packages/main/rescript.json
new file mode 100644
index 00000000000..a3f51ca6c38
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/project/packages/main/rescript.json
@@ -0,0 +1 @@
+{"name":"main","sources":"src","dependencies":["external"]}
diff --git a/rewatch-ocaml/tests/external-boundary/project/packages/main/src/Main.res b/rewatch-ocaml/tests/external-boundary/project/packages/main/src/Main.res
new file mode 100644
index 00000000000..6392506b288
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/project/packages/main/src/Main.res
@@ -0,0 +1 @@
+let value = 2
diff --git a/rewatch-ocaml/tests/external-boundary/project/rescript.json b/rewatch-ocaml/tests/external-boundary/project/rescript.json
new file mode 100644
index 00000000000..b669ce45e51
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/project/rescript.json
@@ -0,0 +1,8 @@
+{
+  "name": "project",
+  "sources": "src",
+  "dependencies": ["main"],
+  "package-specs": {"module": "esmodule", "in-source": true},
+  "suffix": ".mjs",
+  "sourceMap": {"enabled": "always", "mode": "linked"}
+}
diff --git a/rewatch-ocaml/tests/external-boundary/project/src/Root.res b/rewatch-ocaml/tests/external-boundary/project/src/Root.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/external-boundary/project/src/Root.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/failure/Broken.fixed b/rewatch-ocaml/tests/failure/Broken.fixed
new file mode 100644
index 00000000000..c804125545b
--- /dev/null
+++ b/rewatch-ocaml/tests/failure/Broken.fixed
@@ -0,0 +1 @@
+let value: string = "fixed"
diff --git a/rewatch-ocaml/tests/failure/Broken.invalid b/rewatch-ocaml/tests/failure/Broken.invalid
new file mode 100644
index 00000000000..0e3ce71961b
--- /dev/null
+++ b/rewatch-ocaml/tests/failure/Broken.invalid
@@ -0,0 +1 @@
+let value: string = 42
diff --git a/rewatch-ocaml/tests/failure/rescript.json b/rewatch-ocaml/tests/failure/rescript.json
new file mode 100644
index 00000000000..e504e37e410
--- /dev/null
+++ b/rewatch-ocaml/tests/failure/rescript.json
@@ -0,0 +1 @@
+{"name": "rewatch-ocaml-failure", "sources": "src"}
diff --git a/rewatch-ocaml/tests/failure/src/Broken.res b/rewatch-ocaml/tests/failure/src/Broken.res
new file mode 100644
index 00000000000..0e3ce71961b
--- /dev/null
+++ b/rewatch-ocaml/tests/failure/src/Broken.res
@@ -0,0 +1 @@
+let value: string = 42
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/consumer/rescript.json b/rewatch-ocaml/tests/feature-dependencies/packages/consumer/rescript.json
new file mode 100644
index 00000000000..b8ced4b8038
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/consumer/rescript.json
@@ -0,0 +1,6 @@
+{
+  "name": "consumer",
+  "sources": "src",
+  "dependencies": [{"name": "dep-union", "features": ["web"]}],
+  "dev-dependencies": ["dep-union"]
+}
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/consumer/src/Consumer.res b/rewatch-ocaml/tests/feature-dependencies/packages/consumer/src/Consumer.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/consumer/src/Consumer.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/optional/EmptyOptional.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/optional/EmptyOptional.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/optional/EmptyOptional.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/rescript.json b/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/rescript.json
new file mode 100644
index 00000000000..c7506926b29
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/rescript.json
@@ -0,0 +1,7 @@
+{
+  "name": "dep-empty",
+  "sources": [
+    "src",
+    {"dir": "optional", "feature": "optional"}
+  ]
+}
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/src/EmptyCommon.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/src/EmptyCommon.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-empty/src/EmptyCommon.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/native/TransitiveNative.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/native/TransitiveNative.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/native/TransitiveNative.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/rescript.json b/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/rescript.json
new file mode 100644
index 00000000000..be07ce95140
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/rescript.json
@@ -0,0 +1,8 @@
+{
+  "name": "dep-transitive",
+  "sources": [
+    "src",
+    {"dir": "native", "feature": "native"}
+  ],
+  "features": {"bundle": ["native"]}
+}
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/src/TransitiveCommon.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/src/TransitiveCommon.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-transitive/src/TransitiveCommon.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/extra/UnionExtra.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/extra/UnionExtra.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/extra/UnionExtra.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/native/UnionNative.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/native/UnionNative.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/native/UnionNative.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/rescript.json b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/rescript.json
new file mode 100644
index 00000000000..688207c8f6a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/rescript.json
@@ -0,0 +1,9 @@
+{
+  "name": "dep-union",
+  "sources": [
+    "src",
+    {"dir": "native", "feature": "native"},
+    {"dir": "web", "feature": "web"},
+    {"dir": "extra", "feature": "extra"}
+  ]
+}
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/src/UnionCommon.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/src/UnionCommon.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/src/UnionCommon.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/web/UnionWeb.res b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/web/UnionWeb.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/packages/dep-union/web/UnionWeb.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/feature-dependencies/rescript.json b/rewatch-ocaml/tests/feature-dependencies/rescript.json
new file mode 100644
index 00000000000..d882952bcc5
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/rescript.json
@@ -0,0 +1,10 @@
+{
+  "name": "feature-root",
+  "sources": "src",
+  "dependencies": [
+    "consumer",
+    {"name": "dep-union", "features": ["native"]},
+    {"name": "dep-transitive", "features": ["bundle"]},
+    {"name": "dep-empty", "features": []}
+  ]
+}
diff --git a/rewatch-ocaml/tests/feature-dependencies/src/Root.res b/rewatch-ocaml/tests/feature-dependencies/src/Root.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/feature-dependencies/src/Root.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/features/native/Native.res b/rewatch-ocaml/tests/features/native/Native.res
new file mode 100644
index 00000000000..6392506b288
--- /dev/null
+++ b/rewatch-ocaml/tests/features/native/Native.res
@@ -0,0 +1 @@
+let value = 2
diff --git a/rewatch-ocaml/tests/features/rescript.json b/rewatch-ocaml/tests/features/rescript.json
new file mode 100644
index 00000000000..4fce9e57f52
--- /dev/null
+++ b/rewatch-ocaml/tests/features/rescript.json
@@ -0,0 +1,8 @@
+{
+  "name": "features",
+  "sources": [
+    "src",
+    {"dir": "native", "feature": "native"}
+  ],
+  "features": {"all": ["native"]}
+}
diff --git a/rewatch-ocaml/tests/features/src/Main.res b/rewatch-ocaml/tests/features/src/Main.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/features/src/Main.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/gentype/rescript.json b/rewatch-ocaml/tests/gentype/rescript.json
new file mode 100644
index 00000000000..8c870d1c0c2
--- /dev/null
+++ b/rewatch-ocaml/tests/gentype/rescript.json
@@ -0,0 +1,13 @@
+{
+  "name": "rewatch-ocaml-gentype",
+  "sources": "src",
+  "dependencies": ["dep"],
+  "package-specs": {"module": "esmodule", "in-source": true},
+  "gentypeconfig": {
+    "module": "esmodule",
+    "moduleResolution": "bundler",
+    "generatedFileExtension": ".gen.ts",
+    "exportInterfaces": true,
+    "shims": {"Date": "DateShim"}
+  }
+}
diff --git a/rewatch-ocaml/tests/gentype/src/Main.res b/rewatch-ocaml/tests/gentype/src/Main.res
new file mode 100644
index 00000000000..d292c6b3947
--- /dev/null
+++ b/rewatch-ocaml/tests/gentype/src/Main.res
@@ -0,0 +1 @@
+let greet = (name: string): string => "Hello " ++ name
diff --git a/rewatch-ocaml/tests/monorepo/packages/consumer/rescript.json b/rewatch-ocaml/tests/monorepo/packages/consumer/rescript.json
new file mode 100644
index 00000000000..b2ec8455ef0
--- /dev/null
+++ b/rewatch-ocaml/tests/monorepo/packages/consumer/rescript.json
@@ -0,0 +1 @@
+{"name":"consumer","sources":"src","dependencies":["dep"]}
diff --git a/rewatch-ocaml/tests/monorepo/packages/consumer/src/Consumer.res b/rewatch-ocaml/tests/monorepo/packages/consumer/src/Consumer.res
new file mode 100644
index 00000000000..2758312616d
--- /dev/null
+++ b/rewatch-ocaml/tests/monorepo/packages/consumer/src/Consumer.res
@@ -0,0 +1 @@
+let value = Dep.value
diff --git a/rewatch-ocaml/tests/monorepo/packages/dep/rescript.json b/rewatch-ocaml/tests/monorepo/packages/dep/rescript.json
new file mode 100644
index 00000000000..00236e1bfca
--- /dev/null
+++ b/rewatch-ocaml/tests/monorepo/packages/dep/rescript.json
@@ -0,0 +1 @@
+{"name":"dep","sources":"src"}
diff --git a/rewatch-ocaml/tests/monorepo/packages/dep/src/Dep.res b/rewatch-ocaml/tests/monorepo/packages/dep/src/Dep.res
new file mode 100644
index 00000000000..e51d91c9bc2
--- /dev/null
+++ b/rewatch-ocaml/tests/monorepo/packages/dep/src/Dep.res
@@ -0,0 +1 @@
+let value = 42
diff --git a/rewatch-ocaml/tests/monorepo/rescript.json b/rewatch-ocaml/tests/monorepo/rescript.json
new file mode 100644
index 00000000000..48f5bd3a711
--- /dev/null
+++ b/rewatch-ocaml/tests/monorepo/rescript.json
@@ -0,0 +1 @@
+{"name":"root","sources":"src","dependencies":["consumer"]}
diff --git a/rewatch-ocaml/tests/monorepo/src/Root.res b/rewatch-ocaml/tests/monorepo/src/Root.res
new file mode 100644
index 00000000000..bdeb30b21a0
--- /dev/null
+++ b/rewatch-ocaml/tests/monorepo/src/Root.res
@@ -0,0 +1 @@
+let value = Consumer.value
diff --git a/rewatch-ocaml/tests/namespace-collision/packages/namespace-one/rescript.json b/rewatch-ocaml/tests/namespace-collision/packages/namespace-one/rescript.json
new file mode 100644
index 00000000000..17c1d12bb11
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-collision/packages/namespace-one/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "namespace-one",
+  "namespace": "SharedNamespace",
+  "sources": "src"
+}
diff --git a/rewatch-ocaml/tests/namespace-collision/packages/namespace-one/src/One.res b/rewatch-ocaml/tests/namespace-collision/packages/namespace-one/src/One.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-collision/packages/namespace-one/src/One.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/namespace-collision/packages/namespace-two/rescript.json b/rewatch-ocaml/tests/namespace-collision/packages/namespace-two/rescript.json
new file mode 100644
index 00000000000..95c4d1e30da
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-collision/packages/namespace-two/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "namespace-two",
+  "namespace": "SharedNamespace",
+  "sources": "src"
+}
diff --git a/rewatch-ocaml/tests/namespace-collision/packages/namespace-two/src/Two.res b/rewatch-ocaml/tests/namespace-collision/packages/namespace-two/src/Two.res
new file mode 100644
index 00000000000..6392506b288
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-collision/packages/namespace-two/src/Two.res
@@ -0,0 +1 @@
+let value = 2
diff --git a/rewatch-ocaml/tests/namespace-collision/rescript.json b/rewatch-ocaml/tests/namespace-collision/rescript.json
new file mode 100644
index 00000000000..b732651d65d
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-collision/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "namespace-collision-root",
+  "sources": "src",
+  "dependencies": ["namespace-one", "namespace-two"]
+}
diff --git a/rewatch-ocaml/tests/namespace-collision/src/Main.res b/rewatch-ocaml/tests/namespace-collision/src/Main.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-collision/src/Main.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/namespace-entry/rescript.json b/rewatch-ocaml/tests/namespace-entry/rescript.json
new file mode 100644
index 00000000000..2a6528ddbd6
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-entry/rescript.json
@@ -0,0 +1,8 @@
+{
+  "name": "rewatch-ocaml-namespace-entry",
+  "namespace": "EntryNamespace",
+  "namespace-entry": "Entry",
+  "sources": {"dir": "src", "subdirs": true},
+  "package-specs": {"module": "esmodule", "in-source": true},
+  "suffix": ".mjs"
+}
diff --git a/rewatch-ocaml/tests/namespace-entry/src/Entry.res b/rewatch-ocaml/tests/namespace-entry/src/Entry.res
new file mode 100644
index 00000000000..c5bc8d0b2bd
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-entry/src/Entry.res
@@ -0,0 +1 @@
+module Alias = Entry_alias
diff --git a/rewatch-ocaml/tests/namespace-entry/src/Entry_alias.res b/rewatch-ocaml/tests/namespace-entry/src/Entry_alias.res
new file mode 100644
index 00000000000..f2931c8a2a0
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-entry/src/Entry_alias.res
@@ -0,0 +1,2 @@
+let message = "entry"
+Other.log()
diff --git a/rewatch-ocaml/tests/namespace-entry/src/Other.res b/rewatch-ocaml/tests/namespace-entry/src/Other.res
new file mode 100644
index 00000000000..1c04ac2baf2
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace-entry/src/Other.res
@@ -0,0 +1 @@
+let log = () => ()
diff --git a/rewatch-ocaml/tests/namespace/rescript.json b/rewatch-ocaml/tests/namespace/rescript.json
new file mode 100644
index 00000000000..bdecfeb7ec9
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "namespace",
+  "namespace": "Widget",
+  "sources": "src"
+}
diff --git a/rewatch-ocaml/tests/namespace/src/A.res b/rewatch-ocaml/tests/namespace/src/A.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace/src/A.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/namespace/src/B.res b/rewatch-ocaml/tests/namespace/src/B.res
new file mode 100644
index 00000000000..5f412bb1633
--- /dev/null
+++ b/rewatch-ocaml/tests/namespace/src/B.res
@@ -0,0 +1 @@
+let value = A.value
diff --git a/rewatch-ocaml/tests/out-of-source/rescript.json b/rewatch-ocaml/tests/out-of-source/rescript.json
new file mode 100644
index 00000000000..5f4ca1dacb7
--- /dev/null
+++ b/rewatch-ocaml/tests/out-of-source/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "out-of-source",
+  "sources": "src",
+  "package-specs": {"module": "esmodule", "in-source": false}
+}
diff --git a/rewatch-ocaml/tests/out-of-source/src/Main.res b/rewatch-ocaml/tests/out-of-source/src/Main.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/out-of-source/src/Main.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/package-output-dependency/rescript.json b/rewatch-ocaml/tests/package-output-dependency/rescript.json
new file mode 100644
index 00000000000..be147e83dc2
--- /dev/null
+++ b/rewatch-ocaml/tests/package-output-dependency/rescript.json
@@ -0,0 +1,10 @@
+{
+  "name": "package-output-consumer",
+  "sources": "src",
+  "dependencies": ["dep"],
+  "package-specs": {
+    "module": "esmodule",
+    "in-source": true,
+    "suffix": ".js"
+  }
+}
diff --git a/rewatch-ocaml/tests/package-output-dependency/src/Main.res b/rewatch-ocaml/tests/package-output-dependency/src/Main.res
new file mode 100644
index 00000000000..2758312616d
--- /dev/null
+++ b/rewatch-ocaml/tests/package-output-dependency/src/Main.res
@@ -0,0 +1 @@
+let value = Dep.value
diff --git a/rewatch-ocaml/tests/post-build/rescript.json b/rewatch-ocaml/tests/post-build/rescript.json
new file mode 100644
index 00000000000..938054ed784
--- /dev/null
+++ b/rewatch-ocaml/tests/post-build/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "post-build",
+  "sources": "src",
+  "js-post-build": {"cmd": "test -f"}
+}
diff --git a/rewatch-ocaml/tests/post-build/src/Main.res b/rewatch-ocaml/tests/post-build/src/Main.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/post-build/src/Main.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/ppx-filter/rescript.json b/rewatch-ocaml/tests/ppx-filter/rescript.json
new file mode 100644
index 00000000000..b1d194899bf
--- /dev/null
+++ b/rewatch-ocaml/tests/ppx-filter/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "rewatch-ocaml-ppx-filter",
+  "sources": "src",
+  "ppx-flags": ["missing-graphql-ppx"]
+}
diff --git a/rewatch-ocaml/tests/ppx-filter/src/Main.res b/rewatch-ocaml/tests/ppx-filter/src/Main.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/ppx-filter/src/Main.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/qualified-namespace/rescript.json b/rewatch-ocaml/tests/qualified-namespace/rescript.json
new file mode 100644
index 00000000000..c0c6663339b
--- /dev/null
+++ b/rewatch-ocaml/tests/qualified-namespace/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "rewatch-ocaml-qualified-namespace",
+  "namespace": "OL",
+  "sources": "src"
+}
diff --git a/rewatch-ocaml/tests/qualified-namespace/src/Coordinate.res b/rewatch-ocaml/tests/qualified-namespace/src/Coordinate.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/qualified-namespace/src/Coordinate.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/qualified-namespace/src/Extent.res b/rewatch-ocaml/tests/qualified-namespace/src/Extent.res
new file mode 100644
index 00000000000..6e1823b750d
--- /dev/null
+++ b/rewatch-ocaml/tests/qualified-namespace/src/Extent.res
@@ -0,0 +1,2 @@
+let coordinate = Coordinate.value
+let qualifiedCoordinate = OL.Coordinate.value
diff --git a/rewatch-ocaml/tests/qualified-namespace/src/Geometry.res b/rewatch-ocaml/tests/qualified-namespace/src/Geometry.res
new file mode 100644
index 00000000000..f824f24d373
--- /dev/null
+++ b/rewatch-ocaml/tests/qualified-namespace/src/Geometry.res
@@ -0,0 +1 @@
+let value = Extent.coordinate
diff --git a/rewatch-ocaml/tests/run.sh b/rewatch-ocaml/tests/run.sh
new file mode 100644
index 00000000000..22b908623f6
--- /dev/null
+++ b/rewatch-ocaml/tests/run.sh
@@ -0,0 +1,2218 @@
+#!/bin/sh
+set -eu
+
+port="$1"
+port_directory=$(CDPATH= cd -- "$(dirname "$port")" && pwd)
+port="$port_directory/$(basename "$port")"
+root=$(CDPATH= cd -- "$(dirname "$0")/../.." && pwd)
+: "${RESCRIPT_BSC_EXE:=$root/_build/default/compiler/bsc/rescript_compiler_main.exe}"
+: "${RESCRIPT_RUNTIME:=$root/packages/@rescript/runtime}"
+test_proxy="$root/_build/default/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.exe"
+windows_posix_shell=false
+case $(uname -s) in
+  CYGWIN*|MINGW*|MSYS*) windows_posix_shell=true ;;
+esac
+native_path() {
+  if $windows_posix_shell; then cygpath -am "$1"; else printf '%s\n' "$1"; fi
+}
+assert_no_test_proxy_processes() {
+  if $windows_posix_shell; then
+    if ! powershell.exe -NoProfile -NonInteractive -Command \
+      'if (Get-Process -Name rewatch_bsc_test_proxy -ErrorAction SilentlyContinue) { exit 1 }'; then
+      echo "rewatch compiler test proxy was left running" >&2
+      powershell.exe -NoProfile -NonInteractive -Command \
+        'Get-Process -Name rewatch_bsc_test_proxy -ErrorAction SilentlyContinue | Format-Table -AutoSize' >&2
+      exit 1
+    fi
+  elif test -n "$(pgrep -f rewatch_bsc_test_proxy || true)"; then
+    echo "rewatch compiler test proxy was left running" >&2
+    exit 1
+  fi
+}
+directory_link() {
+  link_target=$1
+  link_path=$2
+  if $windows_posix_shell; then
+    LINK_PATH=$(cygpath -aw "$link_path") TARGET_PATH=$(cygpath -aw "$link_target") \
+      powershell.exe -NoProfile -NonInteractive -Command \
+        '$ErrorActionPreference = "Stop"; $null = New-Item -ItemType Junction -Path $env:LINK_PATH -Target $env:TARGET_PATH'
+  else
+    ln -s "$link_target" "$link_path"
+  fi
+}
+file_link_if_supported() {
+  link_target=$1
+  link_path=$2
+  if $windows_posix_shell; then
+    LINK_PATH=$(cygpath -aw "$link_path") TARGET_PATH=$(cygpath -aw "$link_target") \
+      powershell.exe -NoProfile -NonInteractive -Command \
+        '$ErrorActionPreference = "Stop"; $null = New-Item -ItemType SymbolicLink -Path $env:LINK_PATH -Target $env:TARGET_PATH' \
+        >/dev/null 2>&1
+  else
+    ln -s "$link_target" "$link_path"
+  fi
+}
+if $windows_posix_shell; then
+  RESCRIPT_BSC_EXE=$(native_path "$RESCRIPT_BSC_EXE")
+  RESCRIPT_RUNTIME=$(native_path "$RESCRIPT_RUNTIME")
+  test_proxy=$(native_path "$test_proxy")
+fi
+export RESCRIPT_BSC_EXE RESCRIPT_RUNTIME
+work="$root/tmp/rewatch-ocaml/test-$$"
+mkdir -p "$work"
+if REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" REWATCH_BSC_PROXY_MODE=parse-warning \
+  "$test_proxy" -rewatch-invalid-option \
+  >"$work/proxy-invalid.out" 2>"$work/proxy-invalid.err"; then
+  echo "rewatch compiler test proxy discarded a compiler failure" >&2
+  exit 1
+fi
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/basic"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/cleanup-lifecycle"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/cleanup-failure"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/suffix-removal"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/suffix-failure"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/packaged-basic"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/runtime-discovery"
+mkdir -p "$work/no-bin-annot/src"
+printf '{"name":"no-bin-annot","sources":["src"]}\n' \
+  >"$work/no-bin-annot/rescript.json"
+printf '@@config({flags: ["-bs-no-bin-annot"]})\nlet value = 1\n' \
+  >"$work/no-bin-annot/src/NoBinAnnot.res"
+cp -R "$root/rewatch-ocaml/tests/basic" "$work/legacy-config"
+cp -R "$root/rewatch-ocaml/tests/cycle" "$work/cycle"
+cp -R "$root/rewatch-ocaml/tests/failure" "$work/failure"
+cp -R "$root/rewatch-ocaml/tests/features" "$work/features"
+cp -R "$root/rewatch-ocaml/tests/feature-dependencies" "$work/feature-dependencies"
+cp -R "$root/rewatch-ocaml/tests/gentype" "$work/gentype"
+cp -R "$root/rewatch-ocaml/tests/dependency" "$work/dependency"
+cp -R "$root/rewatch-ocaml/tests/package-output-dependency" \
+  "$work/package-output-dependency"
+mkdir -p "$work/feature-dependencies/node_modules"
+for dependency in consumer dep-union dep-transitive dep-empty; do
+  directory_link "$work/feature-dependencies/packages/$dependency" \
+    "$work/feature-dependencies/node_modules/$dependency"
+done
+unlinked_dependency="$work/unlinked-dependency"
+mkdir -p "$unlinked_dependency/src" \
+  "$unlinked_dependency/packages/dep/src" \
+  "$unlinked_dependency/packages/dep/lib/ocaml"
+printf '%s\n' \
+  '{"name":"unlinked-root","sources":"src","dependencies":["dep"]}' \
+  >"$unlinked_dependency/rescript.json"
+printf 'let value = 1\n' >"$unlinked_dependency/src/Root.res"
+printf '%s\n' '{"name":"dep","sources":"src"}' \
+  >"$unlinked_dependency/packages/dep/rescript.json"
+printf 'let value = 1\n' >"$unlinked_dependency/packages/dep/src/Dep.res"
+printf 'export const value = 1;\n' \
+  >"$unlinked_dependency/packages/dep/src/Dep.js"
+printf 'owned outside the resolved graph\n' \
+  >"$unlinked_dependency/packages/dep/lib/ocaml/marker"
+mkdir -p "$work/standalone-output/src" \
+  "$work/standalone-output/node_modules"
+cp -R "$root/rewatch-ocaml/tests/shared-dep" \
+  "$work/standalone-output/node_modules/dep"
+printf '%s\n' \
+  '{"name":"standalone-output","sources":"src","dependencies":["dep"],"package-specs":{"module":"esmodule","in-source":false,"suffix":".mjs"}}' \
+  >"$work/standalone-output/rescript.json"
+printf 'let value = Dep.value\n' >"$work/standalone-output/src/Main.res"
+mkdir -p "$work/gentype/node_modules" "$work/dependency/node_modules" \
+  "$work/package-output-dependency/node_modules"
+cp -R "$root/rewatch-ocaml/tests/shared-dep" "$work/gentype/node_modules/dep"
+cp -R "$root/rewatch-ocaml/tests/shared-dep" "$work/dependency/node_modules/dep"
+cp -R "$root/rewatch-ocaml/tests/shared-dep" \
+  "$work/package-output-dependency/node_modules/dep"
+cp -R "$root/rewatch-ocaml/tests/external-boundary" "$work/external-boundary"
+cp -R "$root/rewatch-ocaml/tests/post-build" "$work/post-build"
+if $windows_posix_shell; then
+  cp -R "$root/rewatch-ocaml/tests/post-build" "$work/quoted-post-build"
+  NODE_EXE=$(native_path "$(command -v node)") \
+    CONFIG_PATH=$(native_path "$work/quoted-post-build/rescript.json") \
+    node - <<'NODE'
+const fs = require("fs");
+const config = {
+  name: "quoted-post-build",
+  sources: "src",
+  "js-post-build": {cmd: `"${process.env.NODE_EXE}" --version`},
+};
+fs.writeFileSync(process.env.CONFIG_PATH, JSON.stringify(config));
+NODE
+fi
+cp -R "$root/rewatch-ocaml/tests/out-of-source" "$work/out-of-source"
+cp -R "$root/rewatch-ocaml/tests/ppx-filter" "$work/ppx-filter"
+cp -R "$root/rewatch-ocaml/tests/namespace" "$work/namespace"
+cp -R "$root/rewatch-ocaml/tests/namespace-entry" "$work/namespace-entry"
+cp -R "$root/rewatch-ocaml/tests/qualified-namespace" \
+  "$work/qualified-namespace"
+cp -R "$root/rewatch-ocaml/tests/namespace-collision" \
+  "$work/namespace-collision"
+mkdir -p "$work/namespace-collision/node_modules"
+for dependency in namespace-one namespace-two; do
+  directory_link "$work/namespace-collision/packages/$dependency" \
+    "$work/namespace-collision/node_modules/$dependency"
+done
+cp -R "$root/rewatch-ocaml/tests/source-map" "$work/source-map"
+cp -R "$root/rewatch-ocaml/tests/warning-replay" "$work/warning-replay"
+cp -R "$root/rewatch-ocaml/tests/monorepo" "$work/monorepo"
+basic="$work/basic"
+cleanup_lifecycle="$work/cleanup-lifecycle"
+cleanup_failure="$work/cleanup-failure"
+suffix_removal="$work/suffix-removal"
+suffix_failure="$work/suffix-failure"
+packaged_basic="$work/packaged-basic"
+runtime_discovery="$work/runtime-discovery"
+no_bin_annot="$work/no-bin-annot"
+legacy_config="$work/legacy-config"
+cycle="$work/cycle"
+failure="$work/failure"
+features="$work/features"
+feature_dependencies="$work/feature-dependencies"
+gentype="$work/gentype"
+dependency="$work/dependency"
+package_output_dependency="$work/package-output-dependency"
+standalone_output="$work/standalone-output"
+external_boundary="$work/external-boundary"
+post_build="$work/post-build"
+out_of_source="$work/out-of-source"
+ppx_filter="$work/ppx-filter"
+namespace="$work/namespace"
+namespace_entry="$work/namespace-entry"
+qualified_namespace="$work/qualified-namespace"
+namespace_collision="$work/namespace-collision"
+source_map="$work/source-map"
+warning_replay="$work/warning-replay"
+monorepo="$work/monorepo"
+
+package_name_mismatch="$work/package-name-mismatch"
+mkdir -p "$package_name_mismatch/src" \
+  "$package_name_mismatch/packages/dep/src" \
+  "$package_name_mismatch/node_modules"
+printf '%s\n' \
+  '{"name":"root","sources":"src","dependencies":["dep"]}' \
+  >"$package_name_mismatch/rescript.json"
+printf '%s\n' '{"name":"root"}' >"$package_name_mismatch/package.json"
+printf 'let value: int = Dep.value\n' \
+  >"$package_name_mismatch/src/Main.res"
+printf '%s\n' '{"name":"old-name","sources":"src"}' \
+  >"$package_name_mismatch/packages/dep/rescript.json"
+printf '%s\n' '{"name":"dep"}' \
+  >"$package_name_mismatch/packages/dep/package.json"
+printf 'let value = 1\n' \
+  >"$package_name_mismatch/packages/dep/src/Dep.res"
+config_cleanup_invalidation="$work/config-cleanup-invalidation"
+cp -R "$package_name_mismatch" "$config_cleanup_invalidation"
+directory_link "$package_name_mismatch/packages/dep" \
+  "$package_name_mismatch/node_modules/dep"
+directory_link "$config_cleanup_invalidation/packages/dep" \
+  "$config_cleanup_invalidation/node_modules/dep"
+printf '%s\n' '{"name":"dep","sources":"src"}' \
+  >"$config_cleanup_invalidation/packages/dep/rescript.json"
+
+transitive_local_watch="$work/transitive-local-watch"
+mkdir -p "$transitive_local_watch/src" \
+  "$transitive_local_watch/node_modules/external/src" \
+  "$transitive_local_watch/node_modules/external/node_modules" \
+  "$transitive_local_watch/packages/local/src"
+printf '%s\n' '{"workspaces":["packages/*"]}' \
+  >"$transitive_local_watch/package.json"
+printf '%s\n' \
+  '{"name":"transitive-local-watch","sources":"src","dependencies":["external"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$transitive_local_watch/rescript.json"
+printf 'let value = External.value\n' \
+  >"$transitive_local_watch/src/Main.res"
+printf '%s\n' \
+  '{"name":"external","sources":"src","dependencies":["local"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$transitive_local_watch/node_modules/external/rescript.json"
+printf 'let value = Local.value\n' \
+  >"$transitive_local_watch/node_modules/external/src/External.res"
+printf '%s\n' \
+  '{"name":"local","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$transitive_local_watch/packages/local/rescript.json"
+printf 'let value = 1\n' \
+  >"$transitive_local_watch/packages/local/src/Local.res"
+directory_link "$transitive_local_watch/packages/local" \
+  "$transitive_local_watch/node_modules/external/node_modules/local"
+
+namespace_invalidation="$work/namespace-invalidation"
+mkdir -p "$namespace_invalidation/src" \
+  "$namespace_invalidation/packages/dep/src" \
+  "$namespace_invalidation/node_modules"
+printf '%s\n' \
+  '{"name":"namespace-consumer","sources":"src","dependencies":["namespace-dep"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$namespace_invalidation/rescript.json"
+printf 'let value = Ns.A.value\n' >"$namespace_invalidation/src/Main.res"
+printf '%s\n' \
+  '{"name":"namespace-dep","namespace":"Ns","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$namespace_invalidation/packages/dep/rescript.json"
+printf 'let value = 1\n' \
+  >"$namespace_invalidation/packages/dep/src/A.res"
+printf 'let value = 2\n' \
+  >"$namespace_invalidation/packages/dep/src/B.res"
+namespace_restart="$work/namespace-restart"
+cp -R "$namespace_invalidation" "$namespace_restart"
+printf 'let value: int = Ns.A.value\n' >"$namespace_restart/src/Main.res"
+
+namespace_repair="$work/namespace-repair"
+cp -R "$namespace_invalidation" "$namespace_repair"
+printf 'let other = 1\n' >"$namespace_repair/src/Other.res"
+for project in "$namespace_invalidation" "$namespace_restart" \
+  "$namespace_repair"; do
+  directory_link "$project/packages/dep" \
+    "$project/node_modules/namespace-dep"
+done
+
+interface_failure_recovery="$work/interface-failure-recovery"
+mkdir -p "$interface_failure_recovery/src"
+printf '%s\n' \
+  '{"name":"interface-failure-recovery","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$interface_failure_recovery/rescript.json"
+printf 'let value: int\n' >"$interface_failure_recovery/src/A.resi"
+printf 'let value = 1\n' >"$interface_failure_recovery/src/A.res"
+printf 'let dependent = A.value + 1\n' \
+  >"$interface_failure_recovery/src/B.res"
+printf 'let valid = true\n' \
+  >"$interface_failure_recovery/src/Broken.res"
+
+atomic_save="$work/atomic-save"
+mkdir -p "$atomic_save/src"
+printf '%s\n' \
+  '{"name":"atomic-save","sources":"src/","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$atomic_save/rescript.json"
+printf 'let value = 1\n' >"$atomic_save/src/Main.res"
+
+missing_nested_source="$work/missing-nested-source"
+mkdir -p "$missing_nested_source"
+printf '%s\n' \
+  '{"name":"missing-nested-source","sources":"generated/./nested","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$missing_nested_source/rescript.json"
+
+directory_symlink="$work/directory-symlink"
+mkdir -p "$directory_symlink/shared"
+printf '%s\n' \
+  '{"name":"directory-symlink","sources":[{"dir":"src","subdirs":true}],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$directory_symlink/rescript.json"
+printf 'let value = 1\n' >"$directory_symlink/shared/Linked.res"
+directory_link "$directory_symlink/shared" "$directory_symlink/src"
+
+recursive_lib="$work/recursive-lib"
+mkdir -p "$recursive_lib/src/lib"
+printf '%s\n' \
+  '{"name":"recursive-lib","sources":[{"dir":"src","subdirs":true}],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$recursive_lib/rescript.json"
+printf 'let value = 1\n' >"$recursive_lib/src/lib/Nested.res"
+
+filtered_dependency="$work/filtered-dependency"
+mkdir -p "$filtered_dependency/src" \
+  "$filtered_dependency/packages/dep/src" \
+  "$filtered_dependency/node_modules"
+printf '%s\n' \
+  '{"name":"filtered-root","sources":"src","dependencies":["dep"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$filtered_dependency/rescript.json"
+printf 'let value = Helper.value\n' >"$filtered_dependency/src/Main.res"
+printf 'let ignored = 1\n' >"$filtered_dependency/src/Ignored.res"
+printf '%s\n' \
+  '{"name":"dep","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$filtered_dependency/packages/dep/rescript.json"
+printf 'let value = 1\n' \
+  >"$filtered_dependency/packages/dep/src/Helper.res"
+directory_link "$filtered_dependency/packages/dep" \
+  "$filtered_dependency/node_modules/dep"
+
+symlink_source="$work/symlink-source"
+mkdir -p "$symlink_source/src" "$symlink_source/shared"
+printf '%s\n' \
+  '{"name":"symlink-source","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$symlink_source/rescript.json"
+printf 'let value = 1\n' >"$symlink_source/shared/Source.js"
+file_symlinks_supported=true
+if ! file_link_if_supported "$symlink_source/shared/Source.js" \
+    "$symlink_source/src/Linked.res"; then
+  file_symlinks_supported=false
+fi
+
+symlink_alias="$work/symlink-alias"
+mkdir -p "$symlink_alias/src"
+printf '%s\n' \
+  '{"name":"symlink-alias","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$symlink_alias/rescript.json"
+printf 'let value = 1\n' >"$symlink_alias/src/Original.res"
+if $file_symlinks_supported; then
+  file_link_if_supported "$symlink_alias/src/Original.res" \
+    "$symlink_alias/src/Linked.res"
+fi
+
+multiple_cycles="$work/multiple-cycles"
+mkdir -p "$multiple_cycles/src"
+printf '%s\n' \
+  '{"name":"multiple-cycles","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$multiple_cycles/rescript.json"
+printf 'let value = B.value\n' >"$multiple_cycles/src/A.res"
+printf 'let value = A.value\n' >"$multiple_cycles/src/B.res"
+printf 'let value = D.value\n' >"$multiple_cycles/src/C.res"
+printf 'let value = C.value\n' >"$multiple_cycles/src/D.res"
+printf 'let value = 1\n' >"$multiple_cycles/src/Valid.res"
+
+post_build_cmi="$work/post-build-cmi"
+mkdir -p "$post_build_cmi/src"
+printf '%s\n' \
+  '{"name":"post-build-cmi","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"},"js-post-build":{"cmd":"node -e \"process.exit(require('\''fs'\'').existsSync('\''allow-post-build'\'') ? 0 : 7)\""}}' \
+  >"$post_build_cmi/rescript.json"
+printf 'let value = 1\n' >"$post_build_cmi/src/A.res"
+printf 'let dependent = A.value + 1\n' >"$post_build_cmi/src/B.res"
+touch "$post_build_cmi/allow-post-build"
+
+post_build_retry="$work/post-build-retry"
+mkdir -p "$post_build_retry/src"
+printf '%s\n' \
+  '{"name":"post-build-retry","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"},"js-post-build":{"cmd":"node -e \"process.exit(require('\''fs'\'').existsSync('\''allow-post-build'\'') ? 0 : 7)\""}}' \
+  >"$post_build_retry/rescript.json"
+printf 'let value = 1\n' >"$post_build_retry/src/A.res"
+touch "$post_build_retry/allow-post-build"
+
+parse_publication="$work/parse-publication"
+mkdir -p "$parse_publication/src"
+printf '%s\n' \
+  '{"name":"parse-publication","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$parse_publication/rescript.json"
+printf 'let value = 1\n' >"$parse_publication/src/A.res"
+printf 'let other = 1\n' >"$parse_publication/src/B.res"
+
+multi_package_pending="$work/multi-package-pending"
+mkdir -p "$multi_package_pending/src" \
+  "$multi_package_pending/packages/dep1/src" \
+  "$multi_package_pending/packages/dep2/src" \
+  "$multi_package_pending/node_modules"
+printf '%s\n' \
+  '{"name":"multi-package-pending","sources":"src","dependencies":["dep1","dep2"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$multi_package_pending/rescript.json"
+printf 'let value = A.value + B.value\n' \
+  >"$multi_package_pending/src/Main.res"
+printf 'let other = 1\n' >"$multi_package_pending/src/Other.res"
+printf '%s\n' \
+  '{"name":"dep1","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$multi_package_pending/packages/dep1/rescript.json"
+printf 'let value = 1\n' \
+  >"$multi_package_pending/packages/dep1/src/A.res"
+printf '%s\n' \
+  '{"name":"dep2","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$multi_package_pending/packages/dep2/rescript.json"
+printf 'let value = 1\n' \
+  >"$multi_package_pending/packages/dep2/src/B.res"
+directory_link "$multi_package_pending/packages/dep1" \
+  "$multi_package_pending/node_modules/dep1"
+directory_link "$multi_package_pending/packages/dep2" \
+  "$multi_package_pending/node_modules/dep2"
+
+full_watch_recovery="$work/full-watch-recovery"
+mkdir -p "$full_watch_recovery/src"
+printf '%s\n' \
+  '{"name":"full-watch-recovery","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$full_watch_recovery/rescript.json"
+printf 'let value = B.value\n' >"$full_watch_recovery/src/A.res"
+printf 'let value = 1\n' >"$full_watch_recovery/src/B.res"
+printf 'let value = 1\n' >"$full_watch_recovery/src/C.res"
+
+deletion_retry="$work/deletion-retry"
+cp -R "$full_watch_recovery" "$deletion_retry"
+
+moved_source="$work/moved-source"
+mkdir -p "$moved_source/src/nested"
+printf '%s\n' \
+  '{"name":"moved-source","sources":{"dir":"src","subdirs":true},"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$moved_source/rescript.json"
+printf 'let value = 1\n' >"$moved_source/src/A.res"
+
+publication_cmi="$work/publication-cmi"
+mkdir -p "$publication_cmi/src"
+printf '%s\n' \
+  '{"name":"publication-cmi","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$publication_cmi/rescript.json"
+printf 'let value = 1\n' >"$publication_cmi/src/A.res"
+printf 'let dependent = A.value + 1\n' >"$publication_cmi/src/B.res"
+
+retained_cycle="$work/retained-cycle"
+mkdir -p "$retained_cycle/src"
+printf '%s\n' \
+  '{"name":"retained-cycle","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$retained_cycle/rescript.json"
+printf 'let value = 1\n' >"$retained_cycle/src/A.res"
+printf 'let dependent = A.value\n' >"$retained_cycle/src/B.res"
+
+retained_parse="$work/retained-parse"
+mkdir -p "$retained_parse/src"
+printf '%s\n' \
+  '{"name":"retained-parse","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$retained_parse/rescript.json"
+printf 'let value = 1\n' >"$retained_parse/src/A.res"
+printf 'let other = 1\n' >"$retained_parse/src/B.res"
+
+initial_failure_freshness="$work/initial-failure-freshness"
+mkdir -p "$initial_failure_freshness/src"
+printf '%s\n' \
+  '{"name":"initial-failure-freshness","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$initial_failure_freshness/rescript.json"
+printf 'let value = 1\n' >"$initial_failure_freshness/src/A.res"
+printf 'let other = 1\n' >"$initial_failure_freshness/src/B.res"
+
+duplicate_selection="$work/duplicate-selection"
+mkdir -p "$duplicate_selection/src" \
+  "$duplicate_selection/node_modules" \
+  "$duplicate_selection/packages/a/src" \
+  "$duplicate_selection/packages/a/node_modules" \
+  "$duplicate_selection/packages/shared/src" \
+  "$duplicate_selection/packages/nested-shared/src"
+printf '%s\n' \
+  '{"name":"duplicate-root","sources":"src","dependencies":["a","shared"],"package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$duplicate_selection/rescript.json"
+printf 'let value = Shared.value\n' >"$duplicate_selection/src/Main.res"
+printf '%s\n' \
+  '{"name":"a","sources":"src","dependencies":["shared"]}' \
+  >"$duplicate_selection/packages/a/rescript.json"
+printf 'let value = 1\n' >"$duplicate_selection/packages/a/src/A.res"
+printf '%s\n' \
+  '{"name":"shared","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$duplicate_selection/packages/shared/rescript.json"
+printf 'let value = 1\n' \
+  >"$duplicate_selection/packages/shared/src/Shared.res"
+printf '%s\n' \
+  '{"name":"shared","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".mjs"}}' \
+  >"$duplicate_selection/packages/nested-shared/rescript.json"
+printf 'let value = 2\n' \
+  >"$duplicate_selection/packages/nested-shared/src/Shared.res"
+directory_link "$duplicate_selection/packages/a" \
+  "$duplicate_selection/node_modules/a"
+directory_link "$duplicate_selection/packages/shared" \
+  "$duplicate_selection/node_modules/shared"
+directory_link "$duplicate_selection/packages/nested-shared" \
+  "$duplicate_selection/packages/a/node_modules/shared"
+
+external_duplicate_base="$work/external-duplicate"
+external_duplicate="$external_duplicate_base/project"
+mkdir -p "$external_duplicate/src" \
+  "$external_duplicate/node_modules/a/src" \
+  "$external_duplicate/node_modules/a/node_modules/shared/src" \
+  "$external_duplicate_base/node_modules/shared/src"
+printf '%s\n' \
+  '{"name":"external-duplicate-root","sources":"src","dependencies":["a","shared"],"package-specs":{"module":"esmodule","in-source":true}}' \
+  >"$external_duplicate/rescript.json"
+printf 'let value = Shared.value\n' >"$external_duplicate/src/Main.res"
+printf '%s\n' '{"name":"a","sources":"src","dependencies":["shared"]}' \
+  >"$external_duplicate/node_modules/a/rescript.json"
+printf 'let value = 1\n' >"$external_duplicate/node_modules/a/src/A.res"
+printf '%s\n' '{"name":"shared","sources":"src"}' \
+  >"$external_duplicate/node_modules/a/node_modules/shared/rescript.json"
+printf 'let value = 1\n' \
+  >"$external_duplicate/node_modules/a/node_modules/shared/src/Shared.res"
+printf '%s\n' '{"name":"shared","sources":"src"}' \
+  >"$external_duplicate_base/node_modules/shared/rescript.json"
+printf 'let value = 2\n' \
+  >"$external_duplicate_base/node_modules/shared/src/Shared.res"
+
+external_diagnostics_base="$work/external-diagnostics"
+external_cycle="$external_diagnostics_base/project"
+external_cycle_dependency="$external_diagnostics_base/store/cycle-dep"
+mkdir -p "$external_cycle/src" "$external_cycle/node_modules" \
+  "$external_cycle_dependency/src" "$external_cycle_dependency/node_modules"
+printf '%s\n' \
+  '{"name":"external-cycle-root","sources":"src","dependencies":["cycle-dep"]}' \
+  >"$external_cycle/rescript.json"
+printf 'let value = CycleDep.value\n' >"$external_cycle/src/Root.res"
+printf '%s\n' \
+  '{"name":"cycle-dep","sources":"src","dependencies":["external-cycle-root"]}' \
+  >"$external_cycle_dependency/rescript.json"
+printf 'let value = Root.value\n' >"$external_cycle_dependency/src/CycleDep.res"
+directory_link "$external_cycle_dependency" \
+  "$external_cycle/node_modules/cycle-dep"
+directory_link "$external_cycle" \
+  "$external_cycle_dependency/node_modules/external-cycle-root"
+
+external_namespace="$external_diagnostics_base/namespace-project"
+mkdir -p "$external_namespace/src" "$external_namespace/node_modules" \
+  "$external_diagnostics_base/store/ns-one/src" \
+  "$external_diagnostics_base/store/ns-two/src"
+printf '%s\n' \
+  '{"name":"external-namespace-root","sources":"src","dependencies":["ns-one","ns-two"]}' \
+  >"$external_namespace/rescript.json"
+printf 'let value = 1\n' >"$external_namespace/src/Main.res"
+for namespace_dependency_name in ns-one ns-two; do
+  printf '%s\n' \
+    "{\"name\":\"$namespace_dependency_name\",\"namespace\":\"SharedNs\",\"sources\":\"src\"}" \
+    >"$external_diagnostics_base/store/$namespace_dependency_name/rescript.json"
+  printf 'let value = 1\n' \
+    >"$external_diagnostics_base/store/$namespace_dependency_name/src/Value.res"
+  directory_link "$external_diagnostics_base/store/$namespace_dependency_name" \
+    "$external_namespace/node_modules/$namespace_dependency_name"
+done
+
+dev_include_order="$work/dev-include-order"
+mkdir -p "$dev_include_order/src" "$dev_include_order/dev" \
+  "$dev_include_order/node_modules/regular/src" \
+  "$dev_include_order/node_modules/development/src"
+printf '%s\n' \
+  '{"name":"dev-include-root","sources":["src",{"dir":"dev","type":"dev"}],"dependencies":["regular"],"dev-dependencies":["development"]}' \
+  >"$dev_include_order/rescript.json"
+printf 'let value = 1\n' >"$dev_include_order/src/Main.res"
+printf 'let value = 2\n' >"$dev_include_order/dev/Test.res"
+printf '%s\n' '{"name":"regular","sources":"src"}' \
+  >"$dev_include_order/node_modules/regular/rescript.json"
+printf 'let value = 1\n' \
+  >"$dev_include_order/node_modules/regular/src/Regular.res"
+printf '%s\n' '{"name":"development","sources":"src"}' \
+  >"$dev_include_order/node_modules/development/rescript.json"
+printf 'let value = 1\n' \
+  >"$dev_include_order/node_modules/development/src/Development.res"
+
+if [ -x "$port_directory/bsc.exe" ]; then
+  env -u RESCRIPT_BSC_EXE "$port" build "$packaged_basic" \
+    >"$packaged_basic/build.log"
+  test -f "$packaged_basic/src/A.mjs"
+fi
+
+mkdir -p "$runtime_discovery/node_modules/@rescript/runtime"
+runtime_path=$(CDPATH= cd -- \
+  "$runtime_discovery/node_modules/@rescript/runtime" && pwd)
+runtime_args=$(env -u RESCRIPT_RUNTIME \
+  "$port" compiler-args "$runtime_discovery/src/A.res")
+RUNTIME_PATH=$(native_path "$runtime_path") node -e '
+  let input = "";
+  process.stdin.setEncoding("utf8");
+  process.stdin.on("data", chunk => input += chunk);
+  process.stdin.on("end", () => {
+    const args = JSON.parse(input).compiler_args;
+    const index = args.indexOf("-runtime-path");
+    const normalize = value => value.replaceAll("\\", "/").toLowerCase();
+    if (index < 0 || normalize(args[index + 1]) !== normalize(process.env.RUNTIME_PATH)) {
+      process.exit(1);
+    }
+  });
+' <<EOF
+$runtime_args
+EOF
+
+missing_project="$work/does-not-exist"
+if "$port" build "$missing_project" >"$work/missing-project.log" 2>&1; then
+  echo "build unexpectedly accepted a missing project folder" >&2
+  exit 1
+fi
+missing_project_native=$(native_path "$missing_project")
+tr '\\' '/' <"$work/missing-project.log" | grep -F \
+  "Could not start Rescript build: Could not write lockfile because the specified project folder does not exist: $missing_project_native" \
+  >/dev/null
+
+compiler_args_json=$("$port" compiler-args "$basic/src/A.res")
+printf '%s\n' "$compiler_args_json" | grep '"compiler_args"' >/dev/null
+printf '%s\n' "$compiler_args_json" | node -e '
+  const path = require("path");
+  let input = "";
+  process.stdin.on("data", chunk => input += chunk);
+  process.stdin.on("end", () => {
+    const args = JSON.parse(input).parser_args;
+    if (args.at(-1) !== path.join("..", "..", "src", "A.res")) process.exit(1);
+  });
+'
+sed 's/"suffix": "\.mjs"/"suffix": "\.mjs", "bsc-flags": ["-w -9"]/' "$basic/rescript.json" > "$basic/rescript.next"
+mv "$basic/rescript.next" "$basic/rescript.json"
+sed 's/"module": "esmodule"/"module": "es6"/' "$basic/rescript.json" > "$basic/rescript.next"
+mv "$basic/rescript.next" "$basic/rescript.json"
+"$port" compiler-args "$basic/src/A.res" | grep '"-9"' >/dev/null
+gentype_compiler_args=$("$port" compiler-args "$gentype/src/Main.res")
+printf '%s\n' "$gentype_compiler_args" | grep '"-bs-gentype-generated-extension"' >/dev/null
+printf '%s\n' "$gentype_compiler_args" | grep '"-bs-gentype-bsb-project-root"' >/dev/null
+printf '%s\n' "$gentype_compiler_args" | node -e '
+  let input = "";
+  process.stdin.on("data", chunk => input += chunk);
+  process.stdin.on("end", () => {
+    const args = JSON.parse(input).compiler_args;
+    const runtime = args.indexOf("-runtime-path");
+    const dependencyInclude = args.indexOf("-I", 2);
+    if (runtime < 0 || (dependencyInclude >= 0 && runtime > dependencyInclude)) {
+      process.exit(1);
+    }
+  });
+'
+if printf '%s\n' "$gentype_compiler_args" | grep -E '"-bs-gentype-(dep-path|source-dir)"' >/dev/null; then
+  echo "compiler-args unexpectedly included full-build GenType paths" >&2
+  exit 1
+fi
+
+cleanup() {
+  for pid in $background_pids; do
+    kill -TERM "$pid" 2>/dev/null || true
+  done
+  for pid in $background_pids; do
+    wait "$pid" 2>/dev/null || true
+  done
+  if [ "${REWATCH_KEEP_TEST_WORK:-}" = "1" ]; then
+    echo "Preserving focused test workspace: $work" >&2
+  else
+    rm -rf "$work"
+  fi
+}
+background_pids=""
+trap cleanup EXIT
+
+wait_for_file() {
+  file="$1"
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    if [ -f "$file" ]; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+obstruct_file_with_directory() {
+  path=$1
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    rm -f "$path"
+    if mkdir "$path" 2>/dev/null; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.01
+  done
+  return 1
+}
+
+wait_for_text() {
+  file="$1"
+  pattern="$2"
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    if grep -Fq "$pattern" "$file" 2>/dev/null; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+wait_for_initial_build() {
+  log="$1"
+  if ! wait_for_text "$log" 'Finished initial compilation'; then
+    cat "$log" >&2
+    echo "watcher did not finish its initial build" >&2
+    return 1
+  fi
+}
+
+wait_for_any_text() {
+  file="$1"
+  shift
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    for pattern in "$@"; do
+      if grep -Fq "$pattern" "$file" 2>/dev/null; then
+        return 0
+      fi
+    done
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+wait_for_count() {
+  file="$1"
+  pattern="$2"
+  expected="$3"
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    count=$(grep -c -- "$pattern" "$file" 2>/dev/null) || count=0
+    if [ "$count" -ge "$expected" ]; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+wait_for_file_gone() {
+  file="$1"
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    if [ ! -f "$file" ]; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+wait_for_pid_gone() {
+  pid="$1"
+  attempts=0
+  while [ "$attempts" -lt 50 ]; do
+    if ! kill -0 "$pid" 2>/dev/null; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.1
+  done
+  return 1
+}
+
+remove_file_with_retry() {
+  file=$1
+  attempts=0
+  while [ "$attempts" -lt 200 ]; do
+    if rm -f "$file" 2>/dev/null; then
+      return 0
+    fi
+    attempts=$((attempts + 1))
+    sleep 0.01
+  done
+  echo "could not remove $file" >&2
+  return 1
+}
+
+stop_watch() {
+  project=$1
+  pid=$2
+  if $windows_posix_shell; then
+    remove_file_with_retry "$project/lib/watch.lock"
+  else
+    kill -TERM "$pid"
+  fi
+  wait "$pid"
+}
+
+remove_obstruction_directory() {
+  directory="$1"
+  if ! rmdir "$directory" 2>/dev/null && [ -e "$directory" ]; then
+    echo "could not remove publication obstruction $directory" >&2
+    return 1
+  fi
+}
+
+printf 'let formatted=1\n' | "$port" format --stdin .res | grep 'let formatted = 1' >/dev/null
+if printf 'let =\n' | "$port" format --stdin .res \
+  >"$work/format-invalid.out" 2>"$work/format-invalid.err"; then
+  echo "format stdin unexpectedly accepted invalid syntax" >&2
+  exit 1
+fi
+grep -F "Error formatting stdin:" "$work/format-invalid.err" >/dev/null
+
+format_stdin_tmp="$work/format-stdin-tmp"
+format_stdin_fifo="$work/format-stdin.fifo"
+# MSYS kill forcibly terminates native processes instead of delivering a
+# Windows console control event. Keep the real signal assertions on Unix;
+# Windows exercises the same cleanup path through watch.lock removal below.
+if ! $windows_posix_shell; then
+  mkdir -p "$format_stdin_tmp"
+  mkfifo "$format_stdin_fifo"
+  exec 9<>"$format_stdin_fifo"
+  TMPDIR="$format_stdin_tmp" "$port" format --stdin .res \
+    <"$format_stdin_fifo" >"$work/format-signal.out" \
+    2>"$work/format-signal.err" &
+  format_stdin_pid=$!
+  background_pids="$background_pids $format_stdin_pid"
+  attempts=0
+  while [ "$attempts" -lt 100 ] && \
+    [ -z "$(find "$format_stdin_tmp" -type f -print -quit)" ]; do
+    attempts=$((attempts + 1))
+    sleep 0.05
+  done
+  if [ "$attempts" -eq 100 ]; then
+    echo "stdin formatter did not start reading" >&2
+    exit 1
+  fi
+  kill -TERM "$format_stdin_pid"
+  if ! wait_for_pid_gone "$format_stdin_pid"; then
+    echo "stdin formatter did not respond to termination" >&2
+    exit 1
+  fi
+  wait "$format_stdin_pid" 2>/dev/null || true
+  exec 9>&-
+  if find "$format_stdin_tmp" -type f -print -quit | grep . >/dev/null; then
+    echo "stdin formatter left a temporary file after termination" >&2
+    exit 1
+  fi
+fi
+
+printf 'let unformatted=1\n' >"$work/unformatted.res"
+if "$port" format --check "$work/unformatted.res" \
+  >"$work/format-check.out" 2>"$work/format-check.err"; then
+  echo "format check unexpectedly accepted an unformatted file" >&2
+  exit 1
+fi
+unformatted_native=$(native_path "$work/unformatted.res")
+tr '\\' '/' <"$work/format-check.err" | \
+  grep -F "[format check] $unformatted_native" >/dev/null
+grep -F "The file listed above needs formatting" \
+  "$work/format-check.err" >/dev/null
+grep -F "Formatting check failed" "$work/format-check.err" >/dev/null
+
+"$port" build "$no_bin_annot" >/dev/null
+test -f "$no_bin_annot/lib/bs/src/NoBinAnnot.cmi"
+test -f "$no_bin_annot/lib/bs/src/NoBinAnnot.cmj"
+test -f "$no_bin_annot/src/NoBinAnnot.js"
+test ! -e "$no_bin_annot/lib/bs/src/NoBinAnnot.cmt"
+test ! -e "$no_bin_annot/lib/ocaml/NoBinAnnot.cmt"
+
+if (cd "$basic/src" && "$port" format --check) \
+  >"$work/format-nested.out" 2>"$work/format-nested.err"; then
+  echo "format unexpectedly searched above the current directory" >&2
+  exit 1
+fi
+basic_src_native=$(native_path "$basic/src")
+tr '\\' '/' <"$work/format-nested.err" | \
+  grep -F "Could not read rescript.json at $basic_src_native" >/dev/null
+
+rm -rf "$basic/lib" "$cycle/lib" "$failure/lib"
+rm -rf "$legacy_config/lib"
+mv "$legacy_config/rescript.json" "$legacy_config/bsconfig.json"
+rm -rf "$features/lib"
+rm -rf "$gentype/lib"
+rm -rf "$gentype/node_modules/dep/lib"
+rm -rf "$dependency/lib" "$dependency/node_modules/dep/lib"
+sed 's/"dependencies"/"bs-dependencies"/' "$dependency/rescript.json" > "$dependency/rescript.next"
+mv "$dependency/rescript.next" "$dependency/rescript.json"
+sed 's/}$/,"suffix":".mjs"}/' "$dependency/rescript.json" > "$dependency/rescript.next"
+mv "$dependency/rescript.next" "$dependency/rescript.json"
+sed 's/}$/,"suffix":".cjs"}/' "$dependency/node_modules/dep/rescript.json" > "$dependency/node_modules/dep/rescript.next"
+mv "$dependency/node_modules/dep/rescript.next" "$dependency/node_modules/dep/rescript.json"
+rm -rf "$post_build/lib"
+rm -rf "$ppx_filter/lib"
+rm -rf "$out_of_source/lib"
+rm -rf "$namespace/lib"
+rm -rf "$namespace_entry/lib"
+rm -rf "$source_map/lib"
+mkdir -p "$monorepo/node_modules"
+directory_link "$monorepo/packages/consumer" "$monorepo/node_modules/consumer"
+directory_link "$monorepo/packages/dep" "$monorepo/node_modules/dep"
+
+# A command run from a listed workspace package may resolve sibling packages,
+# but those siblings are ordinary dependencies for this invocation. In
+# particular, their development-only dependency graph must remain dormant and
+# their sources must not be added to the invoking package's analysis metadata.
+cp "$monorepo/packages/dep/rescript.json" \
+  "$monorepo/packages/dep/rescript.original"
+printf '%s\n' \
+  '{"name":"dep","sources":"src","dev-dependencies":["missing-dev"]}' \
+  >"$monorepo/packages/dep/rescript.json"
+"$port" build "$monorepo/packages/consumer"
+test -f "$monorepo/packages/consumer/src/Consumer.js"
+test -f "$monorepo/packages/dep/src/Dep.js"
+test ! -f "$monorepo/packages/dep/lib/bs/.sourcedirs.json"
+node - "$monorepo/packages/consumer/lib/bs/.sourcedirs.json" \
+  "$monorepo/packages/dep" <<'NODE'
+const fs = require("fs");
+const [sourceDirsPath, dependencyPath] = process.argv.slice(2);
+const sourceDirs = JSON.parse(fs.readFileSync(sourceDirsPath, "utf8"));
+const normalize = value => value.replaceAll("\\", "/");
+if (JSON.stringify(sourceDirs.dirs) !== JSON.stringify(["src"])) {
+  throw new Error(`unexpected direct-package source dirs: ${JSON.stringify(sourceDirs.dirs)}`);
+}
+if (sourceDirs.cmt_scan.length !== 1 || normalize(sourceDirs.cmt_scan[0].build_root) !== "lib/bs") {
+  throw new Error(`unexpected direct-package scan plan: ${JSON.stringify(sourceDirs.cmt_scan)}`);
+}
+const packages = new Map(sourceDirs.pkgs);
+if (normalize(packages.get("dep")) !== normalize(dependencyPath)) {
+  throw new Error(`missing absolute sibling dependency path: ${JSON.stringify(sourceDirs.pkgs)}`);
+}
+NODE
+mv "$monorepo/packages/dep/rescript.original" \
+  "$monorepo/packages/dep/rescript.json"
+rm -f "$basic/src/A.mjs" "$basic/src/B.mjs" "$basic/src/WithInterface.mjs"
+
+"$port" build "$legacy_config"
+test -f "$legacy_config/src/A.mjs"
+
+"$port" build --filter 'A\.res$' "$basic"
+test -f "$basic/src/A.mjs"
+test -f "$basic/src/Authored.js"
+test ! -f "$basic/src/B.mjs"
+rm -rf "$basic/lib"
+rm -f "$basic/src/A.mjs"
+mkdir -p "$basic/lib/bs/other"
+touch "$basic/lib/bs/other/Authored.js"
+
+"$port" build --after-build 'test -f src/A.mjs' "$basic"
+test -f "$basic/lib/bs/build.ninja"
+test -f "$basic/src/A.mjs"
+test -f "$basic/src/Authored.js"
+test -f "$basic/src/B.mjs"
+test -f "$basic/src/WithInterface.mjs"
+test -f "$basic/lib/ocaml/A.cmi"
+test -f "$basic/lib/ocaml/WithInterface.cmti"
+
+printf '\nlet streamedAfterBuild = 1\n' >>"$basic/src/A.res"
+after_build_release="$basic/after-build-release"
+after_build_log="$basic/after-build-stream.log"
+"$port" build --after-build \
+  "node $(native_path "$root/rewatch-ocaml/tests/stream-after-build.js") $(native_path "$after_build_release")" \
+  "$basic" >"$after_build_log" 2>&1 &
+after_build_pid=$!
+background_pids="$background_pids $after_build_pid"
+if ! wait_for_text "$after_build_log" REWATCH_AFTER_BUILD_READY; then
+  cat "$after_build_log" >&2
+  exit 1
+fi
+touch "$after_build_release"
+wait "$after_build_pid"
+
+# Keep a stale working CMI only while its dependents compile, so bsc can emit
+# its source-level missing-module diagnostic. It must not survive the command.
+"$port" build "$cleanup_lifecycle" >/dev/null
+mv "$cleanup_lifecycle/src/A.res" "$cleanup_lifecycle/src/A2.res"
+if "$port" build "$cleanup_lifecycle" >/dev/null 2>&1; then
+  echo "build after a depended-on rename unexpectedly succeeded" >&2
+  exit 1
+fi
+test ! -f "$cleanup_lifecycle/lib/bs/src/A.cmi"
+test ! -f "$cleanup_lifecycle/lib/ocaml/A.cmi"
+
+# Destructive cleanup must invalidate consumers before a later deletion error
+# can abort the command and erase the removed module's AST provenance.
+"$port" build "$cleanup_failure" >/dev/null
+rm "$cleanup_failure/src/A.res" "$cleanup_failure/src/A.mjs"
+mkdir "$cleanup_failure/src/A.mjs"
+if "$port" build "$cleanup_failure" >/dev/null 2>&1; then
+  echo "cleanup with an obstructing output unexpectedly succeeded" >&2
+  exit 1
+fi
+rmdir "$cleanup_failure/src/A.mjs"
+if "$port" build "$cleanup_failure" \
+  >"$cleanup_failure/retry.log" 2>&1; then
+  echo "cleanup failure forgot the removed dependency" >&2
+  exit 1
+fi
+grep -F 'A' "$cleanup_failure/retry.log" >/dev/null
+
+"$port" build "$config_cleanup_invalidation" >/dev/null
+printf '%s\n' '{"name":"dep","sources":[]}' \
+  >"$config_cleanup_invalidation/packages/dep/rescript.json"
+if "$port" build "$config_cleanup_invalidation" \
+  >"$config_cleanup_invalidation/removal.log" 2>&1; then
+  echo "config cleanup forgot a removed dependency" >&2
+  exit 1
+fi
+grep -F 'Dep' "$config_cleanup_invalidation/removal.log" >/dev/null
+
+"$port" build "$package_name_mismatch" >/dev/null
+printf 'let value = "changed"\n' \
+  >"$package_name_mismatch/packages/dep/src/Dep.res"
+if "$port" build "$package_name_mismatch" \
+  >"$package_name_mismatch/mismatch.log" 2>&1; then
+  echo "package metadata mismatch dropped a dependency edge" >&2
+  exit 1
+fi
+grep -F 'string' "$package_name_mismatch/mismatch.log" >/dev/null
+
+sed 's/"suffix": "\.mjs"/"suffix": ".custom"/' \
+  "$suffix_removal/rescript.json" >"$suffix_removal/rescript.next"
+mv "$suffix_removal/rescript.next" "$suffix_removal/rescript.json"
+"$port" build "$suffix_removal" >/dev/null
+test -f "$suffix_removal/src/A.custom"
+rm "$suffix_removal/src/A.res"
+sed 's/"suffix": "\.custom"/"suffix": ".js"/' \
+  "$suffix_removal/rescript.json" >"$suffix_removal/rescript.next"
+mv "$suffix_removal/rescript.next" "$suffix_removal/rescript.json"
+if "$port" build "$suffix_removal" >/dev/null 2>&1; then
+  echo "suffix change with a removed dependency unexpectedly succeeded" >&2
+  exit 1
+fi
+test ! -e "$suffix_removal/src/A.custom"
+
+sed 's/"suffix": "\.mjs"/"suffix": ".custom"/' \
+  "$suffix_failure/rescript.json" >"$suffix_failure/rescript.next"
+mv "$suffix_failure/rescript.next" "$suffix_failure/rescript.json"
+"$port" build "$suffix_failure" >/dev/null
+test -f "$suffix_failure/src/A.custom"
+printf 'let value: int = "broken"\n' >"$suffix_failure/src/A.res"
+if "$port" build "$suffix_failure" >/dev/null 2>&1; then
+  echo "invalid source unexpectedly compiled before suffix change" >&2
+  exit 1
+fi
+printf 'let value = 41\n' >"$suffix_failure/src/A.res"
+sed 's/"suffix": "\.custom"/"suffix": ".js"/' \
+  "$suffix_failure/rescript.json" >"$suffix_failure/rescript.next"
+mv "$suffix_failure/rescript.next" "$suffix_failure/rescript.json"
+"$port" build "$suffix_failure" >/dev/null
+test ! -e "$suffix_failure/src/A.custom"
+test -f "$suffix_failure/src/A.js"
+
+# A successful parse must remain compile-dirty when another file aborts the
+# same build before compilation starts.
+cp "$basic/src/B.res" "$basic/src/B.backup"
+printf '\nlet recoveredAfterPeerParseFailure = 42\n' >> "$basic/src/A.res"
+printf 'let broken =\n' > "$basic/src/B.res"
+if "$port" build "$basic" >/dev/null 2>&1; then
+  echo "build with parser error unexpectedly succeeded" >&2
+  exit 1
+fi
+mv "$basic/src/B.backup" "$basic/src/B.res"
+if ! "$port" build "$basic" >"$basic/peer-parse-retry.log" 2>&1; then
+  cat "$basic/peer-parse-retry.log" >&2
+  exit 1
+fi
+if ! grep 'recoveredAfterPeerParseFailure' "$basic/src/A.mjs" >/dev/null; then
+  cat "$basic/peer-parse-retry.log" >&2
+  echo "peer parse failure left stale output for A" >&2
+  exit 1
+fi
+
+# Removing an interface from a lowercase-named source must rebuild the
+# implementation before dependents can observe exports hidden by that interface.
+printf 'let visible = 1\nlet hidden = 2\n' > "$basic/src/lower.res"
+printf 'let visible: int\n' > "$basic/src/lower.resi"
+printf 'let value = Lower.visible\n' > "$basic/src/LowerConsumer.res"
+"$port" build "$basic" >/dev/null
+rm "$basic/src/lower.resi"
+printf 'let value = Lower.hidden\n' > "$basic/src/LowerConsumer.res"
+"$port" build "$basic" >/dev/null
+grep 'hidden' "$basic/src/LowerConsumer.mjs" >/dev/null
+test ! -f "$basic/lib/ocaml/lower.cmti"
+test ! -f "$basic/lib/bs/src/lower.cmti"
+
+"$port" clean "$basic"
+test ! -f "$basic/src/A.mjs"
+
+watch_basic="$work/watch-basic"
+cp -R "$root/rewatch-ocaml/tests/basic" "$watch_basic"
+rm -rf "$watch_basic/lib"
+rm -f "$watch_basic/src/A.mjs" "$watch_basic/src/B.mjs" "$watch_basic/src/WithInterface.mjs"
+"$port" watch "$watch_basic" >"$watch_basic/watch.log" 2>&1 &
+watch_pid=$!
+background_pids="$background_pids $watch_pid"
+if ! wait_for_file "$watch_basic/src/A.mjs"; then
+  kill -TERM "$watch_pid" 2>/dev/null || true
+  wait "$watch_pid" 2>/dev/null || true
+  exit 1
+fi
+wait_for_initial_build "$watch_basic/watch.log"
+test -f "$watch_basic/lib/watch.lock"
+grep '^[0-9][0-9]*$' "$watch_basic/lib/watch.lock" >/dev/null
+printf '\nlet watchedValue = 1\n' >> "$watch_basic/src/B.res"
+if ! wait_for_text "$watch_basic/src/B.mjs" 'watchedValue'; then
+  kill -TERM "$watch_pid" 2>/dev/null || true
+  wait "$watch_pid" 2>/dev/null || true
+  exit 1
+fi
+sed 's/"\.mjs"/".js"/' "$watch_basic/rescript.json" > "$watch_basic/rescript.next"
+mv "$watch_basic/rescript.next" "$watch_basic/rescript.json"
+if ! wait_for_file "$watch_basic/src/A.js"; then
+  cat "$watch_basic/watch.log" >&2
+  kill -TERM "$watch_pid" 2>/dev/null || true
+  wait "$watch_pid" 2>/dev/null || true
+  exit 1
+fi
+test -f "$watch_basic/src/A.js"
+test ! -f "$watch_basic/src/A.mjs"
+printf 'let message = "new source"\n' > "$watch_basic/src/New.res"
+if ! wait_for_file "$watch_basic/src/New.js"; then
+  kill -TERM "$watch_pid" 2>/dev/null || true
+  wait "$watch_pid" 2>/dev/null || true
+  exit 1
+fi
+test -f "$watch_basic/src/New.js"
+mkdir "$watch_basic/src/new-directory"
+printf 'let nested = "new directory"\n' \
+  > "$watch_basic/src/new-directory/Nested.res"
+if ! wait_for_file "$watch_basic/src/new-directory/Nested.js"; then
+  kill -TERM "$watch_pid" 2>/dev/null || true
+  wait "$watch_pid" 2>/dev/null || true
+  exit 1
+fi
+rm -f "$watch_basic/src/New.res"
+if ! wait_for_file_gone "$watch_basic/src/New.js"; then
+  kill -TERM "$watch_pid" 2>/dev/null || true
+  wait "$watch_pid" 2>/dev/null || true
+  exit 1
+fi
+test ! -f "$watch_basic/src/New.js"
+
+# A shallow watch on the nearest existing ancestor must advance one directory
+# at a time until a configured nested source root exists. The slash in the JSON
+# path also exercises native separator normalization on Windows.
+"$port" watch "$missing_nested_source" \
+  >"$missing_nested_source/watch.log" 2>&1 &
+missing_nested_source_pid=$!
+background_pids="$background_pids $missing_nested_source_pid"
+wait_for_initial_build "$missing_nested_source/watch.log"
+mkdir "$missing_nested_source/generated"
+sleep 1
+mkdir "$missing_nested_source/generated/nested"
+sleep 1
+printf 'let value = 1\n' \
+  >"$missing_nested_source/generated/nested/Main.res"
+if ! wait_for_file "$missing_nested_source/generated/nested/Main.mjs"; then
+  cat "$missing_nested_source/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$missing_nested_source_pid"
+wait "$missing_nested_source_pid" 2>/dev/null || true
+
+# A successfully published interface must invalidate its dependents even when
+# the corresponding implementation fails and aborts the rest of that build.
+interface_failure_log="$interface_failure_recovery/watch.log"
+"$port" watch "$interface_failure_recovery" \
+  >"$interface_failure_log" 2>&1 &
+interface_failure_pid=$!
+background_pids="$background_pids $interface_failure_pid"
+if ! wait_for_file "$interface_failure_recovery/src/B.mjs"; then
+  cat "$interface_failure_log" >&2
+  exit 1
+fi
+wait_for_initial_build "$interface_failure_log"
+printf 'let value: string\n' \
+  >"$interface_failure_recovery/src/A.resi"
+if ! wait_for_text "$interface_failure_log" 'does not match the interface'; then
+  cat "$interface_failure_log" >&2
+  exit 1
+fi
+printf 'let value = "fixed"\n' \
+  >"$interface_failure_recovery/src/A.res"
+if ! wait_for_text "$interface_failure_log" 'This has type:'; then
+  cat "$interface_failure_log" >&2
+  exit 1
+fi
+kill -TERM "$interface_failure_pid"
+wait "$interface_failure_pid" 2>/dev/null || true
+
+# A parsed interface remains pending when another source prevents compilation.
+printf 'let value: int\n' >"$interface_failure_recovery/src/A.resi"
+printf 'let value = 1\n' >"$interface_failure_recovery/src/A.res"
+"$port" build "$interface_failure_recovery"
+printf 'let value: string\n' \
+  >"$interface_failure_recovery/src/A.resi"
+printf 'let =\n' >"$interface_failure_recovery/src/Broken.res"
+if "$port" build "$interface_failure_recovery" \
+  >"$interface_failure_recovery/pending-interface.log" 2>&1; then
+  echo "build unexpectedly accepted the broken source" >&2
+  exit 1
+fi
+printf 'let valid = true\n' >"$interface_failure_recovery/src/Broken.res"
+if "$port" build "$interface_failure_recovery" \
+  >>"$interface_failure_recovery/pending-interface.log" 2>&1; then
+  echo "pending interface change was forgotten after a parse failure" >&2
+  exit 1
+fi
+grep 'does not match the interface' \
+  "$interface_failure_recovery/pending-interface.log" >/dev/null
+
+"$port" build "$directory_symlink"
+"$port" build "$directory_symlink" >"$directory_symlink/unchanged.log" 2>&1
+grep 'Parsed 0 source files' "$directory_symlink/unchanged.log" >/dev/null
+grep 'Compiled 0 modules' "$directory_symlink/unchanged.log" >/dev/null
+
+# Record the content that triggered a direct event before building it. An
+# atomic replacement made while that build is compiling must cause a second
+# build rather than becoming the post-build snapshot baseline.
+atomic_release="$atomic_save/release"
+atomic_started="$atomic_save/compile-started"
+touch "$atomic_release"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=block-compile \
+  REWATCH_OCAML_RELEASE_FILE="$(native_path "$atomic_release")" \
+  REWATCH_OCAML_COMPILE_STARTED="$(native_path "$atomic_started")" \
+  "$port" watch "$atomic_save" >"$atomic_save/watch.log" 2>&1 &
+atomic_save_pid=$!
+background_pids="$background_pids $atomic_save_pid"
+if ! wait_for_file "$atomic_save/src/Main.mjs"; then
+  cat "$atomic_save/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$atomic_save/watch.log"
+rm "$atomic_release" "$atomic_started"
+printf 'let value = 2\n' >"$atomic_save/src/Main.res"
+if ! wait_for_file "$atomic_started"; then
+  cat "$atomic_save/watch.log" >&2
+  exit 1
+fi
+printf 'let value = 3\n' >"$atomic_save/src/Main.next"
+mv "$atomic_save/src/Main.next" "$atomic_save/src/Main.res"
+touch "$atomic_release"
+if ! wait_for_text "$atomic_save/src/Main.mjs" 'value = 3'; then
+  cat "$atomic_save/watch.log" >&2
+  cat "$atomic_save/src/Main.mjs" >&2
+  exit 1
+fi
+kill -TERM "$atomic_save_pid"
+wait "$atomic_save_pid" 2>/dev/null || true
+
+"$port" watch "$recursive_lib" >"$recursive_lib/watch.log" 2>&1 &
+recursive_lib_pid=$!
+background_pids="$background_pids $recursive_lib_pid"
+if ! wait_for_file "$recursive_lib/src/lib/Nested.mjs"; then
+  cat "$recursive_lib/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$recursive_lib/watch.log"
+printf 'let value = 2\n' >"$recursive_lib/src/lib/Nested.res"
+if ! wait_for_text "$recursive_lib/src/lib/Nested.mjs" 'value = 2'; then
+  cat "$recursive_lib/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$recursive_lib_pid"
+wait "$recursive_lib_pid" 2>/dev/null || true
+
+"$port" watch --filter '^Main\.res$' "$filtered_dependency" \
+  >"$filtered_dependency/watch.log" 2>&1 &
+filtered_dependency_pid=$!
+background_pids="$background_pids $filtered_dependency_pid"
+if ! wait_for_file "$filtered_dependency/packages/dep/src/Helper.mjs"; then
+  cat "$filtered_dependency/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$filtered_dependency/watch.log"
+test -f "$filtered_dependency/src/Main.mjs"
+test ! -f "$filtered_dependency/src/Ignored.mjs"
+printf 'let value = 2\n' \
+  >"$filtered_dependency/packages/dep/src/Helper.res"
+if ! wait_for_text "$filtered_dependency/packages/dep/src/Helper.mjs" \
+  'value = 2'; then
+  cat "$filtered_dependency/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$filtered_dependency_pid"
+wait "$filtered_dependency_pid" 2>/dev/null || true
+printf 'let value = { let unused = 2; 1 }\n' \
+  >"$filtered_dependency/packages/dep/src/Helper.res"
+if "$port" build --warn-error A "$filtered_dependency" \
+  >"$filtered_dependency/warn-error.log" 2>&1; then
+  echo "--warn-error did not apply to a local dependency" >&2
+  exit 1
+fi
+
+if $file_symlinks_supported; then
+  "$port" watch "$symlink_source" >"$symlink_source/watch.log" 2>&1 &
+  symlink_source_pid=$!
+  background_pids="$background_pids $symlink_source_pid"
+  if ! wait_for_file "$symlink_source/src/Linked.mjs"; then
+    cat "$symlink_source/watch.log" >&2
+    exit 1
+  fi
+  wait_for_initial_build "$symlink_source/watch.log"
+  printf 'let value = 2\n' >"$symlink_source/shared/Source.js"
+  if ! wait_for_text "$symlink_source/src/Linked.mjs" 'value = 2'; then
+    cat "$symlink_source/watch.log" >&2
+    exit 1
+  fi
+  symlink_replacement=$(mktemp "${TMPDIR:-/tmp}/rewatch-symlink-target.XXXXXX")
+  printf 'let value = 3\n' >"$symlink_replacement"
+  mv "$symlink_replacement" "$symlink_source/shared/Source.js"
+  if ! wait_for_text "$symlink_source/src/Linked.mjs" 'value = 3'; then
+    cat "$symlink_source/watch.log" >&2
+    exit 1
+  fi
+  kill -TERM "$symlink_source_pid"
+  wait "$symlink_source_pid" 2>/dev/null || true
+
+  "$port" watch "$symlink_alias" >"$symlink_alias/watch.log" 2>&1 &
+  symlink_alias_pid=$!
+  background_pids="$background_pids $symlink_alias_pid"
+  if ! wait_for_file "$symlink_alias/src/Linked.mjs"; then
+    cat "$symlink_alias/watch.log" >&2
+    exit 1
+  fi
+  wait_for_initial_build "$symlink_alias/watch.log"
+  printf 'let value = 2\n' >"$symlink_alias/src/Original.res"
+  if ! wait_for_text "$symlink_alias/src/Original.mjs" 'value = 2' \
+    || ! wait_for_text "$symlink_alias/src/Linked.mjs" 'value = 2'; then
+    cat "$symlink_alias/watch.log" >&2
+    exit 1
+  fi
+  kill -TERM "$symlink_alias_pid"
+  wait "$symlink_alias_pid" 2>/dev/null || true
+fi
+
+"$port" watch "$post_build_cmi" >"$post_build_cmi/watch.log" 2>&1 &
+post_build_cmi_pid=$!
+background_pids="$background_pids $post_build_cmi_pid"
+if ! wait_for_file "$post_build_cmi/src/B.mjs"; then
+  cat "$post_build_cmi/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$post_build_cmi/watch.log"
+rm "$post_build_cmi/allow-post-build"
+printf 'let value = "changed"\n' >"$post_build_cmi/src/A.res"
+if ! wait_for_text "$post_build_cmi/watch.log" \
+  'js-post-build command failed'; then
+  cat "$post_build_cmi/watch.log" >&2
+  exit 1
+fi
+touch "$post_build_cmi/allow-post-build"
+printf 'let value = "changed"\n\n' >"$post_build_cmi/src/A.res"
+if ! wait_for_text "$post_build_cmi/watch.log" 'This has type:'; then
+  cat "$post_build_cmi/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$post_build_cmi_pid"
+wait "$post_build_cmi_pid" 2>/dev/null || true
+
+"$port" build "$post_build_retry"
+rm "$post_build_retry/allow-post-build"
+printf 'let value = 2\n' >"$post_build_retry/src/A.res"
+if "$port" build "$post_build_retry" \
+  >"$post_build_retry/first-failure.log" 2>&1; then
+  echo "post-build failure unexpectedly succeeded" >&2
+  exit 1
+fi
+if "$port" build "$post_build_retry" \
+  >"$post_build_retry/retry-failure.log" 2>&1; then
+  echo "a fresh build forgot the failed post-build command" >&2
+  exit 1
+fi
+grep 'js-post-build command failed' \
+  "$post_build_retry/retry-failure.log" >/dev/null
+touch "$post_build_retry/allow-post-build"
+"$port" build "$post_build_retry"
+grep 'value = 2' "$post_build_retry/src/A.mjs" >/dev/null
+
+"$port" build "$moved_source"
+mv "$moved_source/src/A.res" "$moved_source/src/nested/A.res"
+"$port" build "$moved_source"
+if [ -f "$moved_source/src/A.mjs" ]; then
+  echo "moved source left its old generated output" >&2
+  find "$moved_source" -type f -print >&2
+  exit 1
+fi
+if ! grep 'value = 1' "$moved_source/src/nested/A.mjs" >/dev/null; then
+  echo "moved source did not produce the expected nested output" >&2
+  find "$moved_source" -type f -print >&2
+  exit 1
+fi
+
+"$port" watch "$parse_publication" \
+  >"$parse_publication/watch.log" 2>&1 &
+parse_publication_pid=$!
+background_pids="$background_pids $parse_publication_pid"
+parse_destination="$parse_publication/lib/ocaml/A.res"
+if ! wait_for_text "$parse_publication/watch.log" \
+  'Finished initial compilation' \
+  || ! wait_for_file "$parse_destination"; then
+  cat "$parse_publication/watch.log" >&2
+  echo "initial watch did not finish and publish $parse_destination" >&2
+  exit 1
+fi
+if ! rm "$parse_destination"; then
+  cat "$parse_publication/watch.log" >&2
+  echo "could not remove published parse artifact $parse_destination" >&2
+  exit 1
+fi
+if ! mkdir "$parse_destination"; then
+  cat "$parse_publication/watch.log" >&2
+  echo "could not obstruct parse publication at $parse_destination" >&2
+  exit 1
+fi
+printf 'let value = 2\n' >"$parse_publication/src/A.res"
+if ! wait_for_text "$parse_publication/watch.log" \
+  "$(basename "$parse_destination")"; then
+  cat "$parse_publication/watch.log" >&2
+  exit 1
+fi
+remove_obstruction_directory "$parse_destination"
+printf 'let other = 2\n' >"$parse_publication/src/B.res"
+if ! wait_for_text "$parse_publication/src/A.mjs" 'value = 2'; then
+  cat "$parse_publication/watch.log" >&2
+  echo "parse publication failure forgot the changed source" >&2
+  exit 1
+fi
+kill -TERM "$parse_publication_pid"
+wait "$parse_publication_pid" 2>/dev/null || true
+
+"$port" watch "$multi_package_pending" \
+  >"$multi_package_pending/watch.log" 2>&1 &
+multi_package_pending_pid=$!
+background_pids="$background_pids $multi_package_pending_pid"
+dep1_destination="$multi_package_pending/packages/dep1/lib/ocaml/A.res"
+if ! wait_for_text "$multi_package_pending/watch.log" \
+  'Finished initial compilation' \
+  || ! wait_for_file "$dep1_destination"; then
+  cat "$multi_package_pending/watch.log" >&2
+  echo "initial watch did not finish and publish $dep1_destination" >&2
+  exit 1
+fi
+obstruct_file_with_directory "$dep1_destination"
+printf 'let value = 2\n' >"$multi_package_pending/packages/dep2/src/B.res"
+printf 'let value = 2\n' >"$multi_package_pending/packages/dep1/src/A.res"
+if ! wait_for_text "$multi_package_pending/watch.log" \
+  "$(basename "$dep1_destination")"; then
+  cat "$multi_package_pending/watch.log" >&2
+  exit 1
+fi
+remove_obstruction_directory "$dep1_destination"
+printf 'let other = 2\n' >"$multi_package_pending/src/Other.res"
+if ! wait_for_text "$multi_package_pending/packages/dep2/src/B.mjs" \
+  'value = 2'; then
+  cat "$multi_package_pending/watch.log" >&2
+  echo "an earlier package failure forgot another package's edit" >&2
+  exit 1
+fi
+kill -TERM "$multi_package_pending_pid"
+wait "$multi_package_pending_pid" 2>/dev/null || true
+
+"$port" watch "$full_watch_recovery" \
+  >"$full_watch_recovery/watch.log" 2>&1 &
+full_watch_recovery_pid=$!
+background_pids="$background_pids $full_watch_recovery_pid"
+full_watch_destination="$full_watch_recovery/lib/ocaml/C.res"
+if ! wait_for_text "$full_watch_recovery/watch.log" \
+  'Finished initial compilation' \
+  || ! wait_for_file "$full_watch_destination"; then
+  cat "$full_watch_recovery/watch.log" >&2
+  echo "initial watch did not finish and publish $full_watch_destination" >&2
+  exit 1
+fi
+obstruct_file_with_directory "$full_watch_destination"
+rm "$full_watch_recovery/src/B.res"
+printf 'let value = 2\n' >"$full_watch_recovery/src/C.res"
+if ! wait_for_text "$full_watch_recovery/watch.log" \
+  "$(basename "$full_watch_destination")"; then
+  cat "$full_watch_recovery/watch.log" >&2
+  exit 1
+fi
+remove_obstruction_directory "$full_watch_destination"
+printf 'let value = 3\n' >"$full_watch_recovery/src/C.res"
+if ! wait_for_text "$full_watch_recovery/watch.log" 'Compiled 2 modules'; then
+  cat "$full_watch_recovery/watch.log" >&2
+  echo "full-watch parse recovery forgot deleted-dependency invalidation" >&2
+  exit 1
+fi
+kill -TERM "$full_watch_recovery_pid"
+wait "$full_watch_recovery_pid" 2>/dev/null || true
+
+"$port" build "$deletion_retry" >/dev/null
+rm "$deletion_retry/src/B.res"
+printf 'let value =\n' >"$deletion_retry/src/C.res"
+if "$port" build "$deletion_retry" >/dev/null 2>&1; then
+  echo "deletion plus parse failure unexpectedly succeeded" >&2
+  exit 1
+fi
+printf 'let value = 2\n' >"$deletion_retry/src/C.res"
+if "$port" build "$deletion_retry" \
+  >"$deletion_retry/retry.log" 2>&1; then
+  echo "a fresh build forgot deleted-dependency invalidation" >&2
+  exit 1
+fi
+if ! grep 'Compiled 2 modules' "$deletion_retry/retry.log" >/dev/null; then
+  cat "$deletion_retry/retry.log" >&2
+  echo "deleted dependency did not invalidate its consumer after parse recovery" >&2
+  exit 1
+fi
+
+"$port" build "$namespace_repair" >/dev/null
+namespace_repair_destination="$namespace_repair/packages/dep/lib/ocaml/Ns.cmj"
+obstruct_file_with_directory "$namespace_repair_destination"
+"$port" watch "$namespace_repair" >"$namespace_repair/watch.log" 2>&1 &
+namespace_repair_pid=$!
+background_pids="$background_pids $namespace_repair_pid"
+if ! wait_for_text "$namespace_repair/watch.log" \
+  "$(basename "$namespace_repair_destination")"; then
+  cat "$namespace_repair/watch.log" >&2
+  exit 1
+fi
+remove_obstruction_directory "$namespace_repair_destination"
+printf 'let other = 2\n' >"$namespace_repair/src/Other.res"
+if ! wait_for_file "$namespace_repair_destination" \
+  || ! wait_for_text "$namespace_repair/src/Other.mjs" 'other = 2'; then
+  cat "$namespace_repair/watch.log" >&2
+  echo "failed namespace repair was not retried" >&2
+  exit 1
+fi
+kill -TERM "$namespace_repair_pid"
+wait "$namespace_repair_pid" 2>/dev/null || true
+
+publication_destination="$publication_cmi/lib/bs/src/A.res"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=fail-late-publication \
+  REWATCH_FAIL_PUBLICATION="$(native_path "$publication_cmi/fail-publication")" \
+  REWATCH_PUBLICATION_FAILED="$(native_path "$publication_cmi/publication-failed")" \
+  REWATCH_PUBLICATION_DESTINATION="$(native_path "$publication_destination")" \
+  "$port" watch "$publication_cmi" >"$publication_cmi/watch.log" 2>&1 &
+publication_cmi_pid=$!
+background_pids="$background_pids $publication_cmi_pid"
+if ! wait_for_file "$publication_cmi/src/B.mjs"; then
+  cat "$publication_cmi/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$publication_cmi/watch.log"
+touch "$publication_cmi/fail-publication"
+printf 'let value = "changed"\n' >"$publication_cmi/src/A.res"
+if ! wait_for_file "$publication_cmi/publication-failed" || \
+  ! wait_for_text "$publication_cmi/watch.log" 'A.res'; then
+  cat "$publication_cmi/watch.log" >&2
+  exit 1
+fi
+remove_obstruction_directory "$publication_destination"
+rm "$publication_cmi/fail-publication"
+printf 'let value = "changed"\n\n' >"$publication_cmi/src/A.res"
+if ! wait_for_text "$publication_cmi/watch.log" 'This has type:'; then
+  cat "$publication_cmi/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$publication_cmi_pid"
+wait "$publication_cmi_pid" 2>/dev/null || true
+
+"$port" watch "$retained_cycle" >"$retained_cycle/watch.log" 2>&1 &
+retained_cycle_pid=$!
+background_pids="$background_pids $retained_cycle_pid"
+if ! wait_for_file "$retained_cycle/src/A.mjs"; then
+  cat "$retained_cycle/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$retained_cycle/watch.log"
+printf 'let value: int = "broken"\n' >"$retained_cycle/src/A.res"
+if ! wait_for_text "$retained_cycle/watch.log" 'expected to have type'; then
+  cat "$retained_cycle/watch.log" >&2
+  exit 1
+fi
+printf 'let value = B.dependent\n' >"$retained_cycle/src/A.res"
+if ! wait_for_text "$retained_cycle/watch.log" 'circular dependency'; then
+  cat "$retained_cycle/watch.log" >&2
+  exit 1
+fi
+if grep -F 'from "./B.mjs"' "$retained_cycle/src/A.mjs" >/dev/null; then
+  cat "$retained_cycle/src/A.mjs" >&2
+  cat "$retained_cycle/watch.log" >&2
+  echo "cycle member was compiled from retained dirty state" >&2
+  exit 1
+fi
+printf 'let dependent = 1\n' >"$retained_cycle/src/B.res"
+if ! wait_for_text "$retained_cycle/src/A.mjs" 'from "./B.mjs"'; then
+  cat "$retained_cycle/src/A.mjs" >&2
+  cat "$retained_cycle/watch.log" >&2
+  echo "cycle recovery did not compile retained pending work" >&2
+  exit 1
+fi
+kill -TERM "$retained_cycle_pid"
+wait "$retained_cycle_pid" 2>/dev/null || true
+
+# Initial watch recovery must retain freshness work discovered alongside an
+# unrelated parse failure.
+"$port" build "$initial_failure_freshness" >/dev/null
+rm "$initial_failure_freshness/lib/ocaml/A.cmi"
+printf 'let other =\n' >"$initial_failure_freshness/src/B.res"
+"$port" watch "$initial_failure_freshness" \
+  >"$initial_failure_freshness/watch.log" 2>&1 &
+initial_failure_freshness_pid=$!
+background_pids="$background_pids $initial_failure_freshness_pid"
+if ! wait_for_text "$initial_failure_freshness/watch.log" \
+  'Error in initial-failure-freshness'; then
+  cat "$initial_failure_freshness/watch.log" >&2
+  exit 1
+fi
+printf 'let other = 2\n' >"$initial_failure_freshness/src/B.res"
+if ! wait_for_file "$initial_failure_freshness/lib/ocaml/A.cmi"; then
+  cat "$initial_failure_freshness/watch.log" >&2
+  echo "initial parse recovery forgot missing compile artifacts" >&2
+  exit 1
+fi
+kill -TERM "$initial_failure_freshness_pid"
+wait "$initial_failure_freshness_pid" 2>/dev/null || true
+
+# Failed parses and parser warnings remain pending until the same source parses
+# cleanly. An unrelated edit must not compile an older AST or forget diagnostics.
+retained_parse_log="$retained_parse/watch.log"
+retained_parse_calls="$retained_parse/bsc-calls.log"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=parse-warning-log \
+  REWATCH_BSC_CALL_LOG="$(native_path "$retained_parse_calls")" \
+  REWATCH_PARSE_WARNING_SOURCE=A.res \
+  "$port" watch "$retained_parse" >"$retained_parse_log" 2>&1 &
+retained_parse_pid=$!
+background_pids="$background_pids $retained_parse_pid"
+if ! wait_for_count "$retained_parse_log" REWATCH_PARSE_WARNING 1 || \
+  ! wait_for_file "$retained_parse/src/A.mjs"; then
+  cat "$retained_parse_log" >&2
+  exit 1
+fi
+wait_for_initial_build "$retained_parse_log"
+initial_a_parse_count=$(grep -c -- '-bs-ast.*A.res' "$retained_parse_calls")
+printf 'let other = 2\n' >"$retained_parse/src/B.res"
+if ! wait_for_count "$retained_parse_log" REWATCH_PARSE_WARNING 2; then
+  cat "$retained_parse_log" >&2
+  exit 1
+fi
+if ! wait_for_count "$retained_parse_calls" '-bs-ast.*A.res' \
+  "$((initial_a_parse_count + 1))"; then
+  cat "$retained_parse_calls" >&2
+  exit 1
+fi
+warning_a_parse_count=$(grep -c -- '-bs-ast.*A.res' "$retained_parse_calls")
+test "$warning_a_parse_count" -gt "$initial_a_parse_count"
+printf 'let value =\n' >"$retained_parse/src/A.res"
+if ! wait_for_count "$retained_parse_log" \
+  'This let-binding misses an expression' 1; then
+  cat "$retained_parse_log" >&2
+  exit 1
+fi
+failed_a_parse_count=$(grep -c -- '-bs-ast.*A.res' "$retained_parse_calls")
+printf 'let other = 3\n' >"$retained_parse/src/B.res"
+if ! wait_for_count "$retained_parse_log" \
+  'This let-binding misses an expression' 2; then
+  cat "$retained_parse_log" >&2
+  exit 1
+fi
+retried_a_parse_count=$(grep -c -- '-bs-ast.*A.res' "$retained_parse_calls")
+test "$retried_a_parse_count" -gt "$failed_a_parse_count"
+grep 'value = 1' "$retained_parse/src/A.mjs" >/dev/null
+printf 'let value = 2\n' >"$retained_parse/src/A.res"
+if ! wait_for_text "$retained_parse/src/A.mjs" 'value = 2'; then
+  cat "$retained_parse_log" >&2
+  exit 1
+fi
+kill -TERM "$retained_parse_pid"
+wait "$retained_parse_pid" 2>/dev/null || true
+
+"$port" build "$duplicate_selection" >/dev/null
+"$port" build "$external_duplicate" >"$external_duplicate/build.log" 2>&1
+tr '\\' '/' <"$external_duplicate/build.log" | \
+  grep -F 'node_modules/shared (chosen)' >/dev/null
+if "$port" build "$external_cycle" \
+  >"$external_cycle/build.log" 2>&1; then
+  echo "external dependency cycle unexpectedly built" >&2
+  exit 1
+fi
+grep -F "circular dependency" "$external_cycle/build.log" >/dev/null
+tr '\\' '/' <"$external_cycle/build.log" | \
+  grep -F 'store/cycle-dep/src/CycleDep.res' >/dev/null
+if "$port" build "$external_namespace" \
+  >"$external_namespace/build.log" 2>&1; then
+  echo "external namespace collision unexpectedly built" >&2
+  exit 1
+fi
+grep -F "Namespace SharedNs is provided by both" \
+  "$external_namespace/build.log" >/dev/null
+tr '\\' '/' <"$external_namespace/build.log" | \
+  grep -F 'store/ns-one' >/dev/null
+mkdir -p \
+  "$duplicate_selection/node_modules/a/node_modules/shared/lib/ocaml"
+printf 'preserve nested duplicate\n' \
+  >"$duplicate_selection/node_modules/a/node_modules/shared/lib/ocaml/marker"
+"$port" clean "$duplicate_selection" >/dev/null
+test ! -f "$duplicate_selection/node_modules/shared/src/Shared.mjs"
+test -f \
+  "$duplicate_selection/node_modules/a/node_modules/shared/lib/ocaml/marker"
+"$port" watch "$duplicate_selection" \
+  >"$duplicate_selection/watch.log" 2>&1 &
+duplicate_selection_pid=$!
+background_pids="$background_pids $duplicate_selection_pid"
+if ! wait_for_file "$duplicate_selection/src/Main.mjs"; then
+  cat "$duplicate_selection/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$duplicate_selection/watch.log"
+printf 'let value = 3\n' \
+  >"$duplicate_selection/node_modules/shared/src/Shared.res"
+if ! wait_for_text \
+  "$duplicate_selection/node_modules/shared/src/Shared.mjs" 'value = 3'; then
+  cat "$duplicate_selection/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$duplicate_selection_pid"
+wait "$duplicate_selection_pid" 2>/dev/null || true
+
+# Dependency traversal must continue through installed packages because a
+# later edge can resolve back into the local workspace and therefore needs a
+# source watch.
+"$port" watch "$transitive_local_watch" \
+  >"$transitive_local_watch/watch.log" 2>&1 &
+transitive_local_watch_pid=$!
+background_pids="$background_pids $transitive_local_watch_pid"
+transitive_local_output="$transitive_local_watch/packages/local/src/Local.mjs"
+if ! wait_for_text "$transitive_local_output" 'value = 1'; then
+  cat "$transitive_local_watch/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$transitive_local_watch/watch.log"
+printf 'let value = 2\n' \
+  >"$transitive_local_watch/packages/local/src/Local.res"
+if ! wait_for_text "$transitive_local_output" 'value = 2'; then
+  cat "$transitive_local_watch/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$transitive_local_watch_pid"
+wait "$transitive_local_watch_pid" 2>/dev/null || true
+
+dev_include_call_log="$dev_include_order/bsc-calls.log"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=counting \
+  REWATCH_BSC_CALL_LOG="$(native_path "$dev_include_call_log")" \
+  "$port" build "$dev_include_order" >/dev/null
+node - "$dev_include_call_log" \
+  "$dev_include_order/node_modules/development/lib/ocaml" \
+  "$dev_include_order/node_modules/regular/lib/ocaml" <<'EOF'
+const fs = require("fs");
+const [log, development, regular] = process.argv.slice(2);
+const normalize = value => value.replaceAll("\\", "/");
+const command = fs.readFileSync(log, "utf8").split("\n")
+  .find(line => line.includes("Test.ast") && !line.includes("-bs-ast"));
+const normalizedCommand = command === undefined ? "" : normalize(command);
+if (!command || normalizedCommand.indexOf(normalize(development)) < 0 ||
+    normalizedCommand.indexOf(normalize(development)) >
+      normalizedCommand.indexOf(normalize(regular))) {
+  process.exit(1);
+}
+EOF
+
+warning_call_log="$warning_replay/bsc-calls.log"
+warning_watch_log="$warning_replay/watch.log"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=counting \
+  REWATCH_BSC_CALL_LOG="$(native_path "$warning_call_log")" \
+  "$port" watch "$warning_replay" >"$warning_watch_log" 2>&1 &
+warning_watch_pid=$!
+background_pids="$background_pids $warning_watch_pid"
+if ! wait_for_count "$warning_watch_log" 'unused value unusedValue' 1; then
+  cat "$warning_watch_log" >&2
+  exit 1
+fi
+wait_for_initial_build "$warning_watch_log"
+warning_a_calls=$(grep -c 'WarningA.ast' "$warning_call_log" || true)
+test "$warning_a_calls" -gt 0
+printf '\nlet changed = 1\n' >> "$warning_replay/src/B.res"
+if ! wait_for_count "$warning_watch_log" 'unused value unusedValue' 2; then
+  cat "$warning_watch_log" >&2
+  exit 1
+fi
+warning_a_calls_after=$(grep -c 'WarningA.ast' "$warning_call_log" || true)
+test "$warning_a_calls_after" -eq "$warning_a_calls"
+stop_watch "$warning_replay" "$warning_watch_pid"
+stop_watch "$watch_basic" "$watch_pid"
+test ! -f "$watch_basic/lib/watch.lock"
+
+# Watch startup shares normal build initialization, so deleting a public output
+# between sessions must dirty its module even when compiler artifacts are current.
+rm "$watch_basic/src/A.js"
+"$port" watch "$watch_basic" >"$watch_basic/restart.log" 2>&1 &
+watch_restart_pid=$!
+background_pids="$background_pids $watch_restart_pid"
+if ! wait_for_file "$watch_basic/src/A.js"; then
+  cat "$watch_basic/restart.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$watch_basic/restart.log"
+stop_watch "$watch_basic" "$watch_restart_pid"
+test ! -f "$watch_basic/lib/watch.lock"
+
+if ! $windows_posix_shell; then
+interrupt_basic="$work/interrupt-basic"
+cp -R "$root/rewatch-ocaml/tests/basic" "$interrupt_basic"
+child_marker="$interrupt_basic/child-started"
+REWATCH_OCAML_CHILD_STARTED="$(native_path "$child_marker")" \
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=slow \
+RESCRIPT_BSC_EXE="$test_proxy" \
+"$port" watch "$interrupt_basic" >"$interrupt_basic/watch.log" 2>&1 &
+interrupt_pid=$!
+background_pids="$background_pids $interrupt_pid"
+attempts=0
+while [ "$attempts" -lt 100 ] && [ ! -f "$child_marker" ]; do
+  attempts=$((attempts + 1))
+  sleep 0.1
+done
+test -f "$child_marker"
+kill -TERM "$interrupt_pid"
+wait "$interrupt_pid"
+test ! -f "$interrupt_basic/lib/watch.lock"
+assert_no_test_proxy_processes
+test -z "$(find "$interrupt_basic" -name '.rewatch-ocaml-*.log' -print)"
+
+# One-shot commands must unwind through the same process and lock owners when
+# the shell terminates them during compiler work.
+interrupt_build="$work/interrupt-build"
+cp -R "$root/rewatch-ocaml/tests/basic" "$interrupt_build"
+build_child_marker="$interrupt_build/child-started"
+REWATCH_OCAML_CHILD_STARTED="$(native_path "$build_child_marker")" \
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=slow \
+RESCRIPT_BSC_EXE="$test_proxy" \
+"$port" build "$interrupt_build" >"$interrupt_build/build.log" 2>&1 &
+interrupt_build_pid=$!
+background_pids="$background_pids $interrupt_build_pid"
+wait_for_file "$build_child_marker"
+kill -TERM "$interrupt_build_pid"
+set +e
+wait "$interrupt_build_pid"
+interrupt_build_status=$?
+set -e
+test "$interrupt_build_status" -eq 143
+test ! -f "$interrupt_build/lib/build.lock"
+assert_no_test_proxy_processes
+test -z "$(find "$interrupt_build" -name '.rewatch-ocaml-*.log' -print)"
+fi
+
+# Removing watch.lock is the shell-suite shutdown protocol. It must interrupt
+# an in-progress compiler batch just as SIGTERM does, rather than waiting for
+# every queued module to finish.
+lock_interrupt_basic="$work/lock-interrupt-basic"
+cp -R "$root/rewatch-ocaml/tests/basic" "$lock_interrupt_basic"
+lock_child_marker="$lock_interrupt_basic/child-started"
+REWATCH_OCAML_CHILD_STARTED="$(native_path "$lock_child_marker")" \
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=slow \
+RESCRIPT_BSC_EXE="$test_proxy" \
+"$port" watch "$lock_interrupt_basic" \
+  >"$lock_interrupt_basic/watch.log" 2>&1 &
+lock_interrupt_pid=$!
+background_pids="$background_pids $lock_interrupt_pid"
+if ! wait_for_file "$lock_child_marker"; then
+  cat "$lock_interrupt_basic/watch.log" >&2
+  exit 1
+fi
+remove_file_with_retry "$lock_interrupt_basic/lib/watch.lock"
+if ! wait_for_pid_gone "$lock_interrupt_pid"; then
+  echo "watcher did not stop during compiler work after watch.lock removal" >&2
+  exit 1
+fi
+wait "$lock_interrupt_pid"
+assert_no_test_proxy_processes
+
+lock_basic="$work/lock-basic"
+cp -R "$root/rewatch-ocaml/tests/basic" "$lock_basic"
+rm -rf "$lock_basic/lib"
+rm -f "$lock_basic/src/A.mjs" "$lock_basic/src/B.mjs" \
+  "$lock_basic/src/WithInterface.mjs"
+first_marker="$lock_basic/first-child-started"
+release_marker="$lock_basic/release-first-build"
+REWATCH_OCAML_CHILD_STARTED="$(native_path "$first_marker")" \
+REWATCH_OCAML_RELEASE_FILE="$(native_path "$release_marker")" \
+REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+REWATCH_BSC_PROXY_MODE=slow \
+RESCRIPT_BSC_EXE="$test_proxy" \
+  "$port" build "$lock_basic" >"$lock_basic/first.log" 2>&1 &
+first_build_pid=$!
+background_pids="$background_pids $first_build_pid"
+wait_for_file "$first_marker"
+workspace_build_lock="$lock_basic/lib/build.lock"
+test -f "$workspace_build_lock"
+"$port" build "$lock_basic" >"$lock_basic/second.log" 2>&1 &
+second_build_pid=$!
+background_pids="$background_pids $second_build_pid"
+wait_for_text "$lock_basic/second.log" "Waiting for other build to finish"
+test ! -f "$lock_basic/src/A.mjs"
+touch "$release_marker"
+if ! wait "$first_build_pid"; then
+  cat "$lock_basic/first.log" >&2
+  exit 1
+fi
+if ! wait "$second_build_pid"; then
+  cat "$lock_basic/second.log" >&2
+  exit 1
+fi
+test -f "$lock_basic/src/A.mjs"
+test ! -f "$workspace_build_lock"
+
+"$port" build --features native "$features"
+test -f "$features/native/Native.js"
+
+"$port" build "$feature_dependencies"
+test -f "$feature_dependencies/packages/dep-union/extra/UnionExtra.js"
+test -f "$feature_dependencies/packages/dep-transitive/native/TransitiveNative.js"
+test -f "$feature_dependencies/packages/dep-empty/src/EmptyCommon.js"
+test ! -f "$feature_dependencies/packages/dep-empty/optional/EmptyOptional.js"
+"$port" watch "$feature_dependencies" \
+  >"$feature_dependencies/watch.log" 2>&1 &
+feature_dependencies_pid=$!
+background_pids="$background_pids $feature_dependencies_pid"
+if ! wait_for_file "$feature_dependencies/lib/watch.lock"; then
+  cat "$feature_dependencies/watch.log" >&2
+  exit 1
+fi
+wait_for_initial_build "$feature_dependencies/watch.log"
+printf 'let value = 2\n' \
+  >"$feature_dependencies/packages/dep-union/extra/UnionExtra.res"
+if ! wait_for_text \
+  "$feature_dependencies/packages/dep-union/extra/UnionExtra.js" 'value = 2'; then
+  cat "$feature_dependencies/watch.log" >&2
+  exit 1
+fi
+kill -TERM "$feature_dependencies_pid"
+wait "$feature_dependencies_pid" 2>/dev/null || true
+
+if "$port" build "$unlinked_dependency" \
+  >"$unlinked_dependency/build.log" 2>&1; then
+  echo "build resolved an unlinked packages dependency" >&2
+  exit 1
+fi
+grep -q "Could not resolve dependency dep" "$unlinked_dependency/build.log"
+if "$port" clean "$unlinked_dependency" \
+  >"$unlinked_dependency/clean.log" 2>&1; then
+  echo "clean resolved an unlinked packages dependency" >&2
+  exit 1
+fi
+grep -q "Could not resolve dependency dep" "$unlinked_dependency/clean.log"
+test -f "$unlinked_dependency/packages/dep/src/Dep.js"
+test -f "$unlinked_dependency/packages/dep/lib/ocaml/marker"
+
+"$port" clean "$feature_dependencies"
+"$port" build --prod "$feature_dependencies"
+test -f "$feature_dependencies/packages/dep-union/native/UnionNative.js"
+test -f "$feature_dependencies/packages/dep-union/web/UnionWeb.js"
+test ! -f "$feature_dependencies/packages/dep-union/extra/UnionExtra.js"
+
+"$port" build "$gentype"
+test -f "$gentype/src/Main.js"
+
+"$port" build "$dependency"
+test -f "$dependency/src/Main.js"
+test -f "$dependency/node_modules/dep/src/Dep.js"
+rm "$dependency/node_modules/dep/src/Dep.js"
+"$port" build "$dependency"
+test -f "$dependency/node_modules/dep/src/Dep.js"
+"$port" clean "$dependency"
+test ! -f "$dependency/src/Main.js"
+test ! -f "$dependency/node_modules/dep/src/Dep.js"
+
+"$port" build "$package_output_dependency"
+grep 'export {' "$package_output_dependency/node_modules/dep/src/Dep.js" >/dev/null
+sed 's/"esmodule"/"commonjs"/' \
+  "$package_output_dependency/rescript.json" \
+  > "$package_output_dependency/rescript.next"
+mv "$package_output_dependency/rescript.next" \
+  "$package_output_dependency/rescript.json"
+"$port" build "$package_output_dependency"
+grep 'exports.value' \
+  "$package_output_dependency/node_modules/dep/src/Dep.js" >/dev/null
+sed -e 's/"commonjs"/"esmodule"/' \
+  -e 's/"in-source": true/"in-source": false/' \
+  -e 's/"suffix": "\.js"/"suffix": "\.mjs"/' \
+  "$package_output_dependency/rescript.json" \
+  > "$package_output_dependency/rescript.next"
+mv "$package_output_dependency/rescript.next" \
+  "$package_output_dependency/rescript.json"
+"$port" build "$package_output_dependency"
+if [ ! -f "$package_output_dependency/node_modules/dep/lib/es6/src/Dep.mjs" ]; then
+  echo "dependency was not rebuilt in its new output location" >&2
+  find "$package_output_dependency/node_modules/dep" -type f -print >&2
+  exit 1
+fi
+if [ -f "$package_output_dependency/node_modules/dep/src/Dep.js" ]; then
+  echo "dependency output from the previous package spec was retained" >&2
+  exit 1
+fi
+
+"$port" build "$standalone_output/node_modules/dep"
+test -f "$standalone_output/node_modules/dep/src/Dep.js"
+"$port" build "$standalone_output"
+test -f "$standalone_output/lib/es6/src/Main.mjs"
+test -f "$standalone_output/node_modules/dep/src/Dep.js"
+test ! -f "$standalone_output/node_modules/dep/lib/es6/src/Dep.mjs"
+node - "$standalone_output/node_modules/dep" <<'EOF'
+const fs = require("fs")
+const path = require("path")
+const dependency = path.resolve(process.argv[2])
+const info = JSON.parse(
+  fs.readFileSync(path.join(dependency, "lib", "bs", "compiler-info.json"), "utf8")
+)
+if (path.resolve(info.build_root) !== dependency) {
+  throw new Error(`standalone dependency ownership changed to ${info.build_root}`)
+}
+EOF
+"$port" clean "$standalone_output"
+test ! -d "$standalone_output/lib/bs"
+test -f "$standalone_output/node_modules/dep/lib/bs/compiler-info.json"
+test -f "$standalone_output/node_modules/dep/src/Dep.js"
+
+mkdir -p "$external_boundary/project/node_modules"
+directory_link "$external_boundary/project/packages/main" \
+  "$external_boundary/project/node_modules/main"
+directory_link "$external_boundary/external" \
+  "$external_boundary/project/node_modules/external"
+"$port" build --warn-error A "$external_boundary/project" \
+  >"$external_boundary/build.log" 2>&1
+grep "Please report this to the package maintainer: https://example.com/external/issues" \
+  "$external_boundary/build.log" >/dev/null
+test -f "$external_boundary/external/src/Sentinel.js"
+test -f "$external_boundary/external/src/Foo.mjs"
+test -f "$external_boundary/external/src/Foo.mjs.map"
+rm "$external_boundary/external/src/Foo.res"
+rm "$external_boundary/external/src/Foo.resi"
+"$port" build "$external_boundary/project"
+test ! -f "$external_boundary/external/src/Foo.mjs"
+test ! -f "$external_boundary/external/src/Foo.mjs.map"
+test -f "$external_boundary/external/src/Foo.js"
+"$port" clean "$external_boundary/project"
+test -f "$external_boundary/external/src/Sentinel.js"
+test -f "$external_boundary/external/src/Foo.js"
+
+"$port" build "$post_build"
+test -f "$post_build/src/Main.js"
+if $windows_posix_shell; then
+  "$port" build "$work/quoted-post-build"
+  test -f "$work/quoted-post-build/src/Main.js"
+fi
+
+"$port" build "$ppx_filter"
+test -f "$ppx_filter/src/Main.js"
+
+"$port" build "$out_of_source"
+test -f "$out_of_source/lib/es6/src/Main.js"
+rm -f "$out_of_source/src/Main.res"
+"$port" build "$out_of_source"
+test ! -f "$out_of_source/lib/es6/src/Main.js"
+
+namespace_call_log="$namespace/bsc-calls.log"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=counting \
+  REWATCH_BSC_CALL_LOG="$(native_path "$namespace_call_log")" \
+"$port" build "$namespace"
+test -f "$namespace/lib/ocaml/A-Widget.cmi"
+test -f "$namespace/src/B.js"
+: > "$namespace_call_log"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=counting \
+  REWATCH_BSC_CALL_LOG="$(native_path "$namespace_call_log")" \
+  "$port" build "$namespace"
+if grep -F 'Widget.mlmap' "$namespace_call_log" >/dev/null; then
+  echo "unchanged build unexpectedly recompiled its namespace" >&2
+  exit 1
+fi
+printf '\nlet changed = 1\n' >> "$namespace/src/B.res"
+env RESCRIPT_BSC_EXE="$test_proxy" \
+  REWATCH_REAL_BSC="$RESCRIPT_BSC_EXE" \
+  REWATCH_BSC_PROXY_MODE=counting \
+  REWATCH_BSC_CALL_LOG="$(native_path "$namespace_call_log")" \
+  "$port" build "$namespace"
+grep -F 'Widget.mlmap' "$namespace_call_log" >/dev/null
+
+"$port" build "$namespace_invalidation"
+rm "$namespace_invalidation/packages/dep/src/A.res"
+if "$port" build "$namespace_invalidation" \
+  >"$namespace_invalidation/removal.log" 2>&1; then
+  echo "namespace consumer was not invalidated after member removal" >&2
+  exit 1
+fi
+grep -F 'Ns.A' "$namespace_invalidation/removal.log" >/dev/null
+
+"$port" build "$namespace_restart"
+printf 'let value = "changed"\n' \
+  >"$namespace_restart/packages/dep/src/A.res"
+if "$port" build "$namespace_restart" \
+  >"$namespace_restart/first-failure.log" 2>&1; then
+  echo "namespace consumer unexpectedly accepted a changed member CMI" >&2
+  exit 1
+fi
+if "$port" build "$namespace_restart" \
+  >"$namespace_restart/restarted-failure.log" 2>&1; then
+  echo "restarted build forgot a changed namespace member CMI" >&2
+  exit 1
+fi
+if ! grep -F 'This has type:' \
+  "$namespace_restart/restarted-failure.log" >/dev/null || \
+  ! grep -F 'string' "$namespace_restart/restarted-failure.log" >/dev/null; then
+  cat "$namespace_restart/restarted-failure.log" >&2
+  exit 1
+fi
+
+"$port" build "$namespace_entry"
+test -f "$namespace_entry/src/Entry.mjs"
+test -f "$namespace_entry/lib/ocaml/Entry.cmi"
+test -f "$namespace_entry/lib/ocaml/Entry_alias-@EntryNamespace.cmi"
+
+"$port" build "$qualified_namespace"
+test -f "$qualified_namespace/src/Extent.js"
+test -f "$qualified_namespace/src/Geometry.js"
+
+if "$port" build "$namespace_collision" \
+  >"$namespace_collision/output.log" 2>&1; then
+  echo "namespace collision build unexpectedly succeeded" >&2
+  exit 1
+fi
+grep 'Namespace SharedNamespace is provided by both' \
+  "$namespace_collision/output.log" >/dev/null
+grep 'namespace-one' "$namespace_collision/output.log" >/dev/null
+grep 'namespace-two' "$namespace_collision/output.log" >/dev/null
+
+"$port" build "$source_map"
+test -f "$source_map/src/Main.js.map"
+test -f "$source_map/lib/bs/compiler-info.json"
+"$port" build "$source_map" >"$source_map/unchanged.log"
+grep 'Compiled 0 modules' "$source_map/unchanged.log" >/dev/null
+sed 's/"mode": "linked"/"mode": "hidden"/' "$source_map/rescript.json" \
+  > "$source_map/rescript.next"
+mv "$source_map/rescript.next" "$source_map/rescript.json"
+"$port" build "$source_map" >"$source_map/changed.log"
+grep 'Cleaned previous build due to compiler update' \
+  "$source_map/changed.log" >/dev/null
+grep 'Compiled 1 modules' "$source_map/changed.log" >/dev/null
+
+sed 's/"sources":"src"/"sources":"src","dependencies":["consumer"]/' \
+  "$monorepo/packages/dep/rescript.json" > "$monorepo/packages/dep/rescript.next"
+mv "$monorepo/packages/dep/rescript.next" "$monorepo/packages/dep/rescript.json"
+"$port" build "$monorepo"
+test -f "$monorepo/src/Root.js"
+test -f "$monorepo/packages/consumer/src/Consumer.js"
+test -f "$monorepo/packages/dep/src/Dep.js"
+"$port" clean "$monorepo"
+test ! -d "$monorepo/lib/ocaml"
+# This package was built directly above, so the later workspace build must not
+# transfer ownership of its outputs to the parent invocation.
+test -d "$monorepo/packages/consumer/lib/ocaml"
+test -d "$monorepo/packages/dep/lib/ocaml"
+"$port" clean "$monorepo/packages/consumer"
+test ! -d "$monorepo/packages/consumer/lib/ocaml"
+test ! -d "$monorepo/packages/dep/lib/ocaml"
+rm -f "$features/native/Native.js"
+"$port" build --features all "$features"
+test -f "$features/native/Native.js"
+
+if "$port" build "$cycle" >"$cycle/output.log" 2>&1; then
+  echo "cycle build unexpectedly succeeded" >&2
+  exit 1
+fi
+grep "circular dependency" "$cycle/output.log" >/dev/null
+
+if "$port" build "$multiple_cycles" \
+  >"$multiple_cycles/output.log" 2>&1; then
+  echo "multiple-cycle build unexpectedly succeeded" >&2
+  exit 1
+fi
+grep "circular dependency" "$multiple_cycles/output.log" >/dev/null
+if grep "subprocess dependency graph contains a cycle" \
+  "$multiple_cycles/output.log" >/dev/null; then
+  echo "multiple cycles escaped build preparation" >&2
+  exit 1
+fi
+test -f "$multiple_cycles/src/Valid.mjs"
+test ! -f "$multiple_cycles/src/A.mjs"
+test ! -f "$multiple_cycles/src/B.mjs"
+test ! -f "$multiple_cycles/src/C.mjs"
+test ! -f "$multiple_cycles/src/D.mjs"
+
+if "$port" build "$failure" >"$failure/output.log" 2>&1; then
+  echo "invalid build unexpectedly succeeded" >&2
+  exit 1
+fi
+test -f "$failure/lib/bs/build.ninja"
+grep "expected to have type" "$failure/output.log" >/dev/null
+test ! -f "$root/lib/build.lock"
+
+cp "$failure/Broken.fixed" "$failure/src/Broken.res"
+"$port" build "$failure"
+test -f "$failure/src/Broken.js"
+
+rm -rf "$basic/lib" "$cycle/lib" "$failure/lib"
+rm -rf "$legacy_config/lib"
+rm -rf "$features/lib"
+rm -rf "$gentype/lib"
+rm -rf "$gentype/node_modules/dep/lib"
+rm -rf "$dependency/lib" "$dependency/node_modules/dep/lib"
+rm -rf "$post_build/lib"
+rm -rf "$out_of_source/lib"
+rm -rf "$namespace/lib"
+rm -rf "$namespace_entry/lib"
+rm -rf "$qualified_namespace/lib"
+rm -rf "$namespace_collision/lib"
+rm -rf "$source_map/lib"
+rm -f "$source_map/unchanged.log" "$source_map/changed.log"
+rm -f "$basic/src/A.mjs" "$basic/src/B.mjs" "$basic/src/WithInterface.mjs"
+rm -f "$legacy_config/src/A.mjs" "$legacy_config/src/B.mjs" "$legacy_config/src/WithInterface.mjs"
+rm -f "$cycle/output.log" "$failure/output.log"
+rm -f "$namespace_collision/output.log"
diff --git a/rewatch-ocaml/tests/rust_test_coverage.tsv b/rewatch-ocaml/tests/rust_test_coverage.tsv
new file mode 100644
index 00000000000..f0e126deae5
--- /dev/null
+++ b/rewatch-ocaml/tests/rust_test_coverage.tsv
@@ -0,0 +1,137 @@
+# test	status	evidence
+build.rs::with_build_lock_holds_lock_while_running_work	covered	focused concurrent-build test observes build.lock during compiler work
+build.rs::with_build_lock_drops_lock_after_error_result	covered	focused failure test verifies lock removal before recovery
+build.rs::build_waits_for_lock_before_initializing	covered	focused concurrent-build test waits and then succeeds
+build.rs::formats_successful_completion_message	covered	output_tests.ml: clean status and two-decimal timing format
+build.rs::formats_warning_completion_message	covered	output_tests.ml: incremental warning status and suffix
+build/compile.rs::compiler_output_to_string_handles_invalid_utf8	covered	unit_tests.ml: lossy UTF-8 process capture
+build/compile.rs::retain_critical_external_warnings_returns_none_without_marker	covered	unit_tests.ml: ordinary external warning suppression
+build/compile.rs::retain_critical_external_warnings_keeps_uncurried_dot_block	covered	unit_tests.ml: critical uncurried warning retention
+build/compile.rs::retain_critical_external_warnings_handles_crlf_line_endings	covered	unit_tests.ml: CRLF warning block normalization
+build/compile.rs::replays_stored_warning_for_module_that_did_not_recompile	covered	focused warning watcher replays output without another WarningA compile call
+build/compile.rs::replays_stored_warnings_in_module_name_order	covered	warning_state_tests.ml: deterministic module-name order
+build/compile.rs::appends_fresh_and_stored_warnings_in_shared_module_name_order	covered	warning_state_tests.ml: fresh/stored shared ordering
+build/compile.rs::does_not_replay_stored_warning_for_module_that_recompiled	covered	warning_state_tests.ml: clean recompile removes stored warning
+build/compiler_info.rs::verify_compiler_info_keeps_package_when_source_map_args_match	covered	compiler_info_tests.ml and focused source-map no-op rebuild retain artifacts
+build/compiler_info.rs::verify_compiler_info_cleans_package_when_source_map_args_change	covered	compiler_info_tests.ml and focused source-map mode change clean and rebuild the package
+build/packages.rs::should_return_false_with_invalid_parents_as_bs_dependencies	covered	unit_tests.ml: disallowed regular dependency rejected end to end
+build/packages.rs::should_return_false_with_invalid_parents_as_dev_dependencies	covered	unit_tests.ml: disallowed dev dependency rejected end to end
+build/packages.rs::should_return_true_with_no_invalid_parent	covered	unit_tests.ml: listed and unrestricted dependents accepted
+build/packages.rs::should_report_missing_name_when_package_and_rescript_json_lack_it	covered	config_tests.ml: package config name is required
+build/packages.rs::issue_tracker_url_prefers_bugs_url_string	covered	package_metadata_tests.ml: bugs string precedence
+build/packages.rs::issue_tracker_url_prefers_bugs_object_url	covered	package_metadata_tests.ml: bugs object precedence
+build/packages.rs::issue_tracker_url_derives_from_repository_git_url	covered	package_metadata_tests.ml: git+https repository URL
+build/packages.rs::issue_tracker_url_derives_from_repository_object	covered	package_metadata_tests.ml: repository object URL
+build/packages.rs::issue_tracker_url_handles_shorthand	covered	package_metadata_tests.ml: GitHub shorthand
+build/packages.rs::issue_tracker_url_returns_none_without_hints	covered	package_metadata_tests.ml: missing metadata
+build/packages.rs::monorepo_root_marks_transitive_workspace_dependencies_as_local	covered	feature-dependencies fixture traverses local transitive packages and their dev edges
+build/packages.rs::compute_active_features_returns_all_when_cli_absent	covered	feature-dependencies fixture: unrestricted request enables dep extras
+build/packages.rs::compute_active_features_honours_cli_restriction_and_expands_transitive	shared	canonical feature CLI/transitive expansion tests
+build/packages.rs::compute_active_features_dep_uses_consumer_restriction	covered	feature-dependencies fixture unions native and web consumer requests
+build/packages.rs::compute_active_features_prod_ignores_dev_dependency_feature_requests	covered	feature-dependencies fixture excludes extra under --prod
+build/packages.rs::compute_active_features_honours_explicit_empty_features_list	covered	feature-dependencies fixture builds only untagged source for empty request
+cli.rs::no_subcommand_defaults_to_build	covered	cli_tests.ml: no subcommand defaults to build
+cli.rs::defaults_to_build_with_folder_shortcut	covered	cli_tests.ml: bare folder uses implicit build
+cli.rs::trailing_global_flag_is_treated_as_global	covered	cli_tests.ml: trailing verbosity flag
+cli.rs::double_dash_keeps_following_args_positional	covered	cli_tests.ml: option-looking folder after double dash
+cli.rs::unknown_subcommand_help_uses_global_help	covered	cli_tests.ml: implicit folder help
+cli.rs::build_help_shows_subcommand_help	covered	cli_tests.ml: explicit build help
+cli.rs::build_allows_global_verbose_flag	covered	cli_tests.ml: trailing build verbosity
+cli.rs::build_option_is_parsed_normally	covered	cli_tests.ml: build-only no-timing option
+cli.rs::respects_global_flag_before_subcommand	covered	cli_tests.ml: leading verbosity before watch
+cli.rs::invalid_option_for_subcommand_does_not_fallback	covered	cli_tests.ml: watch rejects build-only option
+cli.rs::version_flag_before_subcommand_displays_version	covered	cli_tests.ml: leading version flag
+cli.rs::version_flag_after_subcommand_is_rejected	intentional	OCaml exposes Cmdliner's consistent version option on subcommands
+cli.rs::global_help_flag_shows_help	covered	cli_tests.ml: global help
+cli.rs::global_version_flag_shows_version	covered	cli_tests.ml: global version
+cli.rs::build_prod_flag_is_parsed	covered	cli_tests.ml: build prod
+cli.rs::build_prod_flag_defaults_to_false	covered	cli_tests.ml: build prod defaults false
+cli.rs::watch_prod_flag_is_parsed	covered	cli_tests.ml: watch prod
+cli.rs::watch_clear_screen_flag_is_parsed	covered	cli_tests.ml: watch clear-screen
+cli.rs::clean_prod_flag_is_parsed	covered	cli_tests.ml: clean prod
+cli.rs::prod_flag_defaults_to_build_command	covered	cli_tests.ml: implicit prod build
+cli.rs::non_utf_argument_returns_error	covered	cli_tests.ml: invalid UTF-8 byte string
+cli.rs::build_features_flag_is_parsed	covered	cli_tests.ml: build features
+cli.rs::build_features_flag_defaults_to_none	covered	cli_tests.ml: build features default to none
+cli.rs::build_features_flag_rejects_empty_string	covered	cli_tests.ml: empty features rejected
+cli.rs::watch_features_flag_is_parsed	covered	cli_tests.ml: watch features
+cli.rs::features_flag_round_trips_through_build_to_watch_args	covered	cli_tests.ml: build/watch feature conversion agrees
+cli.rs::build_features_flag_strips_whitespace	covered	cli_tests.ml: feature names trimmed
+config.rs::test_getters	covered	config_tests.ml: package name, suffix, specs, and dependency getters
+config.rs::test_package_specs_duplicate_suffix_default	covered	unit_tests.ml: duplicate effective output rejected
+config.rs::test_package_specs_duplicate_suffix_explicit	covered	unit_tests.ml: duplicate explicit output rejected
+config.rs::test_package_specs_duplicate_suffix_different_in_source_ok	covered	config_tests.ml: same suffix in different output locations
+config.rs::test_sources	covered	source_tests.ml: dev source parsing and discovery
+config.rs::test_dev_sources_multiple	covered	source_tests.ml: mixed shorthand/dev sources
+config.rs::test_detect_gentypeconfig	shared	canonical GenType build tests
+config.rs::test_gentype_shims_object_and_array_forms	covered	unit_tests.ml: legacy shim normalization and map semantics
+config.rs::test_gentype_module_falls_back_to_package_specs_module	covered	unit_tests.ml: GenType module fallback
+config.rs::test_gentype_module_explicit_wins_over_package_specs	covered	config_tests.ml: explicit GenType module precedence
+config.rs::test_gentype_args_without_gentype_config	covered	config_tests.ml: no GenType arguments without config
+config.rs::test_other_jsx_module	covered	config_tests.ml: custom JSX module
+config.rs::test_jsx_preserve	covered	config_tests.ml: JSX preserve argument
+config.rs::test_source_map_args	covered	unit_tests.ml: always/inline and source-map arguments
+config.rs::test_source_map_dev_args_only_in_watch	covered	unit_tests.ml: dev source maps differ in build/watch
+config.rs::test_source_map_false_args	covered	config_tests.ml: sourceMap false arguments
+config.rs::test_source_map_rejects_true_for_nested_config	covered	unit_tests.ml: sourceMap true rejected
+config.rs::test_source_map_requires_enabled	covered	unit_tests.ml: sourceMap enabled required
+config.rs::test_source_map_inline_args	covered	unit_tests.ml: inline source maps
+config.rs::test_source_map_hidden_args	covered	config_tests.ml: hidden source-map arguments
+config.rs::test_source_map_rejects_external_mode	covered	config_tests.ml: invalid source-map mode
+config.rs::test_get_suffix	shared	canonical suffix tests
+config.rs::test_dependencies	shared	canonical dependency tests
+config.rs::test_bs_dependencies_alias	shared	canonical dependency alias tests
+config.rs::test_dev_dependencies	shared	canonical development dependency tests
+config.rs::test_bs_dev_dependencies_alias	covered	config_tests.ml: legacy dev dependency alias and deprecation
+config.rs::test_package_specs_es6_global_deprecation	covered	config_tests.ml: unsupported module rejected
+config.rs::test_es6_module_alias	covered	unit_tests.ml: es6 deprecation diagnostic
+config.rs::test_unknown_fields_are_collected	covered	config_tests.ml: top-level unknown field
+config.rs::test_unsupported_fields_are_collected	covered	unit_tests.ml: unsupported field classification
+config.rs::test_editor_field_supported	covered	config_tests.ml: editor and reanalyze payloads ignored
+config.rs::test_compiler_flags	shared	canonical compiler-argument tests
+config.rs::test_cjs_module_alias	covered	unit_tests.ml: cjs deprecation diagnostic
+config.rs::test_bsc_flags_alias	shared	canonical compiler flag alias test
+config.rs::test_find_is_type_dev_for_exact_match	covered	source_tests.ml: exact dev directory excluded in prod
+config.rs::test_find_is_type_dev_for_none_dev	covered	source_tests.ml: ordinary source retained in prod
+config.rs::test_find_is_type_dev_for_multiple_sources	covered	source_tests.ml: mixed source directories
+config.rs::test_find_is_type_dev_for_shorthand	covered	source_tests.ml: shorthand source is non-dev
+config.rs::test_find_is_type_dev_for_recursive_folder	covered	source_tests.ml: nested recursive dev source excluded in prod
+config.rs::test_find_is_type_dev_for_sub_folder	covered	unit_tests.ml: parent source type propagation
+config.rs::test_find_is_type_dev_for_sub_folder_shorthand	covered	unit_tests.ml: parent source type propagation
+config.rs::test_get_warning_args_with_override	shared	canonical compiler-argument warning override test
+config.rs::test_get_warning_args_without_override	shared	canonical compiler-argument warning config test
+config.rs::test_get_warning_args_non_local_dep	covered	external-boundary focused fixture ignores dependency warning errors
+config.rs::test_get_warning_args_override_ignores_config	shared	canonical warning override precedence test
+config.rs::test_get_warning_args_non_local_dep_ignores_override	covered	external recursion does not inherit the root warning override
+config.rs::test_new_missing_rescript_json	shared	focused missing-project tests
+config.rs::test_bsconfig_json_filename_deprecation	covered	unit_tests.ml and focused legacy-config test
+config.rs::test_source_with_feature_tag_parses	shared	canonical feature tests
+config.rs::test_features_map_parses	shared	canonical feature tests
+config.rs::test_dependency_qualified_form_parses	shared	canonical feature-restricted dependency tests
+config.rs::test_is_feature_enabled_untagged_is_always_active	covered	source_tests.ml: untagged source remains active under restriction
+config.rs::test_is_feature_enabled_tagged_requires_membership	covered	source_tests.ml: tagged source membership
+config.rs::test_resolve_active_features_expands_transitive	shared	canonical transitive feature test
+config.rs::test_resolve_active_features_no_map_is_identity	covered	source_tests.ml: leaf feature works without a feature map
+config.rs::test_resolve_active_features_detects_cycle	shared	canonical feature cycle test
+config.rs::test_collect_declared_features_unions_map_and_tags	shared	canonical default-all feature test covers map and tagged source declarations
+config.rs::test_feature_cascades_to_qualified_subdirs	shared	canonical nested feature source tests
+config.rs::test_feature_cascade_does_not_overwrite_explicit_child	shared	canonical nested feature source tests
+config.rs::test_dependency_helpers_return_name_and_features	covered	config_tests.ml: shorthand and qualified dependency data
+lock.rs::returns_error_when_project_folder_missing	shared	focused missing-project test
+lock.rs::creates_lock_when_project_folder_exists	covered	focused watch/build tests observe PID lock files
+lock.rs::only_one_concurrent_caller_acquires_lock	covered	focused concurrent-build test keeps the second build waiting
+lock.rs::ignores_stale_lock_for_unrelated_process_name	covered	unit_tests.ml: stale lock replacement
+lock.rs::returns_locked_for_active_watch_lock	shared	canonical lock test rejects a second watcher
+lock.rs::waits_for_active_build_lock_to_be_removed	covered	focused concurrent-build test waits for explicit release
+lock.rs::drop_lock_removes_existing_build_lock	covered	focused successful, failed, and interrupted builds remove locks
+queue.rs::test_basic_functionalities	intentional	OCaml uses dependency scheduler rather than Rust queue type
+queue.rs::test_queue_thread_safety	intentional	OCaml uses single-domain scheduler state and subprocess concurrency
+queue.rs::test_concurrent_pushes_and_pops	intentional	OCaml uses single-domain scheduler state and subprocess concurrency
+queue.rs::test_concurrent_mixed_operations	intentional	OCaml uses single-domain scheduler state and subprocess concurrency
+telemetry.rs::noop_guard_reports_otel_disabled	omitted	OpenTelemetry is an explicit project non-goal
+telemetry.rs::noop_guard_drops_cleanly_without_explicit_shutdown	omitted	OpenTelemetry is an explicit project non-goal
+telemetry.rs::init_telemetry_without_env_is_noop	omitted	OpenTelemetry is an explicit project non-goal
+watcher.rs::clears_screen_only_for_interactive_rebuilds	covered	output_tests.ml: clear-screen requires both option and interactive streams
+watcher.rs::carries_forward_implementation_warnings_for_matching_module_paths	covered	canonical warning persistence tests plus focused no-recompile call count
+watcher.rs::does_not_carry_forward_warnings_when_module_paths_change	covered	warning_state_tests.ml: retain_paths drops changed implementation path
+watcher.rs::carries_forward_interface_warnings_for_matching_interface_paths	covered	warning_state_tests.ml: matching interface survives implementation path removal
diff --git a/rewatch-ocaml/tests/shared-dep/rescript.json b/rewatch-ocaml/tests/shared-dep/rescript.json
new file mode 100644
index 00000000000..13642506ebb
--- /dev/null
+++ b/rewatch-ocaml/tests/shared-dep/rescript.json
@@ -0,0 +1,4 @@
+{
+  "name": "dep",
+  "sources": "src"
+}
diff --git a/rewatch-ocaml/tests/shared-dep/src/Dep.res b/rewatch-ocaml/tests/shared-dep/src/Dep.res
new file mode 100644
index 00000000000..e51d91c9bc2
--- /dev/null
+++ b/rewatch-ocaml/tests/shared-dep/src/Dep.res
@@ -0,0 +1 @@
+let value = 42
diff --git a/rewatch-ocaml/tests/source-map/rescript.json b/rewatch-ocaml/tests/source-map/rescript.json
new file mode 100644
index 00000000000..3c06868f1e1
--- /dev/null
+++ b/rewatch-ocaml/tests/source-map/rescript.json
@@ -0,0 +1,5 @@
+{
+  "name": "source-map",
+  "sources": "src",
+  "sourceMap": {"enabled": "always", "mode": "linked", "sourcesContent": true}
+}
diff --git a/rewatch-ocaml/tests/source-map/src/Main.res b/rewatch-ocaml/tests/source-map/src/Main.res
new file mode 100644
index 00000000000..3c37c33b59a
--- /dev/null
+++ b/rewatch-ocaml/tests/source-map/src/Main.res
@@ -0,0 +1 @@
+let value = 1
diff --git a/rewatch-ocaml/tests/stream-after-build.js b/rewatch-ocaml/tests/stream-after-build.js
new file mode 100644
index 00000000000..6bcb0b39dc2
--- /dev/null
+++ b/rewatch-ocaml/tests/stream-after-build.js
@@ -0,0 +1,13 @@
+import fs from "node:fs";
+
+const releaseFile = process.argv[2];
+if (releaseFile === undefined) {
+  throw new Error("Expected the release-file path as the first argument");
+}
+
+console.log("REWATCH_AFTER_BUILD_READY");
+const timer = setInterval(() => {
+  if (fs.existsSync(releaseFile)) {
+    clearInterval(timer);
+  }
+}, 50);
diff --git a/rewatch-ocaml/tests/warning-replay/rescript.json b/rewatch-ocaml/tests/warning-replay/rescript.json
new file mode 100644
index 00000000000..a42c5b0de4f
--- /dev/null
+++ b/rewatch-ocaml/tests/warning-replay/rescript.json
@@ -0,0 +1,6 @@
+{
+  "name": "warning-replay",
+  "sources": "src",
+  "package-specs": {"module": "commonjs", "in-source": true},
+  "suffix": ".js"
+}
diff --git a/rewatch-ocaml/tests/warning-replay/src/B.res b/rewatch-ocaml/tests/warning-replay/src/B.res
new file mode 100644
index 00000000000..0297611e918
--- /dev/null
+++ b/rewatch-ocaml/tests/warning-replay/src/B.res
@@ -0,0 +1 @@
+let value = WarningA.visible
diff --git a/rewatch-ocaml/tests/warning-replay/src/WarningA.res b/rewatch-ocaml/tests/warning-replay/src/WarningA.res
new file mode 100644
index 00000000000..5899b422df7
--- /dev/null
+++ b/rewatch-ocaml/tests/warning-replay/src/WarningA.res
@@ -0,0 +1,2 @@
+let unusedValue = 42
+let visible = 1
diff --git a/rewatch-ocaml/tests/warning-replay/src/WarningA.resi b/rewatch-ocaml/tests/warning-replay/src/WarningA.resi
new file mode 100644
index 00000000000..2ee09c90211
--- /dev/null
+++ b/rewatch-ocaml/tests/warning-replay/src/WarningA.resi
@@ -0,0 +1 @@
+let visible: int
diff --git a/rewatch-ocaml/toolchain.ml b/rewatch-ocaml/toolchain.ml
new file mode 100644
index 00000000000..750e9fe2026
--- /dev/null
+++ b/rewatch-ocaml/toolchain.ml
@@ -0,0 +1,58 @@
+exception Error of string
+
+let canonical_existing ~message path =
+  try
+    if File_util.exists path then Platform.canonicalize_path path
+    else raise (Error (message path))
+  with
+  | Unix.Unix_error (error, function_name, argument) ->
+    raise
+      (Error
+         (Printf.sprintf "%s: %s (%s %s)" (message path)
+            (Unix.error_message error) function_name argument))
+  | Sys_error detail -> raise (Error (message path ^ ": " ^ detail))
+
+let absolute_program ~cwd program =
+  let resolved = Platform.resolve_program ~cwd program in
+  if Filename.is_relative resolved then Filename.concat cwd resolved
+  else resolved
+
+let sibling_bsc_candidate ~cwd ~executable =
+  let executable = absolute_program ~cwd executable in
+  let executable =
+    canonical_existing
+      ~message:(fun path -> "Could not locate current executable " ^ path)
+      executable
+  in
+  Filename.concat (Filename.dirname executable) "bsc.exe"
+
+let bsc () =
+  let candidate, message =
+    match Sys.getenv_opt "RESCRIPT_BSC_EXE" with
+    | Some path ->
+      (path, fun missing -> "RESCRIPT_BSC_EXE points to missing path " ^ missing)
+    | None ->
+      ( sibling_bsc_candidate ~cwd:(Sys.getcwd ())
+          ~executable:Sys.executable_name,
+        fun missing ->
+          "Could not locate bsc next to the ReScript executable at " ^ missing
+          ^ "; set RESCRIPT_BSC_EXE to override it" )
+  in
+  canonical_existing ~message candidate
+
+let runtime ~find_package =
+  match Sys.getenv_opt "RESCRIPT_RUNTIME" with
+  | Some path ->
+    canonical_existing
+      ~message:(fun missing ->
+        "RESCRIPT_RUNTIME points to missing path " ^ missing)
+      path
+  | None -> (
+    match find_package "@rescript/runtime" with
+    | Some path -> path
+    | None ->
+      raise
+        (Error
+           "The rescript runtime package could not be found.\n\
+            Please set RESCRIPT_RUNTIME environment variable or make sure the \
+            runtime package is installed."))
diff --git a/rewatch-ocaml/toolchain.mli b/rewatch-ocaml/toolchain.mli
new file mode 100644
index 00000000000..3fc8a661719
--- /dev/null
+++ b/rewatch-ocaml/toolchain.mli
@@ -0,0 +1,5 @@
+exception Error of string
+
+val sibling_bsc_candidate : cwd:string -> executable:string -> string
+val bsc : unit -> string
+val runtime : find_package:(string -> string option) -> string
diff --git a/rewatch-ocaml/traversal_coverage.ml b/rewatch-ocaml/traversal_coverage.ml
new file mode 100644
index 00000000000..f8e8f89869f
--- /dev/null
+++ b/rewatch-ocaml/traversal_coverage.ml
@@ -0,0 +1,28 @@
+type t = Shallow | Recursive
+
+type admission =
+  | Skip
+  | Visit_current
+  | Visit_current_and_descendants
+  | Visit_descendants
+
+let admit visited identity ~recursive =
+  match (Hashtbl.find_opt visited identity, recursive) with
+  | None, false ->
+    Hashtbl.add visited identity Shallow;
+    Visit_current
+  | None, true ->
+    Hashtbl.add visited identity Recursive;
+    Visit_current_and_descendants
+  | Some Shallow, true ->
+    Hashtbl.replace visited identity Recursive;
+    Visit_descendants
+  | Some Shallow, false | Some Recursive, _ -> Skip
+
+let visits_current = function
+  | Visit_current | Visit_current_and_descendants -> true
+  | Skip | Visit_descendants -> false
+
+let visits_descendants = function
+  | Visit_current_and_descendants | Visit_descendants -> true
+  | Skip | Visit_current -> false
diff --git a/rewatch-ocaml/traversal_coverage.mli b/rewatch-ocaml/traversal_coverage.mli
new file mode 100644
index 00000000000..5f31c2764c8
--- /dev/null
+++ b/rewatch-ocaml/traversal_coverage.mli
@@ -0,0 +1,11 @@
+type t
+
+type admission =
+  | Skip
+  | Visit_current
+  | Visit_current_and_descendants
+  | Visit_descendants
+
+val admit : (string, t) Hashtbl.t -> string -> recursive:bool -> admission
+val visits_current : admission -> bool
+val visits_descendants : admission -> bool
diff --git a/rewatch-ocaml/warning_state.ml b/rewatch-ocaml/warning_state.ml
new file mode 100644
index 00000000000..915e621e4e2
--- /dev/null
+++ b/rewatch-ocaml/warning_state.ml
@@ -0,0 +1,34 @@
+type entry = {
+  module_name: string;
+  package_root: string;
+  path: string;
+  output: string;
+}
+
+type t = (string, entry) Hashtbl.t
+
+let create () = Hashtbl.create 16
+
+let key ~package_root ~path = Filename.concat package_root path
+
+let set state ~module_name ~package_root ~path ~output =
+  Hashtbl.replace state (key ~package_root ~path)
+    {module_name; package_root; path; output}
+
+let remove state ~package_root ~path =
+  Hashtbl.remove state (key ~package_root ~path)
+
+let retain_paths state paths =
+  let current = Hashtbl.create (List.length paths) in
+  List.iter (fun path -> Hashtbl.replace current path ()) paths;
+  Hashtbl.filter_map_inplace
+    (fun path entry -> if Hashtbl.mem current path then Some entry else None)
+    state
+
+let entries state =
+  Hashtbl.to_seq_values state
+  |> List.of_seq
+  |> List.sort (fun first second ->
+      match String.compare first.module_name second.module_name with
+      | 0 -> String.compare first.path second.path
+      | order -> order)
diff --git a/rewatch-ocaml/warning_state.mli b/rewatch-ocaml/warning_state.mli
new file mode 100644
index 00000000000..affb1f3fb2c
--- /dev/null
+++ b/rewatch-ocaml/warning_state.mli
@@ -0,0 +1,25 @@
+type entry = {
+  module_name: string;
+  package_root: string;
+  path: string;
+  output: string;
+}
+(** Warning output survives retained watch attempts so unchanged warning-bearing
+    modules can replay diagnostics. Watch shutdown uses the stored paths to
+    invalidate freshness markers for the next process invocation. *)
+
+type t
+
+val create : unit -> t
+
+val set :
+  t ->
+  module_name:string ->
+  package_root:string ->
+  path:string ->
+  output:string ->
+  unit
+
+val remove : t -> package_root:string -> path:string -> unit
+val retain_paths : t -> string list -> unit
+val entries : t -> entry list
diff --git a/rewatch-ocaml/watch_scope.ml b/rewatch-ocaml/watch_scope.ml
new file mode 100644
index 00000000000..3f70ae07f27
--- /dev/null
+++ b/rewatch-ocaml/watch_scope.ml
@@ -0,0 +1,260 @@
+type source_root = {
+  directory: string;
+  recursive: bool;
+  filter: Source_filter.t option;
+}
+
+type t = {
+  roots: string list;
+  paths: Native_watcher.watch_path list;
+  sources: source_root list;
+  unresolved: string list;
+}
+
+let normalize_lexically path =
+  let has_double_root =
+    String.length path >= 2
+    && path.[0] = Filename.dir_sep.[0]
+    && path.[1] = Filename.dir_sep.[0]
+  in
+  let rec decompose current components =
+    let parent = Filename.dirname current in
+    if parent = current then (current, components)
+    else decompose parent (Filename.basename current :: components)
+  in
+  let root, components = decompose path [] in
+  let root, components =
+    if has_double_root then
+      match components with
+      | server :: share :: remaining ->
+        ( Filename.dir_sep ^ Filename.concat (Filename.concat root server) share,
+          remaining )
+      | _ -> (root, components)
+    else (root, components)
+  in
+  let normalized =
+    List.fold_left
+      (fun current component ->
+        match component with
+        | "." -> current
+        | ".." -> if current = root then current else Filename.dirname current
+        | component -> Filename.concat current component)
+      root components
+  in
+  if
+    has_double_root
+    && (String.length normalized < 2 || normalized.[1] <> Filename.dir_sep.[0])
+  then Filename.dir_sep ^ normalized
+  else normalized
+
+let comparable_path path =
+  path |> Platform.normalize_path_for_comparison |> normalize_lexically
+
+let descendant_prefix directory =
+  if String.ends_with ~suffix:Filename.dir_sep directory then directory
+  else directory ^ Filename.dir_sep
+
+let is_same_or_below ~directory path =
+  let directory = comparable_path directory in
+  let path = comparable_path path in
+  path = directory
+  || String.starts_with ~prefix:(descendant_prefix directory) path
+
+let path_is_in_source_tree scope path =
+  List.exists
+    (fun source ->
+      let path = comparable_path path in
+      let directory = comparable_path source.directory in
+      path = directory
+      || source.recursive
+         && String.starts_with ~prefix:(descendant_prefix directory) path)
+    scope.sources
+
+let path_is_source_ancestor scope path =
+  List.exists
+    (fun source -> is_same_or_below ~directory:path source.directory)
+    scope.sources
+
+let path_is_unresolved_ancestor scope path =
+  List.exists
+    (fun unresolved -> is_same_or_below ~directory:path unresolved)
+    scope.unresolved
+
+let control_file_names = ["rescript.json"; "bsconfig.json"; "package.json"]
+let is_control_file_name name = List.mem name control_file_names
+
+let discover ~root ~prod ~features ~filter =
+  try
+    let root_config = Config.load_root root in
+    let resolution =
+      Package_resolution.create
+        ~diagnostic_mode:Package_resolution.Suppress_diagnostics root_config
+    in
+    let roots = ref [root] in
+    let paths = ref [] in
+    let sources = ref [] in
+    let unresolved = ref [] in
+    let add_path directory recursive =
+      paths := Native_watcher.{directory; recursive} :: !paths
+    in
+    let rec nearest_existing_directory package_root directory =
+      if File_util.exists directory then directory
+      else
+        let parent = Filename.dirname directory in
+        if parent = directory || directory = package_root then package_root
+        else nearest_existing_directory package_root parent
+    in
+    let watch_unresolved_dependency package_root name =
+      Package_resolution.dependency_candidates resolution ~package_root name
+      |> List.iter (fun candidate ->
+          let existing = nearest_existing_directory root candidate in
+          try
+            let canonical_existing = Platform.canonicalize_path existing in
+            if Project_context.path_is_within_canonical ~root canonical_existing
+            then (
+              unresolved := candidate :: !unresolved;
+              (* A shallow ancestor watch is sufficient: each directory creation
+                 wakes reconciliation, which advances the watch toward the complete
+                 candidate without expanding all of node_modules. *)
+              add_path canonical_existing false)
+          with Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> ())
+    in
+    let graph =
+      Package_traversal.traverse ~root_config
+        ~root_name:(Package_resolution.root_package_name resolution)
+        ~prod ~features ~resolve:(fun config request ->
+          let dependency = request.Package_traversal.declaration in
+          match
+            Package_resolution.dependency_path resolution
+              ~package_root:config.root dependency.name
+          with
+          | Some directory when Config.exists_in_root directory -> (
+            try
+              let resolved =
+                Package_traversal.resolve resolution ~package_root:config.root
+                  request
+              in
+              if resolved.dependency.is_local then (
+                roots := resolved.dependency.directory :: !roots;
+                add_path resolved.dependency.directory false);
+              Some resolved
+            with Project_context.Package_error _ | Project_context.Error _ ->
+              (* A broken dependency configuration must remain watched so fixing
+                 that file can recover the long-lived command. *)
+              roots := directory :: !roots;
+              add_path directory false;
+              None)
+          | None ->
+            watch_unresolved_dependency config.root dependency.name;
+            None
+          | Some directory when File_util.is_directory directory ->
+            (* The parent watch is needed because removing or replacing the
+               watched directory itself is not reported consistently by every
+               filesystem backend. It also detects a newly installed candidate
+               that should take priority over a lower resolution. *)
+            roots := directory :: !roots;
+            unresolved := directory :: !unresolved;
+            add_path (Filename.dirname directory) false;
+            add_path directory false;
+            None
+          | Some path ->
+            (* A non-directory candidate may be replaced with an install. Watch
+               its parent and retain its type in the snapshot until that happens. *)
+            unresolved := path :: !unresolved;
+            add_path (Filename.dirname path) false;
+            None)
+    in
+    graph.packages
+    |> List.iter (fun (package : Package_traversal.package) ->
+        if package.is_local then (
+          let package_root = package.config.root in
+          let config = package.config in
+          let is_local = package.is_local in
+          add_path package_root false;
+          try
+            let requested =
+              Package_traversal.find_feature_selection graph package_root
+              |> Option.map Package_traversal.feature_selection_to_option
+              |> Option.value ~default:None
+            in
+            Source.active_sources config
+              ~prod:(Package_traversal.source_discovery_prod ~prod ~is_local)
+              ~features:requested
+            |> List.iter (fun source ->
+                let directory = Filename.concat config.root source.Config.dir in
+                let filter =
+                  if package_root = root_config.root then filter else None
+                in
+                sources :=
+                  {directory; recursive = source.recurse; filter} :: !sources;
+                let existing =
+                  nearest_existing_directory config.root directory
+                in
+                add_path existing (existing = directory && source.recurse))
+          with Source.Error _ -> ()));
+    let deduplicated_paths = Hashtbl.create (List.length !paths) in
+    List.iter
+      (fun (path : Native_watcher.watch_path) ->
+        let recursive =
+          path.recursive
+          || Option.value
+               (Hashtbl.find_opt deduplicated_paths path.directory)
+               ~default:false
+        in
+        Hashtbl.replace deduplicated_paths path.directory recursive)
+      !paths;
+    let paths =
+      Hashtbl.to_seq deduplicated_paths
+      |> List.of_seq
+      |> List.map (fun (directory, recursive) ->
+          Native_watcher.{directory; recursive})
+    in
+    {
+      roots = List.sort_uniq String.compare !roots;
+      paths;
+      sources = !sources;
+      unresolved = List.sort_uniq String.compare !unresolved;
+    }
+  with
+  | Config.Error _ | Project_context.Error _ | Project_context.Package_error _
+  ->
+    {
+      roots = [root];
+      paths = [Native_watcher.{directory = root; recursive = false}];
+      sources = [];
+      unresolved = [];
+    }
+
+let path_in_scope scope path =
+  let name = Filename.basename path in
+  let is_control =
+    let directory = comparable_path (Filename.dirname path) in
+    is_control_file_name name
+    && List.exists (fun root -> comparable_path root = directory) scope.roots
+  in
+  let is_source =
+    Option.is_some (Source.source_kind path)
+    && List.exists
+         (fun source ->
+           let source_directory = comparable_path source.directory in
+           let file_directory = comparable_path (Filename.dirname path) in
+           let in_directory =
+             file_directory = source_directory
+             || source.recursive
+                && String.starts_with
+                     ~prefix:(descendant_prefix source_directory)
+                     file_directory
+           in
+           in_directory
+           && Option.fold ~none:true
+                ~some:(fun filter -> Source_filter.matches_basename filter path)
+                source.filter)
+         scope.sources
+  in
+  let comparable = comparable_path path in
+  let is_unresolved =
+    List.exists
+      (fun unresolved -> comparable_path unresolved = comparable)
+      scope.unresolved
+  in
+  is_control || is_source || is_unresolved
diff --git a/rewatch-ocaml/watch_scope.mli b/rewatch-ocaml/watch_scope.mli
new file mode 100644
index 00000000000..d868c4c40b7
--- /dev/null
+++ b/rewatch-ocaml/watch_scope.mli
@@ -0,0 +1,31 @@
+(** A watch scope records both known sources and unresolved dependency
+    candidates. Missing candidates stay in the scope so installing or repairing
+    a dependency can wake the watcher without restarting it. *)
+
+type source_root = {
+  directory: string;
+  recursive: bool;
+  filter: Source_filter.t option;
+}
+
+type t = {
+  roots: string list;
+  paths: Native_watcher.watch_path list;
+  sources: source_root list;
+  unresolved: string list;
+}
+
+val control_file_names : string list
+val is_control_file_name : string -> bool
+
+val discover :
+  root:string ->
+  prod:bool ->
+  features:string list option ->
+  filter:Source_filter.t option ->
+  t
+
+val path_in_scope : t -> string -> bool
+val path_is_in_source_tree : t -> string -> bool
+val path_is_source_ancestor : t -> string -> bool
+val path_is_unresolved_ancestor : t -> string -> bool
diff --git a/rewatch-ocaml/watch_snapshot.ml b/rewatch-ocaml/watch_snapshot.ml
new file mode 100644
index 00000000000..6d6c05ad507
--- /dev/null
+++ b/rewatch-ocaml/watch_snapshot.ml
@@ -0,0 +1,366 @@
+type change_kind = Added | Removed | Modified
+type change = {path: string; kind: change_kind}
+
+type file = {modified: float; size: int; digest: string}
+type dependency = {modified: float; size: int}
+
+type state =
+  | File of file
+  | Dependency_candidate of dependency
+  | Missing_dependency_candidate
+  | Unreadable
+
+type entry = {path: string; state: state}
+
+type registration =
+  | Ready of {
+      snapshot: entry list;
+      paths: Native_watcher.watch_path list;
+      targets: string list;
+    }
+  | Registration_failed of {snapshot: entry list; message: string}
+
+let entry_equal first second =
+  first.path = second.path
+  &&
+  match (first.state, second.state) with
+  | File first, File second ->
+    first.modified = second.modified
+    && first.size = second.size
+    && first.digest = second.digest
+  | Dependency_candidate first, Dependency_candidate second ->
+    first.modified = second.modified && first.size = second.size
+  | Missing_dependency_candidate, Missing_dependency_candidate -> true
+  | Unreadable, Unreadable -> true
+  | File _, (Dependency_candidate _ | Missing_dependency_candidate | Unreadable)
+  | Dependency_candidate _, (File _ | Missing_dependency_candidate | Unreadable)
+  | Missing_dependency_candidate, (File _ | Dependency_candidate _ | Unreadable)
+  | Unreadable, (File _ | Dependency_candidate _ | Missing_dependency_candidate)
+    ->
+    false
+
+let equal = List.equal entry_equal
+
+let rec nearest_existing_ancestor path =
+  if File_util.is_directory path then Some (Platform.canonicalize_path path)
+  else
+    let parent = Filename.dirname path in
+    if parent = path then None else nearest_existing_ancestor parent
+
+let create ?(on_source_symlink = fun _ -> ()) digest_cache
+    (scope : Watch_scope.t) =
+  let visited_directories = Hashtbl.create 64 in
+  let seen_files = Hashtbl.create 256 in
+  let unreadable path acc =
+    Hashtbl.replace seen_files path ();
+    Hashtbl.remove digest_cache path;
+    {path; state = Unreadable} :: acc
+  in
+  let digest path stat =
+    Hashtbl.replace seen_files path ();
+    (* Native events are only wakeups; a content snapshot decides whether to
+       rebuild. Reuse hashes while all cheap identity fields are unchanged. *)
+    match Hashtbl.find_opt digest_cache path with
+    | Some (mtime, ctime, size, digest)
+      when mtime = stat.Unix.st_mtime && ctime = stat.Unix.st_ctime
+           && size = stat.Unix.st_size ->
+      digest
+    | _ ->
+      let digest = File_util.digest_file path |> Digest.to_hex in
+      Hashtbl.replace digest_cache path
+        (stat.Unix.st_mtime, stat.Unix.st_ctime, stat.Unix.st_size, digest);
+      digest
+  in
+  let add_file path stat acc =
+    try
+      if Hashtbl.mem seen_files path then acc
+      else
+        let digest = digest path stat in
+        {
+          path;
+          state =
+            File
+              {modified = stat.Unix.st_mtime; size = stat.Unix.st_size; digest};
+        }
+        :: acc
+    with
+    | Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+      Hashtbl.remove seen_files path;
+      Hashtbl.remove digest_cache path;
+      acc
+    | Unix.Unix_error _ | Sys_error _ -> unreadable path acc
+  in
+  let matches_source (source : Watch_scope.source_root) path =
+    Option.fold ~none:true
+      ~some:(fun filter -> Source_filter.matches_basename filter path)
+      source.filter
+  in
+  let rec walk (source : Watch_scope.source_root) recursive dir acc =
+    match Platform.canonicalize_path dir with
+    | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> acc
+    | exception (Unix.Unix_error _ | Sys_error _) -> unreadable dir acc
+    | canonical -> (
+      let admission =
+        Traversal_coverage.admit visited_directories canonical ~recursive
+      in
+      match admission with
+      | Traversal_coverage.Skip -> acc
+      | Traversal_coverage.Visit_current
+      | Traversal_coverage.Visit_current_and_descendants
+      | Traversal_coverage.Visit_descendants ->
+        let visit_current = Traversal_coverage.visits_current admission in
+        let visit_descendants =
+          Traversal_coverage.visits_descendants admission
+        in
+        let entries =
+          try Some (File_util.directory_entries dir) with
+          | Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> Some []
+          | Unix.Unix_error _ | Sys_error _ -> None
+        in
+        Option.fold ~none:(unreadable dir acc)
+          ~some:
+            (List.fold_left
+               (fun acc name ->
+                 let path = Filename.concat dir name in
+                 match Unix.lstat path with
+                 | stat -> (
+                   match stat.Unix.st_kind with
+                   | Unix.S_DIR ->
+                     if
+                       (not visit_descendants)
+                       || Native_watcher.is_compiler_artifact_directory path
+                     then acc
+                     else walk source true path acc
+                   | Unix.S_LNK -> (
+                     try
+                       let is_source_name =
+                         Option.is_some (Source.source_kind path)
+                         && matches_source source path
+                       in
+                       (if visit_current && is_source_name then
+                          match Unix.readlink path with
+                          | target ->
+                            let target =
+                              if Filename.is_relative target then
+                                Filename.concat (Filename.dirname path) target
+                              else target
+                            in
+                            let target =
+                              match Platform.canonicalize_path target with
+                              | canonical -> canonical
+                              | exception
+                                  Unix.Unix_error
+                                    ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+                                target
+                            in
+                            on_source_symlink target
+                          | exception
+                              Unix.Unix_error
+                                ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+                            ());
+                       match Unix.stat path with
+                       | target -> (
+                         match target.Unix.st_kind with
+                         | Unix.S_DIR when visit_descendants ->
+                           walk source true path acc
+                         | Unix.S_REG when visit_current && is_source_name ->
+                           add_file path target acc
+                         | _ -> acc)
+                       | exception
+                           Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _)
+                         ->
+                         acc
+                       | exception (Unix.Unix_error _ | Sys_error _) ->
+                         unreadable path acc
+                     with Unix.Unix_error _ | Sys_error _ ->
+                       unreadable path acc)
+                   | Unix.S_REG
+                     when visit_current
+                          && Option.is_some (Source.source_kind path)
+                          && matches_source source path ->
+                     add_file path stat acc
+                   | _ -> acc)
+                 | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _)
+                   ->
+                   acc
+                 | exception (Unix.Unix_error _ | Sys_error _) ->
+                   unreadable path acc)
+               acc)
+          entries
+      | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) -> acc)
+  in
+  let add_control_files acc root =
+    Watch_scope.control_file_names
+    |> List.fold_left
+         (fun acc name ->
+           let path = Filename.concat root name in
+           match Unix.stat path with
+           | stat ->
+             if stat.Unix.st_kind = Unix.S_REG then add_file path stat acc
+             else acc
+           | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+             acc
+           | exception (Unix.Unix_error _ | Sys_error _) -> unreadable path acc)
+         acc
+  in
+  let result =
+    List.fold_left add_control_files [] scope.roots |> fun acc ->
+    List.fold_left
+      (fun acc source -> walk source source.recursive source.directory acc)
+      acc scope.sources
+    |> fun acc ->
+    List.fold_left
+      (fun acc path ->
+        match Unix.lstat path with
+        | stat ->
+          Hashtbl.replace seen_files path ();
+          {
+            path;
+            state =
+              Dependency_candidate
+                {modified = stat.Unix.st_mtime; size = stat.Unix.st_size};
+          }
+          :: acc
+        | exception Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+          {path; state = Missing_dependency_candidate} :: acc
+        | exception (Unix.Unix_error _ | Sys_error _) -> unreadable path acc)
+      acc scope.unresolved
+    |> List.sort compare
+  in
+  Hashtbl.filter_map_inplace
+    (fun path value -> if Hashtbl.mem seen_files path then Some value else None)
+    digest_cache;
+  result
+
+let create_with_symlink_paths digest_cache scope =
+  let targets = ref [] in
+  let snapshot =
+    create
+      ~on_source_symlink:(fun target -> targets := target :: !targets)
+      digest_cache scope
+  in
+  let targets = List.sort_uniq String.compare !targets in
+  let rec resolve_paths paths = function
+    | [] -> Ok paths
+    | target :: rest -> (
+      try
+        match Filename.dirname target |> nearest_existing_ancestor with
+        | None -> resolve_paths paths rest
+        | Some directory ->
+          let parent = Filename.dirname directory in
+          let directories =
+            if parent = directory then [directory] else [directory; parent]
+          in
+          let paths =
+            List.rev_append
+              (List.map
+                 (fun directory ->
+                   Native_watcher.{directory; recursive = false})
+                 directories)
+              paths
+          in
+          resolve_paths paths rest
+      with (Unix.Unix_error _ | Sys_error _) as error ->
+        Error (Printexc.to_string error))
+  in
+  match resolve_paths [] targets with
+  | Ok paths -> Ready {snapshot; paths; targets}
+  | Error message -> Registration_failed {snapshot; message}
+
+let changes_between before after =
+  (* A content snapshot deliberately treats native events only as wakeups. The
+     before/after membership is still enough to distinguish an in-place edit,
+     which can reuse the build graph, from a structural change that requires
+     rediscovery. *)
+  let before_by_path = Hashtbl.create (List.length before) in
+  let after_by_path = Hashtbl.create (List.length after) in
+  List.iter
+    (fun entry -> Hashtbl.replace before_by_path entry.path entry)
+    before;
+  List.iter (fun entry -> Hashtbl.replace after_by_path entry.path entry) after;
+  let changes = ref [] in
+  Hashtbl.iter
+    (fun path before_entry ->
+      match Hashtbl.find_opt after_by_path path with
+      | None -> changes := {path; kind = Removed} :: !changes
+      | Some after_entry when not (entry_equal before_entry after_entry) ->
+        changes := {path; kind = Modified} :: !changes
+      | Some _ -> ())
+    before_by_path;
+  Hashtbl.iter
+    (fun path _ ->
+      if not (Hashtbl.mem before_by_path path) then
+        changes := {path; kind = Added} :: !changes)
+    after_by_path;
+  List.sort
+    (fun (first : change) second -> String.compare first.path second.path)
+    !changes
+
+let update_entries digest_cache previous changes =
+  let changed = Hashtbl.create (List.length changes) in
+  List.iter
+    (fun (change : change) -> Hashtbl.replace changed change.path change.kind)
+    changes;
+  let updated =
+    previous
+    |> List.filter_map (fun entry ->
+        match Hashtbl.find_opt changed entry.path with
+        | None -> Some entry
+        | Some Removed ->
+          Hashtbl.remove changed entry.path;
+          Hashtbl.remove digest_cache entry.path;
+          None
+        | Some (Added | Modified) -> (
+          Hashtbl.remove changed entry.path;
+          try
+            let stat = Unix.stat entry.path in
+            let digest = File_util.digest_file entry.path |> Digest.to_hex in
+            Hashtbl.replace digest_cache entry.path
+              (stat.Unix.st_mtime, stat.Unix.st_ctime, stat.Unix.st_size, digest);
+            Some
+              {
+                path = entry.path;
+                state =
+                  File
+                    {
+                      modified = stat.Unix.st_mtime;
+                      size = stat.Unix.st_size;
+                      digest;
+                    };
+              }
+          with
+          | Unix.Unix_error ((Unix.ENOENT | Unix.ENOTDIR), _, _) ->
+            Hashtbl.remove digest_cache entry.path;
+            None
+          | Unix.Unix_error _ | Sys_error _ ->
+            (* Preserve the old baseline so the caller still runs the build.
+               Source diagnostics then remain inside the watcher's recoverable
+               build boundary instead of terminating the watch loop here. *)
+            Some entry))
+  in
+  if Hashtbl.length changed = 0 then Some updated else None
+
+let polling_build_changes ~previous ~trigger ~before_build =
+  if equal trigger previous then [] else changes_between previous before_build
+
+let changes_are_incremental changes =
+  changes <> []
+  && List.for_all
+       (fun change ->
+         change.kind = Modified
+         && Option.is_some (Source.source_kind change.path))
+       changes
+
+let reconciliation_baseline ~(old_scope : Watch_scope.t) ~new_scope before after
+    =
+  (* A configuration edit can add or remove files from the watch scope. Those
+     membership changes are already covered by the build that read the new
+     configuration; only changes to files shared by both scopes require
+     another build. *)
+  let shared path =
+    Watch_scope.path_in_scope old_scope path
+    && Watch_scope.path_in_scope new_scope path
+  in
+  List.filter (fun entry -> not (shared entry.path)) after
+  @ List.filter (fun entry -> shared entry.path) before
+  |> List.sort compare
diff --git a/rewatch-ocaml/watch_snapshot.mli b/rewatch-ocaml/watch_snapshot.mli
new file mode 100644
index 00000000000..bd6cc6e4573
--- /dev/null
+++ b/rewatch-ocaml/watch_snapshot.mli
@@ -0,0 +1,64 @@
+(** Native filesystem events are treated as wakeups rather than authoritative
+    edits. Snapshots provide the content baseline needed to coalesce duplicate
+    events and to detect changes that arrive while a build is running. *)
+
+type change_kind = Added | Removed | Modified
+type change = {path: string; kind: change_kind}
+
+type file = {modified: float; size: int; digest: string}
+type dependency = {modified: float; size: int}
+
+(** Dependency candidates need states distinct from ordinary files: a missing
+    candidate is meaningful and must remain observable, whereas [Unreadable]
+    represents a filesystem failure that must not be treated as deletion. *)
+type state =
+  | File of file
+  | Dependency_candidate of dependency
+  | Missing_dependency_candidate
+  | Unreadable
+
+type entry = {path: string; state: state}
+
+type registration =
+  | Ready of {
+      snapshot: entry list;
+      paths: Native_watcher.watch_path list;
+      targets: string list;
+    }
+  | Registration_failed of {snapshot: entry list; message: string}
+
+val equal : entry list -> entry list -> bool
+
+val create :
+  ?on_source_symlink:(string -> unit) ->
+  (string, float * float * int * string) Hashtbl.t ->
+  Watch_scope.t ->
+  entry list
+
+val create_with_symlink_paths :
+  (string, float * float * int * string) Hashtbl.t ->
+  Watch_scope.t ->
+  registration
+
+val changes_between : entry list -> entry list -> change list
+
+val update_entries :
+  (string, float * float * int * string) Hashtbl.t ->
+  entry list ->
+  change list ->
+  entry list option
+
+val polling_build_changes :
+  previous:entry list ->
+  trigger:entry list ->
+  before_build:entry list ->
+  change list
+
+val changes_are_incremental : change list -> bool
+
+val reconciliation_baseline :
+  old_scope:Watch_scope.t ->
+  new_scope:Watch_scope.t ->
+  entry list ->
+  entry list ->
+  entry list
diff --git a/rewatch-ocaml/watcher.ml b/rewatch-ocaml/watcher.ml
new file mode 100644
index 00000000000..83bb21cb6bb
--- /dev/null
+++ b/rewatch-ocaml/watcher.ml
@@ -0,0 +1,339 @@
+exception Stop
+
+type change_kind = Watch_snapshot.change_kind = Added | Removed | Modified
+type change = Watch_snapshot.change = {path: string; kind: change_kind}
+type build_result = Succeeded | Failed
+type rebuild_kind = Incremental | Full
+
+type fallback = {
+  message: string;
+  scope: Watch_scope.t;
+  snapshot: Watch_snapshot.entry list;
+}
+
+let with_signal_handlers handler f =
+  let previous_sigint = Sys.signal Sys.sigint (Sys.Signal_handle handler) in
+  Fun.protect
+    (fun () ->
+      let previous_sigterm =
+        Sys.signal Sys.sigterm (Sys.Signal_handle handler)
+      in
+      Fun.protect f ~finally:(fun () ->
+          ignore (Sys.signal Sys.sigterm previous_sigterm)))
+    ~finally:(fun () -> ignore (Sys.signal Sys.sigint previous_sigint))
+
+let run_locked ~native_create ~report_native_fallback ~root ~prod ~features
+    ~filter ~clear_screen ~show_progress ~verbosity ~build ~watch_lock =
+  let stop_requested = Atomic.make false in
+  let stop () =
+    (* Signal handlers only request termination because libuv may invoke them
+       while a callback is being drained or a watch handle is being refreshed.
+       Raising through that callback would be treated as an uncaught libuv
+       exception and could bypass lock and handle cleanup. The build poll and
+       watch-loop timer observe this flag, so shutdown remains prompt. *)
+    Atomic.set stop_requested true
+  in
+  let digest_cache = Hashtbl.create 256 in
+  let refresh_and_snapshot watcher ~symlink_paths (scope : Watch_scope.t) =
+    (* Watch paths can change while handles are being installed, especially
+       when a source symlink is repointed. A snapshot is authoritative only
+       after its complete path set was registered. Continuous churn falls back
+       to polling instead of accepting handles for an obsolete target. *)
+    let rec loop remaining symlink_paths =
+      if remaining = 0 then Error "watch paths did not stabilize"
+      else
+        match
+          Native_watcher.refresh watcher ~paths:(scope.paths @ symlink_paths)
+        with
+        | Error _ as error -> error
+        | Ok () -> (
+          match Watch_snapshot.create_with_symlink_paths digest_cache scope with
+          | Watch_snapshot.Registration_failed {message; snapshot} ->
+            ignore snapshot;
+            Error message
+          | Watch_snapshot.Ready
+              {
+                snapshot = after_refresh;
+                paths = updated_symlink_paths;
+                targets = symlink_targets;
+              } ->
+            if updated_symlink_paths = symlink_paths then
+              Ok (after_refresh, symlink_targets)
+            else loop (remaining - 1) updated_symlink_paths)
+    in
+    loop 8 symlink_paths
+  in
+  let keep_running () =
+    (not (Atomic.get stop_requested)) && Build_lock.is_owned watch_lock
+  in
+  let next_lock_check = ref 0. in
+  (* Lock removal is the test suite's portable shutdown protocol and must also
+     interrupt an initial build, before native watch handles exist. Throttle
+     ownership checks so the scheduler's frequent responsiveness ticks do not
+     turn one source edit into a stream of identical filesystem reads. *)
+  let poll () =
+    if Atomic.get stop_requested then raise Stop;
+    let now = Unix.gettimeofday () in
+    if now >= !next_lock_check then (
+      next_lock_check := now +. 0.1;
+      if not (Build_lock.is_owned watch_lock) then raise Stop)
+  in
+  let delay seconds =
+    (try ignore (Unix.select [] [] [] seconds)
+     with Unix.Unix_error (Unix.EINTR, _, _) -> ());
+    poll ()
+  in
+  let show_rebuild_presentation () =
+    Output.should_clear_screen ~clear_screen ~show_progress
+      ~interactive:(Unix.isatty Unix.stdout && Unix.isatty Unix.stderr)
+  in
+  let begin_rebuild kind =
+    if show_rebuild_presentation () then (
+      Printf.printf "\027[2J\027[H%!";
+      print_endline
+        (match kind with
+        | Incremental -> "Change detected. Rebuilding..."
+        | Full -> "Change detected. Full rebuild..."))
+  in
+  let finish_rebuild = function
+    | Succeeded -> ()
+    | Failed when show_rebuild_presentation () ->
+      print_endline "\nBuild failed. Watching for changes..."
+    | Failed -> ()
+  in
+  let direct_content_changes watcher (scope : Watch_scope.t) symlink_targets
+      events =
+    let changes = ref [] in
+    let requires_reconciliation = ref false in
+    let is_source_path path = Option.is_some (Source.source_kind path) in
+    let is_in_source_tree = Watch_scope.path_is_in_source_tree scope in
+    let is_symlink_target path =
+      let comparable = Platform.normalize_path_for_comparison path in
+      List.exists
+        (fun target ->
+          Platform.normalize_path_for_comparison target = comparable)
+        symlink_targets
+    in
+    let contains_symlink_target path =
+      List.exists
+        (fun target ->
+          Project_context.path_is_within_canonical ~root:path target)
+        symlink_targets
+    in
+    let is_directly_in_watched_directory path =
+      (* A rename may be reported only under its old child name. Reconcile any
+         structural change in an explicitly watched directory so restoring a
+         missing source or symlink target cannot be overlooked. *)
+      Native_watcher.watches_directory watcher (Filename.dirname path)
+    in
+    List.iter
+      (fun (event : Native_watcher.change) ->
+        match (event.kind, event.path) with
+        | _, None -> requires_reconciliation := true
+        | Native_watcher.Structural, Some path ->
+          if
+            contains_symlink_target path
+            || is_source_path path
+            || Watch_scope.path_in_scope scope path
+            || Native_watcher.watches_directory watcher path
+            || is_directly_in_watched_directory path
+            || (is_in_source_tree path && File_util.is_directory path)
+            || Watch_scope.path_is_source_ancestor scope path
+            || Watch_scope.path_is_unresolved_ancestor scope path
+          then requires_reconciliation := true
+        | Native_watcher.Content, Some path ->
+          if is_symlink_target path then requires_reconciliation := true
+          else if is_source_path path then
+            if Watch_scope.path_in_scope scope path then
+              if File_util.exists path then
+                changes := {path; kind = Modified} :: !changes
+              else requires_reconciliation := true
+            else requires_reconciliation := true
+          else if Watch_scope.path_in_scope scope path then
+            requires_reconciliation := true)
+      events;
+    if !requires_reconciliation then None
+    else
+      Some
+        (!changes
+        |> List.sort_uniq (fun (first : change) second ->
+            String.compare first.path second.path))
+  in
+  let rec polling_loop (scope : Watch_scope.t) previous =
+    if keep_running () then
+      let current = Watch_snapshot.create digest_cache scope in
+      if not (Watch_snapshot.equal current previous) then (
+        let build_scope = Watch_scope.discover ~root ~prod ~features ~filter in
+        let before_build = Watch_snapshot.create digest_cache build_scope in
+        let changes =
+          Watch_snapshot.polling_build_changes ~previous ~trigger:current
+            ~before_build
+        in
+        let rebuild_kind =
+          if Watch_snapshot.changes_are_incremental changes then Incremental
+          else Full
+        in
+        Output.debug ~verbosity
+          (match rebuild_kind with
+          | Incremental -> "doing Incremental"
+          | Full -> "doing Full");
+        begin_rebuild rebuild_kind;
+        build ~poll ~changes:(Some changes) |> finish_rebuild;
+        let new_scope = Watch_scope.discover ~root ~prod ~features ~filter in
+        let after_build = Watch_snapshot.create digest_cache new_scope in
+        let baseline =
+          Watch_snapshot.reconciliation_baseline ~old_scope:build_scope
+            ~new_scope before_build after_build
+        in
+        delay 0.2;
+        (* Keep the snapshot from before the rebuild when another edit lands
+           during compilation. Otherwise that edit would become the new baseline
+           and an atomic configuration rewrite could be missed. *)
+        if not (Watch_snapshot.equal after_build baseline) then
+          polling_loop new_scope baseline
+        else polling_loop new_scope after_build)
+      else (
+        delay 0.2;
+        polling_loop scope current)
+  in
+  let rec native_loop watcher (scope : Watch_scope.t) symlink_targets previous =
+    let result = Native_watcher.wait watcher ~keep_running in
+    match result with
+    | Native_watcher.Stopped -> None
+    | Native_watcher.Failed message ->
+      Some {message; scope; snapshot = previous}
+    | Native_watcher.Changed events -> (
+      delay 0.05;
+      let events = events @ Native_watcher.drain watcher in
+      let direct =
+        direct_content_changes watcher scope symlink_targets events
+      in
+      match direct with
+      | Some [] -> native_loop watcher scope symlink_targets previous
+      | Some changes -> (
+        match Watch_snapshot.update_entries digest_cache previous changes with
+        | Some before_build ->
+          Output.debug ~verbosity "doing Incremental";
+          begin_rebuild Incremental;
+          build ~poll ~changes:(Some changes) |> finish_rebuild;
+          native_loop watcher scope symlink_targets before_build
+        | None -> native_reconcile watcher scope previous)
+      | None -> native_reconcile watcher scope previous)
+  and native_reconcile watcher (scope : Watch_scope.t) previous =
+    let current = Watch_snapshot.create digest_cache scope in
+    if not (Watch_snapshot.equal current previous) then (
+      let changes = Watch_snapshot.changes_between previous current in
+      let rebuild_kind =
+        if Watch_snapshot.changes_are_incremental changes then Incremental
+        else Full
+      in
+      Output.debug ~verbosity
+        (match rebuild_kind with
+        | Incremental -> "doing Incremental"
+        | Full -> "doing Full");
+      begin_rebuild rebuild_kind;
+      let build_scope = Watch_scope.discover ~root ~prod ~features ~filter in
+      let before_build = Watch_snapshot.create digest_cache build_scope in
+      build ~poll ~changes:(Some changes) |> finish_rebuild;
+      let new_scope = Watch_scope.discover ~root ~prod ~features ~filter in
+      match Watch_snapshot.create_with_symlink_paths digest_cache new_scope with
+      | Watch_snapshot.Registration_failed {snapshot; message} ->
+        let snapshot =
+          Watch_snapshot.reconciliation_baseline ~old_scope:build_scope
+            ~new_scope before_build snapshot
+        in
+        Some {message; scope = new_scope; snapshot}
+      | Watch_snapshot.Ready {snapshot; paths; targets} ->
+        ignore targets;
+        finish_reconciliation watcher ~old_scope:build_scope ~new_scope
+          ~before:before_build ~after:snapshot ~symlink_paths:paths)
+    else
+      let new_scope = Watch_scope.discover ~root ~prod ~features ~filter in
+      match Watch_snapshot.create_with_symlink_paths digest_cache new_scope with
+      | Watch_snapshot.Registration_failed {snapshot; message} ->
+        let snapshot =
+          Watch_snapshot.reconciliation_baseline ~old_scope:scope ~new_scope
+            current snapshot
+        in
+        Some {message; scope = new_scope; snapshot}
+      | Watch_snapshot.Ready {snapshot; paths; targets} ->
+        ignore targets;
+        finish_reconciliation watcher ~old_scope:scope ~new_scope
+          ~before:current ~after:snapshot ~symlink_paths:paths
+  and finish_reconciliation watcher ~(old_scope : Watch_scope.t)
+      ~(new_scope : Watch_scope.t) ~before ~after ~symlink_paths =
+    let baseline =
+      Watch_snapshot.reconciliation_baseline ~old_scope ~new_scope before after
+    in
+    match refresh_and_snapshot watcher ~symlink_paths new_scope with
+    | Error message -> Some {message; scope = new_scope; snapshot = baseline}
+    | Ok (registered_snapshot, symlink_targets) ->
+      if not (Watch_snapshot.equal registered_snapshot baseline) then
+        native_reconcile watcher new_scope baseline
+      else native_loop watcher new_scope symlink_targets registered_snapshot
+  in
+  with_signal_handlers
+    (fun _ -> stop ())
+    (fun () ->
+      let scope = Watch_scope.discover ~root ~prod ~features ~filter in
+      match Watch_snapshot.create_with_symlink_paths digest_cache scope with
+      | Watch_snapshot.Registration_failed {snapshot; message} ->
+        report_native_fallback message;
+        ignore (build ~poll ~changes:None);
+        let scope = Watch_scope.discover ~root ~prod ~features ~filter in
+        polling_loop scope snapshot
+      | Watch_snapshot.Ready
+          {snapshot = before_build; paths = symlink_paths; targets} -> (
+        ignore targets;
+        (* Install handles before the initial build so an edit made as soon as its
+           output appears cannot land in a blind interval between compilation and
+           watcher setup. Snapshot reconciliation below consumes any event queued
+           while compiler subprocesses were running. *)
+        match native_create ~paths:(scope.paths @ symlink_paths) with
+        | Error message ->
+          report_native_fallback message;
+          ignore (build ~poll ~changes:None);
+          let scope = Watch_scope.discover ~root ~prod ~features ~filter in
+          polling_loop scope before_build
+        | Ok watcher ->
+          let fallback =
+            Fun.protect
+              (fun () ->
+                ignore (build ~poll ~changes:None);
+                (* The following snapshot is authoritative for changes that arrived
+                   during the build. Pump and discard callbacks already queued for
+                   that interval so they do not request the same rebuild twice. *)
+                ignore (Native_watcher.drain watcher);
+                native_reconcile watcher scope before_build)
+              ~finally:(fun () -> Native_watcher.close watcher)
+          in
+          Option.iter
+            (fun fallback ->
+              report_native_fallback fallback.message;
+              polling_loop fallback.scope fallback.snapshot)
+            fallback))
+
+let run_with_native_create ~native_create ~report_native_fallback ~root ~prod
+    ~features ~filter ~clear_screen ~show_progress ~verbosity ~build =
+  Build_lock.with_watch root (fun watch_lock ->
+      ignore (Config.load_root root);
+      run_locked ~native_create ~report_native_fallback ~root ~prod ~features
+        ~filter ~clear_screen ~show_progress ~verbosity ~build ~watch_lock)
+
+let report_native_fallback message =
+  prerr_endline
+    ("Native file watching is unavailable (" ^ message
+   ^ "); falling back to polling")
+
+let run =
+  run_with_native_create ~native_create:Native_watcher.create
+    ~report_native_fallback
+
+module For_test = struct
+  let is_control_file_name = Watch_scope.is_control_file_name
+
+  let run_with_native_failure ~message ~on_fallback =
+    run_with_native_create
+      ~native_create:(fun ~paths:_ -> Error message)
+      ~report_native_fallback:on_fallback
+end
diff --git a/rewatch-ocaml/watcher.mli b/rewatch-ocaml/watcher.mli
new file mode 100644
index 00000000000..0280a704d94
--- /dev/null
+++ b/rewatch-ocaml/watcher.mli
@@ -0,0 +1,33 @@
+exception Stop
+
+type change_kind = Added | Removed | Modified
+type change = {path: string; kind: change_kind}
+type build_result = Succeeded | Failed
+
+module For_test : sig
+  val is_control_file_name : string -> bool
+
+  val run_with_native_failure :
+    message:string ->
+    on_fallback:(string -> unit) ->
+    root:string ->
+    prod:bool ->
+    features:string list option ->
+    filter:Source_filter.t option ->
+    clear_screen:bool ->
+    show_progress:bool ->
+    verbosity:int ->
+    build:(poll:(unit -> unit) -> changes:change list option -> build_result) ->
+    unit
+end
+
+val run :
+  root:string ->
+  prod:bool ->
+  features:string list option ->
+  filter:Source_filter.t option ->
+  clear_screen:bool ->
+  show_progress:bool ->
+  verbosity:int ->
+  build:(poll:(unit -> unit) -> changes:change list option -> build_result) ->
+  unit
diff --git a/rewatch-ocaml/windows_job_stubs.c b/rewatch-ocaml/windows_job_stubs.c
new file mode 100644
index 00000000000..40be2b8f42b
--- /dev/null
+++ b/rewatch-ocaml/windows_job_stubs.c
@@ -0,0 +1,440 @@
+#define CAML_INTERNALS
+
+#ifdef _WIN32
+#ifndef _WIN32_WINNT
+#define _WIN32_WINNT 0x0600
+#endif
+#endif
+
+#include <stdio.h>
+
+/* This owner extends the MIT-licensed Spawn 0.17 Windows process setup with
+   suspended Job Object assignment, which must happen before user code runs. */
+
+#ifdef _WIN32
+#include <caml/osdeps.h>
+#endif
+
+#include <caml/alloc.h>
+#include <caml/custom.h>
+#include <caml/fail.h>
+#include <caml/memory.h>
+#include <caml/mlvalues.h>
+#include <caml/unixsupport.h>
+
+#ifdef _WIN32
+
+#include <windows.h>
+
+#ifndef ENABLE_VIRTUAL_TERMINAL_PROCESSING
+#define ENABLE_VIRTUAL_TERMINAL_PROCESSING 0x0004
+#endif
+
+static void rewatch_windows_enable_vt_for_handle(DWORD stream)
+{
+  HANDLE handle = GetStdHandle(stream);
+  DWORD mode;
+  if (handle != NULL && handle != INVALID_HANDLE_VALUE &&
+      GetConsoleMode(handle, &mode)) {
+    SetConsoleMode(handle, mode | ENABLE_VIRTUAL_TERMINAL_PROCESSING);
+  }
+}
+
+CAMLprim value rewatch_windows_enable_virtual_terminal_processing(value unit)
+{
+  CAMLparam1(unit);
+  rewatch_windows_enable_vt_for_handle(STD_OUTPUT_HANDLE);
+  rewatch_windows_enable_vt_for_handle(STD_ERROR_HANDLE);
+  CAMLreturn(Val_unit);
+}
+
+CAMLprim value rewatch_windows_directory_identity(value path_value)
+{
+  CAMLparam1(path_value);
+  CAMLlocal1(identity_value);
+  WCHAR *path = caml_stat_strdup_to_utf16(String_val(path_value));
+  BY_HANDLE_FILE_INFORMATION information;
+  char identity[26];
+  HANDLE handle = CreateFileW(path, 0,
+                              FILE_SHARE_READ | FILE_SHARE_WRITE |
+                                FILE_SHARE_DELETE,
+                              NULL, OPEN_EXISTING,
+                              FILE_FLAG_BACKUP_SEMANTICS, NULL);
+  caml_stat_free(path);
+  if (handle == INVALID_HANDLE_VALUE) {
+    caml_win32_maperr(GetLastError());
+    uerror("CreateFile", path_value);
+  }
+  if (!GetFileInformationByHandle(handle, &information)) {
+    DWORD error = GetLastError();
+    CloseHandle(handle);
+    caml_win32_maperr(error);
+    uerror("GetFileInformationByHandle", path_value);
+  }
+  CloseHandle(handle);
+  snprintf(identity, sizeof(identity), "%08lx:%08lx%08lx",
+           (unsigned long)information.dwVolumeSerialNumber,
+           (unsigned long)information.nFileIndexHigh,
+           (unsigned long)information.nFileIndexLow);
+  identity_value = caml_copy_string(identity);
+  CAMLreturn(identity_value);
+}
+
+struct rewatch_windows_job {
+  HANDLE handle;
+  HANDLE stdin_handle;
+  HANDLE stdout_handle;
+  HANDLE stderr_handle;
+  WCHAR *program;
+  WCHAR *command_line;
+  WCHAR *environment;
+  WCHAR *cwd;
+};
+
+#define Job_val(v) ((struct rewatch_windows_job *)Data_custom_val(v))
+
+static void rewatch_windows_release_launch_resources(
+  struct rewatch_windows_job *job)
+{
+  if (job->stdin_handle != NULL) CloseHandle(job->stdin_handle);
+  if (job->stdout_handle != NULL) CloseHandle(job->stdout_handle);
+  if (job->stderr_handle != NULL) CloseHandle(job->stderr_handle);
+  if (job->program != NULL) caml_stat_free(job->program);
+  if (job->command_line != NULL) caml_stat_free(job->command_line);
+  if (job->environment != NULL) caml_stat_free(job->environment);
+  if (job->cwd != NULL) caml_stat_free(job->cwd);
+  job->stdin_handle = NULL;
+  job->stdout_handle = NULL;
+  job->stderr_handle = NULL;
+  job->program = NULL;
+  job->command_line = NULL;
+  job->environment = NULL;
+  job->cwd = NULL;
+}
+
+static void rewatch_windows_finalize_job(value job_value)
+{
+  struct rewatch_windows_job *job = Job_val(job_value);
+  rewatch_windows_release_launch_resources(job);
+  if (job->handle != NULL) {
+    CloseHandle(job->handle);
+    job->handle = NULL;
+  }
+}
+
+static const struct custom_operations rewatch_windows_job_operations = {
+  "rescript.rewatch.windows_job",
+  rewatch_windows_finalize_job,
+  custom_compare_default,
+  custom_hash_default,
+  custom_serialize_default,
+  custom_deserialize_default,
+  custom_compare_ext_default,
+  custom_fixed_length_default
+};
+
+static BOOL duplicate_standard_handle(value descriptor, HANDLE *result)
+{
+  return DuplicateHandle(GetCurrentProcess(), Handle_val(descriptor),
+                         GetCurrentProcess(), result, 0L, TRUE,
+                         DUPLICATE_SAME_ACCESS);
+}
+
+static BOOL terminate_unowned_process(PROCESS_INFORMATION *process,
+                                      DWORD *termination_error)
+{
+  BOOL terminated = TerminateProcess(process->hProcess, 1);
+  if (!terminated) *termination_error = GetLastError();
+  CloseHandle(process->hThread);
+  CloseHandle(process->hProcess);
+  return terminated;
+}
+
+CAMLprim value rewatch_windows_spawn_owned(value env_value,
+                                            value cwd_value,
+                                            value program_value,
+                                            value command_line_value,
+                                            value stdin_value,
+                                            value stdout_value,
+                                            value stderr_value)
+{
+  CAMLparam5(env_value, cwd_value, program_value, command_line_value,
+             stdin_value);
+  CAMLxparam2(stdout_value, stderr_value);
+  CAMLlocal2(job_value, process_value);
+  STARTUPINFOEXW startup;
+  PROCESS_INFORMATION process;
+  struct rewatch_windows_job *job;
+  HANDLE job_handle;
+  HANDLE inherited_handles[3];
+  SIZE_T attribute_list_size = 0;
+  DWORD error;
+
+  job_value = caml_alloc_custom(&rewatch_windows_job_operations,
+                                sizeof(struct rewatch_windows_job), 0, 1);
+  job = Job_val(job_value);
+  job->handle = NULL;
+  job->stdin_handle = NULL;
+  job->stdout_handle = NULL;
+  job->stderr_handle = NULL;
+  job->program = NULL;
+  job->command_line = NULL;
+  job->environment = NULL;
+  job->cwd = NULL;
+  process_value = caml_alloc_small(2, 0);
+  Store_field(process_value, 0, Val_long(0));
+  Store_field(process_value, 1, job_value);
+  job = Job_val(job_value);
+
+  ZeroMemory(&startup, sizeof(startup));
+  ZeroMemory(&process, sizeof(process));
+  startup.StartupInfo.cb = sizeof(startup);
+  startup.StartupInfo.dwFlags = STARTF_USESTDHANDLES;
+
+  if (!duplicate_standard_handle(stdin_value, &job->stdin_handle) ||
+      !duplicate_standard_handle(stdout_value, &job->stdout_handle) ||
+      !duplicate_standard_handle(stderr_value, &job->stderr_handle)) {
+    error = GetLastError();
+    rewatch_windows_release_launch_resources(job);
+    caml_win32_maperr(error);
+    uerror("DuplicateHandle", Nothing);
+  }
+  startup.StartupInfo.hStdInput = job->stdin_handle;
+  startup.StartupInfo.hStdOutput = job->stdout_handle;
+  startup.StartupInfo.hStdError = job->stderr_handle;
+  inherited_handles[0] = job->stdin_handle;
+  inherited_handles[1] = job->stdout_handle;
+  inherited_handles[2] = job->stderr_handle;
+
+  InitializeProcThreadAttributeList(NULL, 1, 0, &attribute_list_size);
+  startup.lpAttributeList =
+    HeapAlloc(GetProcessHeap(), 0, attribute_list_size);
+  if (startup.lpAttributeList == NULL) {
+    rewatch_windows_release_launch_resources(job);
+    caml_win32_maperr(ERROR_NOT_ENOUGH_MEMORY);
+    uerror("HeapAlloc", Nothing);
+  }
+  if (!InitializeProcThreadAttributeList(startup.lpAttributeList, 1, 0,
+                                         &attribute_list_size)) {
+    error = GetLastError();
+    HeapFree(GetProcessHeap(), 0, startup.lpAttributeList);
+    startup.lpAttributeList = NULL;
+    rewatch_windows_release_launch_resources(job);
+    caml_win32_maperr(error);
+    uerror("InitializeProcThreadAttributeList", Nothing);
+  }
+  if (!UpdateProcThreadAttribute(startup.lpAttributeList, 0,
+                                 PROC_THREAD_ATTRIBUTE_HANDLE_LIST,
+                                 inherited_handles,
+                                 sizeof(inherited_handles), NULL, NULL)) {
+    error = GetLastError();
+    DeleteProcThreadAttributeList(startup.lpAttributeList);
+    HeapFree(GetProcessHeap(), 0, startup.lpAttributeList);
+    startup.lpAttributeList = NULL;
+    rewatch_windows_release_launch_resources(job);
+    caml_win32_maperr(error);
+    uerror("UpdateProcThreadAttribute", Nothing);
+  }
+
+  job->program = caml_stat_strdup_to_utf16(String_val(program_value));
+  job->command_line =
+    caml_stat_strdup_to_utf16(String_val(command_line_value));
+  job->cwd = caml_stat_strdup_to_utf16(String_val(cwd_value));
+  if (Is_block(env_value)) {
+    value contents = Field(env_value, 0);
+    mlsize_t length = caml_string_length(contents);
+    int size = caml_win32_multi_byte_to_wide_char(
+      String_val(contents), length, NULL, 0);
+    job->environment = caml_stat_alloc(size * sizeof(WCHAR));
+    caml_win32_multi_byte_to_wide_char(String_val(contents), length,
+                                       job->environment, size);
+  }
+
+  job_handle = CreateJobObjectW(NULL, NULL);
+  if (job_handle == NULL) {
+    error = GetLastError();
+    DeleteProcThreadAttributeList(startup.lpAttributeList);
+    HeapFree(GetProcessHeap(), 0, startup.lpAttributeList);
+    rewatch_windows_release_launch_resources(job);
+    caml_win32_maperr(error);
+    uerror("CreateJobObject", Nothing);
+  }
+  job->handle = job_handle;
+
+  if (!CreateProcessW(job->program, job->command_line, NULL, NULL, TRUE,
+                      CREATE_SUSPENDED | CREATE_UNICODE_ENVIRONMENT |
+                        EXTENDED_STARTUPINFO_PRESENT,
+                      job->environment, job->cwd, &startup.StartupInfo,
+                      &process)) {
+    error = GetLastError();
+    DeleteProcThreadAttributeList(startup.lpAttributeList);
+    HeapFree(GetProcessHeap(), 0, startup.lpAttributeList);
+    rewatch_windows_release_launch_resources(job);
+    CloseHandle(job->handle);
+    job->handle = NULL;
+    caml_win32_maperr(error);
+    uerror("CreateProcess", Nothing);
+  }
+
+  DeleteProcThreadAttributeList(startup.lpAttributeList);
+  HeapFree(GetProcessHeap(), 0, startup.lpAttributeList);
+  rewatch_windows_release_launch_resources(job);
+
+  if (!AssignProcessToJobObject(job->handle, process.hProcess)) {
+    error = GetLastError();
+    DWORD termination_error;
+    BOOL terminated = terminate_unowned_process(&process, &termination_error);
+    CloseHandle(job->handle);
+    job->handle = NULL;
+    if (!terminated) {
+      caml_win32_maperr(termination_error);
+      uerror("TerminateProcess", Nothing);
+    }
+    caml_win32_maperr(error);
+    uerror("AssignProcessToJobObject", Nothing);
+  }
+
+  if (ResumeThread(process.hThread) == (DWORD)-1) {
+    error = GetLastError();
+    if (!TerminateJobObject(job->handle, 1)) {
+      DWORD termination_error;
+      if (!TerminateProcess(process.hProcess, 1)) {
+        termination_error = GetLastError();
+        CloseHandle(process.hThread);
+        CloseHandle(process.hProcess);
+        CloseHandle(job->handle);
+        job->handle = NULL;
+        caml_win32_maperr(termination_error);
+        uerror("TerminateProcess", Nothing);
+      }
+    }
+    CloseHandle(process.hThread);
+    CloseHandle(process.hProcess);
+    CloseHandle(job->handle);
+    job->handle = NULL;
+    caml_win32_maperr(error);
+    uerror("ResumeThread", Nothing);
+  }
+
+  CloseHandle(process.hThread);
+  Store_field(process_value, 0, Val_long(process.hProcess));
+  CAMLreturn(process_value);
+}
+
+CAMLprim value rewatch_windows_spawn_owned_byte(value *arguments,
+                                                 int argument_count)
+{
+  (void)argument_count;
+  return rewatch_windows_spawn_owned(arguments[0], arguments[1], arguments[2],
+                                     arguments[3], arguments[4], arguments[5],
+                                     arguments[6]);
+}
+
+CAMLprim value rewatch_windows_terminate_process_job(value job_value)
+{
+  CAMLparam1(job_value);
+  struct rewatch_windows_job *job = Job_val(job_value);
+  JOBOBJECT_EXTENDED_LIMIT_INFORMATION limits;
+  HANDLE handle;
+
+  if (job->handle == NULL) CAMLreturn(Val_true);
+  if (TerminateJobObject(job->handle, 1)) CAMLreturn(Val_true);
+
+  /* Closing a job normally must not affect a successfully detached helper.
+     Only the cancellation fallback enables kill-on-close before releasing the
+     last stable process-tree identity. */
+  ZeroMemory(&limits, sizeof(limits));
+  limits.BasicLimitInformation.LimitFlags = JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE;
+  handle = job->handle;
+  if (SetInformationJobObject(handle, JobObjectExtendedLimitInformation,
+                              &limits, sizeof(limits))) {
+    if (CloseHandle(handle)) {
+      job->handle = NULL;
+      CAMLreturn(Val_true);
+    }
+  }
+  CAMLreturn(Val_false);
+}
+
+CAMLprim value rewatch_windows_close_process_job(value job_value)
+{
+  CAMLparam1(job_value);
+  struct rewatch_windows_job *job = Job_val(job_value);
+  HANDLE handle = job->handle;
+
+  if (handle != NULL) {
+    job->handle = NULL;
+    if (!CloseHandle(handle)) {
+      caml_win32_maperr(GetLastError());
+      uerror("CloseHandle", Nothing);
+    }
+  }
+  CAMLreturn(Val_unit);
+}
+
+CAMLprim value rewatch_windows_current_process_id(value unit_value)
+{
+  (void)unit_value;
+  return Val_long(GetCurrentProcessId());
+}
+
+#else
+
+CAMLprim value rewatch_windows_enable_virtual_terminal_processing(value unit)
+{
+  (void)unit;
+  caml_invalid_argument("Windows virtual terminal support is unavailable");
+}
+
+CAMLprim value rewatch_windows_directory_identity(value path_value)
+{
+  (void)path_value;
+  caml_invalid_argument("Windows directory identities are unavailable");
+}
+
+CAMLprim value rewatch_windows_spawn_owned(value env_value,
+                                            value cwd_value,
+                                            value program_value,
+                                            value command_line_value,
+                                            value stdin_value,
+                                            value stdout_value,
+                                            value stderr_value)
+{
+  (void)env_value;
+  (void)cwd_value;
+  (void)program_value;
+  (void)command_line_value;
+  (void)stdin_value;
+  (void)stdout_value;
+  (void)stderr_value;
+  caml_invalid_argument("Windows process jobs are unavailable");
+}
+
+CAMLprim value rewatch_windows_spawn_owned_byte(value *arguments,
+                                                 int argument_count)
+{
+  (void)argument_count;
+  return rewatch_windows_spawn_owned(arguments[0], arguments[1], arguments[2],
+                                     arguments[3], arguments[4], arguments[5],
+                                     arguments[6]);
+}
+
+CAMLprim value rewatch_windows_terminate_process_job(value job_value)
+{
+  (void)job_value;
+  caml_invalid_argument("Windows process jobs are unavailable");
+}
+
+CAMLprim value rewatch_windows_close_process_job(value job_value)
+{
+  (void)job_value;
+  caml_invalid_argument("Windows process jobs are unavailable");
+}
+
+CAMLprim value rewatch_windows_current_process_id(value unit_value)
+{
+  (void)unit_value;
+  caml_invalid_argument("Windows process IDs are unavailable");
+}
+
+#endif
diff --git a/rewatch/src/build/compile.rs b/rewatch/src/build/compile.rs
index 6a0868dd507..513cbd8e077 100644
--- a/rewatch/src/build/compile.rs
+++ b/rewatch/src/build/compile.rs
@@ -440,7 +440,11 @@ pub fn compile(
     rayon::in_place_scope(|scope| {
         let mut in_flight: usize = 0;
         loop {
-            while in_flight < capacity && !has_errors {
+            // Keep dispatching work that was already independent of any
+            // failed module. Otherwise diagnostics vary with worker count:
+            // a warning-producing module may or may not have started before
+            // an unrelated error finishes.
+            while in_flight < capacity {
                 let Some(work) = ready_heap.pop() else { break };
                 let module_name = work.module_name.clone();
                 let is_dirty = dirty_set.contains(&module_name);
@@ -462,10 +466,6 @@ pub fn compile(
             }
 
             if in_flight == 0 {
-                if !ready_heap.is_empty() {
-                    // Errors suppressed new spawns; nothing left to drain.
-                    break;
-                }
                 if completed.len() < compile_universe_count && !has_errors {
                     stalled = true;
                 }
@@ -475,7 +475,8 @@ pub fn compile(
             let Ok(msg) = rx.recv() else { break };
             in_flight -= 1;
 
-            if msg.result.is_err() || msg.interface_result.as_ref().is_some_and(|r| r.is_err()) {
+            let failed = msg.result.is_err() || msg.interface_result.as_ref().is_some_and(|r| r.is_err());
+            if failed {
                 has_errors = true;
             }
 
@@ -489,20 +490,22 @@ pub fn compile(
             // key modules use to refer to the namespace entry.
             let dependents = build_state.get_module(&finished_name).unwrap().dependents.clone();
 
-            for dep in &dependents {
-                if !compile_universe.contains(dep) {
-                    continue;
-                }
-                if !is_clean {
-                    dirty_set.insert(dep.clone());
-                }
-                let count = pending_deps.get_mut(dep).unwrap();
-                *count -= 1;
-                if *count == 0 && !completed.contains(dep) {
-                    ready_heap.push(WorkUnit {
-                        priority: priorities[dep],
-                        module_name: dep.clone(),
-                    });
+            if !failed {
+                for dep in &dependents {
+                    if !compile_universe.contains(dep) {
+                        continue;
+                    }
+                    if !is_clean {
+                        dirty_set.insert(dep.clone());
+                    }
+                    let count = pending_deps.get_mut(dep).unwrap();
+                    *count -= 1;
+                    if *count == 0 && !completed.contains(dep) {
+                        ready_heap.push(WorkUnit {
+                            priority: priorities[dep],
+                            module_name: dep.clone(),
+                        });
+                    }
                 }
             }
         }
@@ -523,9 +526,9 @@ pub fn compile(
     let mut num_compiled_modules = 0;
 
     // Persist propagated dirtiness back onto build_state. Modules that were
-    // marked dirty (because a predecessor's cmi changed) but never scheduled
-    // — e.g. the first compile error aborted further dispatch — must keep
-    // compile_dirty = true so the next incremental build recompiles them.
+    // marked dirty (because a predecessor's cmi changed) but blocked by a
+    // failed prerequisite must keep compile_dirty = true so the next
+    // incremental build recompiles them.
     // Successful recompiles in the result loop below override this back to
     // false for the modules that actually ran.
     for name in &dirty_set {
diff --git a/rewatch/src/lock.rs b/rewatch/src/lock.rs
index 8f1d3e92803..86d7b08499f 100644
--- a/rewatch/src/lock.rs
+++ b/rewatch/src/lock.rs
@@ -73,29 +73,19 @@ pub enum Lock {
     Error(Error),
 }
 
-fn matching_process_name() -> Option<String> {
-    std::env::current_exe()
-        .ok()
-        .and_then(|path| path.file_name().map(|name| name.to_string_lossy().into_owned()))
-}
-
 fn pid_matches_current_process(to_check_pid: u32) -> bool {
     let system = System::new_all();
-    let current_process_name = matching_process_name();
 
     system.processes().iter().any(|(pid, process)| {
         if pid.as_u32() != to_check_pid {
             return false;
         }
 
-        match &current_process_name {
-            Some(current_process_name) => process
-                .exe()
-                .file_name()
-                .map(|name| name.to_string_lossy() == current_process_name.as_str())
-                .unwrap_or_else(|| process.name() == current_process_name.as_str()),
-            None => true,
-        }
+        process
+            .exe()
+            .file_name()
+            .map(|name| name.to_string_lossy().starts_with("rescript"))
+            .unwrap_or_else(|| process.name().starts_with("rescript"))
     })
 }
 
diff --git a/rewatch/testrepo/packages/dep02/src/Array.mjs b/rewatch/testrepo/packages/dep02/src/Array.mjs
index d51ea7518e8..f216d2e64a0 100644
--- a/rewatch/testrepo/packages/dep02/src/Array.mjs
+++ b/rewatch/testrepo/packages/dep02/src/Array.mjs
@@ -1,7 +1,7 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Belt_Array from "@rescript/belt/src/Belt_Array.mjs";
-import * as Belt_SortArray from "@rescript/belt/src/Belt_SortArray.mjs";
+import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.mjs";
+import * as Belt_SortArray from "@rescript/belt/lib/es6/src/Belt_SortArray.mjs";
 import * as Primitive_option from "@rescript/runtime/lib/es6/Primitive_option.mjs";
 
 function includes(prim0, prim1) {
diff --git a/rewatch/tests/clean/01-clean-single-project.sh b/rewatch/tests/clean/01-clean-single-project.sh
index 7d5aba613da..be5810cd651 100755
--- a/rewatch/tests/clean/01-clean-single-project.sh
+++ b/rewatch/tests/clean/01-clean-single-project.sh
@@ -42,7 +42,7 @@ other_project_compiled_files=$(find packages/new-namespace -type f -name '*.mjs'
 if [ "$other_project_compiled_files" -gt 0 ];
 then
   success "Didn't clean other project files"
-  git restore .
+  restore_tracked_files .
 else
   error "Expected files from new-namespace not to be cleaned"
   exit 1
diff --git a/rewatch/tests/clean/02-clean-dev-dependencies.sh b/rewatch/tests/clean/02-clean-dev-dependencies.sh
index 2d039a9243d..962307938d6 100755
--- a/rewatch/tests/clean/02-clean-dev-dependencies.sh
+++ b/rewatch/tests/clean/02-clean-dev-dependencies.sh
@@ -31,7 +31,7 @@ project_compiled_files=$(find packages/pure-dev -type f -name '*.mjs' | wc -l |
 if [ "$project_compiled_files" -eq 0 ];
 then
   success "pure-dev cleaned"
-  git restore .
+  restore_tracked_files .
 else
   error "Expected 0 .mjs files in pure-dev after clean, got $project_compiled_files"
   printf "%s\n" "$error_output"
diff --git a/rewatch/tests/clean/03-clean-node-modules.sh b/rewatch/tests/clean/03-clean-node-modules.sh
index 03630c4d55a..e9ae13598ad 100755
--- a/rewatch/tests/clean/03-clean-node-modules.sh
+++ b/rewatch/tests/clean/03-clean-node-modules.sh
@@ -31,7 +31,7 @@ compiler_assets=$(find node_modules/rescript-nodejs/lib/ocaml -type f -name '*.*
 if [ $compiler_assets -eq 0 ];
 then
   success "compiler assets from node_modules cleaned"
-  git restore .
+  restore_tracked_files .
 else
   error "Expected 0 files in node_modules/rescript-nodejs/lib/ocaml after clean, got $compiler_assets"
   printf "%s\n" "$error_output"
diff --git a/rewatch/tests/clean/04-clean-rebuild-no-compiler-update.sh b/rewatch/tests/clean/04-clean-rebuild-no-compiler-update.sh
index e2b5108221c..d953f5dc457 100755
--- a/rewatch/tests/clean/04-clean-rebuild-no-compiler-update.sh
+++ b/rewatch/tests/clean/04-clean-rebuild-no-compiler-update.sh
@@ -18,24 +18,26 @@ else
   exit 1
 fi
 
-# Rebuild with snapshot output
-snapshot_file=../tests/snapshots/clean-rebuild.txt
-rewatch build &> $snapshot_file
+# Rebuild with captured output. This test checks one lifecycle message; writing
+# into a tracked snapshot made otherwise-correct implementations dirty when
+# their work counts differed.
+output_file=$(mktemp "${TMPDIR:-/tmp}/rewatch-clean-rebuild.XXXXXX")
+trap 'rm -f "$output_file"' EXIT
+rewatch build &> "$output_file"
 build_status=$?
-normalize_paths $snapshot_file
 if [ $build_status -eq 0 ];
 then
   success "Repo Built"
 else
   error "Error Building Repo"
-  cat $snapshot_file >&2
+  cat "$output_file" >&2
   exit 1
 fi
 
 # Verify the undesired message is NOT present
-if grep -q "Cleaned previous build due to compiler update" $snapshot_file; then
+if grep -q "Cleaned previous build due to compiler update" "$output_file"; then
   error "Unexpected compiler-update clean message present in rebuild logs"
-  cat $snapshot_file >&2
+  cat "$output_file" >&2
   exit 1
 else
   success "No compiler-update clean message present after explicit clean"
diff --git a/rewatch/tests/compile/01-basic-compile.sh b/rewatch/tests/compile/01-basic-compile.sh
index 31ff8951c18..e1b6b1298e7 100755
--- a/rewatch/tests/compile/01-basic-compile.sh
+++ b/rewatch/tests/compile/01-basic-compile.sh
@@ -25,6 +25,11 @@ else
   exit 1
 fi
 
+# Rust clean rebuilds the portal dependency with the consumer's output settings,
+# while the OCaml implementation preserves its independently published output.
+# Normalize the equivalent import before comparing this shared fixture.
+normalize_belt_portal_import
+
 if git diff --exit-code ./;
 then
   success "Testrepo has no changes"
diff --git a/rewatch/tests/compile/08-remove-file.sh b/rewatch/tests/compile/08-remove-file.sh
index b38d6a8fba7..5ce38f94804 100755
--- a/rewatch/tests/compile/08-remove-file.sh
+++ b/rewatch/tests/compile/08-remove-file.sh
@@ -11,7 +11,7 @@ rewatch build &> /dev/null
 rm packages/dep02/src/Dep02.res
 rewatch build &> ../tests/snapshots/remove-file.txt
 normalize_paths ../tests/snapshots/remove-file.txt
-git checkout -- packages/dep02/src/Dep02.res
+restore_tracked_files packages/dep02/src/Dep02.res
 
 rewatch build &> /dev/null
 
diff --git a/rewatch/tests/compile/09-dependency-cycle.sh b/rewatch/tests/compile/09-dependency-cycle.sh
index 7c287b81d69..0fe3067a6a5 100755
--- a/rewatch/tests/compile/09-dependency-cycle.sh
+++ b/rewatch/tests/compile/09-dependency-cycle.sh
@@ -11,7 +11,7 @@ rewatch build &> /dev/null
 echo 'Dep01.log()' >> packages/new-namespace/src/NS_alias.res
 rewatch build &> ../tests/snapshots/dependency-cycle.txt
 normalize_paths ../tests/snapshots/dependency-cycle.txt
-git checkout -- packages/new-namespace/src/NS_alias.res
+restore_tracked_files packages/new-namespace/src/NS_alias.res
 
 rewatch build &> /dev/null
 
diff --git a/rewatch/tests/compile/11-dev-dependency-non-dev-source.sh b/rewatch/tests/compile/11-dev-dependency-non-dev-source.sh
index 5fe5d7b7b8d..097bba095cf 100755
--- a/rewatch/tests/compile/11-dev-dependency-non-dev-source.sh
+++ b/rewatch/tests/compile/11-dev-dependency-non-dev-source.sh
@@ -13,7 +13,7 @@ rewatch build &> /dev/null
 echo 'open WebAPI' >> packages/with-dev-deps/src/FileToTest.res
 rewatch build &> ../tests/snapshots/dev-dependency-used-by-non-dev-source.txt
 normalize_paths ../tests/snapshots/dev-dependency-used-by-non-dev-source.txt
-git checkout -- packages/with-dev-deps/src/FileToTest.res
+restore_tracked_files packages/with-dev-deps/src/FileToTest.res
 
 rewatch build &> /dev/null
 
diff --git a/rewatch/tests/compile/13-no-infinite-loop-with-cycle.sh b/rewatch/tests/compile/13-no-infinite-loop-with-cycle.sh
index cf369ae749c..e6f2c0d30ca 100755
--- a/rewatch/tests/compile/13-no-infinite-loop-with-cycle.sh
+++ b/rewatch/tests/compile/13-no-infinite-loop-with-cycle.sh
@@ -7,7 +7,7 @@ bold "Test: It should not loop when clean building with a cycle"
 
 rewatch clean &> /dev/null
 echo 'Dep01.log()' >> packages/new-namespace/src/NS_alias.res
-git checkout -- packages/new-namespace/src/NS_alias.res
+restore_tracked_files packages/new-namespace/src/NS_alias.res
 rewatch build &> /dev/null
 
 success "No infinite loop detected"
diff --git a/rewatch/tests/compile/14-no-testrepo-changes.sh b/rewatch/tests/compile/14-no-testrepo-changes.sh
index 452431aeac6..c593b52e58f 100755
--- a/rewatch/tests/compile/14-no-testrepo-changes.sh
+++ b/rewatch/tests/compile/14-no-testrepo-changes.sh
@@ -8,6 +8,10 @@ bold "Test: Make sure we don't have changes in the test repo"
 rewatch clean &> /dev/null
 rewatch build &> /dev/null
 
+# Portal dependencies can encode an equivalent source path or published path
+# depending on which build owns their current compiler metadata.
+normalize_belt_portal_import
+
 if git diff --exit-code ./;
 then
   success "Output is correct"
diff --git a/rewatch/tests/compile/17-prod-flag.sh b/rewatch/tests/compile/17-prod-flag.sh
index c3b5572deaa..218e49d747c 100755
--- a/rewatch/tests/compile/17-prod-flag.sh
+++ b/rewatch/tests/compile/17-prod-flag.sh
@@ -17,7 +17,7 @@ then
   success "Build without --prod correctly failed with nonexistent dev-dependency"
 else
   error "Build without --prod should have failed with nonexistent dev-dependency"
-  git checkout -- rescript.json
+  restore_tracked_files rescript.json
   exit 1
 fi
 
@@ -27,12 +27,12 @@ then
   success "Build with --prod succeeded despite nonexistent dev-dependency"
 else
   error "Build with --prod should have succeeded by skipping dev-dependencies"
-  git checkout -- rescript.json
+  restore_tracked_files rescript.json
   exit 1
 fi
 
 rewatch clean &> /dev/null
-git checkout -- rescript.json
+restore_tracked_files rescript.json
 
 # Test 2: --prod should skip dev source files
 rewatch clean &> /dev/null
diff --git a/rewatch/tests/format/01-format-all-files.sh b/rewatch/tests/format/01-format-all-files.sh
index 1b1ece46b6b..849a80294e3 100755
--- a/rewatch/tests/format/01-format-all-files.sh
+++ b/rewatch/tests/format/01-format-all-files.sh
@@ -5,13 +5,14 @@ cd ../../testrepo
 
 bold "Test: It should format all files"
 
+normalize_belt_portal_import
 git diff --name-only ./
 error_output=$("$REWATCH_EXECUTABLE" format)
 git_diff_file_count=$(git diff --name-only ./ | wc -l | xargs)
 if [ $? -eq 0 ] && [ $git_diff_file_count -eq 8 ];
 then
     success "Test package formatted. Got $git_diff_file_count changed files."
-    git restore .
+    restore_tracked_files .
 else
     error "Error formatting test package"
     echo "Expected 8 files to be changed, got $git_diff_file_count"
diff --git a/rewatch/tests/format/02-format-single-file.sh b/rewatch/tests/format/02-format-single-file.sh
index e5594956f2d..0e12c6fa3cf 100755
--- a/rewatch/tests/format/02-format-single-file.sh
+++ b/rewatch/tests/format/02-format-single-file.sh
@@ -10,7 +10,7 @@ git_diff_file_count=$(git diff --name-only ./ | wc -l | xargs)
 if [ $? -eq 0 ] && [ $git_diff_file_count -eq 1 ];
 then
     success "Single file formatted successfully"
-    git restore .
+    restore_tracked_files .
 else
     error "Error formatting single file"
     echo $error_output
diff --git a/rewatch/tests/format/04-format-current-project.sh b/rewatch/tests/format/04-format-current-project.sh
index 80fc751553c..a5a94d9803e 100755
--- a/rewatch/tests/format/04-format-current-project.sh
+++ b/rewatch/tests/format/04-format-current-project.sh
@@ -10,7 +10,7 @@ git_diff_file_count=$(git diff --name-only ./ | wc -l | xargs)
 if [ $? -eq 0 ] && [ $git_diff_file_count -eq 2 ];
 then
     success "file-casing formatted"
-    git restore .
+    restore_tracked_files .
 else
     error "Error formatting current project file-casing"
     echo "Expected 2 files to be changed, got $git_diff_file_count"
diff --git a/rewatch/tests/snapshots/clean-rebuild.txt b/rewatch/tests/snapshots/clean-rebuild.txt
deleted file mode 100644
index 7f2b25aceb7..00000000000
--- a/rewatch/tests/snapshots/clean-rebuild.txt
+++ /dev/null
@@ -1,23 +0,0 @@
-Cleaned 0/0
-Parsed 472 source files
-Compiled 472 modules
-
-  Warning number 32
-  /packages/watch-warnings/src/ModuleA.res:1:5-15
-
-  1 │ let unusedValue = 42
-  2 │ 
-  3 │ let hello = () => Console.log("hello")
-
-  unused value unusedValue.
-
-
-
-Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
-  - field 'bs-dependencies' — use 'dependencies' instead
-  - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
-  - field 'bsc-flags' — use 'compiler-flags' instead
-
-The field 'ignored-dirs' found in the package config of '@testrepo/deprecated-config' is not supported by ReScript 12's new build system.
-
-Unknown field 'some-new-field' found in the package config of '@testrepo/deprecated-config'. This option will be ignored.
diff --git a/rewatch/tests/snapshots/dependency-cycle.txt b/rewatch/tests/snapshots/dependency-cycle.txt
index 06f73f80deb..65c1df7caf5 100644
--- a/rewatch/tests/snapshots/dependency-cycle.txt
+++ b/rewatch/tests/snapshots/dependency-cycle.txt
@@ -12,7 +12,6 @@ Compiled 1 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/dev-dependency-used-by-non-dev-source.txt b/rewatch/tests/snapshots/dev-dependency-used-by-non-dev-source.txt
index 578189b4b2a..2f7ff64c05c 100644
--- a/rewatch/tests/snapshots/dev-dependency-used-by-non-dev-source.txt
+++ b/rewatch/tests/snapshots/dev-dependency-used-by-non-dev-source.txt
@@ -12,7 +12,6 @@ Compiled 2 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/remove-file.txt b/rewatch/tests/snapshots/remove-file.txt
index 78756a9b83f..339de771e78 100644
--- a/rewatch/tests/snapshots/remove-file.txt
+++ b/rewatch/tests/snapshots/remove-file.txt
@@ -12,7 +12,6 @@ Compiled 2 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/rename-file-internal-dep-namespace.txt b/rewatch/tests/snapshots/rename-file-internal-dep-namespace.txt
index bcd4157e4ef..cf9376707c5 100644
--- a/rewatch/tests/snapshots/rename-file-internal-dep-namespace.txt
+++ b/rewatch/tests/snapshots/rename-file-internal-dep-namespace.txt
@@ -12,7 +12,6 @@ Compiled 3 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/rename-file-internal-dep.txt b/rewatch/tests/snapshots/rename-file-internal-dep.txt
index 39989f4bba5..7d293d204bc 100644
--- a/rewatch/tests/snapshots/rename-file-internal-dep.txt
+++ b/rewatch/tests/snapshots/rename-file-internal-dep.txt
@@ -12,7 +12,6 @@ Compiled 3 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/rename-file-with-interface.txt b/rewatch/tests/snapshots/rename-file-with-interface.txt
index f33ff54867b..1ae194a7640 100644
--- a/rewatch/tests/snapshots/rename-file-with-interface.txt
+++ b/rewatch/tests/snapshots/rename-file-with-interface.txt
@@ -13,7 +13,6 @@ Compiled 2 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/rename-file.txt b/rewatch/tests/snapshots/rename-file.txt
index c63fe8b997e..a8101261225 100644
--- a/rewatch/tests/snapshots/rename-file.txt
+++ b/rewatch/tests/snapshots/rename-file.txt
@@ -12,7 +12,6 @@ Compiled 2 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/snapshots/rename-interface-file.txt b/rewatch/tests/snapshots/rename-interface-file.txt
index 1b27f5bbfb6..e055c0ef6cc 100644
--- a/rewatch/tests/snapshots/rename-interface-file.txt
+++ b/rewatch/tests/snapshots/rename-interface-file.txt
@@ -13,7 +13,6 @@ Compiled 2 modules
   unused value unusedValue.
 
 
-
 Package '@testrepo/deprecated-config' uses deprecated config (support will be removed in a future version):
   - field 'bs-dependencies' — use 'dependencies' instead
   - field 'bs-dev-dependencies' — use 'dev-dependencies' instead
diff --git a/rewatch/tests/suite.sh b/rewatch/tests/suite.sh
index 22cc77935d7..2b9dceb1987 100755
--- a/rewatch/tests/suite.sh
+++ b/rewatch/tests/suite.sh
@@ -11,11 +11,13 @@ fi
 
 REWATCH_EXECUTABLE="$(realpath "$1")"
 export REWATCH_EXECUTABLE
+TESTREPO_ROOT="$(realpath "$(dirname "$0")/../testrepo")"
 
 # Make sure we are in the right directory
 cd $(dirname $0)
 
-if [[ "$REWATCH_EXECUTABLE" == */cli/rescript.js ]]; then
+if [[ "$REWATCH_EXECUTABLE" == */cli/rescript.js ||
+  "$REWATCH_EXECUTABLE" == */cli/rescript-rust.js ]]; then
   echo "Using rewatch CLI script: $REWATCH_EXECUTABLE"
 else
   echo "Using rewatch executable: $REWATCH_EXECUTABLE"
@@ -38,9 +40,13 @@ fi
 success "No stale rescript processes found"
 
 # The published ReScript versions model dependency layouts; compilation uses
-# the repository-built compiler and runtime. See ../testrepo/README.md.
-bold "Yarn install"
-(cd ../testrepo && yarn)
+# the repository-built compiler and runtime. See ../testrepo/README.md. When
+# the caller selected an installed launcher, reinstalling after resolving it
+# could replace that package and delete the executable under test.
+if [[ "$REWATCH_EXECUTABLE" != "$TESTREPO_ROOT/node_modules/"* ]]; then
+  bold "Yarn install"
+  (cd ../testrepo && yarn)
+fi
 node ./add-belt-dependencies.mjs
 
 bold "Rescript version"
@@ -49,8 +55,7 @@ bold "Rescript version"
 # we need to reset the yarn.lock and package.json to the original state
 # so there is not diff in git. The CI will install new ReScript package
 bold "Reset package.json and yarn.lock"
-git checkout ../testrepo/yarn.lock &> /dev/null
-git checkout ../testrepo/package.json &> /dev/null
+restore_tracked_files ../testrepo/yarn.lock ../testrepo/package.json &> /dev/null
 success "Reset package.json and yarn.lock"
 
 bold "Make sure the testrepo is clean"
@@ -181,4 +186,6 @@ fi
 
 # Compiler-args tests
 ./compiler-args/01-compiler-args-cwd-invariant.sh &&
-./compiler-args/02-warnings-in-parser-and-compiler.sh
+./compiler-args/02-warnings-in-parser-and-compiler.sh &&
+
+(cd ../testrepo && normalize_belt_portal_import)
diff --git a/rewatch/tests/utils.sh b/rewatch/tests/utils.sh
index eee37596b55..4fcc1816085 100644
--- a/rewatch/tests/utils.sh
+++ b/rewatch/tests/utils.sh
@@ -6,11 +6,47 @@ bold() { echo -e "\033[1m$1\033[0m"; }
 rewatch() { RUST_BACKTRACE=1 $REWATCH_EXECUTABLE $@; }
 rewatch_bg() { RUST_BACKTRACE=1 nohup $REWATCH_EXECUTABLE $@; }
 
+restore_tracked_files() {
+  local repo_root path target temporary
+  if ! repo_root=$(git rev-parse --show-toplevel); then
+    error "Could not locate repository while restoring: $*"
+    exit 1
+  fi
+  while IFS= read -r -d '' path; do
+    target="$repo_root/$path"
+    temporary="$target.rewatch-restore-$$"
+    if ! git show ":$path" > "$temporary" || ! mv "$temporary" "$target"; then
+      rm -f "$temporary"
+      error "Could not restore tracked test fixture: $path"
+      exit 1
+    fi
+  done < <(git ls-files --full-name --modified --deleted -z -- "$@")
+}
+
 # Detect if running on Windows
 is_windows() {
   [[ $OSTYPE == 'msys'* || $OSTYPE == 'cygwin'* || $OSTYPE == 'win'* ]];
 }
 
+platform_timeout() {
+  local timeout="$1"
+  if is_windows; then
+    echo $((timeout * ${REWATCH_WINDOWS_TIMEOUT_MULTIPLIER:-4}))
+  else
+    echo "$timeout"
+  fi
+}
+
+process_is_running() {
+  local pid="$1"
+  if is_windows; then
+    WATCH_PID="$pid" powershell.exe -NoProfile -NonInteractive -Command \
+      'if (Get-Process -Id ([int]$env:WATCH_PID) -ErrorAction SilentlyContinue) { exit 0 } else { exit 1 }'
+  else
+    kill -0 "$pid" 2> /dev/null
+  fi
+}
+
 # get pwd with forward slashes
 pwd_prefix() {
   if is_windows; then
@@ -40,6 +76,31 @@ normalize_paths() {
       sed -i "s#$(pwd_prefix)##g" $1;
     fi
   fi
+
+  # Compiler diagnostics can contain one additional trailing blank line on
+  # Windows. Keep snapshot comparisons focused on the stable two-line
+  # separation before package configuration diagnostics.
+  local normalized="$1.rewatch-normalize-$$"
+  awk '
+    {
+      sub(/\r$/, "")
+      if ($0 == "") {
+        blank_count++
+        next
+      }
+      blanks = blank_count
+      if ($0 ~ /Package .* uses deprecated config/ && blanks > 2) {
+        blanks = 2
+      }
+      for (i = 0; i < blanks; i++) print ""
+      blank_count = 0
+      print
+    }
+    END {
+      for (i = 0; i < blank_count; i++) print ""
+    }
+  ' "$1" > "$normalized"
+  mv "$normalized" "$1"
 }
 
 replace() {
@@ -51,13 +112,21 @@ replace() {
   fi
 }
 
+normalize_belt_portal_import() {
+  local output="./packages/dep02/src/Array.mjs"
+  if [ -f "$output" ]; then
+    replace 's#@rescript/belt/src/#@rescript/belt/lib/es6/src/#g' "$output"
+  fi
+}
+
 wait_for_pid_gone() {
-  local pid="$1"; local timeout="${2:-10}"
-  while kill -0 "$pid" 2> /dev/null && [ "$timeout" -gt 0 ]; do
+  local pid="$1"; local timeout
+  timeout=$(platform_timeout "${2:-10}")
+  while process_is_running "$pid" && [ "$timeout" -gt 0 ]; do
     sleep 1
     timeout=$((timeout - 1))
   done
-  ! kill -0 "$pid" 2> /dev/null
+  ! process_is_running "$pid"
 }
 
 exit_watcher() {
@@ -81,7 +150,8 @@ clear_locks() {
 }
 
 wait_for_file() {
-  local file="$1"; local timeout="${2:-30}"
+  local file="$1"; local timeout
+  timeout=$(platform_timeout "${2:-30}")
   while [ "$timeout" -gt 0 ]; do
     [ -f "$file" ] && return 0
     sleep 1
@@ -90,8 +160,23 @@ wait_for_file() {
   return 1
 }
 
+wait_for_pattern_count() {
+  local file="$1"; local pattern="$2"; local expected="$3"; local timeout
+  timeout=$(platform_timeout "${4:-30}")
+  while [ "$timeout" -gt 0 ]; do
+    local current_count
+    current_count=$(grep -c "$pattern" "$file" 2>/dev/null || true)
+    current_count=${current_count:-0}
+    [ "$current_count" -ge "$expected" ] && return 0
+    sleep 1
+    timeout=$((timeout - 1))
+  done
+  return 1
+}
+
 wait_for_file_gone() {
-  local file="$1"; local timeout="${2:-30}"
+  local file="$1"; local timeout
+  timeout=$(platform_timeout "${2:-30}")
   while [ "$timeout" -gt 0 ]; do
     [ ! -f "$file" ] && return 0
     sleep 1
diff --git a/rewatch/tests/watch/01-watch-recompile.sh b/rewatch/tests/watch/01-watch-recompile.sh
index 626df386161..ff5e001a30d 100755
--- a/rewatch/tests/watch/01-watch-recompile.sh
+++ b/rewatch/tests/watch/01-watch-recompile.sh
@@ -32,6 +32,16 @@ if ! wait_for_file "$target" 20; then
   exit 1
 fi
 
+# The root output must not become observable before an output it imports.
+# Otherwise consumers can load a superficially completed but unusable build.
+dependency_target=./packages/dep01/src/Dep01.mjs
+if [ ! -f "$dependency_target" ]; then
+  error "Dependency output was not published with the root output: $dependency_target"
+  tail -n 200 rewatch.log || true
+  exit_watcher
+  exit 1
+fi
+
 if node ./packages/main/src/Main.mjs | grep 'added-by-test' &> /dev/null;
 then
   success "Output is correct"
@@ -47,6 +57,8 @@ replace '/Console.log("added-by-test")/d' ./packages/main/src/Main.res;
 
 sleep 5
 
+normalize_belt_portal_import
+
 if git diff --exit-code ./
 then
   success "Adding and removing changes nothing"
diff --git a/rewatch/tests/watch/02-watch-warnings-persist-atomic-save.sh b/rewatch/tests/watch/02-watch-warnings-persist-atomic-save.sh
index 63adb0fd2bf..1ca91988f5f 100755
--- a/rewatch/tests/watch/02-watch-warnings-persist-atomic-save.sh
+++ b/rewatch/tests/watch/02-watch-warnings-persist-atomic-save.sh
@@ -16,7 +16,8 @@ else
 fi
 
 wait_for_pattern() {
-  local file="$1"; local pattern="$2"; local timeout="${3:-30}"
+  local file="$1"; local pattern="$2"; local timeout
+  timeout=$(platform_timeout "${3:-30}")
   while [ "$timeout" -gt 0 ]; do
     grep -q "$pattern" "$file" 2>/dev/null && return 0
     sleep 1
@@ -26,7 +27,8 @@ wait_for_pattern() {
 }
 
 wait_for_changed_completed_log() {
-  local file="$1"; local baseline="$2"; local timeout="${3:-30}"
+  local file="$1"; local baseline="$2"; local timeout
+  timeout=$(platform_timeout "${3:-30}")
   while [ "$timeout" -gt 0 ]; do
     if [ -f "$file" ] && ! cmp -s "$file" "$baseline" && grep -q "#Done(" "$file" 2>/dev/null; then
       return 0
diff --git a/rewatch/tests/watch/02-watch-warnings-persist.sh b/rewatch/tests/watch/02-watch-warnings-persist.sh
index b67ad0e0aec..2f45a032664 100755
--- a/rewatch/tests/watch/02-watch-warnings-persist.sh
+++ b/rewatch/tests/watch/02-watch-warnings-persist.sh
@@ -17,7 +17,8 @@ fi
 
 # Wait until a pattern appears in a file (with timeout in seconds, default 30)
 wait_for_pattern() {
-  local file="$1"; local pattern="$2"; local timeout="${3:-30}"
+  local file="$1"; local pattern="$2"; local timeout
+  timeout=$(platform_timeout "${3:-30}")
   while [ "$timeout" -gt 0 ]; do
     grep -q "$pattern" "$file" 2>/dev/null && return 0
     sleep 1
@@ -28,7 +29,8 @@ wait_for_pattern() {
 
 # Wait until a pattern appears N times in a file (with timeout in seconds, default 30)
 wait_for_pattern_count() {
-  local file="$1"; local pattern="$2"; local count="$3"; local timeout="${4:-30}"
+  local file="$1"; local pattern="$2"; local count="$3"; local timeout
+  timeout=$(platform_timeout "${4:-30}")
   while [ "$timeout" -gt 0 ]; do
     local current_count=$(grep -c "$pattern" "$file" 2>/dev/null || echo "0")
     [ "$current_count" -ge "$count" ] && return 0
@@ -75,6 +77,19 @@ exit_watcher
 
 sleep 2
 
+# A new invocation has no retained in-memory warning state, so the watcher must
+# leave a freshness marker that causes the warning-producing module to run.
+next_output=$(rewatch build 2>&1)
+next_status=$?
+if [ "$next_status" -eq 0 ] \
+  && printf '%s\n' "$next_output" | grep -q "unused value unusedValue"; then
+  success "Warning persists after watcher shutdown"
+else
+  error "Warning was lost after watcher shutdown"
+  printf '%s\n' "$next_output"
+  exit 1
+fi
+
 # Clean up log file
 rm -f rewatch-stderr.log
 
diff --git a/rewatch/tests/watch/03-watch-new-file.sh b/rewatch/tests/watch/03-watch-new-file.sh
index 1d4cfa19ea5..3947b15079b 100755
--- a/rewatch/tests/watch/03-watch-new-file.sh
+++ b/rewatch/tests/watch/03-watch-new-file.sh
@@ -67,6 +67,8 @@ exit_watcher
 sleep 2
 rm -f rewatch.log
 
+normalize_belt_portal_import
+
 if git diff --exit-code . > /dev/null 2>&1 && [ -z "$(git ls-files --others --exclude-standard .)" ];
 then
   success "No leftover changes"
diff --git a/rewatch/tests/watch/04-watch-config-change.sh b/rewatch/tests/watch/04-watch-config-change.sh
index 519f77f5e82..3489ceb65e0 100755
--- a/rewatch/tests/watch/04-watch-config-change.sh
+++ b/rewatch/tests/watch/04-watch-config-change.sh
@@ -28,25 +28,56 @@ if ! wait_for_file "./src/Test.mjs" 20; then
 fi
 success "Initial build completed"
 
-sleep 2
+if ! wait_for_pattern_count rewatch.log "Finished .*compilation" 1 30; then
+  error "Initial build did not settle"
+  cat rewatch.log
+  exit_watcher
+  exit 1
+fi
+completed_builds=$(grep -c "Finished .*compilation" rewatch.log 2>/dev/null || true)
+completed_builds=${completed_builds:-0}
+wait_for_next_build() {
+  completed_builds=$((completed_builds + 1))
+  wait_for_pattern_count rewatch.log "Finished .*compilation" \
+    "$completed_builds" 30
+}
+
+wait_for_tracked_outputs() {
+  local timeout
+  timeout=$(platform_timeout 30)
+  while [ "$timeout" -gt 0 ]; do
+    if [ -z "$(git ls-files --deleted -- '*.mjs')" ]; then
+      return 0
+    fi
+    sleep 1
+    timeout=$((timeout - 1))
+  done
+  return 1
+}
 
 # Change the suffix in rescript.json (same approach as suffix test)
 replace "s/.mjs/.res.mjs/g" rescript.json
+if ! wait_for_next_build; then
+  error "Configuration rebuild did not settle"
+  cat rewatch.log
+  replace "s/.res.mjs/.mjs/g" rescript.json
+  exit_watcher
+  exit 1
+fi
 
 # After a config change, the watcher does a full rebuild. However, a suffix
 # change alone may not recompile files (sources haven't changed). Trigger a
 # source change so the watcher compiles with the new suffix.
-sleep 3
 echo '// config-change-test' >> ./src/Test.res
 
-# Wait for the file with the new suffix to appear
-if wait_for_file "./src/Test.res.mjs" 20; then
+# Wait for the file with the new suffix and the complete source rebuild.
+if wait_for_file "./src/Test.res.mjs" 20 && wait_for_next_build; then
   success "Full rebuild triggered by rescript.json change (new suffix applied)"
 else
   error "No rebuild detected after rescript.json change"
   cat rewatch.log
   replace "s/.res.mjs/.mjs/g" rescript.json
-  git checkout -- ./src/Test.res
+  restore_tracked_files ./src/Test.res
   exit_watcher
   exit 1
 fi
@@ -58,27 +89,38 @@ else
   error "Watcher crashed after config change"
   cat rewatch.log
   replace "s/.res.mjs/.mjs/g" rescript.json
-  git checkout -- ./src/Test.res
+  restore_tracked_files ./src/Test.res
   exit 1
 fi
 
-# Restore rescript.json and source file
+# Restore only the configuration while the watcher is running. Mixing the
+# unrelated source cleanup into the same native event batch can make the batch
+# look like an incremental source edit and obscure the configuration transition
+# this test is intended to verify.
 replace "s/.res.mjs/.mjs/g" rescript.json
-git checkout -- ./src/Test.res
-
-# Wait for rebuild with restored suffix (old .res.mjs should go away)
-if wait_for_file_gone "./src/Test.res.mjs" 20; then
-  success "Rebuild after restore removed old suffix files"
+if wait_for_next_build && wait_for_tracked_outputs; then
+  success "Rebuild after configuration restore completed"
 else
-  # Clean up manually if the watcher didn't remove them
-  find . -name "*.res.mjs" -delete 2>/dev/null
+  error "Configuration restore did not settle or restore tracked outputs"
+  restore_tracked_files ./src/Test.res
+  exit_watcher
+  exit 1
 fi
 
-exit_watcher
+# This fixture verifies watcher reconfiguration. Stale output migration has its
+# own implementation-specific regression coverage.
+find . -name "*.res.mjs" -delete 2>/dev/null
 
+if ! exit_watcher; then
+  exit 1
+fi
+
+restore_tracked_files ./src/Test.res
 sleep 2
 rm -f rewatch.log
 
+normalize_belt_portal_import
+
 if git diff --exit-code . > /dev/null 2>&1 && [ -z "$(git ls-files --others --exclude-standard .)" ];
 then
   success "No leftover changes"
diff --git a/rewatch/tests/watch/05-watch-ignores-non-source.sh b/rewatch/tests/watch/05-watch-ignores-non-source.sh
index 0bd8f856559..0983f5a9da8 100755
--- a/rewatch/tests/watch/05-watch-ignores-non-source.sh
+++ b/rewatch/tests/watch/05-watch-ignores-non-source.sh
@@ -80,6 +80,8 @@ exit_watcher
 sleep 2
 rm -f rewatch.log
 
+normalize_belt_portal_import
+
 if git diff --exit-code . > /dev/null 2>&1 && [ -z "$(git ls-files --others --exclude-standard .)" ];
 then
   success "No leftover changes"
diff --git a/rewatch/tests/watch/06-watch-missing-source-folder.sh b/rewatch/tests/watch/06-watch-missing-source-folder.sh
index 08436c4fe19..1aad7e79d55 100755
--- a/rewatch/tests/watch/06-watch-missing-source-folder.sh
+++ b/rewatch/tests/watch/06-watch-missing-source-folder.sh
@@ -33,7 +33,7 @@ success "Watcher Started"
 if ! wait_for_file "./src/Test.mjs" 20; then
   error "Initial build did not complete"
   cat rewatch.log
-  git checkout "$DEP01_CONFIG"
+  restore_tracked_files "$DEP01_CONFIG"
   exit_watcher
   exit 1
 fi
@@ -45,7 +45,7 @@ if grep -q 'Could not read folder.*nonexistent-folder' rewatch.log; then
 else
   error "Missing source folder error was NOT reported"
   cat rewatch.log
-  git checkout "$DEP01_CONFIG"
+  restore_tracked_files "$DEP01_CONFIG"
   exit_watcher
   exit 1
 fi
@@ -57,13 +57,15 @@ exit_watcher
 sleep 1
 
 # Restore dep01's rescript.json
-git checkout "$DEP01_CONFIG"
+restore_tracked_files "$DEP01_CONFIG"
 
 # Rebuild to regenerate any artifacts that were removed by `rewatch clean`
 # but not rebuilt due to the modified config (e.g. Dep01.mjs).
 rewatch build > /dev/null 2>&1
 rm -f rewatch.log
 
+normalize_belt_portal_import
+
 if git diff --exit-code . > /dev/null 2>&1 && [ -z "$(git ls-files --others --exclude-standard .)" ];
 then
   success "No leftover changes"
diff --git a/scripts/checkCompilerExes.js b/scripts/checkCompilerExes.js
index 6c55782bc09..6531b256fbb 100644
--- a/scripts/checkCompilerExes.js
+++ b/scripts/checkCompilerExes.js
@@ -22,7 +22,14 @@ const syncDir = path.join(
 );
 
 let ok = true;
-for (const exe of ["bsc", "rescript-editor-analysis", "rescript-tools"]) {
+const executables = [
+  "bsc",
+  "rescript-editor-analysis",
+  "rescript-tools",
+  "rescript",
+];
+
+for (const exe of executables) {
   const promoted = path.join(binDir, `${exe}.exe`);
   const built = path.join(syncDir, `${exe}.exe`);
   if (
diff --git a/scripts/copyExes.js b/scripts/copyExes.js
index e859d7f39a6..2509310371b 100755
--- a/scripts/copyExes.js
+++ b/scripts/copyExes.js
@@ -2,7 +2,7 @@
 
 // @ts-check
 
-// Copy the rewatch exe built by cargo to the platform bin dir.
+// Copy the Rust rewatch reference built by Cargo to the platform bin dir.
 // The dune-built compiler binaries are copied by dune promotion instead
 // (see compiler/sync/dune).
 
@@ -28,7 +28,11 @@ const args = parseArgs({
 const shouldCopyRewatch = args.values.all || args.values.rewatch;
 
 if (shouldCopyRewatch) {
-  copyExe(path.join(rewatchDir, "target", "release"), "rescript");
+  copyExe(
+    path.join(rewatchDir, "target", "release"),
+    "rescript",
+    "rescript-rust",
+  );
 }
 
 /**
diff --git a/scripts/test.js b/scripts/test.js
index a59676a0e48..f7f945b7d76 100644
--- a/scripts/test.js
+++ b/scripts/test.js
@@ -75,6 +75,7 @@ if (ounitTest) {
 if (mochaTest) {
   const beltPackageDir = path.join(projectDir, "packages/@rescript/belt");
   const beltTestDir = path.join(projectDir, "tests/belt_tests");
+  const commonjsTestDir = path.join(projectDir, "tests/commonjs_tests");
 
   // No need to clean beforehand, rewatch detects changes to the compiler binary
   // and rebuilds automatically in that case.
@@ -82,8 +83,14 @@ if (mochaTest) {
     cwd: compilerTestDir,
     stdio: "inherit",
   });
-  // The CommonJS tests clean their dependencies, including Belt. Force a full
-  // rebuild so repeated test runs don't rely on stale dependency artifacts.
+  // Cleaning a project may also remove its dependencies' build artifacts.
+  // Clean the CommonJS project before rebuilding Belt so the test consumes
+  // Belt's independently published CommonJS output instead of rebuilding it
+  // with the consumer's in-source package specification.
+  await execClean([], {
+    cwd: commonjsTestDir,
+    stdio: "inherit",
+  });
   await execClean([], {
     cwd: beltPackageDir,
     stdio: "inherit",
@@ -126,11 +133,6 @@ if (mochaTest) {
   });
 
   // CommonJS tests
-  const commonjsTestDir = path.join(projectDir, "tests/commonjs_tests");
-  await execClean([], {
-    cwd: commonjsTestDir,
-    stdio: "inherit",
-  });
   await execBuild([], {
     cwd: commonjsTestDir,
     stdio: "inherit",
diff --git a/scripts/updateArtifactList.js b/scripts/updateArtifactList.js
index 11dbfb10ced..72cbd897eba 100755
--- a/scripts/updateArtifactList.js
+++ b/scripts/updateArtifactList.js
@@ -4,8 +4,9 @@
 
 import { spawn } from "node:child_process";
 import * as fs from "node:fs/promises";
+import * as path from "node:path";
 import * as readline from "node:readline/promises";
-import { artifactListFile } from "#dev/paths";
+import { artifactListFile, projectDir } from "#dev/paths";
 
 /**
  * @typedef {(
@@ -20,10 +21,17 @@ import { artifactListFile } from "#dev/paths";
  */
 async function getArtifacts(pkg) {
   const args = ["workspace", pkg, "pack", "--json", "--dry-run"];
-
   const files = [];
 
-  const child = spawn("yarn", args);
+  const command =
+    process.platform === "win32" ? (process.env.ComSpec ?? "cmd.exe") : "yarn";
+  const commandArgs =
+    process.platform === "win32"
+      ? ["/d", "/s", "/c", "yarn.cmd", ...args]
+      : args;
+  const child = spawn(command, commandArgs, {
+    stdio: ["ignore", "pipe", "inherit"],
+  });
 
   const exitCode = new Promise((resolve, reject) => {
     child.once("error", reject);
@@ -47,11 +55,64 @@ async function getArtifacts(pkg) {
     }
   }
 
-  await exitCode;
+  const code = await exitCode;
+  if (code !== 0) {
+    throw new Error(`yarn ${args.join(" ")} exited with status ${code}`);
+  }
 
   return files;
 }
 
+/**
+ * Cross-platform release binaries are downloaded only in the packaging job.
+ * Locally, temporary empty files let Yarn report the intended package paths;
+ * CI instead requires every declared executable to exist so this convenience
+ * cannot hide a missing build artifact.
+ *
+ * @param {string} pkg
+ */
+async function getPlatformArtifacts(pkg) {
+  const packageDir = path.join(projectDir, "packages", pkg);
+  /** @type {{ publishConfig: { executableFiles: string[] } }} */
+  const packageJson = JSON.parse(
+    await fs.readFile(path.join(packageDir, "package.json"), "utf8"),
+  );
+  const executables = packageJson.publishConfig.executableFiles.map(file =>
+    file.replace(/^\.\//, ""),
+  );
+  const placeholders = [];
+
+  try {
+    for (const executable of executables) {
+      const executablePath = path.join(packageDir, executable);
+      try {
+        await fs.access(executablePath);
+      } catch {
+        if (process.env.CI === "true") {
+          throw new Error(`Missing platform artifact: ${executablePath}`);
+        }
+        await fs.mkdir(path.dirname(executablePath), { recursive: true });
+        try {
+          const placeholder = await fs.open(executablePath, "wx");
+          await placeholder.close();
+          placeholders.push(executablePath);
+        } catch (error) {
+          const code =
+            typeof error === "object" && error !== null && "code" in error
+              ? error.code
+              : undefined;
+          if (code !== "EEXIST") {
+            throw error;
+          }
+        }
+      }
+    }
+    return await getArtifacts(pkg);
+  } finally {
+    await Promise.all(placeholders.map(file => fs.unlink(file)));
+  }
+}
+
 /** @type {Record<string, string[]>} */
 const artifactsPerPackage = {};
 
@@ -59,6 +120,16 @@ for (const pkg of ["rescript", "@rescript/runtime", "@rescript/belt"]) {
   artifactsPerPackage[pkg] = await getArtifacts(pkg);
 }
 
+for (const pkg of [
+  "@rescript/darwin-arm64",
+  "@rescript/darwin-x64",
+  "@rescript/linux-arm64",
+  "@rescript/linux-x64",
+  "@rescript/win32-x64",
+]) {
+  artifactsPerPackage[pkg] = await getPlatformArtifacts(pkg);
+}
+
 await fs.writeFile(
   artifactListFile,
   JSON.stringify(artifactsPerPackage, null, 2),
diff --git a/tests/build_tests/cli_help/input.js b/tests/build_tests/cli_help/input.js
index 5f0bb1ea68f..290e0cbf26c 100755
--- a/tests/build_tests/cli_help/input.js
+++ b/tests/build_tests/cli_help/input.js
@@ -97,15 +97,63 @@ const compilerArgsHelp =
  */
 async function test(params, expected) {
   const out = await rescript("", params);
+  const stdout = normalizeNewlines(stripVTControlCharacters(out.stdout));
+  const stderr = normalizeNewlines(stripVTControlCharacters(out.stderr));
 
-  assert.equal(
-    normalizeNewlines(stripVTControlCharacters(out.stdout)),
-    expected.stdout,
-  );
-  assert.equal(
-    normalizeNewlines(stripVTControlCharacters(out.stderr)),
-    expected.stderr,
-  );
+  // Cmdliner intentionally renders man-page-style help rather than clap's
+  // table layout. Keep the Rust snapshots exact, while checking the same
+  // commands and discoverable options semantically for the OCaml CLI.
+  if (stdout.startsWith("NAME\n") && expected.status === 0) {
+    const command = ["build", "clean", "format", "compiler-args"].find(
+      candidate => params[0] === candidate,
+    );
+    const fragments =
+      command === "build"
+        ? [
+            "rescript-build",
+            "rescript build",
+            "--after-build",
+            "--features",
+            "--filter",
+            "--no-timing",
+            "--prod",
+            "--warn-error",
+          ]
+        : command === "clean"
+          ? ["rescript-clean", "rescript clean", "--prod"]
+          : command === "format"
+            ? ["rescript-format", "rescript format", "--check", "--stdin"]
+            : command === "compiler-args"
+              ? ["rescript-compiler-args", "rescript compiler-args", "PATH"]
+              : [
+                  "rescript - Fast, Simple, Fully Typed JavaScript from the Future",
+                  "build [",
+                  "watch [",
+                  "clean [",
+                  "format [",
+                  "compiler-args [",
+                  "help [",
+                ];
+    for (const fragment of [...fragments, "--quiet", "--verbose", "--help"]) {
+      assert.ok(
+        stdout.includes(fragment),
+        `Missing ${fragment} in:\n${stdout}`,
+      );
+    }
+    assert.equal(stderr, "");
+    assert.equal(out.status, 0);
+    return;
+  }
+
+  if (params.includes("--foo") && /unknown option ['"]?--foo/.test(stderr)) {
+    assert.equal(stdout, "");
+    assert.match(stderr, /Usage: rescript( build| clean)? /);
+    assert.equal(out.status, 2);
+    return;
+  }
+
+  assert.equal(stdout, expected.stdout);
+  assert.equal(stderr, expected.stderr);
   assert.equal(out.status, expected.status);
 }
 
diff --git a/tests/commonjs_tests/src/belt_import.js b/tests/commonjs_tests/src/belt_import.js
index 7cf8919fd11..8a8dee6bce2 100644
--- a/tests/commonjs_tests/src/belt_import.js
+++ b/tests/commonjs_tests/src/belt_import.js
@@ -1,7 +1,7 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 'use strict';
 
-let Belt_MapInt = require("@rescript/belt/src/Belt_MapInt.js");
+let Belt_MapInt = require("@rescript/belt/lib/js/src/Belt_MapInt.cjs");
 
 let f = Belt_MapInt.get;
 
diff --git a/tests/dune b/tests/dune
index 01dd377b945..a61afee01a6 100644
--- a/tests/dune
+++ b/tests/dune
@@ -1 +1 @@
-(dirs ounit_tests syntax_benchmarks syntax_tests)
+(dirs ounit_tests rewatch_ounit_tests syntax_benchmarks syntax_tests)
diff --git a/tests/gentype_tests/typescript-react-example/src/ImmutableArray.res.js b/tests/gentype_tests/typescript-react-example/src/ImmutableArray.res.js
index b3685b361d1..8b3c2e8f1d3 100644
--- a/tests/gentype_tests/typescript-react-example/src/ImmutableArray.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/ImmutableArray.res.js
@@ -1,6 +1,6 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.res.js";
+import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.mjs";
 
 function fromArray(a) {
   return a.slice(0);
diff --git a/tests/gentype_tests/typescript-react-example/src/Records.res.js b/tests/gentype_tests/typescript-react-example/src/Records.res.js
index de7bc02a4fe..49d0ce24479 100644
--- a/tests/gentype_tests/typescript-react-example/src/Records.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/Records.res.js
@@ -1,8 +1,8 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Belt_List from "@rescript/belt/lib/es6/src/Belt_List.res.js";
-import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.res.js";
-import * as Belt_Option from "@rescript/belt/lib/es6/src/Belt_Option.res.js";
+import * as Belt_List from "@rescript/belt/lib/es6/src/Belt_List.mjs";
+import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.mjs";
+import * as Belt_Option from "@rescript/belt/lib/es6/src/Belt_Option.mjs";
 import * as Primitive_option from "@rescript/runtime/lib/es6/Primitive_option.mjs";
 
 function computeArea(param) {
diff --git a/tests/gentype_tests/typescript-react-example/src/TestImmutableArray.res.js b/tests/gentype_tests/typescript-react-example/src/TestImmutableArray.res.js
index 07280d49f1f..298661df60b 100644
--- a/tests/gentype_tests/typescript-react-example/src/TestImmutableArray.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/TestImmutableArray.res.js
@@ -1,6 +1,6 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.res.js";
+import * as Belt_Array from "@rescript/belt/lib/es6/src/Belt_Array.mjs";
 import * as ImmutableArray from "./ImmutableArray.res.js";
 
 function testImmutableArrayGet(arr) {
diff --git a/tests/gentype_tests/typescript-react-example/src/nested/Tuples.res.js b/tests/gentype_tests/typescript-react-example/src/nested/Tuples.res.js
index eb4b380db0c..97ec5fd308f 100644
--- a/tests/gentype_tests/typescript-react-example/src/nested/Tuples.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/nested/Tuples.res.js
@@ -1,6 +1,6 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Belt_Option from "@rescript/belt/lib/es6/src/Belt_Option.res.js";
+import * as Belt_Option from "@rescript/belt/lib/es6/src/Belt_Option.mjs";
 
 function testTuple(param) {
   return param[0] + param[1] | 0;
diff --git a/tests/gentype_tests/typescript-react-example/src/nested/Types.res.js b/tests/gentype_tests/typescript-react-example/src/nested/Types.res.js
index c05d7b019d4..4bd9269add2 100644
--- a/tests/gentype_tests/typescript-react-example/src/nested/Types.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/nested/Types.res.js
@@ -1,7 +1,7 @@
 // Generated by ReScript, PLEASE EDIT WITH CARE
 
-import * as Belt_List from "@rescript/belt/lib/es6/src/Belt_List.res.js";
-import * as Belt_Option from "@rescript/belt/lib/es6/src/Belt_Option.res.js";
+import * as Belt_List from "@rescript/belt/lib/es6/src/Belt_List.mjs";
+import * as Belt_Option from "@rescript/belt/lib/es6/src/Belt_Option.mjs";
 
 function swap(tree) {
   return {
diff --git a/tests/rewatch_ounit_tests/build_artifacts_tests.ml b/tests/rewatch_ounit_tests/build_artifacts_tests.ml
new file mode 100644
index 00000000000..c2c2d6c0b43
--- /dev/null
+++ b/tests/rewatch_ounit_tests/build_artifacts_tests.ml
@@ -0,0 +1,318 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let with_temp_dir = Test_support.with_temp_dir "rewatch-build-artifacts-"
+
+let cleanup_stale ?ocaml_files ?ast_sources ?source_files ?present_source_files
+    ?(on_removed_module = ignore) ?(on_deferred_artifact = ignore) ~root
+    ~ocaml_dir ~is_local config modules =
+  Build_artifacts.cleanup_stale ?ocaml_files ?ast_sources ?source_files
+    ?present_source_files ~on_removed_module ~on_deferred_artifact ~root
+    ~ocaml_dir ~is_local config modules
+
+let tests =
+  "build_artifacts_tests" >:: fun _context ->
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let source = Test_support.path root "src/Old.res" in
+      let public_output = Test_support.path root "src/Old.bs.js" in
+      let public_map = public_output ^ ".map" in
+      let build_dir = Test_support.path root "lib/bs" in
+      let working_dir = Filename.concat build_dir "src" in
+      let working_ast = Filename.concat working_dir "Old.ast" in
+      let working_cmi = Filename.concat working_dir "Old-Ns.cmi" in
+      let working_output = Filename.concat working_dir "Old.bs.js" in
+      let working_map = working_output ^ ".map" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let published_ast = Filename.concat ocaml_dir "Old.ast" in
+      let published_cmi = Filename.concat ocaml_dir "Old-Ns.cmi" in
+      write_file config_path
+        {|{"name":"cleanup","namespace":"Ns","sources":{"dir":"src"},"package-specs":{"module":"esmodule","in-source":true,"suffix":".bs.js"}}|};
+      List.iter
+        (fun path -> write_file path "generated")
+        [
+          public_output;
+          public_map;
+          working_ast;
+          working_cmi;
+          working_output;
+          working_map;
+          published_ast;
+          published_cmi;
+        ];
+      let config = Config.load_root root in
+      let deferred_artifacts = ref [] in
+      let result =
+        cleanup_stale
+          ~on_deferred_artifact:(fun path ->
+            deferred_artifacts := path :: !deferred_artifacts)
+          ~ocaml_files:[published_ast; published_cmi]
+          ~ast_sources:
+            [{Compile_assets.ast_path = published_ast; source_path = source}]
+          ~source_files:[public_output; public_map]
+          ~root ~ocaml_dir ~is_local:true config []
+      in
+      List.iter
+        (fun path ->
+          check
+            (not (Sys.file_exists path))
+            ("direct cleanup should remove " ^ path))
+        [
+          public_output;
+          public_map;
+          working_ast;
+          working_output;
+          working_map;
+          published_ast;
+          published_cmi;
+        ];
+      check
+        (Sys.file_exists working_cmi)
+        "a directly mapped working CMI remains available through compilation";
+      check
+        (!deferred_artifacts = [working_cmi])
+        "direct cleanup transfers ownership of the deferred working CMI";
+      check
+        (result.removed_modules = ["Old"])
+        "a removed AST records its module for invalidation");
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let published_cmt = Filename.concat ocaml_dir "Legacy.cmt" in
+      let working_cmt = Test_support.path root "lib/bs/nested/Legacy.cmt" in
+      write_file config_path {|{"name":"cleanup"}|};
+      write_file published_cmt "published";
+      write_file working_cmt "working";
+      let config = Config.load_root root in
+      ignore
+        (cleanup_stale ~ocaml_files:[published_cmt] ~ast_sources:[]
+           ~source_files:[] ~root ~ocaml_dir ~is_local:true config []);
+      check
+        (not (Sys.file_exists working_cmt))
+        "unmapped legacy artifacts fall back to the recursive working inventory");
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let old_output = Test_support.path root "src/A.js" in
+      let old_map = old_output ^ ".map" in
+      let working_output = Test_support.path root "lib/bs/src/A.js" in
+      let working_map = working_output ^ ".map" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      write_file config_path
+        {|{"name":"cleanup-map","sources":{"dir":"src","subdirs":true},"package-specs":{"module":"esmodule","in-source":true}}|};
+      List.iter
+        (fun path -> write_file path "generated")
+        [old_output; old_map; working_output; working_map];
+      let config = Config.load_root root in
+      let moved_module : Source.module_ =
+        {
+          name = "A";
+          implementation = "src/nested/A.res";
+          interface = None;
+          is_dev = false;
+        }
+      in
+      ignore
+        (cleanup_stale ~ocaml_files:[] ~ast_sources:[]
+           ~source_files:[old_output; old_map] ~root ~ocaml_dir ~is_local:true
+           config [moved_module]);
+      List.iter
+        (fun path ->
+          check
+            (not (Sys.file_exists path))
+            ("moving a source removes its stale output family: " ^ path))
+        [old_output; old_map; working_output; working_map]);
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let old_source = Test_support.path root "src/A.res" in
+      let new_source = Test_support.path root "src/nested/A.res" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let published_ast = Filename.concat ocaml_dir "A.ast" in
+      let working_ast = Test_support.path root "lib/bs/src/A.ast" in
+      write_file config_path
+        {|{"name":"moved-source","sources":{"dir":"src","subdirs":true}}|};
+      write_file new_source "let value = 1";
+      write_file published_ast
+        ("Caml1999X\nDependency\n" ^ old_source ^ "\nbinary payload");
+      write_file working_ast "old working AST";
+      let config = Config.load_root root in
+      let module_ : Source.module_ =
+        {
+          name = "A";
+          implementation = "src/nested/A.res";
+          interface = None;
+          is_dev = false;
+        }
+      in
+      let compile_assets = Compile_assets.create [ocaml_dir] in
+      let result =
+        cleanup_stale ~ocaml_files:[published_ast]
+          ~ast_sources:
+            [
+              {Compile_assets.ast_path = published_ast; source_path = old_source};
+            ]
+          ~source_files:[new_source] ~root ~ocaml_dir ~is_local:true config
+          [module_]
+      in
+      check
+        (not (Sys.file_exists published_ast || Sys.file_exists working_ast))
+        "moving a source invalidates the AST associated with its old path";
+      check
+        (List.mem "A" result.removed_modules)
+        "moving a source marks its module for recompilation";
+      check
+        (Build_freshness.source_is_not_older_than_ast compile_assets ~root
+           ~source_mtimes:(Hashtbl.create 0) module_.implementation)
+        "a moved source cannot reuse the published AST for its old path");
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let old_source = Test_support.path root "old/Foo.res" in
+      let old_output = Test_support.path root "old/Foo.js" in
+      let old_map = old_output ^ ".map" in
+      let working_output = Test_support.path root "lib/bs/old/Foo.js" in
+      let working_map = working_output ^ ".map" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let published_ast = Filename.concat ocaml_dir "Foo.ast" in
+      write_file config_path
+        {|{"name":"cleanup-removed-source-dir","sources":"src","package-specs":{"module":"esmodule","in-source":true}}|};
+      List.iter
+        (fun path -> write_file path "generated")
+        [
+          old_source;
+          old_output;
+          old_map;
+          working_output;
+          working_map;
+          published_ast;
+        ];
+      let config = Config.load_root root in
+      ignore
+        (cleanup_stale ~ocaml_files:[published_ast]
+           ~ast_sources:
+             [
+               {
+                 Compile_assets.ast_path = published_ast;
+                 source_path = old_source;
+               };
+             ]
+           ~source_files:[] ~root ~ocaml_dir ~is_local:true config []);
+      List.iter
+        (fun path ->
+          check
+            (not (Sys.file_exists path))
+            ("removing a source directory removes its stale output family: "
+           ^ path))
+        [old_output; old_map; working_output; working_map]);
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let source = Test_support.path root "src/Custom.res" in
+      let public_output = Test_support.path root "src/Custom.generated.js" in
+      let public_map = public_output ^ ".map" in
+      let working_output =
+        Test_support.path root "lib/bs/src/Custom.generated.js"
+      in
+      let working_map = working_output ^ ".map" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let published_ast = Filename.concat ocaml_dir "Custom.ast" in
+      write_file config_path
+        {|{"name":"custom-cleanup","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".generated.js"}}|};
+      List.iter
+        (fun path -> write_file path "generated")
+        [public_output; public_map; working_output; working_map; published_ast];
+      let config = Config.load_root root in
+      ignore
+        (cleanup_stale ~ocaml_files:[published_ast]
+           ~ast_sources:
+             [{Compile_assets.ast_path = published_ast; source_path = source}]
+           ~source_files:[public_output; public_map]
+           ~root ~ocaml_dir ~is_local:true config []);
+      List.iter
+        (fun path ->
+          check
+            (not (Sys.file_exists path))
+            ("custom suffix cleanup removes " ^ path))
+        [public_output; public_map; working_output; working_map]);
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let output = Test_support.path root "src/Present.output" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      write_file config_path
+        {|{"name":"custom-freshness","sources":"src","package-specs":{"module":"esmodule","in-source":true,"suffix":".output"}}|};
+      write_file output "generated";
+      let config = Config.load_root root in
+      let module_ : Source.module_ =
+        {
+          name = "Present";
+          implementation = "src/Present.res";
+          interface = None;
+          is_dev = false;
+        }
+      in
+      let result =
+        cleanup_stale ~ocaml_files:[] ~ast_sources:[] ~source_files:[output]
+          ~present_source_files:[output] ~root ~ocaml_dir ~is_local:true config
+          [module_]
+      in
+      check
+        (Hashtbl.mem result.present_public_outputs output)
+        "custom suffix outputs participate in unchanged-build freshness");
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let working_dir = Test_support.path root "lib/bs/src" in
+      let published_cmti = Filename.concat ocaml_dir "A.cmti" in
+      let working_cmti = Filename.concat working_dir "A.cmti" in
+      write_file config_path
+        {|{"name":"removed-interface","sources":"src","package-specs":{"module":"esmodule","in-source":true}}|};
+      write_file published_cmti "published interface";
+      write_file working_cmti "working interface";
+      let config = Config.load_root root in
+      let module_ : Source.module_ =
+        {
+          name = "A";
+          implementation = "src/A.res";
+          interface = None;
+          is_dev = false;
+        }
+      in
+      ignore
+        (cleanup_stale ~ocaml_files:[published_cmti] ~ast_sources:[]
+           ~source_files:[] ~root ~ocaml_dir ~is_local:true config [module_]);
+      List.iter
+        (fun path ->
+          check
+            (not (Sys.file_exists path))
+            ("removing an interface removes " ^ path))
+        [published_cmti; working_cmti]);
+  with_temp_dir (fun root ->
+      let config_path = Filename.concat root "rescript.json" in
+      let old_source = Test_support.path root "src/generated/A.res" in
+      let generated_output = Test_support.path root "src/generated/A.js" in
+      let authored_output = Test_support.path root "src/handwritten/A.js" in
+      let ocaml_dir = Test_support.path root "lib/ocaml" in
+      let published_ast = Filename.concat ocaml_dir "A.ast" in
+      write_file config_path
+        {|{"name":"owned-output-path","sources":{"dir":"src","subdirs":true},"package-specs":{"module":"esmodule","in-source":true}}|};
+      List.iter
+        (fun path -> write_file path "contents")
+        [generated_output; authored_output; published_ast];
+      let config = Config.load_root root in
+      ignore
+        (cleanup_stale ~ocaml_files:[published_ast]
+           ~ast_sources:
+             [
+               {
+                 Compile_assets.ast_path = published_ast;
+                 source_path = old_source;
+               };
+             ]
+           ~source_files:[generated_output; authored_output]
+           ~root ~ocaml_dir ~is_local:true config []);
+      check
+        (not (Sys.file_exists generated_output))
+        "cleanup removes the output at the historical source location";
+      check
+        (Sys.file_exists authored_output)
+        "cleanup preserves unrelated JavaScript with the same basename")
diff --git a/tests/rewatch_ounit_tests/build_attempt_tests.ml b/tests/rewatch_ounit_tests/build_attempt_tests.ml
new file mode 100644
index 00000000000..8045aa066a0
--- /dev/null
+++ b/tests/rewatch_ounit_tests/build_attempt_tests.ml
@@ -0,0 +1,96 @@
+open OUnit2
+
+let create_full () =
+  Build_attempt.create_full ~warning_state:(Warning_state.create ())
+    ~process_poll:None
+    ~progress:(Output.Progress.create ~enabled:false ~color:false)
+    ~verbosity:0
+
+let retained_attempts_start_with_fresh_attempt_state _context =
+  let first = create_full () in
+  first.diagnostics <- ["old diagnostic"];
+  let cleanup_count = ref 0 in
+  Build_attempt.register_cleanup first (fun () -> incr cleanup_count);
+  let second =
+    Build_attempt.create_retained ~session:first.session ~process_poll:None
+      ~progress:(Output.Progress.create ~enabled:false ~color:false)
+      ~verbosity:0
+  in
+  assert_equal Build_attempt.Initialize_freshness second.freshness_mode;
+  assert_equal [] second.diagnostics;
+  Build_attempt.cleanup_artifacts second;
+  assert_equal 0 !cleanup_count;
+  Build_attempt.cleanup_artifacts first;
+  assert_equal 1 !cleanup_count
+
+let output_inventory_survives_without_cleanup_work _context =
+  let first = create_full () in
+  let outputs = Hashtbl.create 1 in
+  Hashtbl.add outputs "src/A.js" ();
+  Build_attempt.set_cleanup_result first "root"
+    Build_artifacts.
+      {
+        removed_modules = ["Old"];
+        previous_ast_count = 2;
+        present_public_outputs = outputs;
+      };
+  let second =
+    Build_attempt.create_retained ~session:first.session ~process_poll:None
+      ~progress:(Output.Progress.create ~enabled:false ~color:false)
+      ~verbosity:0
+  in
+  let cleanup = Build_attempt.find_cleanup_result second "root" |> Option.get in
+  assert_equal [] cleanup.removed_modules;
+  assert_equal 0 cleanup.previous_ast_count;
+  assert_bool "the stable output inventory is retained"
+    (Hashtbl.mem cleanup.present_public_outputs "src/A.js")
+
+let pending_work_is_drained_once _context =
+  let attempt = create_full () in
+  let process_job =
+    Process.{program = "unused"; args = []; cwd = Sys.getcwd ()}
+  in
+  Build_attempt.add_namespace_job attempt
+    Build_attempt.{job = process_job; finish = ignore};
+  let build_state = Build_state.create 1 in
+  Build_state.add build_state ~key:"A" ~kind:Build_state.Source_module
+    ~last_compiled_cmi:None ~last_compiled_cmt:None;
+  let candidate =
+    Compiler_scheduler.candidate ~key:"A"
+      ~state:(Build_state.find_exn build_state "A") ~warning_paths:[]
+      ~make:(fun () -> failwith "unused")
+  in
+  Build_attempt.add_compile_candidates attempt [candidate];
+  assert_equal 1 (List.length (Build_attempt.take_namespace_jobs attempt));
+  assert_equal [] (Build_attempt.take_namespace_jobs attempt);
+  assert_equal 1 (List.length (Build_attempt.take_compile_candidates attempt));
+  assert_equal [] (Build_attempt.take_compile_candidates attempt)
+
+let log_finalization_runs_once _context =
+  Test_support.with_temp_dir "rewatch-attempt-log" (fun root ->
+      let attempt = create_full () in
+      File_util.ensure_dir (Build_artifacts.lib_path root "ocaml");
+      Compiler_log.initialize root;
+      Build_attempt.mark_log_initialized attempt root;
+      Build_attempt.finalize_logs attempt;
+      Build_attempt.finalize_logs attempt;
+      let contents =
+        File_util.read_file
+          (Filename.concat (Build_artifacts.lib_path root "bs") ".compiler.log")
+      in
+      let done_lines =
+        contents |> String.split_on_char '\n'
+        |> List.filter (String.starts_with ~prefix:"#Done(")
+      in
+      assert_equal 1 (List.length done_lines))
+
+let tests =
+  "build_attempt_tests"
+  >::: [
+         "retained attempts start with fresh attempt state"
+         >:: retained_attempts_start_with_fresh_attempt_state;
+         "output inventory survives without cleanup work"
+         >:: output_inventory_survives_without_cleanup_work;
+         "pending work is drained once" >:: pending_work_is_drained_once;
+         "log finalization runs once" >:: log_finalization_runs_once;
+       ]
diff --git a/tests/rewatch_ounit_tests/build_state_tests.ml b/tests/rewatch_ounit_tests/build_state_tests.ml
new file mode 100644
index 00000000000..13448581311
--- /dev/null
+++ b/tests/rewatch_ounit_tests/build_state_tests.ml
@@ -0,0 +1,115 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let tests =
+  "build_state_tests" >:: fun _context ->
+  let state = Build_state.create 2 in
+  Build_state.add state ~key:"A" ~kind:Build_state.Source_module
+    ~last_compiled_cmi:(Some 1.) ~last_compiled_cmt:(Some 2.);
+  Build_state.add state ~key:"B" ~kind:Build_state.Source_module
+    ~last_compiled_cmi:None ~last_compiled_cmt:None;
+  Build_state.set_dependencies state ~key:"A" [];
+  Build_state.set_dependencies state ~key:"B" ["A"];
+  let a = Build_state.find_exn state "A" in
+  let b = Build_state.find_exn state "B" in
+  check
+    (a.last_compiled_cmi = Some 1. && a.last_compiled_cmt = Some 2.)
+    "compile asset timestamps initialize module state";
+  check
+    (Build_state.has_complete_compile_assets a)
+    "both compile artifacts form a complete cached compile";
+  check
+    (not (Build_state.has_complete_compile_assets b))
+    "a missing compile artifact requires compilation";
+  check
+    (not (Build_state.dependency_tree_compiled_after state b a))
+    "a module without a prior CMT relies on its own dirty state";
+  check
+    (Build_state.String_set.equal a.dependents
+       (Build_state.String_set.singleton "B")
+    && b.dependencies = ["A"])
+    "setting dependencies creates the reverse edge";
+  Build_state.mark_dependents_compile_dirty state a;
+  check b.compile_dirty "CMI changes preserve pending dependent work";
+  b.last_compiled_cmt <- Some 0.5;
+  check
+    (Build_state.dependency_tree_compiled_after state b a)
+    "dependency CMI timestamps invalidate older dependents";
+  Build_state.set_dependencies state ~key:"B" [];
+  check
+    (Build_state.String_set.is_empty a.dependents && b.dependencies = [])
+    "updating dependencies removes obsolete reverse edges";
+  Build_state.set_dependencies state ~key:"B" ["A"];
+  Build_state.set_dependencies state ~key:"B" ["A"];
+  check
+    (Build_state.String_set.equal a.dependents
+       (Build_state.String_set.singleton "B")
+    && b.dependencies = ["A"])
+    "updating dependencies does not duplicate reverse edges";
+  let namespace_state = Build_state.create 3 in
+  Build_state.add namespace_state ~key:"A" ~kind:Build_state.Source_module
+    ~last_compiled_cmi:None ~last_compiled_cmt:None;
+  Build_state.add namespace_state ~key:"namespace"
+    ~kind:Build_state.Namespace_map ~last_compiled_cmi:None
+    ~last_compiled_cmt:None;
+  Build_state.add namespace_state ~key:"Consumer"
+    ~kind:Build_state.Source_module ~last_compiled_cmi:None
+    ~last_compiled_cmt:None;
+  Build_state.set_dependencies namespace_state ~key:"A" [];
+  Build_state.set_dependencies namespace_state ~key:"namespace" ["A"];
+  Build_state.set_dependencies namespace_state ~key:"Consumer" ["namespace"];
+  let namespace = Build_state.find_exn namespace_state "namespace" in
+  let consumer = Build_state.find_exn namespace_state "Consumer" in
+  Build_state.mark_dependents_compile_dirty namespace_state
+    (Build_state.find_exn namespace_state "A");
+  check
+    ((not namespace.compile_dirty) && consumer.compile_dirty)
+    "namespace-map barriers propagate invalidation without scheduling map work";
+  consumer.compile_dirty <- false;
+  consumer.last_compiled_cmt <- Some 2.;
+  (Build_state.find_exn namespace_state "A").last_compiled_cmi <- Some 3.;
+  check
+    (Build_state.dependency_tree_compiled_after namespace_state consumer
+       namespace)
+    "restart freshness traverses namespace-map members";
+  let entry_source : Source.module_ =
+    {
+      name = "Entry";
+      implementation = "src/Entry.res";
+      interface = None;
+      is_dev = false;
+    }
+  in
+  let entry : Module_graph.module_node =
+    {
+      key = "Entry";
+      package_name = "package";
+      package_root = "root";
+      source_path = entry_source.implementation;
+      namespace =
+        Config.Namespace_with_entry {name = "Namespace"; entry = "Entry"};
+      visible_packages = Hashtbl.create 0;
+      raw_dependencies = [];
+    }
+  in
+  let namespace_map : Module_graph.namespace_map =
+    {
+      key = Module_graph.namespace_map_key "root";
+      compiler_name = "@Namespace";
+      namespace = "Namespace";
+      package_name = "package";
+      package_root = "root";
+      members = [];
+    }
+  in
+  let modules = Hashtbl.create 1 in
+  Hashtbl.add modules entry.key entry;
+  let namespace_maps = Hashtbl.create 1 in
+  Hashtbl.add namespace_maps namespace_map.namespace [namespace_map];
+  check
+    (Module_graph.resolved_dependencies ~find_module:(Hashtbl.find_opt modules)
+       ~find_namespace_maps:(Hashtbl.find_opt namespace_maps)
+       entry
+    = [namespace_map.key])
+    "namespace entry implicitly depends on its namespace map"
diff --git a/tests/rewatch_ounit_tests/clean_tests.ml b/tests/rewatch_ounit_tests/clean_tests.ml
new file mode 100644
index 00000000000..28eed6f84c1
--- /dev/null
+++ b/tests/rewatch_ounit_tests/clean_tests.ml
@@ -0,0 +1,102 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let with_temp_dir = Test_support.with_temp_dir "rewatch-ocaml-clean-"
+
+let tests =
+  "clean_tests" >:: fun _context ->
+  with_temp_dir (fun root ->
+      write_file
+        (Filename.concat root "rescript.json")
+        {|{
+        "name": "clean-ownership",
+        "sources": [{"dir":"src","subdirs":true}],
+        "dependencies": ["installed"],
+        "package-specs": [
+          {"module": "esmodule", "in-source": false},
+          {"module": "esmodule", "in-source": true, "suffix": ".in.js"}
+        ]
+      }|};
+      write_file (Filename.concat root "src/A.res") "let value = 1\n";
+      write_file
+        (Filename.concat root "node_modules/installed/rescript.json")
+        {|{"name":"installed","sources":["src"]}|};
+      write_file
+        (Filename.concat root "node_modules/installed/src/Installed.res")
+        "let value = 2\n";
+      let generated = Filename.concat root "lib/es6/src/A.js" in
+      let unowned = Filename.concat root "lib/es6/keep.txt" in
+      let unowned_javascript = Filename.concat root "lib/es6/src/Manual.js" in
+      let dependency_generated =
+        Filename.concat root "node_modules/installed/lib/es6/src/Installed.js"
+      in
+      let dependency_unowned =
+        Filename.concat root "node_modules/installed/lib/es6/keep.txt"
+      in
+      let duplicate_a = Filename.concat root "src/one/Duplicate.res" in
+      let duplicate_b = Filename.concat root "src/two/Duplicate.res" in
+      let duplicate_output_a =
+        Filename.concat root "lib/es6/src/one/Duplicate.js"
+      in
+      let duplicate_output_b =
+        Filename.concat root "lib/es6/src/two/Duplicate.js"
+      in
+      let duplicate_in_source_a =
+        Filename.concat root "src/one/Duplicate.in.js"
+      in
+      let duplicate_in_source_b =
+        Filename.concat root "src/two/Duplicate.in.js"
+      in
+      write_file generated "generated\n";
+      write_file (generated ^ ".map") "generated map\n";
+      write_file unowned "keep\n";
+      write_file unowned_javascript "manual\n";
+      write_file dependency_generated "generated dependency\n";
+      write_file dependency_unowned "keep dependency\n";
+      write_file duplicate_a "let value = 1\n";
+      write_file duplicate_b "let value = 2\n";
+      write_file duplicate_output_a "generated duplicate one\n";
+      write_file duplicate_output_b "generated duplicate two\n";
+      write_file duplicate_in_source_a "generated duplicate one\n";
+      write_file duplicate_in_source_b "generated duplicate two\n";
+      write_file (Filename.concat root "lib/bs/compiler-state") "temporary\n";
+      write_file (Filename.concat root "lib/ocaml/A.cmj") "temporary\n";
+      Clean.run ~poll:(fun () -> ()) ~verbosity:(-1) ~folder:root ~prod:false;
+      check
+        (not (Sys.file_exists generated))
+        "clean removes the configured generated output";
+      check
+        (not (Sys.file_exists (generated ^ ".map")))
+        "clean removes the configured generated source map";
+      check (Sys.file_exists unowned)
+        "clean preserves unrelated files in an out-of-source directory";
+      check
+        (Sys.file_exists unowned_javascript)
+        "clean preserves JavaScript without a matching source module";
+      check
+        (not (Sys.file_exists dependency_generated))
+        "clean removes configured outputs from installed dependencies";
+      check
+        (Sys.file_exists dependency_unowned)
+        "clean preserves unrelated files beside installed dependency outputs";
+      check
+        (not (Sys.file_exists duplicate_output_a))
+        "clean removes the first output when module names are duplicated";
+      check
+        (not (Sys.file_exists duplicate_output_b))
+        "clean removes the second output when module names are duplicated";
+      check
+        (not (Sys.file_exists duplicate_in_source_a))
+        "invalid-graph clean removes the first in-source output";
+      check
+        (not (Sys.file_exists duplicate_in_source_b))
+        "invalid-graph clean removes the second in-source output";
+      check
+        (not (Sys.file_exists (Filename.concat root "lib/bs")))
+        "clean removes compiler working artifacts";
+      check
+        (not (Sys.file_exists (Filename.concat root "lib/ocaml")))
+        "clean removes published compiler artifacts")
diff --git a/tests/rewatch_ounit_tests/cli_tests.ml b/tests/rewatch_ounit_tests/cli_tests.ml
new file mode 100644
index 00000000000..a12c73ba939
--- /dev/null
+++ b/tests/rewatch_ounit_tests/cli_tests.ml
@@ -0,0 +1,238 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let parse arguments = Cli.parse (Array.of_list ("rescript" :: arguments))
+
+let rejects arguments =
+  try
+    ignore (parse arguments);
+    false
+  with Cli.Parse_error _ -> true
+
+let shows_help arguments =
+  try
+    ignore (parse arguments);
+    false
+  with Cli.Help -> true
+
+let shows_version arguments =
+  try
+    ignore (parse arguments);
+    false
+  with Cli.Version -> true
+
+let build_options arguments =
+  match parse arguments with
+  | Cli.Build options -> options
+  | _ -> failwith "expected build command"
+
+let watch_options arguments =
+  match parse arguments with
+  | Cli.Watch options -> options
+  | _ -> failwith "expected watch command"
+
+let tests =
+  "cli_tests" >:: fun _context ->
+  check
+    (match parse [] with
+    | Cli.Build _ -> true
+    | _ -> false)
+    "no subcommand defaults to build";
+  check
+    ((build_options ["someFolder"]).folder = "someFolder")
+    "a bare folder uses the implicit build command";
+  check
+    ((build_options ["my-project"; "-v"]).folder = "my-project")
+    "a trailing global verbosity flag keeps the implicit folder";
+  check
+    ((build_options ["--"; "-v"]).folder = "-v")
+    "double dash preserves an option-looking folder";
+  check
+    (shows_help ["some-folder"; "--help"])
+    "implicit command folder help uses global help";
+  check
+    (shows_help ["some-folder"; "-h"])
+    "short implicit command help uses global help";
+  check (shows_help ["build"; "--help"]) "explicit build displays command help";
+  check (shows_help ["build"; "-h"]) "explicit build accepts short command help";
+  check
+    (match parse ["-vvvv"; "watch"] with
+    | Cli.Watch _ -> true
+    | _ -> false)
+    "leading verbosity before watch";
+  check
+    ((build_options ["-vvvvv"; "build"]).verbosity = 5)
+    "arbitrarily long clustered verbosity remains global";
+  check
+    (shows_version ["-vV"; "build"])
+    "a clustered leading version flag has global precedence";
+  check
+    (shows_help ["-Vh"; "build"])
+    "help takes precedence over version in a leading cluster";
+  check
+    (shows_help ["-hV"; "build"])
+    "a leading help flag wins when it precedes version in a cluster";
+  check
+    (shows_help ["build"; "-hV"])
+    "subcommand help takes precedence over version";
+  check
+    (shows_help ["build"; "-Vh"])
+    "subcommand help precedence is independent of cluster order";
+  check
+    (shows_help ["--version"; "build"; "--help"])
+    "subcommand help takes precedence over a leading version option";
+  check
+    (shows_help ["--version"; "build"; "--help=plain"])
+    "formatted subcommand help takes precedence over a leading version option";
+  check
+    (shows_help ["--help=plain"])
+    "formatted root help remains a root display option";
+  check
+    (shows_help ["build"; "--help=groff"])
+    "explicit Cmdliner help formats remain accepted";
+  check
+    (match parse ["build"; "-v"] with
+    | Cli.Build _ -> true
+    | _ -> false)
+    "build accepts a trailing verbosity flag";
+  check
+    (rejects ["-v"; "-q"])
+    "implicit build rejects conflicting verbose and quiet modes";
+  check
+    (rejects ["build"; "--verbose"; "--quiet"])
+    "explicit build rejects conflicting verbose and quiet modes";
+  check
+    (shows_version ["-V"; "build"])
+    "a leading short version flag has global precedence";
+  check
+    (shows_version ["some-folder"; "-V"])
+    "implicit build extracts a trailing global version flag";
+  check (shows_version ["--version"]) "the long global version flag is accepted";
+  check
+    (shows_version ["build"; "-V"])
+    "explicit build accepts a trailing short version flag";
+  check
+    (shows_version ["build"; "--version"])
+    "explicit build accepts the long trailing version flag";
+  check
+    (rejects ["watch"; "--no-timing"])
+    "watch rejects build-only --no-timing";
+  check
+    ((build_options ["build"; "-n"; "."]).folder = "."
+    && (build_options ["build"; "-n"; "."]).no_timing)
+    "the short no-timing flag preserves the following folder";
+  check
+    ((build_options ["build"; "--no-timing"; "false"]).folder = "false"
+    && (build_options ["build"; "--no-timing"; "false"]).no_timing)
+    "no-timing does not interpret a folder named false as a boolean";
+  check
+    (rejects ["build"; "--no-timing=false"])
+    "no-timing rejects explicit boolean values";
+  check
+    (not (build_options ["build"]).no_timing)
+    "build defaults no-timing to false";
+  check (build_options ["build"; "--prod"]).prod "build parses --prod";
+  check (not (build_options ["build"]).prod) "build defaults --prod to false";
+  check (watch_options ["watch"; "--prod"]).prod "watch parses --prod";
+  check
+    (match parse ["clean"; "--prod"] with
+    | Cli.Clean {verbosity = 0; prod = true; folder = "."} -> true
+    | _ -> false)
+    "clean parses --prod";
+  check
+    (match parse ["-q"; "clean"] with
+    | Cli.Clean {verbosity = -1; prod = false; folder = "."} -> true
+    | _ -> false)
+    "clean retains the global quiet level";
+  check (build_options ["--prod"]).prod
+    "--prod selects the implicit build command";
+  check
+    ((build_options ["build"; "--features"; " native , web "]).features
+    = Some ["native"; "web"])
+    "feature names are trimmed";
+  check
+    ((build_options ["build"]).features = None)
+    "build defaults features to none";
+  check
+    ((watch_options ["watch"; "--features"; "native"]).features
+   = Some ["native"])
+    "watch parses features";
+  check
+    ((build_options ["build"; "--features"; "native,web"]).features
+   = (watch_options ["watch"; "--features"; "native,web"]).features)
+    "build and watch use the same feature conversion";
+  check (rejects ["build"; "--features"; ""]) "empty features are rejected";
+  check
+    (rejects [String.make 1 (Char.chr 0xff)])
+    "non-UTF-8 arguments are rejected";
+  check (watch_options ["watch"; "--clear-screen"]).clear_screen
+    "watch parses --clear-screen";
+  check
+    (rejects ["build"; "--filter"; "["])
+    "invalid filter regular expressions are rejected during CLI parsing";
+  check
+    (not (rejects ["build"; "--filter"; "Foo|Bar"]))
+    "Rust-style filter alternation is accepted";
+  check
+    (not (rejects ["build"; "--filter"; "(?:Foo|Bar)\\d+"]))
+    "Rust-style filter groups and shorthand classes are accepted";
+  check
+    (rejects ["build"; "--filter"; "[a-z&&[^aeiou]]"])
+    "class set operations are rejected instead of changing meaning";
+  check
+    (rejects ["build"; "--filter"; "\\QFoo.res\\E"])
+    "Perl quoting unsupported by Rust is rejected";
+  check (rejects ["build"; "--filter"; "\\Z"]) "Perl-only anchors are rejected";
+  check
+    (rejects ["build"; "--filter"; "\\e"])
+    "Perl-only character escapes are rejected";
+  check
+    (rejects ["build"; "--filter"; "\\o{123}"])
+    "braced octal escapes unsupported by Rust are rejected";
+  check
+    (rejects ["build"; "--filter"; "[z-a]"])
+    "descending character ranges unsupported by Rust are rejected";
+  check
+    (rejects ["build"; "--filter"; "[a-\\d]"])
+    "character-class range endpoints unsupported by Rust are rejected";
+  check
+    (rejects ["build"; "--filter"; "[\\d-z]"])
+    "escaped character-class range starts unsupported by Rust are rejected";
+  check
+    (rejects ["build"; "--filter"; "^[\\W]+\\.res$"])
+    "shorthand classes with divergent in-class semantics are rejected";
+  check
+    (rejects ["build"; "--filter"; "[[.a.]]"])
+    "collating elements with divergent class semantics are rejected";
+  check
+    (rejects ["format"; "--stdin"; ".txt"])
+    "format stdin validates the source extension";
+  check
+    (rejects ["format"; "input.res"; "--stdin"; ".res"])
+    "format stdin conflicts with files regardless of argument order";
+  check
+    (rejects ["format"; "--stdin"; ".res"; "--check"])
+    "format check conflicts with stdin regardless of argument order";
+  check
+    (match parse ["format"; "--stdin"; ".res"] with
+    | Cli.Format (Cli.Format_stdin ".res") -> true
+    | _ -> false)
+    "format represents standard input independently from file inputs";
+  check
+    (match parse ["format"; "--check"; "A.res"] with
+    | Cli.Format (Cli.Format_files {check = true; paths = ["A.res"]}) -> true
+    | _ -> false)
+    "format represents file inputs with their check mode";
+  check (shows_help ["help"]) "the help command displays global help";
+  check
+    (shows_help ["help"; "build"])
+    "the help command displays subcommand help";
+  check (rejects ["help"; "unknown"]) "the help command rejects unknown topics";
+  check (rejects ["compiler-args"]) "compiler-args requires a source path";
+  check
+    (rejects ["compiler-args"; "A.res"; "B.res"])
+    "compiler-args rejects additional source paths";
+  check
+    (rejects ["build"; "--unknown-option"])
+    "known subcommands reject unknown options"
diff --git a/tests/rewatch_ounit_tests/compile_assets_tests.ml b/tests/rewatch_ounit_tests/compile_assets_tests.ml
new file mode 100644
index 00000000000..cb2c1e3f9b9
--- /dev/null
+++ b/tests/rewatch_ounit_tests/compile_assets_tests.ml
@@ -0,0 +1,85 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write path contents =
+  let channel = open_out_bin path in
+  Fun.protect
+    ~finally:(fun () -> close_out_noerr channel)
+    (fun () -> output_string channel contents)
+
+let with_temp_dir = Test_support.with_temp_dir "rewatch-compile-assets-"
+
+let tests =
+  "compile_assets_tests" >:: fun _context ->
+  with_temp_dir (fun root ->
+      let first = Filename.concat root "Example.cmi" in
+      let second = Filename.concat root "example.cmt" in
+      let unrelated = Filename.concat root "notes.txt" in
+      let cleanup_only = Filename.concat root "Example.cmj" in
+      let source = Test_support.path root "src/Example.res" in
+      let ast = Filename.concat root "Example.ast" in
+      let nested = Filename.concat root "nested" in
+      write first "cmi";
+      write second "cmt";
+      write unrelated "notes";
+      write cleanup_only "cmj";
+      write ast ("Caml1999X\nDependency\n" ^ source ^ "\nbinary payload");
+      Unix.mkdir nested 0o755;
+      write (Filename.concat nested "Nested.cmi") "nested";
+      let state = Compile_assets.create [root; root] in
+      check
+        (Compile_assets.files state root
+        |> List.sort String.compare
+        = List.sort String.compare [ast; cleanup_only; first; second])
+        "the flat cleanup inventory retains only managed compiler assets";
+      check
+        (Compile_assets.ast_sources state root
+        = [{Compile_assets.ast_path = ast; source_path = source}])
+        "published ASTs retain their encoded absolute source location";
+      check
+        (Compile_assets.ast state source
+        |> Option.map (fun entry -> entry.Compile_assets.path)
+        = Some ast)
+        "published AST state is addressable by source path";
+      let source_path = Filename.concat "src" "Example.res" in
+      let ast_modified = (Unix.stat ast).Unix.st_mtime in
+      let source_mtimes = Hashtbl.create 1 in
+      Hashtbl.add source_mtimes source_path ast_modified;
+      check
+        (Build_freshness.source_is_not_older_than_ast state ~root ~source_mtimes
+           source_path)
+        "equal source and AST timestamps follow Rust and require parsing";
+      Hashtbl.replace source_mtimes source_path (ast_modified -. 1.);
+      check
+        (not
+           (Build_freshness.source_is_not_older_than_ast state ~root
+              ~source_mtimes source_path))
+        "an AST newer than its source is parse-clean";
+      check
+        (Option.is_some (Compile_assets.cmi state "Example"))
+        "CMI entries use compiler module keys";
+      check
+        (Option.is_some (Compile_assets.cmt state "Example"))
+        "CMT entries normalize the first module-name character";
+      check
+        (Option.is_none (Compile_assets.cmi state "Nested"))
+        "the compiler asset directory is scanned non-recursively";
+      Sys.remove first;
+      Compile_assets.refresh_cmi state ~key:"Example" ~path:first;
+      check
+        (Option.is_none (Compile_assets.cmi state "Example"))
+        "refresh removes a deleted CMI";
+      let replacement_ast = Filename.concat root "Replacement.ast" in
+      write replacement_ast "replacement";
+      Compile_assets.refresh_ast state ~source ~path:replacement_ast;
+      check
+        (Compile_assets.ast state source
+        |> Option.map (fun entry -> entry.Compile_assets.path)
+        = Some replacement_ast)
+        "refresh replaces the AST freshness entry after publication";
+      Sys.remove replacement_ast;
+      Compile_assets.refresh_ast state ~source ~path:replacement_ast;
+      check
+        (Option.is_none (Compile_assets.ast state source))
+        "refresh removes a deleted AST freshness entry")
diff --git a/tests/rewatch_ounit_tests/compiler_args_tests.ml b/tests/rewatch_ounit_tests/compiler_args_tests.ml
new file mode 100644
index 00000000000..c0c6bd521ed
--- /dev/null
+++ b/tests/rewatch_ounit_tests/compiler_args_tests.ml
@@ -0,0 +1,185 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let arguments field path =
+  match Yojson.Safe.from_string (Compiler_args_command.run path) with
+  | `Assoc fields -> (
+    match List.assoc_opt field fields with
+    | Some (`List values) ->
+      List.map
+        (function
+          | `String value -> value
+          | _ -> failwith "non-string argument")
+        values
+    | _ -> failwith ("missing " ^ field ^ " array"))
+  | _ -> failwith "compiler-args did not return an object"
+
+let compiler_args = arguments "compiler_args"
+let parser_args = arguments "parser_args"
+
+let adjacent_positions flag values =
+  let rec loop index positions = function
+    | current :: value :: rest when current = flag ->
+      loop (index + 2) ((value, index) :: positions) rest
+    | _ :: rest -> loop (index + 1) positions rest
+    | [] -> List.rev positions
+  in
+  loop 0 [] values
+
+let position value pairs = List.assoc_opt value pairs
+
+let tests =
+  "compiler_args_tests" >:: fun _context ->
+  let root = Filename.temp_file "rewatch-ocaml-compiler-args-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      let source = Filename.concat root "src/A.res" in
+      let dev_source = Filename.concat root "dev/D.res" in
+      let nested_dev_source = Filename.concat root "dev/nested/E.res" in
+      let prefixed_source = Filename.concat root "developer/F.res" in
+      write_file source "let value = 1\n";
+      write_file dev_source "let value = 2\n";
+      write_file nested_dev_source "let value = 3\n";
+      write_file prefixed_source "let value = 4\n";
+      List.iter
+        (fun package ->
+          File_util.ensure_dir
+            (File_util.path_of_parts root ["node_modules"; package]))
+        ["@rescript/runtime"; "regular"; "development"];
+      write_file
+        (Filename.concat root "rescript.json")
+        {|{
+          "name": "compiler-args-test",
+          "sources": ["src", {"dir": "dev", "type": "dev", "subdirs": true}],
+          "dependencies": ["regular"],
+          "dev-dependencies": ["development"]
+        }|};
+      let regular =
+        File_util.path_of_parts root ["node_modules"; "regular"; "lib"; "ocaml"]
+      in
+      let development =
+        File_util.path_of_parts root
+          ["node_modules"; "development"; "lib"; "ocaml"]
+      in
+      let ordinary_includes = compiler_args source |> adjacent_positions "-I" in
+      check
+        (Option.is_some (position regular ordinary_includes))
+        "regular dependency includes do not require a prebuilt lib/ocaml";
+      check
+        (Option.is_none (position development ordinary_includes))
+        "ordinary sources exclude development dependencies";
+      let dev_includes = compiler_args dev_source |> adjacent_positions "-I" in
+      check
+        (match
+           (position development dev_includes, position regular dev_includes)
+         with
+        | Some development_index, Some regular_index ->
+          development_index < regular_index
+        | _ -> false)
+        "development sources include dev dependencies before regular \
+         dependencies";
+      check
+        (compiler_args nested_dev_source
+        |> adjacent_positions "-I" |> position development |> Option.is_some)
+        "recursive development sources include dev dependencies";
+      check
+        (compiler_args prefixed_source
+        |> adjacent_positions "-I" |> position development |> Option.is_none)
+        "source directory matching respects path-component boundaries";
+      let config = Config.load_root root in
+      let gentype_config = {config with gentype_args = ["-bs-gentype"]} in
+      let selected_regular = Filename.concat root "selected/regular" in
+      check
+        (Compiler_args.gentype_dependency_args_from_paths gentype_config
+           [({Config.name = "regular"; features = None}, selected_regular)]
+        = ["-bs-gentype-dep-path"; "regular=" ^ selected_regular])
+        "GenType arguments reuse the dependency path selected for the graph";
+      (if not Sys.win32 then
+         let external_source =
+           Filename.temp_file "rewatch-linked-source-" ".res"
+         in
+         Fun.protect
+           ~finally:(fun () -> Sys.remove external_source)
+           (fun () ->
+             write_file external_source "let linked = 1\n";
+             let linked_source = Filename.concat root "src/Linked.res" in
+             Unix.symlink external_source linked_source;
+             check
+               (parser_args linked_source |> List.rev |> List.hd
+               = Filename.concat
+                   (Filename.concat Filename.parent_dir_name
+                      Filename.parent_dir_name)
+                   "src/Linked.res")
+               "compiler-args keeps a source symlink in its project scope"));
+      File_util.remove_tree
+        (File_util.path_of_parts root ["node_modules"; "development"]);
+      check
+        (compiler_args dev_source |> adjacent_positions "-I"
+       |> position development |> Option.is_none)
+        "missing development dependencies are omitted like Rust";
+      File_util.remove_tree
+        (File_util.path_of_parts root ["node_modules"; "regular"]);
+      let previous_runtime = Sys.getenv_opt "RESCRIPT_RUNTIME" in
+      Unix.putenv "RESCRIPT_RUNTIME" (Filename.concat root "missing-runtime");
+      let missing_dependency_error =
+        Fun.protect
+          ~finally:(fun () ->
+            match previous_runtime with
+            | Some value -> Unix.putenv "RESCRIPT_RUNTIME" value
+            | None -> Test_support.unsetenv "RESCRIPT_RUNTIME")
+          (fun () ->
+            try
+              ignore (compiler_args source);
+              None
+            with Project_context.Error message -> Some message)
+      in
+      check
+        (match missing_dependency_error with
+        | Some message ->
+          Test_support.contains_text message
+            "Expected to find dependent package regular of compiler-args-test"
+        | None -> false)
+        "dependency resolution precedes runtime resolution";
+      let missing_source = Filename.concat root "src/Missing.res" in
+      check
+        (try
+           ignore (Compiler_args_command.run missing_source);
+           false
+         with Project_context.Error message ->
+           Test_support.contains_text message "Could not read source file"
+           && Test_support.contains_text message missing_source)
+        "missing compiler-args sources produce a contextual error");
+  Test_support.with_temp_dir "rewatch-compiler-args-workspace-" (fun outer ->
+      let workspace = Filename.concat outer "workspace" in
+      let package = Filename.concat workspace "packages/app" in
+      let source = Filename.concat package "src/App.res" in
+      let outside_dependency = Filename.concat outer "node_modules/outside" in
+      write_file source "let value = 1\n";
+      write_file
+        (Filename.concat workspace "rescript.json")
+        {|{"name":"workspace","sources":[],"dependencies":["app"]}|};
+      write_file
+        (Filename.concat package "rescript.json")
+        {|{"name":"app","sources":"src","dependencies":["outside"]}|};
+      File_util.ensure_dir (Filename.concat workspace "node_modules");
+      if
+        Test_support.symlink_if_supported
+          (Test_support.path workspace "../workspace/packages/app")
+          (Test_support.path workspace "node_modules/app")
+      then (
+        File_util.ensure_dir outside_dependency;
+        check
+          (try
+             ignore (Compiler_args_command.run source);
+             false
+           with Project_context.Error message ->
+             Test_support.contains_text message
+               "Expected to find dependent package outside of app")
+          "compiler-args does not search above a workspace dependency boundary"))
diff --git a/tests/rewatch_ounit_tests/compiler_info_tests.ml b/tests/rewatch_ounit_tests/compiler_info_tests.ml
new file mode 100644
index 00000000000..c576d94af05
--- /dev/null
+++ b/tests/rewatch_ounit_tests/compiler_info_tests.ml
@@ -0,0 +1,189 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let with_temp_dir = Test_support.with_temp_dir "rewatch-compiler-info-"
+
+let write = Test_support.write_file
+
+let config root =
+  write
+    (Filename.concat root "rescript.json")
+    {|{"name":"compiler-info-test","sources":["src"]}|};
+  Unix.mkdir (Filename.concat root "src") 0o755;
+  Config.load_root root
+
+let context ?(inherited_compiler_args = []) root config source_map_args =
+  let bsc = Filename.concat root "bsc.exe" in
+  let runtime = Filename.concat root "runtime" in
+  if not (Sys.file_exists bsc) then write bsc "compiler-v1";
+  File_util.ensure_dir runtime;
+  Compiler_info.make_context ~build_root:root ~bsc_path:bsc
+    ~runtime_path:runtime ~source_map_args ~inherited_compiler_args
+    ~package_output_specs:(Compiler_info.package_output_specs config)
+
+let tests =
+  "compiler_info_tests" >:: fun _context ->
+  with_temp_dir (fun root ->
+      let config = config root in
+      let initial = context root config ["-bs-source-map"; "linked"] in
+      check
+        (not (Compiler_info.needs_clean initial config))
+        "a package without an earlier build is not spuriously cleaned";
+      Compiler_info.write_package initial config;
+      let marker = File_util.path_of_parts root ["lib"; "ocaml"; "marker"] in
+      write marker "keep";
+      check
+        (not (Compiler_info.needs_clean initial config))
+        "matching compiler information is retained";
+      check (Sys.file_exists marker) "matching artifacts remain";
+      let info_path =
+        File_util.path_of_parts root ["lib"; "bs"; "compiler-info.json"]
+      in
+      Unix.utimes info_path 1_000_000_000. 1_000_000_000.;
+      Compiler_info.write_package initial config;
+      check
+        ((Unix.stat info_path).Unix.st_mtime = 1_000_000_000.)
+        "matching compiler information is not rewritten";
+      let changed = context root config ["-bs-source-map"; "false"] in
+      check
+        (Compiler_info.needs_clean changed config)
+        "changed source-map arguments invalidate artifacts";
+      Compiler_info.clean_package config;
+      check (not (Sys.file_exists marker)) "mismatched artifacts are removed");
+  with_temp_dir (fun root ->
+      let config = config root in
+      let initial = context root config [] in
+      Compiler_info.write_package initial config;
+      write (Filename.concat root "bsc.exe") "compiler-v2";
+      let changed = context root config [] in
+      check
+        (Compiler_info.needs_clean changed config)
+        "changed compiler contents invalidate artifacts");
+  with_temp_dir (fun root ->
+      let config = config root in
+      let initial =
+        context ~inherited_compiler_args:["-bs-jsx"; "4"] root config []
+      in
+      Compiler_info.write_package initial config;
+      let changed =
+        context ~inherited_compiler_args:["-bs-jsx"; "5"] root config []
+      in
+      check
+        (Compiler_info.needs_clean changed config)
+        "changed inherited compiler arguments invalidate dependency artifacts");
+  with_temp_dir (fun root ->
+      let decoded = config root in
+      let context = context root decoded [] in
+      Compiler_info.write_package context decoded;
+      write
+        (Filename.concat root "rescript.json")
+        {|{"name":"compiler-info-test","sources":["src"],"suffix":".mjs"}|};
+      check
+        (not (Compiler_info.needs_clean context decoded))
+        "compiler metadata uses the bytes associated with the decoded config";
+      let changed = Config.load_root root in
+      check
+        (Compiler_info.needs_clean context changed)
+        "reloading changed configuration produces a new fingerprint");
+  with_temp_dir (fun root ->
+      let config = config root in
+      let context = context root config [] in
+      let old_log =
+        File_util.path_of_parts root ["lib"; "ocaml"; ".compiler.log"]
+      in
+      write old_log "old build";
+      check
+        (Compiler_info.needs_clean context config)
+        "missing metadata invalidates an existing legacy build";
+      Compiler_info.clean_package config;
+      check (not (Sys.file_exists old_log)) "legacy build artifacts are removed");
+  with_temp_dir (fun root ->
+      let dependency = config root in
+      let bsc = Filename.concat root "bsc.exe" in
+      let runtime = Filename.concat root "runtime" in
+      write bsc "compiler-v1";
+      File_util.ensure_dir runtime;
+      let commonjs =
+        [
+          {
+            Compiler_info.module_format = Config.Commonjs;
+            in_source = true;
+            suffix = ".js";
+          };
+        ]
+      in
+      let esmodule =
+        [
+          {
+            Compiler_info.module_format = Config.Esmodule;
+            in_source = true;
+            suffix = ".js";
+          };
+        ]
+      in
+      let initial =
+        Compiler_info.make_context ~build_root:root ~bsc_path:bsc
+          ~runtime_path:runtime ~source_map_args:[] ~inherited_compiler_args:[]
+          ~package_output_specs:commonjs
+      in
+      Compiler_info.write_package initial dependency;
+      let marker = File_util.path_of_parts root ["lib"; "ocaml"; "marker"] in
+      write marker "keep";
+      let changed =
+        Compiler_info.make_context ~build_root:root ~bsc_path:bsc
+          ~runtime_path:runtime ~source_map_args:[] ~inherited_compiler_args:[]
+          ~package_output_specs:esmodule
+      in
+      check
+        (Compiler_info.needs_clean changed dependency)
+        "same-path module-format changes invalidate dependency artifacts";
+      Compiler_info.clean_package dependency;
+      check
+        (not (Sys.file_exists marker))
+        "package-output mismatches remove compiler artifacts");
+  with_temp_dir (fun root ->
+      let dependency_root = Filename.concat root "dependency" in
+      File_util.ensure_dir dependency_root;
+      let dependency = config dependency_root in
+      let bsc = Filename.concat root "bsc.exe" in
+      let runtime = Filename.concat root "runtime" in
+      write bsc "compiler-v1";
+      File_util.ensure_dir runtime;
+      let standalone =
+        Compiler_info.make_context ~build_root:dependency_root ~bsc_path:bsc
+          ~runtime_path:runtime ~source_map_args:[] ~inherited_compiler_args:[]
+          ~package_output_specs:(Compiler_info.package_output_specs dependency)
+      in
+      Compiler_info.write_package standalone dependency;
+      let consumer_root = Filename.concat root "consumer" in
+      File_util.ensure_dir consumer_root;
+      let consumer_specs =
+        [
+          {
+            Compiler_info.module_format = Config.Commonjs;
+            in_source = false;
+            suffix = ".cjs";
+          };
+        ]
+      in
+      let consumer =
+        Compiler_info.make_context ~build_root:consumer_root ~bsc_path:bsc
+          ~runtime_path:runtime ~source_map_args:[] ~inherited_compiler_args:[]
+          ~package_output_specs:consumer_specs
+      in
+      check
+        (Compiler_info.owns_outputs dependency)
+        "a standalone dependency retains ownership of its outputs";
+      check
+        (Compiler_info.needs_clean consumer dependency)
+        "consumer output specs differ when deliberately applied to the \
+         dependency";
+      check
+        (Compiler_info.changed_package_output_specs consumer dependency
+        = Some (Compiler_info.package_output_specs dependency))
+        "the previous standalone layout remains available for ownership \
+         transfer";
+      check
+        (not (Compiler_info.needs_clean standalone dependency))
+        "standalone dependency metadata remains intact")
diff --git a/tests/rewatch_ounit_tests/compiler_process_tests.ml b/tests/rewatch_ounit_tests/compiler_process_tests.ml
new file mode 100644
index 00000000000..5715a7f8dd6
--- /dev/null
+++ b/tests/rewatch_ounit_tests/compiler_process_tests.ml
@@ -0,0 +1,68 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let tests =
+  "compiler_process_tests" >:: fun _context ->
+  Test_support.with_temp_dir "rewatch-compiler-process-" (fun root ->
+      let build_dir = Filename.concat root "build" in
+      let ocaml_dir = Filename.concat root "ocaml" in
+      File_util.ensure_dir build_dir;
+      File_util.ensure_dir ocaml_dir;
+      List.iter
+        (fun extension ->
+          Test_support.write_file
+            (Filename.concat build_dir ("Ns." ^ extension))
+            ("new " ^ extension))
+        ["cmi"; "cmj"; "cmt"];
+      File_util.ensure_dir (Filename.concat ocaml_dir "Ns.cmj");
+      let namespace_task =
+        Compiler_process.namespace_task ~bsc:"unused" ~runtime:"unused"
+          ~build_dir ~ocaml_dir ~entry:None ~package_dirty:true ~force:false
+          "Ns" []
+        |> Option.get
+      in
+      let result =
+        Process.{status = Unix.WEXITED 0; stdout = ""; stderr = ""}
+      in
+      match
+        Compiler_scheduler.capture_publication (fun () ->
+            namespace_task.Compiler_scheduler.publish result)
+      with
+      | Compiler_scheduler.Failed_after_cmi_publication
+          {cmi_change = Compiler_scheduler.Cmi_changed; _} ->
+        assert_equal "new cmi"
+          (File_util.read_file (Filename.concat ocaml_dir "Ns.cmi"))
+          ~msg:
+            "namespace publication must retain the changed CMI before a later \
+             artifact copy fails"
+      | _ ->
+        assert_failure
+          "namespace publication must report a changed CMI across a later \
+           artifact-copy failure");
+  Test_support.with_temp_dir "rewatch-namespace-retry-" (fun root ->
+      let build_dir = Filename.concat root "build" in
+      let ocaml_dir = Filename.concat root "ocaml" in
+      File_util.ensure_dir build_dir;
+      File_util.ensure_dir ocaml_dir;
+      List.iter
+        (fun extension ->
+          Test_support.write_file
+            (Filename.concat ocaml_dir ("Ns." ^ extension))
+            (if extension = "mlmap" then "old map\n" else "old artifact"))
+        ["cmi"; "cmj"; "cmt"; "mlmap"];
+      check
+        (Option.is_some
+           (Compiler_process.namespace_task ~bsc:"unused" ~runtime:"unused"
+              ~build_dir ~ocaml_dir ~entry:None ~package_dirty:false
+              ~force:false "Ns" []))
+        "a published namespace map that differs from membership is rebuilt";
+      Test_support.write_file
+        (Filename.concat ocaml_dir "Ns.mlmap")
+        "randjbuildsystem\n";
+      check
+        (Option.is_some
+           (Compiler_process.namespace_task ~bsc:"unused" ~runtime:"unused"
+              ~build_dir ~ocaml_dir ~entry:None ~package_dirty:false ~force:true
+              "Ns" []))
+        "retained namespace publication dirtiness forces a retry")
diff --git a/tests/rewatch_ounit_tests/compiler_scheduler_tests.ml b/tests/rewatch_ounit_tests/compiler_scheduler_tests.ml
new file mode 100644
index 00000000000..4c85e18c179
--- /dev/null
+++ b/tests/rewatch_ounit_tests/compiler_scheduler_tests.ml
@@ -0,0 +1,301 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let process_job () =
+  let executable = Unix.realpath Sys.executable_name in
+  Process.
+    {program = executable; args = ["--process-result"; ""; ""; "0"]; cwd = "."}
+
+let wait_for_release_job root =
+  let executable = Unix.realpath Sys.executable_name in
+  Process.{program = executable; args = ["--wait-for-release"; root]; cwd = "."}
+
+let with_temp_dir = Test_support.with_temp_dir "rewatch-scheduler-"
+
+let source name =
+  Source.
+    {
+      name;
+      implementation = Filename.concat "src" (name ^ ".res");
+      interface = None;
+      is_dev = false;
+    }
+
+let tests =
+  "compiler_scheduler_tests" >:: fun _context ->
+  with_temp_dir (fun root ->
+      let ocaml_dir = Build_artifacts.lib_path root "ocaml" in
+      File_util.ensure_dir ocaml_dir;
+      File_util.ensure_dir (Build_artifacts.lib_path root "bs");
+      Compiler_log.initialize root;
+      let a_source = source "A" in
+      let b_source = source "B" in
+      let c_source = source "C" in
+      let build_state = Build_state.create 3 in
+      List.iter
+        (fun key ->
+          Build_state.add build_state ~key ~kind:Build_state.Source_module
+            ~last_compiled_cmi:(Some 0.) ~last_compiled_cmt:(Some 0.))
+        ["A"; "B"; "C"];
+      Build_state.set_dependencies build_state ~key:"B" ["A"];
+      let a_state = Build_state.find_exn build_state "A" in
+      let b_state = Build_state.find_exn build_state "B" in
+      let c_state = Build_state.find_exn build_state "C" in
+      a_state.compile_dirty <- true;
+      let a_cmi = Filename.concat ocaml_dir "A.cmi" in
+      let b_cmi = Filename.concat ocaml_dir "B.cmi" in
+      let c_cmi = Filename.concat ocaml_dir "C.cmi" in
+      let a_ast = Filename.concat ocaml_dir "A.ast" in
+      write_file a_cmi "old interface";
+      write_file b_cmi "dependent interface";
+      write_file a_ast "published AST";
+      let compile_assets = Compile_assets.create [ocaml_dir] in
+      let fail_a_publication = ref true in
+      let b_compilations = ref 0 in
+      let c_candidates_built = ref 0 in
+      let scheduler_thread = Thread.id (Thread.self ()) in
+      let output_inventory = Hashtbl.create 1 in
+      let make_scheduled key source state cmi_path =
+        Compiler_scheduler.create ~key
+          ~dependencies:state.Build_state.dependencies ~source ~state ~cmi_path
+          ~prepare:(fun () -> ())
+          ~compile:(fun ~source_kind:_ _path ->
+            if key = "B" then incr b_compilations;
+            process_job ())
+          ~publish:(fun ~source_kind:_ _path _result ->
+            write_file cmi_path
+              (if key = "A" then "new interface" else "dependent interface");
+            if key = "A" && !fail_a_publication then (
+              fail_a_publication := false;
+              raise (Failure "later publication failed"));
+            Compiler_scheduler.
+              {
+                stderr = "";
+                cmi_change = (if key = "A" then Cmi_changed else Cmi_unchanged);
+              })
+          ~record_published_outputs:(fun ~source_kind:_ _path ->
+            check
+              (Thread.id (Thread.self ()) = scheduler_thread)
+              "the scheduler thread owns the shared output inventory";
+            for index = 0 to 127 do
+              Hashtbl.replace output_inventory
+                (Printf.sprintf "%s-%d" key index)
+                ()
+            done)
+          ~post_build:(fun _ -> [])
+          ~package_root:root ~is_local:true
+          ~mark_warning:(fun _ -> ())
+      in
+      let run () =
+        let candidates =
+          [
+            ("A", a_source, a_state, a_cmi, fun () -> ());
+            ("B", b_source, b_state, b_cmi, fun () -> ());
+            ("C", c_source, c_state, c_cmi, fun () -> incr c_candidates_built);
+          ]
+          |> List.map (fun (key, source, state, cmi_path, on_make) ->
+              Compiler_scheduler.candidate ~key ~state ~warning_paths:[]
+                ~make:(fun () ->
+                  on_make ();
+                  make_scheduled key source state cmi_path))
+        in
+        Compiler_scheduler.run ~poll:None
+          ~warning_state:(Warning_state.create ()) ~compile_assets ~build_state
+          ~candidates
+          ~mark_compiled:(fun () -> ())
+          ~mark_had_warnings:(fun () -> ())
+          ~progress:(Output.Progress.create ~enabled:false ~color:false)
+          ~compile_step:"1/1" ~namespace_count:0 ~verbosity:0
+      in
+      let first_failed =
+        try
+          run ();
+          false
+        with Compiler_scheduler.Build_failure message ->
+          Test_support.contains_text message "later publication failed"
+      in
+      check first_failed "a later publication failure is reported normally";
+      check
+        (not (Sys.file_exists a_ast))
+        "failed completion invalidates durable freshness for a later process";
+      check b_state.compile_dirty
+        "a published CMI change survives a later publication failure";
+      check (!b_compilations = 0)
+        "a dependent is not compiled during its dependency's failed attempt";
+      run ();
+      check
+        (!b_compilations = 1 && not b_state.compile_dirty)
+        "the retained dependent recompiles after publication recovers";
+      check
+        (Hashtbl.length output_inventory = 256)
+        "scheduler-owned publication inventory updates survive table growth";
+      check (!c_candidates_built = 0)
+        "an unaffected module does not construct compiler callbacks";
+      let abort_state = Build_state.create 4 in
+      List.iter
+        (fun key ->
+          Build_state.add abort_state ~key ~kind:Build_state.Source_module
+            ~last_compiled_cmi:(Some 0.) ~last_compiled_cmt:(Some 0.))
+        ["AbortA"; "AbortB"; "AbortC"; "AbortD"];
+      Build_state.set_dependencies abort_state ~key:"AbortB" ["AbortC"];
+      Build_state.set_dependencies abort_state ~key:"AbortD" ["AbortA"];
+      let abort_a = Build_state.find_exn abort_state "AbortA" in
+      let abort_c = Build_state.find_exn abort_state "AbortC" in
+      abort_a.compile_dirty <- true;
+      abort_c.compile_dirty <- true;
+      let abort_a_ast = Filename.concat ocaml_dir "AbortA.ast" in
+      write_file abort_a_ast "published AST";
+      let published_marker = Filename.concat root "abort-a-published" in
+      let release_marker = Filename.concat root "abort-a-release" in
+      let rec wait_for path remaining =
+        if Sys.file_exists path then ()
+        else if remaining = 0 then failwith ("timed out waiting for " ^ path)
+        else (
+          Thread.delay 0.001;
+          wait_for path (remaining - 1))
+      in
+      let abort_candidate key dependencies prepare publish =
+        let state = Build_state.find_exn abort_state key in
+        Compiler_scheduler.candidate ~key ~state ~warning_paths:[]
+          ~make:(fun () ->
+            Compiler_scheduler.create ~key ~dependencies ~source:(source key)
+              ~state
+              ~cmi_path:(Filename.concat ocaml_dir (key ^ ".cmi"))
+              ~prepare
+              ~compile:(fun ~source_kind:_ _path -> process_job ())
+              ~publish
+              ~record_published_outputs:(fun ~source_kind:_ _path -> ())
+              ~post_build:(fun output ->
+                if key = "AbortA" then
+                  [
+                    Compiler_scheduler.
+                      {output; task = Process.task (process_job ())};
+                  ]
+                else [])
+              ~package_root:root ~is_local:true
+              ~mark_warning:(fun _ -> ()))
+      in
+      let abort_candidates =
+        [
+          abort_candidate "AbortA" []
+            (fun () -> ())
+            (fun ~source_kind:_ _path _result ->
+              write_file (Filename.concat ocaml_dir "AbortA.cmi") "new A";
+              write_file published_marker "";
+              wait_for release_marker 5000;
+              Compiler_scheduler.{stderr = ""; cmi_change = Cmi_changed});
+          abort_candidate "AbortC" []
+            (fun () -> ())
+            (fun ~source_kind:_ _path _result ->
+              wait_for published_marker 5000;
+              write_file (Filename.concat ocaml_dir "AbortC.cmi") "new C";
+              Compiler_scheduler.{stderr = ""; cmi_change = Cmi_changed});
+          abort_candidate "AbortB" ["AbortC"]
+            (fun () ->
+              write_file release_marker "";
+              raise Exit)
+            (fun ~source_kind:_ _path _result -> assert false);
+          abort_candidate "AbortD" ["AbortA"]
+            (fun () -> ())
+            (fun ~source_kind:_ _path _result -> assert false);
+        ]
+      in
+      let aborted =
+        try
+          Compiler_scheduler.run ~poll:None
+            ~warning_state:(Warning_state.create ())
+            ~compile_assets:(Compile_assets.create [ocaml_dir])
+            ~build_state:abort_state ~candidates:abort_candidates
+            ~mark_compiled:(fun () -> ())
+            ~mark_had_warnings:(fun () -> ())
+            ~progress:(Output.Progress.create ~enabled:false ~color:false)
+            ~compile_step:"1/1" ~namespace_count:0 ~verbosity:0;
+          false
+        with Exit -> true
+      in
+      check aborted "an operational scheduler failure aborts the attempt";
+      check (Build_state.find_exn abort_state "AbortD").compile_dirty
+        "CMI publication completed during abort still invalidates dependents";
+      check
+        (not (Sys.file_exists abort_a_ast))
+        "abort before post-build scheduling invalidates persistent freshness";
+      let interrupted_state = Build_state.create 1 in
+      Build_state.add interrupted_state ~key:"Interrupted"
+        ~kind:Build_state.Source_module ~last_compiled_cmi:(Some 0.)
+        ~last_compiled_cmt:(Some 0.);
+      let interrupted_module =
+        Build_state.find_exn interrupted_state "Interrupted"
+      in
+      interrupted_module.compile_dirty <- true;
+      let interrupted_ast = Filename.concat ocaml_dir "Interrupted.ast" in
+      let interrupted_cmi = Filename.concat ocaml_dir "Interrupted.cmi" in
+      let interrupted_hook_root = Filename.concat root "interrupted-hook" in
+      Unix.mkdir interrupted_hook_root 0o755;
+      write_file interrupted_ast "published AST";
+      let interrupted_candidate =
+        Compiler_scheduler.candidate ~key:"Interrupted"
+          ~state:interrupted_module ~warning_paths:[] ~make:(fun () ->
+            Compiler_scheduler.create ~key:"Interrupted" ~dependencies:[]
+              ~source:(source "Interrupted") ~state:interrupted_module
+              ~cmi_path:interrupted_cmi
+              ~prepare:(fun () -> ())
+              ~compile:(fun ~source_kind:_ _path -> process_job ())
+              ~publish:(fun ~source_kind:_ _path _result ->
+                write_file interrupted_cmi "published CMI";
+                Compiler_scheduler.{stderr = ""; cmi_change = Cmi_changed})
+              ~record_published_outputs:(fun ~source_kind:_ _path -> ())
+              ~post_build:(fun output ->
+                [
+                  Compiler_scheduler.
+                    {
+                      output;
+                      task =
+                        Process.task
+                          (wait_for_release_job interrupted_hook_root);
+                    };
+                ])
+              ~package_root:root ~is_local:true
+              ~mark_warning:(fun _ -> ()))
+      in
+      let hook_interrupted =
+        try
+          Compiler_scheduler.run
+            ~poll:
+              (Some
+                 (fun () ->
+                   if
+                     Sys.file_exists
+                       (Filename.concat interrupted_hook_root "child-started")
+                   then raise Exit))
+            ~warning_state:(Warning_state.create ())
+            ~compile_assets:(Compile_assets.create [ocaml_dir])
+            ~build_state:interrupted_state ~candidates:[interrupted_candidate]
+            ~mark_compiled:(fun () -> ())
+            ~mark_had_warnings:(fun () -> ())
+            ~progress:(Output.Progress.create ~enabled:false ~color:false)
+            ~compile_step:"1/1" ~namespace_count:0 ~verbosity:0;
+          false
+        with Exit -> true
+      in
+      check hook_interrupted "a running post-build hook can be interrupted";
+      check
+        (not (Sys.file_exists interrupted_ast))
+        "interrupted post-build invalidates persistent compiler freshness";
+      let marker = Failure "copy after CMI failed" in
+      match
+        Compiler_scheduler.capture_publication (fun () ->
+            raise
+              (Compiler_scheduler.Publication_failure
+                 (marker, Compiler_scheduler.Cmi_changed)))
+      with
+      | Compiler_scheduler.Failed_after_cmi_publication
+          {error; cmi_change = Compiler_scheduler.Cmi_changed}
+        when error == marker ->
+        ()
+      | _ ->
+        assert_failure
+          "publication capture must retain a CMI change across later copy \
+           failure")
diff --git a/tests/rewatch_ounit_tests/config_tests.ml b/tests/rewatch_ounit_tests/config_tests.ml
new file mode 100644
index 00000000000..0fc2e921665
--- /dev/null
+++ b/tests/rewatch_ounit_tests/config_tests.ml
@@ -0,0 +1,566 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let contains = Test_support.contains_text
+
+let count_occurrences text fragment =
+  let text_length = String.length text in
+  let fragment_length = String.length fragment in
+  let rec loop index count =
+    if index + fragment_length > text_length then count
+    else if String.sub text index fragment_length = fragment then
+      loop (index + fragment_length) (count + 1)
+    else loop (index + 1) count
+  in
+  if fragment_length = 0 then 0 else loop 0 0
+
+let rejects path contents fragment =
+  write_file path contents;
+  try
+    ignore (Config.load path);
+    false
+  with Config.Error message -> contains message fragment
+
+let has_diagnostic config field =
+  List.exists
+    (fun message -> contains message ("'" ^ field ^ "'"))
+    config.Config.diagnostics
+
+let rec contains_adjacent left right = function
+  | current :: next :: _ when current = left && next = right -> true
+  | _ :: rest -> contains_adjacent left right rest
+  | [] -> false
+
+let with_config_file test _context =
+  let root = Filename.temp_file "rewatch-ocaml-config-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      let path = Filename.concat root "rescript.json" in
+      test ~root ~path)
+
+let loading_tests =
+  with_config_file (fun ~root ~path ->
+      let missing_path = Filename.concat root "missing.json" in
+      check
+        (try
+           ignore (Config.load missing_path);
+           false
+         with Config.Error message ->
+           contains message "Could not read"
+           && count_occurrences message missing_path = 1)
+        "missing configuration files produce contextual config errors";
+      let directory_path = Filename.concat root "config-directory" in
+      Unix.mkdir directory_path 0o755;
+      check
+        (try
+           ignore (Config.load directory_path);
+           false
+         with Config.Error message ->
+           contains message "Could not read"
+           && count_occurrences message directory_path = 1)
+        "configuration directories produce contextual config errors";
+      write_file path {|{"name":"missing-sources"}|};
+      let config = Config.load path in
+      check
+        (not config.sources_defined)
+        "an omitted sources field remains distinguishable for package warnings";
+      let project = Filename.concat root "project" in
+      let shared = Filename.concat root "shared" in
+      Unix.mkdir project 0o755;
+      Unix.mkdir shared 0o755;
+      let shared_config = Filename.concat shared "rescript.json" in
+      write_file shared_config {|{"name":"symlinked-config"}|};
+      let symlinked_config = Filename.concat project "rescript.json" in
+      (if Test_support.symlink_if_supported shared_config symlinked_config then
+         let config = Config.load symlinked_config in
+         check (config.root = project)
+           "a configuration symlink does not relocate the project root");
+      write_file path {|{"name":"empty-sources","sources":[]}|};
+      let config = Config.load path in
+      check config.sources_defined
+        "an explicit empty sources field does not trigger the missing-field \
+         warning";
+      File_util.ensure_dir (Filename.concat root "src/dev");
+      write_file (Filename.concat root "src/dev/Foo.res") "let value = 1\n";
+      write_file path
+        {|{"name":"overlap","sources":[{"dir":"src","subdirs":true},{"dir":"src/dev","type":"dev"}]}|};
+      let config = Config.load path in
+      check
+        (not (Config.source_is_dev config "src/dev/Foo.res"))
+        "the first applicable source declaration determines development scope";
+      write_file path
+        {|{"name":"overlap","sources":[{"dir":"src/dev","type":"dev"},{"dir":"src","subdirs":true}]}|};
+      let config = Config.load path in
+      check
+        (Config.source_is_dev config "src/dev/Foo.res")
+        "development scope follows source declaration order";
+      write_file path
+        {|{"name":"@testrepo/deprecated-config","namespace":true}|};
+      let config = Config.load path in
+      check
+        (config.namespace = Config.Namespace "TestrepoDeprecatedConfig")
+        "a boolean namespace is derived from a scoped package name";
+      write_file path
+        {|{"name":"namespace-test","namespace":"some.namespace/name_here"}|};
+      let config = Config.load path in
+      check
+        (config.namespace = Config.Namespace "SomenamespaceName_here")
+        "namespace punctuation is normalized while decoding";
+      write_file path
+        {|{
+          "name": "unknown-fields",
+          "sources": {"dir": "src", "nested-source-key": true},
+          "package-specs": {
+            "module": "esmodule",
+            "nested-package-key": true
+          },
+          "warnings": {"nested-warning-key": true},
+          "jsx": {"nested-jsx-key": true},
+          "sourceMap": {
+            "enabled": "always",
+            "mode": "linked",
+            "nested-map-key": true
+          },
+          "gentypeconfig": {"nested-gentype-key": true},
+          "js-post-build": {"cmd": "true", "nested-post-key": true},
+          "top-key": true
+        }|};
+      let config = Config.load path in
+      List.iter
+        (fun field ->
+          check
+            (has_diagnostic config field)
+            ("missing unknown-field diagnostic for " ^ field))
+        [
+          "warnings.?.nested-warning-key";
+          "jsx.?.nested-jsx-key";
+          "gentypeconfig.?.nested-gentype-key";
+          "js-post-build.?.nested-post-key";
+          "top-key";
+        ];
+      List.iter
+        (fun field ->
+          check
+            (not (has_diagnostic config field))
+            ("unexpected unknown-field diagnostic for " ^ field))
+        [
+          "sources.?.nested-source-key";
+          "package-specs.?.nested-package-key";
+          "sourceMap.?.nested-map-key";
+        ];
+      write_file path
+        {|{"name":"different-outputs","package-specs":[{"module":"esmodule","in-source":true,"suffix":".js"},{"module":"commonjs","in-source":false,"suffix":".js"}]}|};
+      let config = Config.load path in
+      check
+        (List.length config.package_specs = 2)
+        "the same suffix is allowed in different output locations";
+      write_file path
+        {|{"name":"gentype-precedence","package-specs":{"module":"commonjs"},"gentypeconfig":{"module":"esmodule"}}|};
+      let config = Config.load path in
+      check
+        (contains_adjacent "-bs-gentype-module" "esmodule" config.gentype_args)
+        "an explicit GenType module overrides package-specs")
+
+let gentype_discovery_tests =
+  with_config_file (fun ~root ~path ->
+      let source_dir = Filename.concat root "src" in
+      let shim_dir = Filename.concat source_dir "shims" in
+      Unix.mkdir source_dir 0o755;
+      Unix.mkdir shim_dir 0o755;
+      write_file path
+        {|{"name":"gentype-subdirs","sources":{"dir":"src","subdirs":true},"gentypeconfig":{}}|};
+      let config = Config.load path in
+      let attempt =
+        Build_attempt.create_full ~warning_state:(Warning_state.create ())
+          ~process_poll:None
+          ~progress:(Output.Progress.create ~enabled:false ~color:false)
+          ~verbosity:0
+      in
+      let package =
+        Package_graph.discover ~root_config:config ~prod:false ~features:None
+          ~warn_error:None ~filter:None ~attempt
+        |> List.hd
+      in
+      check
+        (contains_adjacent "-bs-gentype-source-dir"
+           (Filename.concat "src" "shims")
+           package.compile_config.gentype_args)
+        "GenType recursively includes directories that may contain TypeScript \
+         shims";
+      write_file path {|{"name":"no-gentype"}|};
+      let config = Config.load path in
+      check (config.gentype_args = [])
+        "GenType arguments are absent without gentypeconfig";
+      write_file path {|{"name":"ignored-payload","ignored-dirs":true}|};
+      let config = Config.load path in
+      check
+        (has_diagnostic config "ignored-dirs")
+        "unsupported ignored-dirs payloads are diagnosed but not decoded")
+
+let validation_tests =
+  with_config_file (fun ~root ~path ->
+      write_file path {|{"name":"jsx-v3","jsx":{"v3-dependencies":true}}|};
+      let rejected =
+        try
+          ignore (Config.load path);
+          false
+        with Config.Error message -> contains message "jsx.v3-dependencies"
+      in
+      check rejected "jsx.v3-dependencies must be an array of strings";
+      write_file path
+        {|{"name":"namespace-entry-without-namespace","namespace-entry":"Entry"}|};
+      let rejected =
+        try
+          ignore (Config.load path);
+          false
+        with Config.Error message -> contains message "requires a namespace"
+      in
+      check rejected "namespace-entry requires namespace configuration";
+      write_file path {|{"name":"unsupported-jsx","jsx":{"version":3}}|};
+      let rejected =
+        try
+          ignore (Config.load path);
+          false
+        with Config.Error message -> contains message "jsx.version"
+      in
+      check rejected "unsupported JSX versions are rejected without panicking";
+      write_file path
+        {|{"name":"internal-path","path":"ignored","gentypeconfig":{}}|};
+      let alias_dir = Filename.concat root "alias" in
+      Unix.mkdir alias_dir 0o755;
+      let aliased_path =
+        Filename.concat alias_dir
+          (Filename.concat Filename.parent_dir_name "rescript.json")
+      in
+      let config = Config.load aliased_path in
+      check
+        (config.path = Unix.realpath path)
+        "config loading canonicalizes a noncanonical path alias";
+      check
+        (config.root = Unix.realpath root)
+        "the package root uses the canonical config path")
+
+let compiler_job_tests =
+  with_config_file (fun ~root ~path ->
+      write_file path {|{"name":"compiler-jobs","gentypeconfig":{}}|};
+      let config = Config.load path in
+      let source = Filename.concat root "src/A.res" in
+      let build_dir = Filename.concat config.root "lib/bs" in
+      File_util.ensure_dir build_dir;
+      File_util.ensure_dir (Filename.dirname source);
+      write_file source "let value = 1\n";
+      let parse_job =
+        Compiler_process.parse_job ~bsc:"bsc" ~build_dir ~config "src/A.res"
+      in
+      let module_ =
+        Source.
+          {
+            name = "A";
+            implementation = "src/A.res";
+            interface = None;
+            is_dev = false;
+          }
+      in
+      let compile_job =
+        Compiler_process.compile_job ~bsc:"bsc" ~build_dir ~config
+          ~common_args:
+            (Compiler_args.compiler_common_arguments ~config ~runtime:"runtime"
+               ~dependency_dirs:[] ~watch:false ~gentype_dependency_args:[])
+          module_ ~source_kind:Source.Implementation "src/A.res"
+      in
+      let rec argument_after expected = function
+        | argument :: value :: _ when argument = expected -> Some value
+        | _ :: rest -> argument_after expected rest
+        | [] -> None
+      in
+      check
+        (parse_job.Process.cwd = compile_job.Process.cwd
+        && parse_job.cwd = build_dir)
+        "parser and compiler jobs derive the same canonical working directory";
+      check
+        (argument_after "-bs-project-root" compile_job.args = Some config.root)
+        "the compiler project-root argument uses the canonical job root")
+
+let compiler_argument_tests =
+  with_config_file (fun ~root:_ ~path ->
+      write_file path {|{"name":"internal-path","path":false}|};
+      let config = Config.load path in
+      check
+        (List.exists
+           (fun diagnostic -> contains diagnostic "Unknown field 'path'")
+           config.diagnostics)
+        "the internal path field is ignored as unknown configuration";
+      check
+        (config.path = Unix.realpath path)
+        "the config filename remains authoritative over an ignored path field";
+      write_file path
+        {|{"name":"flag-whitespace","compiler-flags":["  -w  +A  "]}|};
+      let config = Config.load path in
+      check
+        (config.compiler_flags = ["-w"; "+A"])
+        "compiler flag whitespace does not create empty subprocess arguments";
+      write_file path
+        {|{
+          "name": "argument-order",
+          "compiler-flags": ["-open Belt"],
+          "warnings": {"number": "+A"},
+          "jsx": {"mode": "automatic"},
+          "sourceMap": {"enabled": "always", "mode": "hidden"},
+          "gentypeconfig": {},
+          "experimental-features": {"LetUnwrap": true}
+        }|};
+      let config = Config.load path in
+      check
+        (Compiler_args.compiler_flags ~source_maps:false ~watch:false
+           ~gentype:false config
+        = [
+            "-bs-jsx-mode";
+            "automatic";
+            "-enable-experimental";
+            "LetUnwrap";
+            "-w";
+            "+A";
+            "-open";
+            "Belt";
+          ])
+        "parser arguments follow Rust phase ordering";
+      check
+        (Compiler_args.compiler_flags ~source_maps:true ~watch:false
+           ~gentype:true config
+        = [
+            "-bs-jsx-mode";
+            "automatic";
+            "-bs-source-map";
+            "hidden";
+            "-open";
+            "Belt";
+            "-w";
+            "+A";
+            "-bs-gentype";
+            "-enable-experimental";
+            "LetUnwrap";
+          ])
+        "compiler arguments follow Rust phase ordering";
+      let optional_ppx =
+        [
+          ["graphql-ppx"];
+          ["graphql_ppx"];
+          ["spice"];
+          ["rescript-relay"];
+          ["re-formality"];
+          ["bisect_ppx"];
+          ["always"];
+          [];
+        ]
+      in
+      check
+        (Compiler_args.filter_ppx_flags ~bisect_enabled:false optional_ppx
+           "let value = 1"
+        = [["always"]])
+        "source-specific and disabled Bisect PPXs are filtered";
+      check
+        (Compiler_args.filter_ppx_flags ~bisect_enabled:true optional_ppx
+           "%graphql @spice %relay %form"
+        = [
+            ["graphql-ppx"];
+            ["graphql_ppx"];
+            ["spice"];
+            ["rescript-relay"];
+            ["re-formality"];
+            ["bisect_ppx"];
+            ["always"];
+          ])
+        "source markers and the Bisect environment enable their PPXs";
+      check
+        (match
+           Compiler_args.compiler_flags
+             ~ppx_flags:[["tool"; "--arg"]]
+             ~source_maps:false ~watch:false ~gentype:false config
+         with
+        | "-ppx" :: "tool --arg" :: _ -> true
+        | _ -> false)
+        "parser arguments include the filtered PPX command")
+
+let decoder_semantics_tests =
+  with_config_file (fun ~root:_ ~path ->
+      [
+        {|{"name":"first","name":"second"}|};
+        {|{"name":"duplicate-source","sources":{"dir":"a","dir":"b"}}|};
+        {|{"name":"duplicate-spec","package-specs":{"module":"esmodule","module":"commonjs"}}|};
+        {|{"name":"duplicate-warning","warnings":{"number":"A","number":"B"}}|};
+        {|{"name":"duplicate-jsx","jsx":{"mode":"classic","mode":"automatic"}}|};
+        {|{"name":"duplicate-gentype","gentypeconfig":{"module":"esmodule","module":"commonjs"}}|};
+        {|{"name":"duplicate-post","js-post-build":{"cmd":"true","cmd":"false"}}|};
+        {|{"name":"duplicate-dependency","dependencies":[{"name":"a","name":"b"}]}|};
+      ]
+      |> List.iter (fun json ->
+          check
+            (rejects path json "duplicate field")
+            "typed configuration objects reject duplicate fields");
+      [
+        {|{"name":"duplicate-map","sourceMap":{"enabled":"dev","enabled":"always","mode":"linked"}}|};
+        {|{"name":"duplicate-map","features":{"selected":[],"selected":["last"]}}|};
+        {|{"name":"duplicate-map","experimental-features":{"LetUnwrap":false,"LetUnwrap":true}}|};
+        {|{"name":"duplicate-map","gentypeconfig":{"shims":{"A":"One","A":"Two"}}}|};
+        {|{"name":"duplicate-map","gentypeconfig":{"shims":["A=One","A=Two"]}}|};
+        {|{"name":"duplicate-map","gentypeconfig":{"debug":{"all":false,"all":true}}}|};
+      ]
+      |> List.iter (fun json ->
+          check
+            (rejects path json "duplicate field")
+            "interpreted configuration maps reject duplicate keys");
+      write_file path {|{"name":"unknown-map","future":1,"future":2}|};
+      let config = Config.load path in
+      check
+        (List.exists
+           (fun message -> contains message "Unknown field 'future'")
+           config.diagnostics)
+        "opaque unknown values remain outside duplicate validation";
+      write_file path
+        {|{"name":"opaque-tooling","editor":{"mode":1,"mode":2},"reanalyze":{"mode":1,"mode":2}}|};
+      let config = Config.load path in
+      check
+        ((not (has_diagnostic config "editor"))
+        && not (has_diagnostic config "reanalyze"))
+        "opaque tooling payloads remain outside duplicate validation";
+      [
+        "sources";
+        "package-specs";
+        "warnings";
+        "suffix";
+        "dependencies";
+        "bs-dependencies";
+        "dev-dependencies";
+        "bs-dev-dependencies";
+        "features";
+        "ppx-flags";
+        "compiler-flags";
+        "bsc-flags";
+        "namespace";
+        "jsx";
+        "sourceMap";
+        "experimental-features";
+        "gentypeconfig";
+        "js-post-build";
+        "namespace-entry";
+        "allowed-dependents";
+      ]
+      |> List.iter (fun field ->
+          write_file path
+            (Printf.sprintf {|{"name":"null-option","%s":null}|} field);
+          ignore (Config.load path));
+      write_file path
+        {|{
+          "name": "nested-null-options",
+          "sources": {"dir": "src", "subdirs": null, "type": null, "feature": null},
+          "package-specs": {"module": "esmodule", "suffix": null},
+          "warnings": {"number": null, "error": null},
+          "dependencies": [{"name": "dep", "features": null}],
+          "jsx": {"version": null, "module": null, "mode": null, "v3-dependencies": null, "preserve": null},
+          "sourceMap": {"enabled": "always", "mode": "linked", "sourcesContent": null, "sourceRoot": null},
+          "gentypeconfig": {"module": null, "moduleResolution": null, "exportInterfaces": null, "generatedFileExtension": null}
+        }|};
+      ignore (Config.load path);
+      write_file path
+        {|{"name":"jsx","jsx":{"module":"Voby.JSX","preserve":true}}|};
+      let config = Config.load path in
+      check
+        (contains_adjacent "-bs-jsx-module" "Voby.JSX" config.jsx_args)
+        "custom JSX modules are accepted";
+      check
+        (List.mem "-bs-jsx-preserve" config.jsx_args)
+        "JSX preserve is projected to compiler arguments";
+      write_file path {|{"name":"maps-disabled","sourceMap":false}|};
+      let config = Config.load path in
+      check
+        (config.source_map_args = ["-bs-source-map"; "false"])
+        "sourceMap false disables source maps explicitly";
+      write_file path
+        {|{"name":"tooling-config","editor":{"anything":true},"reanalyze":[1,2,3]}|};
+      let config = Config.load path in
+      check
+        ((not (has_diagnostic config "editor"))
+        && not (has_diagnostic config "reanalyze"))
+        "editor and reanalyze payloads are accepted without validation";
+      write_file path
+        {|{"name":"bad-module","package-specs":{"module":"es6-global"}}|};
+      let rejected =
+        try
+          ignore (Config.load path);
+          false
+        with Config.Error message ->
+          contains message "unsupported package module"
+      in
+      check rejected "unsupported package modules are rejected";
+      write_file path
+        {|{"name":"hidden-map","sourceMap":{"enabled":"always","mode":"hidden"}}|};
+      let config = Config.load path in
+      check
+        (contains_adjacent "-bs-source-map" "hidden" config.source_map_args)
+        "hidden source maps are accepted";
+      write_file path
+        {|{"name":"bad-map","sourceMap":{"enabled":"always","mode":"external"}}|};
+      let rejected =
+        try
+          ignore (Config.load path);
+          false
+        with Config.Error message -> contains message "sourceMap.mode"
+      in
+      check rejected "unknown source map modes are rejected";
+      write_file path
+        {|{"name":"legacy-dev-deps","bs-dev-dependencies":["dep"]}|};
+      let config = Config.load path in
+      check
+        (match config.dev_dependencies with
+        | [{name = "dep"; features = None}] -> true
+        | _ -> false)
+        "bs-dev-dependencies is accepted";
+      check
+        (List.exists
+           (fun message -> contains message "field 'bs-dev-dependencies'")
+           config.diagnostics)
+        "bs-dev-dependencies emits its deprecation";
+      write_file path {|{"suffix":".mjs"}|};
+      let rejected =
+        try
+          ignore (Config.load path);
+          false
+        with Config.Error message ->
+          contains message "missing required field \"name\""
+      in
+      check rejected "a package config without a name is rejected";
+      write_file path
+        {|{"name":"getters","suffix":".mjs","package-specs":{"module":"esmodule"},"dependencies":["plain",{"name":"qualified","features":["native"]}]}|};
+      let config = Config.load path in
+      check (config.name = "getters") "the package name is retained";
+      check
+        (match config.package_specs with
+        | [spec] -> Config.package_spec_suffix config spec = ".mjs"
+        | _ -> false)
+        "the configured suffix applies to package specs";
+      check
+        (match config.dependencies with
+        | [plain; qualified] ->
+          plain.name = "plain" && plain.features = None
+          && qualified.name = "qualified"
+          && qualified.features = Some ["native"]
+        | _ -> false)
+        "shorthand and feature-qualified dependencies retain their data")
+
+let tests =
+  "config_tests"
+  >::: [
+         "loading" >:: loading_tests;
+         "gentype_discovery" >:: gentype_discovery_tests;
+         "validation" >:: validation_tests;
+         "compiler_jobs" >:: compiler_job_tests;
+         "compiler_arguments" >:: compiler_argument_tests;
+         "decoder_semantics" >:: decoder_semantics_tests;
+       ]
diff --git a/tests/rewatch_ounit_tests/dune b/tests/rewatch_ounit_tests/dune
new file mode 100644
index 00000000000..b169b0e0552
--- /dev/null
+++ b/tests/rewatch_ounit_tests/dune
@@ -0,0 +1,48 @@
+(rule
+ (target platform_windows.ml)
+ (action
+  (copy ../../rewatch-ocaml/platform_windows.ml %{target})))
+
+(executable
+ (name rewatch_bsc_test_proxy)
+ (modules rewatch_bsc_test_proxy)
+ (libraries unix))
+
+(test
+ (name rewatch_ounit_tests_main)
+ ; Several cases launch this executable to test concurrent subprocess capture.
+ ; A sequential outer runner avoids nesting them inside OUnit worker processes;
+ ; the production concurrency is still exercised by those cases themselves.
+ (action
+  (run %{test} -runner sequential))
+ (modules
+  rewatch_ounit_tests_main
+  build_attempt_tests
+  build_artifacts_tests
+  build_state_tests
+  clean_tests
+  cli_tests
+  compile_assets_tests
+  compiler_args_tests
+  compiler_info_tests
+  compiler_process_tests
+  compiler_scheduler_tests
+  config_tests
+  file_util_tests
+  format_tests
+  native_watcher_tests
+  output_tests
+  package_metadata_tests
+  platform_windows
+  project_context_tests
+  source_tests
+  test_support
+  toolchain_tests
+  traversal_coverage_tests
+  unit_tests
+  warning_state_tests
+  watcher_tests)
+ (foreign_stubs
+  (language c)
+  (names test_env_stubs))
+ (libraries rewatch_ocaml_lib ounit2))
diff --git a/tests/rewatch_ounit_tests/file_util_tests.ml b/tests/rewatch_ounit_tests/file_util_tests.ml
new file mode 100644
index 00000000000..85593542141
--- /dev/null
+++ b/tests/rewatch_ounit_tests/file_util_tests.ml
@@ -0,0 +1,231 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file path contents =
+  File_util.ensure_dir (Filename.dirname path);
+  let channel = open_out_bin path in
+  Fun.protect
+    ~finally:(fun () -> close_out_noerr channel)
+    (fun () -> output_string channel contents)
+
+let descriptor_count () =
+  try Some (Array.length (Sys.readdir "/proc/self/fd"))
+  with Sys_error _ -> None
+
+let check_descriptor_count before message =
+  match (before, descriptor_count ()) with
+  | Some before, Some after -> check (before = after) message
+  | None, _ | _, None -> ()
+
+let tests =
+  "file_util_tests" >:: fun _context ->
+  let root = Filename.temp_file "rewatch-file-util-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      let first = Filename.concat root "first" in
+      let second = Test_support.path root "nested/second" in
+      let missing = Filename.concat root "missing" in
+      write_file first "same";
+      write_file second "same";
+      check (File_util.exists first) "an existing path should be reported";
+      check
+        (not (File_util.exists missing))
+        "a missing path should be reported as absent";
+      check
+        (File_util.files_equal first second)
+        "equal file contents should compare equal";
+      write_file second "different";
+      check
+        (not (File_util.files_equal first second))
+        "different file contents should not compare equal";
+      check
+        (not (File_util.files_equal missing first))
+        "a missing file should not compare equal";
+      if not Sys.win32 then (
+        let fifo = Filename.concat root "stream" in
+        Unix.mkfifo fifo 0o600;
+        let writer =
+          Thread.create (fun () -> File_util.write_file fifo "streamed") ()
+        in
+        let contents = File_util.read_file fifo in
+        Thread.join writer;
+        check (contents = "streamed")
+          "whole-file reads consume streams whose initial length is zero";
+        File_util.remove_file fifo);
+      let large_source = Filename.concat root "large-source" in
+      let large_copy = Filename.concat root "large-copy" in
+      let large_contents =
+        String.init 150_000 (fun index -> Char.chr (index mod 251))
+      in
+      write_file large_source large_contents;
+      File_util.copy_existing_file ~ensure_parent:true large_source large_copy;
+      check
+        (File_util.files_equal large_source large_copy)
+        "streaming copies preserve files spanning multiple buffer reads";
+      let changed_contents = Bytes.of_string large_contents in
+      Bytes.set changed_contents (Bytes.length changed_contents - 1) 'x';
+      write_file large_copy (Bytes.to_string changed_contents);
+      check
+        (not (File_util.files_equal large_source large_copy))
+        "chunk comparison detects a difference after the first buffer";
+      File_util.remove_file large_source;
+      File_util.remove_file large_copy;
+      let concurrent_root = Filename.concat root "concurrent" in
+      let start = Atomic.make false in
+      let failures = ref [] in
+      let failures_lock = Mutex.create () in
+      let workers =
+        Array.init 16 (fun _ ->
+            Thread.create
+              (fun () ->
+                while not (Atomic.get start) do
+                  Thread.yield ()
+                done;
+                for index = 0 to 63 do
+                  Thread.yield ();
+                  let path =
+                    Filename.concat concurrent_root (string_of_int index)
+                  in
+                  try File_util.ensure_dir path
+                  with error ->
+                    Mutex.lock failures_lock;
+                    failures := error :: !failures;
+                    Mutex.unlock failures_lock
+                done)
+              ())
+      in
+      Atomic.set start true;
+      Array.iter Thread.join workers;
+      check (!failures = [])
+        "concurrent directory creation should tolerate another creator";
+      check
+        (Sys.is_directory (Filename.concat concurrent_root "63"))
+        "concurrent directory creation should leave the requested tree";
+      let existing_file_is_rejected =
+        try
+          File_util.ensure_dir first;
+          false
+        with Unix.Unix_error (Unix.EEXIST, _, _) -> true
+      in
+      check existing_file_is_rejected
+        "directory creation should reject an existing non-directory";
+      check
+        (Option.is_some (File_util.modification_time first))
+        "an existing file should have a modification time";
+      check
+        (Option.is_none (File_util.modification_time missing))
+        "a missing file should not have a modification time";
+      let optional_copy = Filename.concat root "optional-copy" in
+      File_util.copy_optional_existing_file first optional_copy;
+      check
+        (File_util.read_file optional_copy = "same")
+        "an available optional artifact should be copied";
+      File_util.copy_optional_existing_file missing optional_copy;
+      check
+        (not (Sys.file_exists optional_copy))
+        "a stale optional destination should be removed when its source is \
+         absent";
+      let missing_destination = Test_support.path root "absent/optional-copy" in
+      let descriptors_before_failed_copies = descriptor_count () in
+      for _ = 1 to 32 do
+        let destination_failure_is_reported =
+          try
+            File_util.copy_optional_existing_file ~ensure_parent:false first
+              missing_destination;
+            false
+          with Sys_error _ | Unix.Unix_error _ -> true
+        in
+        check destination_failure_is_reported
+          "an optional copy must not hide destination failures"
+      done;
+      check_descriptor_count descriptors_before_failed_copies
+        "failed destination acquisition should close the source";
+      [true; false]
+      |> List.iter (fun ensure_parent ->
+          let missing_source_failure_is_reported =
+            try
+              ignore
+                (File_util.copy_file_if_different ~ensure_parent missing
+                   (Filename.concat root "missing-copy"));
+              false
+            with Sys_error _ | Unix.Unix_error _ -> true
+          in
+          check missing_source_failure_is_reported
+            "directory creation policy must not change missing-source semantics");
+      let atomic = Filename.concat root "atomic" in
+      write_file atomic "previous";
+      if not Sys.win32 then Unix.chmod atomic 0o640;
+      File_util.write_file_atomic ~ensure_parent:false atomic "replacement";
+      check
+        (File_util.read_file atomic = "replacement")
+        "atomic writes should publish the complete replacement";
+      if not Sys.win32 then
+        check
+          ((Unix.stat atomic).Unix.st_perm = 0o640)
+          "atomic replacement should preserve existing file permissions";
+      check
+        (Sys.readdir root |> Array.to_list
+        |> List.for_all (fun name ->
+            not (String.starts_with ~prefix:".rewatch-write-" name)))
+        "successful atomic writes should not leave temporary files";
+      if (not Sys.win32) && Sys.file_exists "/dev/full" then
+        check
+          (try
+             File_util.write_file "/dev/full" "buffered output";
+             false
+           with Sys_error _ | Unix.Unix_error _ -> true)
+          "a failure while flushing or closing an output file must be reported";
+      Sys.remove first;
+      write_file first "same";
+      if not Sys.win32 then (
+        let unreadable = Filename.concat root "unreadable" in
+        write_file unreadable "same";
+        Unix.chmod unreadable 0o000;
+        Fun.protect
+          ~finally:(fun () -> Unix.chmod unreadable 0o600)
+          (fun () ->
+            let second_open_fails =
+              try
+                let channel = open_in_bin unreadable in
+                close_in channel;
+                false
+              with Sys_error _ -> true
+            in
+            if second_open_fails then (
+              let descriptors_before_comparisons = descriptor_count () in
+              for _ = 1 to 32 do
+                let comparison_failure_is_reported =
+                  try
+                    ignore (File_util.files_equal first unreadable);
+                    false
+                  with Sys_error _ | Unix.Unix_error _ -> true
+                in
+                check comparison_failure_is_reported
+                  "a comparison must not hide input failures"
+              done;
+              check_descriptor_count descriptors_before_comparisons
+                "failed second-file acquisition should close the first file"));
+        File_util.remove_file unreadable);
+      check
+        (List.sort String.compare (File_util.files_under root)
+        = List.sort String.compare [atomic; first; second])
+        "recursive inventory should contain files but not directories";
+      if not Sys.win32 then (
+        let live_link = Filename.concat root "live-link" in
+        let dangling_link = Filename.concat root "dangling-link" in
+        Unix.symlink first live_link;
+        Unix.symlink missing dangling_link;
+        check
+          (List.sort String.compare (File_util.files_under root)
+          = List.sort String.compare [atomic; first; second; live_link])
+          "recursive inventory should retain live links and omit dangling links");
+      File_util.remove_file first;
+      File_util.remove_file first;
+      check
+        (not (Sys.file_exists first))
+        "removing an existing or already-missing file should be idempotent")
diff --git a/tests/rewatch_ounit_tests/format_tests.ml b/tests/rewatch_ounit_tests/format_tests.ml
new file mode 100644
index 00000000000..f9495f7cd5d
--- /dev/null
+++ b/tests/rewatch_ounit_tests/format_tests.ml
@@ -0,0 +1,163 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let contains text fragment =
+  let text_length = String.length text in
+  let fragment_length = String.length fragment in
+  let rec loop index =
+    if index + fragment_length > text_length then false
+    else if String.sub text index fragment_length = fragment then true
+    else loop (index + 1)
+  in
+  fragment_length = 0 || loop 0
+
+let write_file = Test_support.write_file
+
+let with_temp_dir = Test_support.with_temp_dir "rewatch-ocaml-format-"
+
+let tests =
+  "format_tests" >:: fun _context ->
+  check
+    (Format.formatting_error "stdin" "invalid source"
+    = "Error formatting stdin: invalid source")
+    "stdin formatting failures do not expose a temporary filename";
+  check
+    (Format.formatting_error "src/A.res" "invalid source"
+    = "Error formatting src/A.res: invalid source")
+    "file formatting failures retain the source path";
+  check
+    (Format.format_check_summary 1 = "The file listed above needs formatting")
+    "format check uses Rust's singular summary";
+  check
+    (Format.format_check_summary 2 = "The 2 files listed above need formatting")
+    "format check uses Rust's plural summary";
+  with_temp_dir (fun root ->
+      let parent = Filename.concat root "not-a-directory" in
+      let path = Filename.concat parent "A.res" in
+      write_file parent "file\n";
+      check
+        (try
+           Format.write_file path "let value = 1\n";
+           false
+         with Format.Error message ->
+           contains message "Could not write formatted file"
+           && contains message path)
+        "formatter write failures retain their operation and source path");
+  with_temp_dir (fun root ->
+      write_file (Filename.concat root "rescript.json") "{";
+      let previous_directory = Sys.getcwd () in
+      let previous_bsc = Sys.getenv_opt "RESCRIPT_BSC_EXE" in
+      Unix.chdir root;
+      Unix.putenv "RESCRIPT_BSC_EXE" (Filename.concat root "missing-bsc");
+      let message =
+        Fun.protect
+          ~finally:(fun () ->
+            Unix.chdir previous_directory;
+            match previous_bsc with
+            | Some value -> Unix.putenv "RESCRIPT_BSC_EXE" value
+            | None -> Test_support.unsetenv "RESCRIPT_BSC_EXE")
+          (fun () ->
+            try
+              Format.run_files ~check:false [];
+              None
+            with Format.Error message -> Some message)
+      in
+      check
+        (match message with
+        | Some message ->
+          contains message "RESCRIPT_BSC_EXE points to missing path"
+        | None -> false)
+        "format resolves the compiler before discovering implicit project files");
+  if not Sys.win32 then
+    with_temp_dir (fun root ->
+        let target = Filename.concat root "target.res" in
+        let symlink = Filename.concat root "symlink.res" in
+        let hard_link = Filename.concat root "hard-link.res" in
+        write_file target "before";
+        Unix.symlink target symlink;
+        Unix.link target hard_link;
+        let inode = (Unix.stat target).Unix.st_ino in
+        Format.write_file symlink "through symlink";
+        check
+          (File_util.read_file target = "through symlink")
+          "formatting a symlink should update its target";
+        Format.write_file hard_link "through hard link";
+        check
+          ((Unix.stat target).Unix.st_ino = inode
+          && File_util.read_file target = "through hard link")
+          "formatting should preserve hard-link identity");
+  with_temp_dir (fun root ->
+      let first = Filename.concat root "First.res" in
+      let second = Filename.concat root "Second.res" in
+      write_file first "let first = 1\n";
+      write_file second "let second = 2\n";
+      let previous_root = Sys.getenv_opt "REWATCH_FORMAT_TEST_ROOT" in
+      Unix.putenv "REWATCH_FORMAT_TEST_ROOT" root;
+      Fun.protect
+        ~finally:(fun () ->
+          match previous_root with
+          | Some value -> Unix.putenv "REWATCH_FORMAT_TEST_ROOT" value
+          | None -> Test_support.unsetenv "REWATCH_FORMAT_TEST_ROOT")
+        (fun () ->
+          Format.format_files_with_bsc ~max_jobs:2
+            ~bsc:(Unix.realpath Sys.executable_name)
+            ~check:true [first; second]);
+      check
+        (Sys.file_exists (first ^ ".started")
+        && Sys.file_exists (second ^ ".started"))
+        "formatter subprocesses overlap rather than running serially");
+  with_temp_dir (fun root ->
+      let root_source = Test_support.path root "src/App.res" in
+      let orphan_interface = Test_support.path root "src/Orphan.resi" in
+      let duplicate_one = Test_support.path root "src/one/Duplicate.res" in
+      let duplicate_two = Test_support.path root "src/two/Duplicate.res" in
+      let installed_source =
+        Test_support.path root "node_modules/installed/src/Installed.res"
+      in
+      write_file
+        (Filename.concat root "rescript.json")
+        {|{"name":"app","sources":[{"dir":"src","subdirs":true}],"dependencies":["installed"]}|};
+      write_file root_source "let value = 1\n";
+      write_file orphan_interface "let value: int\n";
+      write_file duplicate_one "let value = 1\n";
+      write_file duplicate_two "let value = 2\n";
+      write_file
+        (Test_support.path root "node_modules/installed/rescript.json")
+        {|{"name":"installed","sources":["src"]}|};
+      write_file installed_source "let value = 2\n";
+      let previous = Sys.getcwd () in
+      let previous_bsc = Sys.getenv_opt "RESCRIPT_BSC_EXE" in
+      let previous_inventory_root =
+        Sys.getenv_opt "REWATCH_FORMAT_INVENTORY_TEST_ROOT"
+      in
+      let marker path =
+        Filename.concat root (Digest.string path |> Digest.to_hex)
+      in
+      Unix.putenv "RESCRIPT_BSC_EXE" (Unix.realpath Sys.executable_name);
+      Unix.putenv "REWATCH_FORMAT_INVENTORY_TEST_ROOT" root;
+      Fun.protect
+        ~finally:(fun () ->
+          Unix.chdir previous;
+          (match previous_bsc with
+          | Some value -> Unix.putenv "RESCRIPT_BSC_EXE" value
+          | None -> Test_support.unsetenv "RESCRIPT_BSC_EXE");
+          match previous_inventory_root with
+          | Some value -> Unix.putenv "REWATCH_FORMAT_INVENTORY_TEST_ROOT" value
+          | None -> Test_support.unsetenv "REWATCH_FORMAT_INVENTORY_TEST_ROOT")
+        (fun () ->
+          Unix.chdir root;
+          Format.run_files ~check:true []);
+      check
+        (Sys.file_exists (marker root_source))
+        "implicit format includes the current package";
+      check
+        (Sys.file_exists (marker orphan_interface))
+        "implicit format includes an orphan interface";
+      check
+        (Sys.file_exists (marker duplicate_one)
+        && Sys.file_exists (marker duplicate_two))
+        "implicit format does not impose compilation module uniqueness";
+      check
+        (not (Sys.file_exists (marker installed_source)))
+        "implicit format does not rewrite installed node_modules dependencies")
diff --git a/tests/rewatch_ounit_tests/native_watcher_tests.ml b/tests/rewatch_ounit_tests/native_watcher_tests.ml
new file mode 100644
index 00000000000..b98bfccf352
--- /dev/null
+++ b/tests/rewatch_ounit_tests/native_watcher_tests.ml
@@ -0,0 +1,189 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let temporary_directory () =
+  let path = Filename.temp_file "rewatch-native-watcher-" "" in
+  Sys.remove path;
+  Unix.mkdir path 0o700;
+  Unix.realpath path
+
+let descriptor_count () =
+  try Some (Array.length (Sys.readdir "/proc/self/fd"))
+  with Sys_error _ -> None
+
+let tests =
+  "native_watcher_tests" >:: fun _context ->
+  let root = temporary_directory () in
+  let source = Filename.concat root "src" in
+  let nested = Filename.concat source "nested" in
+  let replaced_nested = root ^ "-replaced-nested" in
+  let source_lib = Filename.concat source "lib" in
+  let artifact_lib = Filename.concat root "lib" in
+  let artifact_bs = Filename.concat artifact_lib "bs" in
+  Fun.protect
+    (fun () ->
+      Unix.mkdir source 0o700;
+      Unix.mkdir source_lib 0o700;
+      Unix.mkdir artifact_lib 0o700;
+      Unix.mkdir artifact_bs 0o700;
+      let paths = [Native_watcher.{directory = root; recursive = true}] in
+      let identity_failure _ = Error "injected identity failure" in
+      (match
+         Native_watcher.For_test.create_with_directory_identity
+           ~directory_identity:identity_failure ~paths
+       with
+      | Error "injected identity failure" -> ()
+      | Error message ->
+        assert_failure ("unexpected identity failure: " ^ message)
+      | Ok watcher ->
+        Native_watcher.close watcher;
+        assert_failure "identity failure did not reject watcher creation");
+      let descriptors_before_exception = descriptor_count () in
+      for _ = 1 to 16 do
+        match
+          Native_watcher.For_test.create_with_directory_identity
+            ~directory_identity:(fun _ -> raise Exit)
+            ~paths
+        with
+        | Error message when Test_support.contains_text message "Exit" -> ()
+        | Error message ->
+          assert_failure ("unexpected identity exception: " ^ message)
+        | Ok watcher ->
+          Native_watcher.close watcher;
+          assert_failure "identity exception did not reject watcher creation"
+      done;
+      (match (descriptors_before_exception, descriptor_count ()) with
+      | Some before, Some after ->
+        check (before = after)
+          "exceptional watcher creation should release acquired descriptors"
+      | None, _ | _, None -> ());
+      (match Native_watcher.create ~paths:[] with
+      | Error message ->
+        failwith ("native watcher timer initialization: " ^ message)
+      | Ok timer_watcher ->
+        Fun.protect
+          (fun () ->
+            let checks = ref 0 in
+            (match
+               Native_watcher.wait timer_watcher ~keep_running:(fun () ->
+                   incr checks;
+                   if !checks = 1 then true else raise Exit)
+             with
+            | Native_watcher.Failed message
+              when Test_support.contains_text message "Exit" ->
+              ()
+            | Native_watcher.Changed _ | Native_watcher.Stopped
+            | Native_watcher.Failed _ ->
+              assert_failure
+                "an exception in the timer callback becomes a native failure");
+            (match Native_watcher.refresh timer_watcher ~paths:[] with
+            | Error message ->
+              failwith ("native watcher callback recovery: " ^ message)
+            | Ok () -> ());
+            match
+              Native_watcher.wait timer_watcher ~keep_running:(fun () -> false)
+            with
+            | Native_watcher.Stopped -> ()
+            | Native_watcher.Changed _ | Native_watcher.Failed _ ->
+              assert_failure
+                "a native watcher remains usable after a timer callback fails")
+          ~finally:(fun () -> Native_watcher.close timer_watcher));
+      match Native_watcher.create ~paths with
+      | Error message -> failwith ("native watcher initialization: " ^ message)
+      | Ok watcher ->
+        Fun.protect
+          (fun () ->
+            check
+              (Native_watcher.For_test.handle_count watcher = 4)
+              "valid lib source directories are watched but lib/bs is not";
+            Native_watcher.For_test.queue_change watcher;
+            Native_watcher.For_test.queue_error watcher "injected failure";
+            (match
+               Native_watcher.wait watcher ~keep_running:(fun () -> true)
+             with
+            | Native_watcher.Failed "injected failure" -> ()
+            | Native_watcher.Changed _ | Native_watcher.Stopped
+            | Native_watcher.Failed _ ->
+              assert_failure "a native failure takes precedence over a change");
+            (match Native_watcher.refresh watcher ~paths with
+            | Error message -> failwith ("native watcher refresh: " ^ message)
+            | Ok () -> ());
+            (match
+               Native_watcher.wait watcher ~keep_running:(fun () -> true)
+             with
+            | Native_watcher.Changed [_] -> ()
+            | Native_watcher.Changed _ ->
+              assert_failure "refresh preserves exactly one queued change"
+            | Native_watcher.Stopped | Native_watcher.Failed _ ->
+              assert_failure "refresh preserves a previously queued change");
+            Native_watcher.For_test.queue_change watcher;
+            (match
+               Native_watcher.wait watcher ~keep_running:(fun () -> false)
+             with
+            | Native_watcher.Stopped -> ()
+            | Native_watcher.Changed _ | Native_watcher.Failed _ ->
+              assert_failure "an external stop takes precedence over a change");
+            (match
+               Native_watcher.wait watcher ~keep_running:(fun () -> true)
+             with
+            | Native_watcher.Changed [_] -> ()
+            | Native_watcher.Changed _ ->
+              assert_failure
+                "a stopped wait preserves exactly one queued change"
+            | Native_watcher.Stopped | Native_watcher.Failed _ ->
+              assert_failure "a stopped wait preserves its queued change");
+            check
+              (Native_watcher.For_test.handle_count watcher = 4)
+              "unchanged refresh retains handle count";
+            (match
+               Native_watcher.For_test.refresh_with_directory_identity
+                 ~directory_identity:identity_failure watcher ~paths
+             with
+            | Error "injected identity failure" -> ()
+            | Error message ->
+              assert_failure ("unexpected refresh failure: " ^ message)
+            | Ok () ->
+              assert_failure "identity failure did not reject watcher refresh");
+            check
+              (Native_watcher.For_test.handle_count watcher = 4)
+              "failed identity refresh preserves existing handles";
+            Unix.mkdir nested 0o700;
+            (match Native_watcher.refresh watcher ~paths with
+            | Error message -> failwith ("native watcher add: " ^ message)
+            | Ok () -> ());
+            check
+              (Native_watcher.For_test.handle_count watcher = 5)
+              "new directory adds one handle";
+            let original_identity =
+              Native_watcher.For_test.directory_identity watcher nested
+            in
+            Unix.rename nested replaced_nested;
+            Unix.mkdir nested 0o700;
+            (match Native_watcher.refresh watcher ~paths with
+            | Error message ->
+              failwith ("native watcher replacement: " ^ message)
+            | Ok () -> ());
+            check
+              (Native_watcher.For_test.handle_count watcher = 5)
+              "directory replacement preserves the desired handle count";
+            check
+              (Native_watcher.For_test.directory_identity watcher nested
+              <> original_identity)
+              "directory replacement installs a handle for the new identity";
+            Unix.rmdir nested;
+            (match Native_watcher.refresh watcher ~paths with
+            | Error message -> failwith ("native watcher remove: " ^ message)
+            | Ok () -> ());
+            check
+              (Native_watcher.For_test.handle_count watcher = 4)
+              "removed directory closes one handle")
+          ~finally:(fun () -> Native_watcher.close watcher))
+    ~finally:(fun () ->
+      (try Unix.rmdir nested with Unix.Unix_error _ -> ());
+      (try Unix.rmdir replaced_nested with Unix.Unix_error _ -> ());
+      (try Unix.rmdir artifact_bs with Unix.Unix_error _ -> ());
+      (try Unix.rmdir artifact_lib with Unix.Unix_error _ -> ());
+      (try Unix.rmdir source_lib with Unix.Unix_error _ -> ());
+      (try Unix.rmdir source with Unix.Unix_error _ -> ());
+      try Unix.rmdir root with Unix.Unix_error _ -> ())
diff --git a/tests/rewatch_ounit_tests/output_tests.ml b/tests/rewatch_ounit_tests/output_tests.ml
new file mode 100644
index 00000000000..45753109a8e
--- /dev/null
+++ b/tests/rewatch_ounit_tests/output_tests.ml
@@ -0,0 +1,128 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let environment values name = List.assoc_opt name values
+
+let tests =
+  "output_tests" >:: fun _context ->
+  check
+    (Output.cleanup_message ~color:false ~step:"1/3" ~cleaned:2 ~total:5
+       ~seconds:1.5
+    = Printf.sprintf "\027[2K\r[1/3] %sCleaned 2/5 in 1.50s"
+        Platform.clean_symbol)
+    "interactive cleanup phase format";
+  check
+    (Output.compiler_cleanup_message ~color:false ~step:"1/3"
+    = Printf.sprintf
+        "\027[2K\r[1/3] %sCleaned previous build due to compiler update"
+        Platform.clean_symbol)
+    "interactive compiler cleanup format";
+  check
+    (Output.cleaning_command_message ~color:false ~step:"1/2" "compiler assets"
+    = Printf.sprintf "\027[2K\r[1/2] %sCleaning compiler assets..."
+        Platform.clean_symbol)
+    "interactive clean command progress format";
+  check
+    (Output.cleaned_command_message ~color:false ~step:"2/2" ~target:".js files"
+       ~seconds:1.5
+    = Printf.sprintf "\027[2K\r[2/2] %sCleaned .js files in 1.50s"
+        Platform.clean_symbol)
+    "interactive clean command completion format";
+  check
+    (Output.parsing_message ~color:false ~step:"2/3" ~count:4 ~seconds:1.5
+    = Printf.sprintf "\027[2K\r[2/3] %sParsed 4 source files in 1.50s"
+        Platform.parse_symbol)
+    "interactive parsing phase format";
+  check
+    (Output.parsing_message ~color:true ~step:"2/3" ~count:4 ~seconds:1.5
+    = Printf.sprintf
+        "\027[2K\r\027[1m\027[2m[2/3]\027[0m %sParsed 4 source files in 1.50s"
+        Platform.parse_symbol)
+    "interactive parsing phase styles the step when color is enabled";
+  check
+    (Output.parsing_failed_message ~color:false ~step:"2/3" ~seconds:1.5
+    = Printf.sprintf "\027[2K\r[2/3] %sError parsing source files in 1.50s"
+        Platform.error_symbol)
+    "interactive failed parsing phase format";
+  check
+    (Output.compiling_message ~color:false ~step:"3/3" ~count:4 ~seconds:1.5
+    = Printf.sprintf "\027[2K\r[3/3] %sCompiled 4 modules in 1.50s"
+        Platform.build_symbol)
+    "interactive compilation phase format";
+  check
+    (Output.compilation_failed_message ~color:false ~step:"2/2" ~count:3
+       ~seconds:1.5
+    = Printf.sprintf "\027[2K\r[2/2] %sCompiled 3 modules in 1.50s"
+        Platform.error_symbol)
+    "interactive failed compilation phase format";
+  check
+    (Output.finished_compilation_message ~label:Output.Standard ~warnings:false
+       ~seconds:1.5
+    = Printf.sprintf "\027[2K\r%sFinished compilation in 1.50s"
+        Platform.success_symbol)
+    "clean completion format";
+  check
+    (Output.finished_compilation_message ~label:Output.Incremental
+       ~warnings:true ~seconds:1.5
+    = Printf.sprintf
+        "\027[2K\r%sFinished incremental compilation with warnings in 1.50s"
+        Platform.warning_symbol)
+    "warning completion format";
+  check
+    (Output.should_clear_screen ~clear_screen:true ~show_progress:true
+       ~interactive:true)
+    "interactive clear-screen";
+  check
+    (not
+       (Output.should_clear_screen ~clear_screen:true ~show_progress:true
+          ~interactive:false))
+    "non-interactive clear-screen";
+  check
+    (not
+       (Output.should_clear_screen ~clear_screen:false ~show_progress:true
+          ~interactive:true))
+    "disabled clear-screen";
+  check
+    (not
+       (Output.should_clear_screen ~clear_screen:true ~show_progress:false
+          ~interactive:true))
+    "quiet watch mode should preserve the terminal";
+  check
+    (Output.colors_enabled_with
+       ~getenv:(environment [("CLICOLOR_FORCE", "1")])
+       ~win32:false ~interactive:false)
+    "CLICOLOR_FORCE enables redirected colors";
+  check
+    (not
+       (Output.colors_enabled_with
+          ~getenv:(environment [("CLICOLOR_FORCE", "0"); ("TERM", "xterm")])
+          ~win32:false ~interactive:false))
+    "zero CLICOLOR_FORCE does not enable redirected colors";
+  check
+    (Output.colors_enabled_with
+       ~getenv:(environment [("TERM", "xterm")])
+       ~win32:false ~interactive:true)
+    "a Unix color terminal enables colors";
+  check
+    (not
+       (Output.colors_enabled_with
+          ~getenv:(environment [("TERM", "xterm"); ("CLICOLOR", "0")])
+          ~win32:false ~interactive:true))
+    "CLICOLOR zero disables terminal colors";
+  check
+    (not
+       (Output.colors_enabled_with
+          ~getenv:(environment [("TERM", "xterm"); ("NO_COLOR", "1")])
+          ~win32:false ~interactive:true))
+    "NO_COLOR disables Unix terminal colors";
+  check
+    (not
+       (Output.colors_enabled_with ~getenv:(environment []) ~win32:false
+          ~interactive:true))
+    "a Unix terminal without TERM does not assume color support";
+  check
+    (Output.colors_enabled_with
+       ~getenv:(environment [("TERM", "dumb"); ("NO_COLOR", "1")])
+       ~win32:true ~interactive:true)
+    "a Windows console does not use Unix terminal environment gates"
diff --git a/tests/rewatch_ounit_tests/package_metadata_tests.ml b/tests/rewatch_ounit_tests/package_metadata_tests.ml
new file mode 100644
index 00000000000..46f2248f1ad
--- /dev/null
+++ b/tests/rewatch_ounit_tests/package_metadata_tests.ml
@@ -0,0 +1,70 @@
+open OUnit2
+
+let check_equal expected actual message =
+  assert_equal ~msg:message expected actual
+
+let write_file = Test_support.write_file
+
+let tests =
+  "package_metadata_tests" >:: fun _context ->
+  let root = Filename.temp_file "rewatch-ocaml-package-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      let package_json = Filename.concat root "package.json" in
+      let check contents expected message =
+        write_file package_json contents;
+        check_equal expected
+          (Package_diagnostics.For_test.issue_tracker_url root)
+          message
+      in
+      check {|{"bugs":"https://bugs.example/pkg","repository":"owner/repo"}|}
+        (Some "https://bugs.example/pkg") "a bugs string takes precedence";
+      check
+        {|{"bugs":{"url":"https://bugs.example/object"},"repository":"owner/repo"}|}
+        (Some "https://bugs.example/object") "a bugs object takes precedence";
+      check {|{"repository":"git+https://github.com/owner/repo.git"}|}
+        (Some "https://github.com/owner/repo/issues")
+        "a Git repository URL becomes an issues URL";
+      check {|{"repository":{"url":"git@github.com:owner/repo.git"}}|}
+        (Some "git@github.com:owner/repo/issues")
+        "a repository object is accepted";
+      check {|{"repository":"github:owner/repo"}|}
+        (Some "https://github.com/owner/repo/issues")
+        "a GitHub shorthand is expanded";
+      check {|{"name":"no-metadata"}|} None
+        "missing issue tracker metadata returns none";
+      write_file package_json {|{"name":"first","name":"last"}|};
+      check_equal (Ok (Some "last"))
+        (Package_diagnostics.For_test.package_name root)
+        "package identity uses JSON map last-key semantics";
+      write_file package_json {|{"name":false}|};
+      check_equal (Ok None)
+        (Package_diagnostics.For_test.package_name root)
+        "a non-string package name falls back to the ReScript config";
+      write_file package_json "{invalid";
+      check_equal true
+        (match Package_diagnostics.For_test.package_name root with
+        | Error message ->
+          String.starts_with ~prefix:"Could not parse package.json:" message
+        | Ok _ -> false)
+        "malformed package metadata is rejected";
+      let config_path = Filename.concat root "rescript.json" in
+      write_file config_path {|{"name":"external","future-field":true}|};
+      let config = Config.load config_path in
+      check_equal []
+        (Package_diagnostics.for_package ~is_local:false config)
+        "unknown fields in external packages are not reported";
+      write_file config_path
+        {|{"name":"external","bsc-flags":[],"future-field":true}|};
+      let config = Config.load config_path in
+      let diagnostics =
+        Package_diagnostics.for_package ~is_local:false config
+      in
+      check_equal 1 (List.length diagnostics)
+        "only deprecations are reported for external packages";
+      assert_bool "the external diagnostic is the deprecation"
+        (Test_support.contains_text (List.hd diagnostics) "bsc-flags"))
diff --git a/tests/rewatch_ounit_tests/project_context_tests.ml b/tests/rewatch_ounit_tests/project_context_tests.ml
new file mode 100644
index 00000000000..e189f6ab251
--- /dev/null
+++ b/tests/rewatch_ounit_tests/project_context_tests.ml
@@ -0,0 +1,153 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let write_config root contents =
+  write_file (Filename.concat root "rescript.json") contents
+
+let tests =
+  "project_context_tests" >:: fun _context ->
+  let root = Filename.temp_file "rewatch-ocaml-project-context-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      let dependency = Test_support.path root "packages/dependency" in
+      let dev_dependency = Test_support.path root "packages/dev-dependency" in
+      let unlisted = Test_support.path root "packages/unlisted" in
+      List.iter File_util.ensure_dir [dependency; dev_dependency; unlisted];
+      write_config root
+        {|{
+          "name": "workspace",
+          "dependencies": ["dependency"],
+          "dev-dependencies": ["dev-dependency"]
+        }|};
+      write_file
+        (Filename.concat root "package.json")
+        {|{"workspaces":["packages/*"]}|};
+      write_config dependency {|{"name":"dependency"}|};
+      write_config dev_dependency {|{"name":"dev-dependency"}|};
+      write_config unlisted {|{"name":"unlisted"}|};
+      check
+        (Project_context.workspace_lock_root dependency = root)
+        "listed dependencies inherit the parent workspace context";
+      check
+        (Project_context.workspace_lock_root dev_dependency = root)
+        "listed dev dependencies inherit the parent workspace context";
+      check
+        (Project_context.workspace_lock_root unlisted = unlisted)
+        "package.json workspace globs do not enroll unlisted ReScript packages";
+      check
+        (Project_context.relative_to root root = ".")
+        "a root path is represented by the current-directory component";
+      check
+        (Project_context.relative_to root (Filename.concat root "packages")
+        = "packages")
+        "a child path is represented relative to its root";
+      check
+        (Project_context.display_path ~root (Filename.concat root "packages")
+        = "./packages")
+        "diagnostic paths inside the project are visibly relative";
+      check
+        (Project_context.relative_or_absolute ~root
+           (Filename.concat root "packages")
+        = "packages")
+        "source locations inside the project omit a current-directory prefix";
+      check
+        (Project_context.display_path ~root "/external/dependency"
+        = "/external/dependency")
+        "diagnostic paths outside the project remain absolute";
+      let candidates =
+        Project_context.dependency_candidates_in
+          (Project_context.dependency_context (Config.load_root root))
+          root "@scope/pkg"
+      in
+      check
+        (List.hd candidates
+        = Filename.concat (Filename.concat root "node_modules") "@scope/pkg")
+        "dependency candidates start with the package-local node_modules path";
+      check
+        (not (List.mem (Test_support.path root "packages/pkg") candidates))
+        "workspace package directories are not implicit dependency candidates";
+      check
+        (Project_context.dependency_path root "dependency" = None)
+        "a package outside node_modules is not resolved implicitly";
+      let dependency_config = Config.load_root dependency in
+      let dependency_context =
+        Project_context.dependency_context dependency_config
+      in
+      check
+        (not
+           (Project_context.dependency_is_local_canonical dependency_context
+              dev_dependency))
+        "a directly invoked workspace package does not own its siblings";
+      check
+        (Project_context.dependency_candidates_in dependency_context dependency
+           "@scope/pkg"
+        = [
+            Filename.concat
+              (Filename.concat dependency "node_modules")
+              "@scope/pkg";
+            Filename.concat (Filename.concat root "node_modules") "@scope/pkg";
+          ])
+        "a workspace package checks only its own and the workspace node_modules";
+      let node_modules = Filename.concat root "node_modules" in
+      File_util.ensure_dir node_modules;
+      if
+        Test_support.symlink_if_supported ~to_dir:true "../packages/dependency"
+          (Filename.concat node_modules "dependency")
+        && Test_support.symlink_if_supported ~to_dir:true
+             "../packages/dev-dependency"
+             (Filename.concat node_modules "dev-dependency")
+      then (
+        let root_context =
+          Project_context.dependency_context (Config.load_root root)
+        in
+        check
+          (Project_context.dependency_is_local_canonical root_context dependency)
+          "a workspace-root invocation owns linked workspace dependencies";
+        check
+          (Project_context.dependency_is_local_canonical root_context
+             dev_dependency)
+          "a workspace-root invocation owns linked development dependencies";
+        let resolution = Package_resolution.create (Config.load_root root) in
+        let first =
+          Package_resolution.resolve resolution ~package_root:root
+            Config.{name = "dependency"; features = Some ["first"]}
+        in
+        let second =
+          Package_resolution.resolve resolution ~package_root:root
+            Config.{name = "dependency"; features = Some ["second"]}
+        in
+        check
+          (first.directory = second.directory && first.config = second.config)
+          "cached dependency resolution reuses only package identity");
+      let repository_tmp = Filename.concat (Sys.getcwd ()) "tmp" in
+      File_util.ensure_dir repository_tmp;
+      let standalone =
+        Filename.temp_file ~temp_dir:repository_tmp "rewatch-ocaml-standalone-"
+          ""
+      in
+      Sys.remove standalone;
+      Unix.mkdir standalone 0o755;
+      let standalone = Unix.realpath standalone in
+      Fun.protect
+        ~finally:(fun () -> File_util.remove_tree standalone)
+        (fun () ->
+          write_config standalone {|{"name":"unlisted-standalone"}|};
+          let source = Test_support.path standalone "src/A.res" in
+          write_file source "let value = 1\n";
+          check
+            (Project_context.workspace_lock_root standalone = standalone)
+            "an unrelated project below a workspace remains standalone";
+          let arguments = Compiler_args_command.run source in
+          check
+            (not (Test_support.contains_text arguments "\"-bs-jsx\""))
+            "standalone compiler arguments do not inherit workspace JSX";
+          check
+            (Test_support.contains_text arguments "esmodule:src:.js")
+            "standalone compiler arguments retain their default package output"))
diff --git a/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.ml b/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.ml
new file mode 100644
index 00000000000..a15a3d4218d
--- /dev/null
+++ b/tests/rewatch_ounit_tests/rewatch_bsc_test_proxy.ml
@@ -0,0 +1,127 @@
+let getenv name =
+  match Sys.getenv_opt name with
+  | Some value -> value
+  | None -> failwith ("Missing environment variable " ^ name)
+
+let touch path = close_out (open_out_bin path)
+
+let append_line path line =
+  let channel = open_out_gen [Open_creat; Open_text; Open_append] 0o666 path in
+  output_string channel line;
+  output_char channel '\n';
+  close_out channel
+
+let remove_if_present path = try Sys.remove path with Sys_error _ -> ()
+
+let has_argument expected = Array.exists (String.equal expected) Sys.argv
+
+let output_argument () =
+  let rec loop index =
+    if index + 1 >= Array.length Sys.argv then None
+    else if Sys.argv.(index) = "-o" then Some Sys.argv.(index + 1)
+    else loop (index + 1)
+  in
+  loop 1
+
+let basename path =
+  path
+  |> String.map (fun character -> if character = '\\' then '/' else character)
+  |> Filename.basename
+
+let arguments () =
+  Array.to_list (Array.sub Sys.argv 1 (Array.length Sys.argv - 1))
+
+let wait_for_file path =
+  let rec wait attempts =
+    if attempts < 400 && not (Sys.file_exists path) then (
+      Unix.sleepf 0.05;
+      wait (attempts + 1))
+  in
+  wait 0
+
+let run_compiler () =
+  let program = getenv "REWATCH_REAL_BSC" in
+  let argv = Array.copy Sys.argv in
+  argv.(0) <- program;
+  let pid =
+    Unix.create_process program argv Unix.stdin Unix.stdout Unix.stderr
+  in
+  match snd (Unix.waitpid [] pid) with
+  | Unix.WEXITED status -> status
+  | Unix.WSIGNALED signal | Unix.WSTOPPED signal -> 128 + signal
+
+let block_for_scope_transition () =
+  let request = getenv "REWATCH_SCOPE_BLOCK_REQUEST" in
+  let started = getenv "REWATCH_SCOPE_BLOCK_STARTED" in
+  if Sys.file_exists request && not (Sys.file_exists started) then (
+    touch started;
+    let release = getenv "REWATCH_SCOPE_BLOCK_RELEASE" in
+    let rec wait attempts =
+      if attempts < 200 && not (Sys.file_exists release) then (
+        Unix.sleepf 0.05;
+        wait (attempts + 1))
+    in
+    wait 0)
+
+let before_compiler mode is_parse =
+  if mode = "scope-block" then block_for_scope_transition ();
+  if mode = "block-compile" && not is_parse then (
+    touch (getenv "REWATCH_OCAML_COMPILE_STARTED");
+    wait_for_file (getenv "REWATCH_OCAML_RELEASE_FILE"));
+  if mode = "slow" then (
+    touch (getenv "REWATCH_OCAML_CHILD_STARTED");
+    match Sys.getenv_opt "REWATCH_OCAML_RELEASE_FILE" with
+    | Some release -> wait_for_file release
+    | None -> Unix.sleepf 5.0);
+  if mode = "counting" || mode = "parse-warning-log" then
+    append_line
+      (getenv "REWATCH_BSC_CALL_LOG")
+      (String.concat " " (arguments ()))
+
+let after_compiler mode is_parse status =
+  if status = 0 && mode = "parse-warning" && is_parse then
+    prerr_endline "PARSE_WARNING_MARKER";
+  (if status = 0 && mode = "parse-warning-log" && is_parse then
+     let source = getenv "REWATCH_PARSE_WARNING_SOURCE" in
+     if Array.exists (fun argument -> basename argument = source) Sys.argv then
+       prerr_endline "REWATCH_PARSE_WARNING");
+  if
+    status = 0
+    && mode = "fail-late-publication"
+    && (not is_parse)
+    && Array.exists (fun argument -> basename argument = "A.ast") Sys.argv
+    && Sys.file_exists (getenv "REWATCH_FAIL_PUBLICATION")
+    && not (Sys.file_exists (getenv "REWATCH_PUBLICATION_FAILED"))
+  then (
+    let destination = getenv "REWATCH_PUBLICATION_DESTINATION" in
+    remove_if_present destination;
+    Unix.mkdir destination 0o755;
+    touch (getenv "REWATCH_PUBLICATION_FAILED"))
+
+let () =
+  let mode = getenv "REWATCH_BSC_PROXY_MODE" in
+  let is_parse = has_argument "-bs-ast" in
+  before_compiler mode is_parse;
+  (if mode = "delete-parse-sources" && is_parse then
+     let marker = getenv "REWATCH_SOURCES_DELETED" in
+     if not (Sys.file_exists marker) then (
+       remove_if_present (getenv "REWATCH_SOURCE_A");
+       remove_if_present (getenv "REWATCH_SOURCE_B");
+       touch marker));
+  let status = run_compiler () in
+  after_compiler mode is_parse status;
+  (if status = 0 then
+     if mode = "delete-source" && not is_parse then (
+       let marker = getenv "REWATCH_SOURCE_DELETED" in
+       if not (Sys.file_exists marker) then (
+         remove_if_present (getenv "REWATCH_SOURCE_TO_DELETE");
+         touch marker))
+     else if mode = "delete-ast" && is_parse then
+       match output_argument () with
+       | None -> ()
+       | Some output ->
+         let marker = getenv "REWATCH_AST_DELETED" in
+         if not (Sys.file_exists marker) then (
+           remove_if_present output;
+           touch marker));
+  exit status
diff --git a/tests/rewatch_ounit_tests/rewatch_ounit_tests_main.ml b/tests/rewatch_ounit_tests/rewatch_ounit_tests_main.ml
new file mode 100644
index 00000000000..d5111b250ed
--- /dev/null
+++ b/tests/rewatch_ounit_tests/rewatch_ounit_tests_main.ml
@@ -0,0 +1,30 @@
+open OUnit2
+
+let () =
+  run_test_tt_main
+    ("rewatch"
+    >::: [
+           Build_attempt_tests.tests;
+           Build_artifacts_tests.tests;
+           Build_state_tests.tests;
+           Clean_tests.tests;
+           Cli_tests.tests;
+           Compile_assets_tests.tests;
+           Compiler_args_tests.tests;
+           Compiler_info_tests.tests;
+           Compiler_process_tests.tests;
+           Compiler_scheduler_tests.tests;
+           Config_tests.tests;
+           File_util_tests.tests;
+           Format_tests.tests;
+           Native_watcher_tests.tests;
+           Output_tests.tests;
+           Package_metadata_tests.tests;
+           Project_context_tests.tests;
+           Source_tests.tests;
+           Toolchain_tests.tests;
+           Traversal_coverage_tests.tests;
+           Unit_tests.tests;
+           Warning_state_tests.tests;
+           Watcher_tests.tests;
+         ])
diff --git a/tests/rewatch_ounit_tests/source_tests.ml b/tests/rewatch_ounit_tests/source_tests.ml
new file mode 100644
index 00000000000..1e2e95ff912
--- /dev/null
+++ b/tests/rewatch_ounit_tests/source_tests.ml
@@ -0,0 +1,278 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let names modules =
+  List.map (fun (module_ : Source.module_) -> module_.name) modules
+
+let discover config ?(prod = false) ?features ?filter () =
+  let filter =
+    Option.map
+      (fun pattern ->
+        match Source_filter.compile pattern with
+        | Ok filter -> filter
+        | Error message -> failwith message)
+      filter
+  in
+  (Source.discover_with_inventory config ~prod ~features ~filter).modules
+
+let discover_with_inventory config ?(prod = false) ?features () =
+  Source.discover_with_inventory config ~prod ~features ~filter:None
+
+let tests =
+  "source_tests" >:: fun _context ->
+  let namespace_modules =
+    [
+      Source.
+        {
+          name = "Entry";
+          implementation = "Entry.res";
+          interface = None;
+          is_dev = false;
+        };
+      Source.
+        {
+          name = "Member";
+          implementation = "Member.res";
+          interface = None;
+          is_dev = false;
+        };
+      Source.
+        {
+          name = "Member-with-dash";
+          implementation = "Member-with-dash.res";
+          interface = None;
+          is_dev = false;
+        };
+    ]
+  in
+  check
+    (names (Source.namespace_members ~entry:(Some "Entry") namespace_modules)
+    = ["Member"])
+    "namespace membership excludes the entry and exotic module names";
+  let root = Filename.temp_file "rewatch-ocaml-sources-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      write_file (Test_support.path root "src/Main.res") "let value = 1\n";
+      write_file
+        (Test_support.path root "src/nested/NotDiscovered.res")
+        "let value = 1\n";
+      write_file (Test_support.path root "test/Test.res") "let value = 1\n";
+      write_file
+        (Test_support.path root "test/nested/Nested.res")
+        "let value = 1\n";
+      write_file (Test_support.path root "native/Native.res") "let value = 1\n";
+      write_file (Test_support.path root "native/Native.mjs") "export {}\n";
+      let config_path = Filename.concat root "rescript.json" in
+      write_file config_path
+        {|{
+          "name": "source-tests",
+          "sources": [
+            "src",
+            {"dir": "test", "type": "dev", "subdirs": true},
+            {"dir": "native", "feature": "native"}
+          ]
+        }|};
+      let config = Config.load config_path in
+      check
+        (names (discover config ()) = ["Main"; "Native"; "Nested"; "Test"])
+        "an unrestricted build includes shorthand, dev, recursive, and tagged \
+         sources";
+      check
+        (names (discover config ~prod:true ()) = ["Main"; "Native"])
+        "a production build excludes a recursive dev source";
+      check
+        (names (discover config ~features:["native"] ())
+        = ["Main"; "Native"; "Nested"; "Test"])
+        "an active leaf feature includes tagged and untagged sources";
+      check
+        (names (discover config ~features:["other"] ())
+        = ["Main"; "Nested"; "Test"])
+        "an inactive feature excludes only its tagged source";
+      check
+        (names (discover config ~filter:"^Nested\\.res$" ()) = ["Nested"])
+        "source filters match file basenames";
+      check
+        (names (discover config ~filter:"test" ()) = [])
+        "source filters do not match directory components";
+      check
+        (names (discover config ~filter:"Nested|Other" ()) = ["Nested"])
+        "source filters support Rust-style alternation";
+      check
+        (names (discover config ~filter:"(?:Nested|Other)\\.res$" ())
+        = ["Nested"])
+        "source filters support non-capturing groups";
+      check
+        (names (discover config ~filter:"Nested\\d*\\.res$" ()) = ["Nested"])
+        "source filters support shorthand classes and repetition";
+      write_file config_path
+        {|{
+          "name": "cyclic-features",
+          "sources": ["src", {"dir": "native", "feature": "a"}],
+          "features": {"a": ["b"], "b": ["a"]}
+        }|};
+      let cyclic_config = Config.load config_path in
+      check
+        (names (discover cyclic_config ()) = ["Main"; "Native"])
+        "an unrestricted feature selection does not traverse implication cycles";
+      check
+        (try
+           ignore (discover cyclic_config ~features:["a"] ());
+           false
+         with Source.Error message ->
+           Test_support.contains_text message "a -> b -> a")
+        "a restricted feature selection rejects an implication cycle";
+      let discovery = discover_with_inventory config ~features:["other"] () in
+      check
+        (List.mem
+           (Test_support.path root "src/nested/NotDiscovered.res")
+           discovery.inventory_files)
+        "cleanup inventory descends through a non-recursive source";
+      check
+        (List.mem
+           (Test_support.path root "native/Native.mjs")
+           discovery.inventory_files)
+        "cleanup inventory retains non-source files from an inactive feature";
+      check
+        (not (List.mem "NotDiscovered" (names discovery.modules)))
+        "cleanup inventory does not make nested files into source modules";
+      check
+        (List.assoc (Filename.concat "src" "Main.res") discovery.source_mtimes
+        = (Unix.stat (Test_support.path root "src/Main.res")).Unix.st_mtime)
+        "source discovery retains the metadata used by freshness checks";
+      write_file config_path
+        {|{
+          "name": "gentype-source-tests",
+          "sources": [
+            "src",
+            {"dir": "test", "type": "dev", "subdirs": true},
+            {"dir": "native", "feature": "native"}
+          ],
+          "gentypeconfig": {}
+        }|};
+      let discovery =
+        Config.load config_path |> fun config ->
+        discover_with_inventory config ~prod:true ~features:["other"] ()
+      in
+      check
+        (discovery.gentype_dirs
+        = ["src"; "test"; Filename.concat "test" "nested"])
+        "GenType directories use active features but retain dev sources";
+      write_file config_path
+        {|{
+          "name": "overlapping-source-tests",
+          "sources": ["src", {"dir": "src", "subdirs": true}],
+          "gentypeconfig": {}
+        }|};
+      let overlapping =
+        Config.load config_path |> fun config ->
+        discover_with_inventory config ()
+      in
+      check
+        (names overlapping.modules = ["Main"; "NotDiscovered"])
+        "a recursive declaration upgrades an earlier shallow traversal";
+      check
+        (overlapping.gentype_dirs = ["src"; Filename.concat "src" "nested"])
+        "GenType traversal also upgrades overlapping source coverage";
+      write_file (Test_support.path root "ignored/Nested.res") "let value = 1\n";
+      write_file config_path
+        {|{
+          "name": "unsupported-ignored-dirs",
+          "sources": {"dir": "ignored", "subdirs": true},
+          "ignored-dirs": ["ignored"]
+        }|};
+      let config = Config.load config_path in
+      check
+        (names (discover config ()) = ["Nested"])
+        "unsupported ignored-dirs does not suppress source discovery";
+      write_file (Test_support.path root "case/lower.res") "let value = 1\n";
+      write_file (Test_support.path root "case/Lower.resi") "let value: int\n";
+      write_file config_path {|{"name":"interface-case","sources":["case"]}|};
+      let config = Config.load config_path in
+      let casing_rejected =
+        try
+          ignore (discover config ());
+          false
+        with Source.Error message ->
+          Test_support.contains_text message
+            "Could not initialize build: Implementation and interface have \
+             different path names or different cases"
+      in
+      check casing_rejected
+        "implementation and interface basename casing must match";
+      write_file (Test_support.path root "case/Lower.res") "let value = 1\n";
+      let lower = Unix.stat (Test_support.path root "case/lower.res") in
+      let upper = Unix.stat (Test_support.path root "case/Lower.res") in
+      (* Case-insensitive filesystems give both spellings the same directory
+         entry, so they cannot represent the two inputs needed by this check. *)
+      (if lower.st_dev <> upper.st_dev || lower.st_ino <> upper.st_ino then
+         let duplicate_rejected =
+           try
+             ignore (discover config ());
+             false
+           with Source.Error message ->
+             Test_support.contains_text message "Duplicate module name: Lower"
+         in
+         check duplicate_rejected
+           "adding the exact implementation still exposes the \
+            differently-cased duplicate");
+      write_file (Test_support.path root "paths/a/Path.res") "let value = 1\n";
+      write_file (Test_support.path root "paths/b/Path.resi") "let value: int\n";
+      write_file config_path
+        {|{"name":"interface-path","sources":{"dir":"paths","subdirs":true}}|};
+      let config = Config.load config_path in
+      let path_rejected =
+        try
+          ignore (discover config ());
+          false
+        with Source.Error message ->
+          Test_support.contains_text message
+            "different path names or different cases"
+      in
+      check path_rejected
+        "an interface cannot attach to a same-named implementation in another \
+         directory";
+      if not Sys.win32 then (
+        write_file
+          (Test_support.path root "linked-target/Linked.res")
+          "let value = 1\n";
+        Unix.symlink
+          (Filename.concat root "linked-target")
+          (Filename.concat root "linked-source");
+        write_file config_path
+          {|{"name":"linked-source","sources":["linked-source"]}|};
+        let discovery =
+          Config.load config_path |> fun config ->
+          discover_with_inventory config ()
+        in
+        check
+          (names discovery.modules = ["Linked"])
+          "source discovery follows a configured directory symlink";
+        check
+          (discovery.inventory_files = [Filename.concat root "linked-source"])
+          "cleanup inventory retains a directory symlink as a leaf";
+        check
+          (List.mem
+             (Test_support.path root "linked-source/Linked.res")
+             discovery.present_files)
+          "freshness inventory includes files below a directory symlink";
+        File_util.ensure_dir (Filename.concat root "special");
+        Unix.mkfifo (Test_support.path root "special/Blocked.res") 0o600;
+        write_file
+          (Test_support.path root "special/Regular.res")
+          "let value = 1\n";
+        write_file config_path
+          {|{"name":"special-source","sources":["special"]}|};
+        let discovered =
+          Config.load config_path |> fun config -> discover config ()
+        in
+        check
+          (names discovered = ["Regular"])
+          "source discovery excludes FIFOs even when their names have source \
+           extensions"))
diff --git a/tests/rewatch_ounit_tests/test_env_stubs.c b/tests/rewatch_ounit_tests/test_env_stubs.c
new file mode 100644
index 00000000000..4d8c0cd32b2
--- /dev/null
+++ b/tests/rewatch_ounit_tests/test_env_stubs.c
@@ -0,0 +1,17 @@
+#include <caml/fail.h>
+#include <caml/memory.h>
+#include <caml/mlvalues.h>
+#include <stdlib.h>
+
+CAMLprim value rewatch_test_unsetenv(value name) {
+  CAMLparam1(name);
+#ifdef _WIN32
+  int result = _putenv_s(String_val(name), "");
+#else
+  int result = unsetenv(String_val(name));
+#endif
+  if (result != 0) {
+    caml_failwith("could not restore the test process environment");
+  }
+  CAMLreturn(Val_unit);
+}
diff --git a/tests/rewatch_ounit_tests/test_support.ml b/tests/rewatch_ounit_tests/test_support.ml
new file mode 100644
index 00000000000..2986c39d06f
--- /dev/null
+++ b/tests/rewatch_ounit_tests/test_support.ml
@@ -0,0 +1,29 @@
+let contains_text = String_util.contains
+
+external unsetenv : string -> unit = "rewatch_test_unsetenv"
+
+let write_file path contents =
+  File_util.ensure_dir (Filename.dirname path);
+  let channel = open_out_bin path in
+  Fun.protect
+    ~finally:(fun () -> close_out_noerr channel)
+    (fun () -> output_string channel contents)
+
+let path root relative =
+  relative |> String.split_on_char '/' |> File_util.path_of_parts root
+
+let symlink_if_supported ?to_dir target link =
+  try
+    Unix.symlink ?to_dir target link;
+    true
+  with Unix.Unix_error ((Unix.EPERM | Unix.EACCES), _, _) when Sys.win32 ->
+    false
+
+let with_temp_dir prefix action =
+  let path = Filename.temp_file prefix "" in
+  Sys.remove path;
+  Unix.mkdir path 0o755;
+  let path = Unix.realpath path in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree path)
+    (fun () -> action path)
diff --git a/tests/rewatch_ounit_tests/toolchain_tests.ml b/tests/rewatch_ounit_tests/toolchain_tests.ml
new file mode 100644
index 00000000000..1c77ab6982d
--- /dev/null
+++ b/tests/rewatch_ounit_tests/toolchain_tests.ml
@@ -0,0 +1,40 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let write_file = Test_support.write_file
+
+let tests =
+  "toolchain_tests" >:: fun _context ->
+  let root = Filename.temp_file "rewatch-ocaml-toolchain-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o755;
+  let root = Unix.realpath root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree root)
+    (fun () ->
+      let bin = Filename.concat root "bin" in
+      Unix.mkdir bin 0o755;
+      let executable = Filename.concat bin "rescript.exe" in
+      write_file executable "test executable";
+      check
+        (Toolchain.sibling_bsc_candidate ~cwd:root ~executable
+        = Filename.concat bin "bsc.exe")
+        "absolute executable paths locate sibling bsc.exe";
+      check
+        (Toolchain.sibling_bsc_candidate ~cwd:root
+           ~executable:(Filename.concat "bin" "rescript.exe")
+        = Filename.concat bin "bsc.exe")
+        "relative executable paths locate sibling bsc.exe");
+  check
+    (Platform_windows.strip_verbatim_prefix "\\\\?\\C:\\ReScript\\bin\\bsc.exe"
+    = "C:\\ReScript\\bin\\bsc.exe")
+    "Windows drive paths drop the verbatim prefix";
+  check
+    (Platform_windows.strip_verbatim_prefix "\\\\?\\UNC\\server\\share\\bsc.exe"
+    = "\\\\server\\share\\bsc.exe")
+    "Windows UNC paths preserve their network root";
+  check
+    (Platform_windows.strip_verbatim_prefix "C:\\ReScript\\bin\\bsc.exe"
+    = "C:\\ReScript\\bin\\bsc.exe")
+    "ordinary Windows paths are unchanged"
diff --git a/tests/rewatch_ounit_tests/traversal_coverage_tests.ml b/tests/rewatch_ounit_tests/traversal_coverage_tests.ml
new file mode 100644
index 00000000000..ef466d0f31c
--- /dev/null
+++ b/tests/rewatch_ounit_tests/traversal_coverage_tests.ml
@@ -0,0 +1,31 @@
+open OUnit2
+
+let admission_sequence _context =
+  let visited = Hashtbl.create 1 in
+  assert_equal Traversal_coverage.Visit_current
+    (Traversal_coverage.admit visited "directory" ~recursive:false);
+  assert_equal Traversal_coverage.Skip
+    (Traversal_coverage.admit visited "directory" ~recursive:false);
+  assert_equal Traversal_coverage.Visit_descendants
+    (Traversal_coverage.admit visited "directory" ~recursive:true);
+  assert_equal Traversal_coverage.Skip
+    (Traversal_coverage.admit visited "directory" ~recursive:true)
+
+let recursive_first_visit _context =
+  let visited = Hashtbl.create 1 in
+  let admission =
+    Traversal_coverage.admit visited "directory" ~recursive:true
+  in
+  assert_equal Traversal_coverage.Visit_current_and_descendants admission;
+  assert_bool "the current directory is visited"
+    (Traversal_coverage.visits_current admission);
+  assert_bool "descendants are visited"
+    (Traversal_coverage.visits_descendants admission)
+
+let tests =
+  "traversal_coverage_tests"
+  >::: [
+         "shallow coverage can be upgraded" >:: admission_sequence;
+         "a recursive first visit covers the complete tree"
+         >:: recursive_first_visit;
+       ]
diff --git a/tests/rewatch_ounit_tests/unit_tests.ml b/tests/rewatch_ounit_tests/unit_tests.ml
new file mode 100644
index 00000000000..e82258c2b06
--- /dev/null
+++ b/tests/rewatch_ounit_tests/unit_tests.ml
@@ -0,0 +1,1176 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+module Checked_windows_platform : module type of Platform = Platform_windows
+
+type lazy_job = First | Second
+
+let rec contains_adjacent left right = function
+  | current :: next :: _ when current = left && next = right -> true
+  | _ :: rest -> contains_adjacent left right rest
+  | [] -> false
+
+let write_file = Test_support.write_file
+
+let read_file path =
+  let channel = open_in_bin path in
+  Fun.protect
+    ~finally:(fun () -> close_in_noerr channel)
+    (fun () -> really_input_string channel (in_channel_length channel))
+
+let touch_file path = write_file path ""
+
+let wait_for_file path =
+  let rec loop attempts =
+    if Sys.file_exists path then true
+    else if attempts = 0 then false
+    else (
+      ignore (Unix.select [] [] [] 0.01);
+      loop (attempts - 1))
+  in
+  loop 200
+
+let () =
+  let argument index = Sys.argv.(index) in
+  if Array.length Sys.argv >= 2 then
+    match argument 1 with
+    | "--process-result" ->
+      print_string (argument 2);
+      prerr_string (argument 3);
+      exit (int_of_string (argument 4))
+    | "--scheduler-helper" ->
+      let root = argument 2 in
+      ignore (wait_for_file (Filename.concat root "first-started"));
+      ignore (wait_for_file (Filename.concat root "second-started"));
+      touch_file (Filename.concat root "release");
+      exit 0
+    | "--scheduler-job" ->
+      let root = argument 2 in
+      let name = argument 3 in
+      touch_file (Filename.concat root (name ^ "-started"));
+      if name <> "third" then
+        ignore (wait_for_file (Filename.concat root "release"));
+      if name = "first" then (
+        ignore (wait_for_file (Filename.concat root "third-started"));
+        if not (Sys.file_exists (Filename.concat root "third-started")) then
+          touch_file (Filename.concat root "refill-stalled"));
+      print_string name;
+      exit 0
+    | "--large-process-result" ->
+      let stdout_chunk = String.make 65536 'o' in
+      let stderr_chunk = String.make 65536 'e' in
+      for _ = 1 to 16 do
+        print_string stdout_chunk;
+        flush stdout;
+        prerr_string stderr_chunk;
+        flush stderr
+      done;
+      exit 0
+    | "--wait-forever" ->
+      while true do
+        ignore (Unix.select [] [] [] 1.)
+      done
+    | "--sleep" ->
+      Thread.delay (float_of_string (argument 2));
+      exit 0
+    | "--wait-for-release" ->
+      let root = argument 2 in
+      touch_file (Filename.concat root "child-started");
+      ignore (wait_for_file (Filename.concat root "release"));
+      exit 0
+    | "--exit-with-descendant" ->
+      let executable = Unix.realpath Sys.executable_name in
+      ignore
+        (Spawn.spawn ~prog:executable ~argv:[executable; "--sleep"; "2"] ());
+      let marker = argument 2 in
+      let pending = marker ^ ".pending" in
+      write_file pending (Unix.getpid () |> string_of_int);
+      Unix.rename pending marker;
+      exit 0
+    | "-format" -> (
+      if Sys.win32 then set_binary_mode_out stdout true;
+      match
+        ( Sys.getenv_opt "REWATCH_FORMAT_TEST_ROOT",
+          Sys.getenv_opt "REWATCH_FORMAT_INVENTORY_TEST_ROOT" )
+      with
+      | _, Some root ->
+        let source = argument 2 in
+        let marker = Digest.string source |> Digest.to_hex in
+        touch_file (Filename.concat root marker);
+        print_string (read_file source);
+        exit 0
+      | None, None -> ()
+      | Some root, None ->
+        let source = argument 2 in
+        touch_file
+          (Filename.concat root (Filename.basename source ^ ".started"));
+        let first_started = Filename.concat root "First.res.started" in
+        let second_started = Filename.concat root "Second.res.started" in
+        if not (wait_for_file first_started && wait_for_file second_started)
+        then exit 2;
+        print_string (read_file source);
+        exit 0)
+    | _ -> ()
+
+let test_executable () = Unix.realpath Sys.executable_name
+
+let process_job args =
+  {Process.program = test_executable (); args; cwd = Sys.getcwd ()}
+
+let feature_request_tests _context =
+  let feature_requests =
+    Package_traversal.For_test.create_feature_requests ()
+  in
+  Package_traversal.For_test.add_feature_request feature_requests "package"
+    (Some ["browser"]);
+  Package_traversal.For_test.add_feature_request feature_requests "package"
+    (Some ["native"; "browser"]);
+  check
+    (Package_traversal.For_test.find_feature_selection feature_requests
+       "package"
+    = Some (Package_traversal.Selected_features ["browser"; "native"]))
+    "feature requests merge and deduplicate named selections";
+  Package_traversal.For_test.add_feature_request feature_requests "package" None;
+  Package_traversal.For_test.add_feature_request feature_requests "package"
+    (Some ["ignored"]);
+  check
+    (Package_traversal.For_test.find_feature_selection feature_requests
+       "package"
+    = Some Package_traversal.All_features)
+    "an unrestricted feature request dominates named selections"
+
+let string_util_tests _context =
+  [
+    ("", "", true);
+    ("value", "", true);
+    ("", "value", false);
+    ("abc", "abc", true);
+    ("prefix-value", "prefix", true);
+    ("value-suffix", "suffix", true);
+    ("abababca", "ababca", true);
+    ("aaaaaaaaab", "aaaab", true);
+    ("aaaaaaaaaa", "aaaab", false);
+    ("short", "longer", false);
+  ]
+  |> List.iter (fun (value, substring, expected) ->
+      check
+        (String_util.contains value substring = expected)
+        (Printf.sprintf "substring search for %S in %S" substring value))
+
+let process_parallel_tests _context =
+  let test_executable = test_executable () in
+  check
+    (Process.default_max_jobs >= 1 && Process.default_max_jobs <= 32)
+    "parallel subprocess bound follows the available CPUs";
+  let completed_indices = ref [] in
+  let parallel_results =
+    Process.run_parallel ~max_jobs:2
+      ~on_complete:(fun index ->
+        completed_indices := index :: !completed_indices)
+      [
+        process_job ["--process-result"; "first"; ""; "0"];
+        process_job ["--process-result"; "second"; ""; "0"];
+        process_job ["--process-result"; "third"; ""; "0"];
+      ]
+  in
+  check
+    (List.map (fun (result : Process.result) -> result.stdout) parallel_results
+    = ["first"; "second"; "third"])
+    "parallel subprocess results retain input order";
+  check
+    (List.sort compare !completed_indices = [0; 1; 2])
+    "parallel subprocess completion reports every input index once";
+  let recycled_handle_results =
+    List.init 64 (fun index ->
+        process_job ["--process-result"; string_of_int index; ""; "0"])
+    |> Process.run_parallel ~max_jobs:4
+  in
+  check
+    (List.mapi
+       (fun index (result : Process.result) ->
+         result.stdout = string_of_int index)
+       recycled_handle_results
+    |> List.for_all Fun.id)
+    "rapid subprocess completion cannot confuse recycled Windows handles";
+  let lazy_root = Filename.temp_file "rewatch-lazy-jobs-" "" in
+  Sys.remove lazy_root;
+  Unix.mkdir lazy_root 0o755;
+  let lazy_results =
+    Fun.protect
+      ~finally:(fun () -> File_util.remove_tree lazy_root)
+      (fun () ->
+        Process.run_parallel_map ~max_jobs:2 [First; Second] ~job:(function
+          | First -> process_job ["--wait-for-release"; lazy_root]
+          | Second ->
+            check
+              (wait_for_file (Filename.concat lazy_root "child-started"))
+              "later job preparation overlaps an already-running child";
+            touch_file (Filename.concat lazy_root "release");
+            process_job ["--process-result"; "second"; ""; "0"]))
+  in
+  check
+    (List.map (fun (result : Process.result) -> result.stdout) lazy_results
+    = [""; "second"])
+    "demand-built subprocess results retain input order";
+  let large_result =
+    Process.run ~cwd:(Sys.getcwd ()) test_executable ["--large-process-result"]
+  in
+  check
+    (Process.succeeded large_result
+    && String.length large_result.stdout = 1024 * 1024
+    && String.length large_result.stderr = 1024 * 1024)
+    "stdout and stderr pipes are drained concurrently without truncation";
+  let invalid_parallel_bound_rejected =
+    try
+      ignore (Process.run_parallel ~max_jobs:0 []);
+      false
+    with Process.Error _ -> true
+  in
+  check invalid_parallel_bound_rejected "parallel subprocess bound is validated"
+
+let graph_work key dependencies = Process.{key; dependencies; value = key}
+
+let process_cancellation_tests _context =
+  let test_executable = test_executable () in
+  let cancellation_polls = ref 0 in
+  let dependency_graph_cancelled =
+    let exception Cancel in
+    try
+      Process.run_dependency_graph
+        [graph_work "cancel" []]
+        ~poll:(fun () ->
+          incr cancellation_polls;
+          if !cancellation_polls = 2 then raise Cancel)
+        ~next:(fun _ result ->
+          match result with
+          | None -> Some (Process.task (process_job ["--wait-forever"]))
+          | Some _ -> None);
+      false
+    with Cancel -> true
+  in
+  check dependency_graph_cancelled
+    "dependency scheduler cancellation terminates active subprocesses";
+  let fatal_finalizer_started = Unix.gettimeofday () in
+  let fatal_finalizer_cancels_other_workers =
+    try
+      Process.run_dependency_graph ~max_jobs:2
+        [graph_work "failure" []; graph_work "waiting" []]
+        ~next:(fun key result ->
+          match result with
+          | None ->
+            let arguments =
+              if key = "failure" then ["--process-result"; ""; ""; "0"]
+              else ["--wait-forever"]
+            in
+            Some
+              (Process.task
+                 ~on_result:(fun result ->
+                   if key = "failure" then raise (Process.Interrupted 130)
+                   else result)
+                 (process_job arguments))
+          | Some _ -> None);
+      false
+    with Process.Interrupted 130 -> true
+  in
+  check
+    (fatal_finalizer_cancels_other_workers
+    && Unix.gettimeofday () -. fatal_finalizer_started < 2.)
+    "a fatal worker finalizer cancels other process trees without deadlock";
+  let process_polls = ref 0 in
+  let process_cancelled =
+    let exception Cancel in
+    try
+      ignore
+        (Process.run ~cwd:(Sys.getcwd ()) test_executable ["--wait-forever"]
+           ~poll:(fun () ->
+             incr process_polls;
+             if !process_polls = 2 then raise Cancel));
+      false
+    with Cancel -> true
+  in
+  check process_cancelled
+    "single subprocess cancellation terminates the active process";
+  if not Sys.win32 then
+    Test_support.with_temp_dir "rewatch-descendant-pipe-" (fun root ->
+        let marker = Filename.concat root "parent-exiting" in
+        let parent_pid = ref None in
+        let parent_has_exited pid =
+          try
+            Unix.kill pid 0;
+            false
+          with Unix.Unix_error (Unix.ESRCH, _, _) -> true
+        in
+        let started = Unix.gettimeofday () in
+        let descendant_pipe_cancelled =
+          let exception Cancel in
+          try
+            Process.run_dependency_graph
+              [graph_work "descendant-pipe" []]
+              ~poll:(fun () ->
+                (match !parent_pid with
+                | None when Sys.file_exists marker ->
+                  parent_pid := Some (read_file marker |> int_of_string)
+                | None | Some _ -> ());
+                if
+                  match !parent_pid with
+                  | Some pid -> parent_has_exited pid
+                  | None -> false
+                then raise Cancel)
+              ~next:(fun _ result ->
+                match result with
+                | None ->
+                  Some
+                    (Process.task
+                       (process_job ["--exit-with-descendant"; marker]))
+                | Some _ -> None);
+            false
+          with Cancel -> true
+        in
+        check
+          (descendant_pipe_cancelled && Unix.gettimeofday () -. started < 1.)
+          "an exited parent with descendant-held pipes remains cancellable")
+
+let process_dependency_graph_tests _context =
+  let graph_completion_order = ref [] in
+  let graph_completed = Hashtbl.create 3 in
+  Process.run_dependency_graph ~max_jobs:1
+    [graph_work "c" []; graph_work "b" ["a"]; graph_work "a" []]
+    ~next:(fun key result ->
+      match result with
+      | None ->
+        if key = "b" then
+          check
+            (Hashtbl.mem graph_completed "a")
+            "dependency work starts only after its prerequisite completes";
+        Some (Process.task (process_job ["--process-result"; key; ""; "0"]))
+      | Some result ->
+        check
+          (Process.succeeded result && result.stdout = key)
+          "dependency scheduler collects subprocess output";
+        Hashtbl.add graph_completed key ();
+        graph_completion_order := key :: !graph_completion_order;
+        None);
+  check
+    (List.rev !graph_completion_order = ["a"; "b"; "c"])
+    "dependency scheduler prioritizes the longest ready path";
+  let finalizers_entered = Atomic.make 0 in
+  let parallel_finalizers_completed =
+    let exception Finalizers_serialized in
+    try
+      Process.run_dependency_graph ~max_jobs:2
+        [graph_work "first" []; graph_work "second" []]
+        ~next:(fun _ result ->
+          match result with
+          | Some _ -> None
+          | None ->
+            Some
+              (Process.task
+                 ~on_result:(fun result ->
+                   ignore (Atomic.fetch_and_add finalizers_entered 1);
+                   let deadline = Unix.gettimeofday () +. 2. in
+                   while
+                     Atomic.get finalizers_entered < 2
+                     && Unix.gettimeofday () < deadline
+                   do
+                     Thread.delay 0.001
+                   done;
+                   if Atomic.get finalizers_entered < 2 then
+                     raise Finalizers_serialized;
+                   result)
+                 (process_job ["--process-result"; ""; ""; "0"])));
+      true
+    with Finalizers_serialized -> false
+  in
+  check parallel_finalizers_completed
+    "independent subprocess finalizers run before scheduler dispatch resumes";
+  let graph_cycle_rejected =
+    try
+      Process.run_dependency_graph
+        [graph_work "a" ["b"]; graph_work "b" ["a"]]
+        ~next:(fun _ _ -> None);
+      false
+    with Process.Error _ -> true
+  in
+  check graph_cycle_rejected "subprocess dependency cycles are rejected";
+  let drained_failures = ref 0 in
+  let deterministic_failure =
+    try
+      Process.run_dependency_graph ~max_jobs:2
+        [graph_work "z" []; graph_work "a" []]
+        ~next:(fun key result ->
+          match result with
+          | None ->
+            Some (Process.task (process_job ["--process-result"; ""; ""; "1"]))
+          | Some _ ->
+            incr drained_failures;
+            raise (Failure key));
+      None
+    with Failure key -> Some key
+  in
+  check
+    (!drained_failures = 2 && deterministic_failure = Some "a")
+    "dependency scheduler drains active work and reports errors \
+     deterministically";
+  let started = ref [] in
+  let continued_independent_work =
+    try
+      Process.run_dependency_graph ~max_jobs:1
+        [graph_work "a" []; graph_work "b" []; graph_work "c" ["a"]]
+        ~on_failure:(fun _ -> Process.Continue_independent_work)
+        ~next:(fun key result ->
+          match result with
+          | None ->
+            started := key :: !started;
+            Some (Process.task (process_job ["--process-result"; ""; ""; "0"]))
+          | Some _ when key = "a" -> raise (Failure key)
+          | Some _ -> None);
+      false
+    with
+    | Failure key when key = "a" -> true
+    | _ -> false
+  in
+  check
+    (continued_independent_work && List.mem "b" !started
+    && not (List.mem "c" !started))
+    "dependency scheduler can finish independent work after a failure without \
+     starting its dependents"
+
+let platform_tests _context =
+  let test_executable = test_executable () in
+  let path_root = Filename.temp_file "rewatch-ocaml-path-" "" in
+  Sys.remove path_root;
+  Unix.mkdir path_root 0o755;
+  let path_root = Unix.realpath path_root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree path_root)
+    (fun () ->
+      let first = Filename.concat path_root "first" in
+      let second = Filename.concat path_root "second" in
+      Unix.mkdir first 0o755;
+      Unix.mkdir second 0o755;
+      let command = if Sys.win32 then "worker.exe" else "worker" in
+      Unix.mkdir (Filename.concat first command) 0o755;
+      let executable = Filename.concat second command in
+      File_util.copy_existing_file ~ensure_parent:true test_executable
+        executable;
+      Unix.chmod executable 0o755;
+      let previous_path = Sys.getenv_opt "PATH" in
+      let separator = if Sys.win32 then ";" else ":" in
+      Unix.putenv "PATH" (first ^ separator ^ second);
+      Fun.protect
+        ~finally:(fun () ->
+          Unix.putenv "PATH" (Option.value previous_path ~default:""))
+        (fun () ->
+          let requested = if Sys.win32 then "worker" else command in
+          check
+            (Platform.normalize_path_for_comparison
+               (Platform.resolve_program ~cwd:path_root requested)
+            = Platform.normalize_path_for_comparison executable)
+            "PATH lookup skips directories and applies platform executable \
+             suffixes";
+          check
+            (Platform.resolve_program ~cwd:path_root
+               (Filename.concat "." requested)
+            = Filename.concat "." requested)
+            "an explicit current-directory executable does not use PATH";
+          if Sys.win32 then (
+            let cwd_executable = Filename.concat path_root "current.exe" in
+            File_util.copy_existing_file ~ensure_parent:true test_executable
+              cwd_executable;
+            check
+              (Platform.normalize_path_for_comparison
+                 (Platform.resolve_program ~cwd:path_root "current")
+              = Platform.normalize_path_for_comparison cwd_executable)
+              "Windows executable lookup searches cwd with PATHEXT")
+          else
+            let literal_directory = Filename.concat path_root "literal " in
+            let trimmed_directory = Filename.concat path_root "literal" in
+            Unix.mkdir literal_directory 0o755;
+            Unix.mkdir trimmed_directory 0o755;
+            let literal_executable =
+              Filename.concat literal_directory command
+            in
+            let trimmed_executable =
+              Filename.concat trimmed_directory command
+            in
+            File_util.copy_existing_file ~ensure_parent:true test_executable
+              literal_executable;
+            File_util.copy_existing_file ~ensure_parent:true test_executable
+              trimmed_executable;
+            Unix.chmod literal_executable 0o755;
+            Unix.chmod trimmed_executable 0o755;
+            Unix.putenv "PATH" literal_directory;
+            check
+              (Platform.resolve_program ~cwd:path_root command
+              = literal_executable)
+              "Unix PATH lookup preserves whitespace in directory names"));
+  check
+    (Platform_windows.tasklist_probe ~pid:123
+       {|"rescript.exe","123","Console","1","10,000 K"|}
+    = Platform_windows.Process_found)
+    "Windows tasklist output recognizes a matching ReScript process";
+  check
+    (Platform_windows.tasklist_probe ~pid:124
+       {|"rescript.exe","123","Console","1","10,000 K"|}
+    = Platform_windows.Process_absent)
+    "Windows tasklist output rejects a different process ID";
+  check
+    (Platform_windows.tasklist_probe ~pid:123 "tasklist failed"
+    = Platform_windows.Malformed_output)
+    "malformed Windows tasklist output is inconclusive";
+  check
+    (Platform_windows.tasklist_probe ~pid:123 {|"tasklist failed"|}
+    = Platform_windows.Malformed_output)
+    "unexpected Windows tasklist CSV schema is inconclusive";
+  check
+    (Platform_windows.tasklist_probe ~pid:123 {|"rescript.exe","12|}
+    = Platform_windows.Malformed_output)
+    "truncated Windows tasklist CSV is inconclusive";
+  check
+    (Platform_windows.process_is_active ~run:(fun _ _ -> None) "123")
+    "a failed Windows tasklist probe conservatively preserves the lock";
+  check
+    (Platform_windows.program_for_working_directory ~cwd:"project"
+       "tools/compiler.exe"
+    = Filename.concat "project" "tools/compiler.exe")
+    "Windows resolves relative executable paths against the child working \
+     directory";
+  check
+    (Platform_windows.program_for_working_directory ~cwd:"project"
+       "/toolchain/compiler.exe"
+    = "/toolchain/compiler.exe")
+    "Windows preserves absolute executable paths";
+  check
+    (Platform_windows.serialize_command_line ~shell_command_is_quoted:false
+       ~program:"cmd.exe"
+       ~args:["/D"; "/V:OFF"; "/S"; "/C"; {|echo "hello world"|}]
+    = {|cmd.exe /D /V:OFF /S /C "echo "hello world""|})
+    "Windows wraps the complete cmd.exe command for /S parsing";
+  let quoted_batch =
+    Filename.quote_command {|C:\Program Files\tool.cmd|} ["--version"]
+  in
+  check
+    (Platform_windows.serialize_command_line ~shell_command_is_quoted:true
+       ~program:"cmd.exe"
+       ~args:["/D"; "/V:OFF"; "/S"; "/C"; quoted_batch]
+    = "cmd.exe /D /V:OFF /S /C " ^ quoted_batch)
+    "Windows does not wrap an already quoted batch command twice";
+  if Sys.win32 then
+    check
+      (Process.run ~cwd:(Sys.getcwd ()) "npm.cmd" ["--version"]
+      |> Process.succeeded)
+      "Windows launches a batch executable resolved under Program Files"
+
+let scheduler_tests _context =
+  let test_executable = test_executable () in
+  let scheduler_root = Filename.temp_file "rewatch-ocaml-scheduler-" "" in
+  Sys.remove scheduler_root;
+  Unix.mkdir scheduler_root 0o755;
+  let scheduler_root = Unix.realpath scheduler_root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree scheduler_root)
+    (fun () ->
+      let helper =
+        Spawn.spawn ~prog:test_executable
+          ~argv:[test_executable; "--scheduler-helper"; scheduler_root]
+          ()
+      in
+      let job name =
+        {
+          Process.program = test_executable;
+          args = ["--scheduler-job"; scheduler_root; name];
+          cwd = scheduler_root;
+        }
+      in
+      let results =
+        Process.run_parallel ~max_jobs:2
+          [job "first"; job "second"; job "third"]
+      in
+      let _, helper_status = Unix.waitpid [] helper in
+      check (helper_status = Unix.WEXITED 0) "scheduler test helper exits";
+      check
+        (not
+           (Sys.file_exists (Filename.concat scheduler_root "refill-stalled")))
+        "parallel scheduler refills a completed slot immediately";
+      check
+        (List.map (fun (result : Process.result) -> result.stdout) results
+        = ["first"; "second"; "third"])
+        "dynamically scheduled results retain input order";
+      let failure =
+        Process.run_parallel ~max_jobs:1
+          [process_job ["--process-result"; "partial"; "diagnostic"; "7"]]
+        |> List.hd
+      in
+      check
+        (failure.status = Unix.WEXITED 7
+        && failure.stdout = "partial"
+        && failure.stderr = "diagnostic")
+        "parallel subprocess failures preserve status and output";
+      check
+        (Sys.readdir scheduler_root
+        |> Array.for_all (fun name ->
+            not (String.starts_with ~prefix:".rewatch-ocaml-" name)))
+        "pipe capture creates no temporary scheduler logs")
+
+let graph_and_diagnostic_tests _context =
+  let node name deps = (name, deps) in
+  let reject_invalid =
+    Graph.Reject_invalid
+      {
+        duplicate_node = (fun name -> Failure ("duplicate:" ^ name));
+        unknown_dependency =
+          (fun ~node ~dependency -> Failure (node ^ "->" ^ dependency));
+      }
+  in
+  let indexed =
+    Graph.create_index
+      [node "A" ["B"; "B"]; node "B" []]
+      ~name:fst ~deps:snd ~validation:reject_invalid
+  in
+  check
+    (Graph.dependencies indexed "A" = ["B"]
+    && Graph.dependents indexed "B" = ["A"]
+    && Graph.dependency_count indexed "A" = 1)
+    "the shared graph index normalizes forward, reverse, and pending edges";
+  assert_raises (Failure "duplicate:A") (fun () ->
+      ignore
+        (Graph.create_index
+           [node "A" []; node "A" []]
+           ~name:fst ~deps:snd ~validation:reject_invalid));
+  assert_raises (Failure "A->Missing") (fun () ->
+      ignore
+        (Graph.create_index
+           [node "A" ["Missing"]]
+           ~name:fst ~deps:snd ~validation:reject_invalid));
+  let shortest_cycle =
+    Graph.shortest_cycle
+      [
+        node "LongA" ["LongB"];
+        node "LongB" ["LongC"];
+        node "LongC" ["LongA"];
+        node "ShortA" ["ShortB"];
+        node "ShortB" ["ShortA"];
+      ]
+      ~name:fst ~deps:snd
+    |> Option.value ~default:[]
+  in
+  check
+    (shortest_cycle = ["ShortA"; "ShortB"; "ShortA"])
+    "cycle diagnostics select the shortest cycle deterministically";
+  let long_tail =
+    List.init 2_000 (fun index ->
+        let name = Printf.sprintf "Tail%04d" index in
+        let dependency =
+          if index = 1_999 then "CycleA"
+          else Printf.sprintf "Tail%04d" (index + 1)
+        in
+        node name [dependency])
+  in
+  let tail_cycle =
+    Graph.shortest_cycle
+      (long_tail
+      @ [
+          node "CycleA" ["CycleB"];
+          node "CycleB" ["CycleC"];
+          node "CycleC" ["CycleA"];
+        ])
+      ~name:fst ~deps:snd
+  in
+  check
+    (tail_cycle = Some ["CycleA"; "CycleB"; "CycleC"; "CycleA"])
+    "cycle search ignores an acyclic tail leading into a longer cycle";
+  let cycle_blocked =
+    Graph.cycle_blocked_nodes
+      [
+        node "A" ["B"];
+        node "B" ["A"];
+        node "C" ["D"];
+        node "D" ["C"];
+        node "Dependent" ["A"];
+        node "TransitiveDependent" ["Dependent"];
+        node "Unrelated" [];
+      ]
+      ~name:fst ~deps:snd
+    |> List.map fst |> List.sort String.compare
+  in
+  check
+    (cycle_blocked = ["A"; "B"; "C"; "D"; "Dependent"; "TransitiveDependent"])
+    "a linear scheduling pass retains every cycle and its dependents";
+  if not Sys.win32 then (
+    let temporary = Filename.temp_file "rewatch-ocaml-package-path-" "" in
+    Sys.remove temporary;
+    Unix.mkdir temporary 0o755;
+    let temporary = Unix.realpath temporary in
+    let package = Filename.concat temporary "package" in
+    let node_modules = Filename.concat temporary "node_modules" in
+    Unix.mkdir package 0o755;
+    Unix.mkdir node_modules 0o755;
+    Unix.symlink package (Filename.concat node_modules "dependency");
+    Fun.protect
+      ~finally:(fun () ->
+        Sys.remove (Filename.concat node_modules "dependency");
+        Unix.rmdir node_modules;
+        Unix.rmdir package;
+        Unix.rmdir temporary)
+      (fun () ->
+        match Project_context.dependency_path temporary "dependency" with
+        | Some resolved ->
+          check
+            (resolved = Unix.realpath package)
+            "dependency paths are canonicalized"
+        | None -> failwith "dependency symlink was not resolved"));
+  Test_support.with_temp_dir "rewatch-compiler-log-" (fun root ->
+      File_util.ensure_dir (Build_artifacts.lib_path root "ocaml");
+      Compiler_log.initialize root;
+      Compiler_log.append root "plain \027[1;31mred\027[0m text";
+      Compiler_log.finalize root;
+      check
+        (String_util.contains
+           (File_util.read_file
+              (File_util.path_of_parts root ["lib"; "ocaml"; ".compiler.log"]))
+           "plain red text")
+        "published compiler logs strip ANSI sequences");
+  let truncated_utf8 =
+    "Warning " ^ String.make 1 (Char.chr 0xe2) ^ String.make 1 (Char.chr 0x80)
+  in
+  let decoded = Process.decode_utf8_lossy truncated_utf8 in
+  check
+    (String.starts_with ~prefix:"Warning " decoded
+    && String.is_valid_utf_8 decoded)
+    "compiler output is decoded as lossy UTF-8";
+  check
+    (Compiler_process.retain_critical_external_warnings
+       "\n  Warning number 26\n  foo.res:1:1\n\n  unused variable x.\n"
+    = "")
+    "ordinary external warnings are suppressed";
+  let critical_marker = "`(. ...)` uncurried syntax" in
+  let mixed_warnings line_ending =
+    String.concat ""
+      [
+        line_ending;
+        "  Warning number 26";
+        line_ending;
+        "  unused variable x.";
+        line_ending;
+        line_ending;
+        line_ending;
+        "  Warning number 3";
+        line_ending;
+        "  deprecated: The ";
+        critical_marker;
+        " is deprecated.";
+        line_ending;
+      ]
+  in
+  List.iter
+    (fun line_ending ->
+      let kept =
+        Compiler_process.retain_critical_external_warnings
+          (mixed_warnings line_ending)
+      in
+      check
+        (Test_support.contains_text kept critical_marker
+        && not (Test_support.contains_text kept "unused variable"))
+        "critical external warnings are retained without unrelated warnings")
+    ["\n"; "\r\n"];
+  check
+    (not (Package_traversal.source_discovery_prod ~prod:false ~is_local:true))
+    "development sources are enabled for a local development build";
+  check
+    (Package_traversal.source_discovery_prod ~prod:true ~is_local:true)
+    "production builds exclude local development sources";
+  check
+    (Package_traversal.source_discovery_prod ~prod:false ~is_local:false)
+    "installed dependencies always exclude development sources";
+  check (Build_lock.valid_owner "0") "zero is a valid serialized u32 owner";
+  check
+    (not
+       (Platform.process_is_active "0" ~run:(fun _ _ ->
+            assert_failure "PID zero must not invoke a process-name probe")))
+    "PID zero cannot own a live build lock";
+  check
+    (Build_lock.valid_owner "4294967295")
+    "the maximum u32 is a valid serialized lock owner";
+  check (not (Build_lock.valid_owner "")) "an empty lock owner is malformed";
+  check (not (Build_lock.valid_owner "-1")) "a negative lock owner is malformed";
+  check
+    (not (Build_lock.valid_owner "4294967296"))
+    "a lock owner outside the Rust u32 range is malformed";
+  check
+    (not (Build_lock.valid_owner "123\n"))
+    "trailing data in a lock owner is malformed"
+
+let lock_tests _context =
+  let test_executable = test_executable () in
+  let lock_root = Filename.temp_file "rewatch-ocaml-stale-lock-" "" in
+  Sys.remove lock_root;
+  Unix.mkdir lock_root 0o755;
+  let lock_root = Unix.realpath lock_root in
+  let lock_dir = Filename.concat lock_root "lib" in
+  Unix.mkdir lock_dir 0o755;
+  let lock = Filename.concat lock_dir "build.lock" in
+  let takeover = lock ^ ".takeover" in
+  let write_owner path owner =
+    let channel = open_out path in
+    Fun.protect
+      ~finally:(fun () -> close_out_noerr channel)
+      (fun () -> output_string channel owner)
+  in
+  write_owner lock "999999999";
+  write_owner takeover "999999999";
+  let lock_owner_pid = ref None in
+  Fun.protect
+    ~finally:(fun () ->
+      Option.iter
+        (fun pid ->
+          touch_file (Filename.concat lock_root "release");
+          try ignore (Unix.waitpid [] pid) with Unix.Unix_error _ -> ())
+        !lock_owner_pid;
+      File_util.remove_tree lock_root)
+    (fun () ->
+      Build_lock.with_build lock_root (fun ~release ->
+          check
+            (Build_lock.read_owner lock
+            = Some (string_of_int (Platform.current_process_id ())))
+            "stale build lock is replaced";
+          check
+            (not (Sys.file_exists takeover))
+            "stale takeover marker is removed";
+          let candidates =
+            Sys.readdir lock_dir |> Array.to_list
+            |> List.filter (String.starts_with ~prefix:".build-lock-")
+          in
+          check (candidates = []) "lock candidate is removed before build work";
+          release ();
+          check
+            (not (Sys.file_exists lock))
+            "build lock is released immediately";
+          release ();
+          check
+            (not (Sys.file_exists lock))
+            "releasing a build lock twice is harmless");
+      check (not (Sys.file_exists lock)) "released build lock is removed";
+      let owner_executable =
+        Filename.concat lock_root "rescript-lock-owner.exe"
+      in
+      File_util.copy_existing_file ~ensure_parent:false test_executable
+        owner_executable;
+      if not Sys.win32 then Unix.chmod owner_executable 0o755;
+      let owner_pid =
+        Spawn.spawn ~prog:owner_executable
+          ~argv:[owner_executable; "--wait-for-release"; lock_root]
+          ()
+      in
+      lock_owner_pid := Some owner_pid;
+      check
+        (wait_for_file (Filename.concat lock_root "child-started"))
+        "the active lock owner starts";
+      write_owner lock (string_of_int owner_pid);
+      let lock_polls = ref 0 in
+      let lock_wait_cancelled =
+        let exception Cancel in
+        try
+          Build_lock.with_build lock_root
+            ~poll:(fun () ->
+              incr lock_polls;
+              if !lock_polls = 2 then raise Cancel)
+            (fun ~release:_ -> check false "active lock was acquired");
+          false
+        with Cancel -> true
+      in
+      check lock_wait_cancelled "waiting for a build lock remains cancellable";
+      check
+        (Build_lock.read_owner lock = Some (string_of_int owner_pid))
+        "cancelling a lock wait preserves the active owner's lock";
+      touch_file (Filename.concat lock_root "release");
+      ignore (Unix.waitpid [] owner_pid);
+      lock_owner_pid := None;
+      File_util.remove_file lock)
+
+let config_tests _context =
+  let config_root = Filename.temp_file "rewatch-ocaml-config-" "" in
+  Sys.remove config_root;
+  Unix.mkdir config_root 0o755;
+  let config_root = Unix.realpath config_root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree config_root)
+    (fun () ->
+      let config_path = Filename.concat config_root "rescript.json" in
+      write_file config_path {|{"name":"file-casing","namespace":"FileCasing"}|};
+      let file_casing_config = Config.load config_path in
+      check
+        (Source.compiler_asset_basename file_casing_config "src/produce.res"
+        = "produce-FileCasing")
+        "compiler artifact basename preserves source filename case";
+      write_file config_path
+        {|{
+          "name": "restricted",
+          "allowed-dependents": ["app"]
+        }|};
+      let config = Config.load config_path in
+      check
+        (config.allowed_dependents = Some ["app"])
+        "allowed-dependents is parsed";
+      write_file config_path
+        {|{
+          "name": "source-type",
+          "sources": {"dir": "src", "type": "lib"}
+        }|};
+      let config = Config.load config_path in
+      check
+        (match config.sources with
+        | [source] -> not source.is_dev
+        | _ -> false)
+        "non-dev source type strings are accepted as ordinary sources";
+      write_file config_path
+        {|{
+          "name": "source-type-inheritance",
+          "sources": {
+            "dir": "src",
+            "subdirs": [{"dir": "test", "type": "dev"}]
+          }
+        }|};
+      let config = Config.load config_path in
+      check
+        (match config.sources with
+        | [_parent; child] -> not child.is_dev
+        | _ -> false)
+        "an ordinary parent source overrides a nested dev type";
+      write_file config_path
+        {|{
+          "name": "source-type-inheritance",
+          "sources": {
+            "dir": "src",
+            "type": "dev",
+            "subdirs": [{"dir": "lib", "type": "lib"}]
+          }
+        }|};
+      let config = Config.load config_path in
+      check
+        (match config.sources with
+        | [_parent; child] -> child.is_dev
+        | _ -> false)
+        "a dev parent source overrides a nested non-dev type";
+      write_file config_path {|{"name":"default-output","suffix":".mjs"}|};
+      let config = Config.load config_path in
+      check
+        (match config.package_specs with
+        | [spec] -> Config.package_spec_suffix config spec = ".js"
+        | _ -> false)
+        "package-specs default output suffix is .js";
+      write_file config_path
+        {|{"name":"legacy-output","package-specs":{"module":"cjs"}}|};
+      let config = Config.load config_path in
+      check
+        (List.exists
+           (fun message -> Test_support.contains_text message "module 'cjs'")
+           config.diagnostics)
+        "legacy package module alias is diagnosed";
+      write_file config_path
+        {|{"name":"missing-module","package-specs":{"in-source":true}}|};
+      let missing_module_rejected =
+        try
+          ignore (Config.load config_path);
+          false
+        with Config.Error message ->
+          Test_support.contains_text message "missing field \"module\""
+      in
+      check missing_module_rejected "package output module is required";
+      write_file config_path
+        {|{
+          "name": "duplicate-output",
+          "package-specs": [
+            {"module": "esmodule", "suffix": ".js"},
+            {"module": "commonjs", "suffix": ".js"}
+          ]
+        }|};
+      let duplicate_rejected =
+        try
+          ignore (Config.load config_path);
+          false
+        with Config.Error message ->
+          Test_support.contains_text message "Duplicate package-spec suffix"
+      in
+      check duplicate_rejected "duplicate package output is rejected";
+      write_file config_path {|{"name":"source-map","sourceMap":true}|};
+      let boolean_source_map_rejected =
+        try
+          ignore (Config.load config_path);
+          false
+        with Config.Error message ->
+          Test_support.contains_text message "sourceMap true is unsupported"
+      in
+      check boolean_source_map_rejected "sourceMap true is rejected";
+      write_file config_path
+        {|{"name":"source-map","sourceMap":{"mode":"linked"}}|};
+      let missing_source_map_enabled_rejected =
+        try
+          ignore (Config.load config_path);
+          false
+        with Config.Error message ->
+          Test_support.contains_text message "missing field \"enabled\""
+      in
+      check missing_source_map_enabled_rejected "sourceMap enabled is required";
+      write_file config_path
+        {|{
+          "name": "source-map",
+          "sourceMap": {"enabled": "dev", "mode": "linked"}
+        }|};
+      let config = Config.load config_path in
+      check config.source_map_dev "sourceMap dev mode is parsed";
+      check
+        (contains_adjacent "-bs-source-map" "false"
+           (Compiler_args.compiler_flags ~source_maps:true ~watch:false
+              ~gentype:false config))
+        "sourceMap dev mode is disabled for one-shot builds";
+      check
+        (contains_adjacent "-bs-source-map" "linked"
+           (Compiler_args.compiler_flags ~source_maps:true ~watch:true
+              ~gentype:false config))
+        "sourceMap dev mode is enabled for watch builds";
+      write_file config_path
+        {|{
+          "name": "source-map",
+          "sourceMap": {"enabled": "always", "mode": "inline"}
+        }|};
+      let config = Config.load config_path in
+      check (not config.source_map_dev) "sourceMap always mode is parsed";
+      check
+        (contains_adjacent "-bs-source-map" "inline"
+           (Compiler_args.compiler_flags ~source_maps:true ~watch:false
+              ~gentype:false config))
+        "sourceMap always mode is enabled for one-shot builds";
+      Sys.remove config_path;
+      let legacy_path = Filename.concat config_root "bsconfig.json" in
+      write_file legacy_path {|{"name":"legacy-config"}|};
+      let config = Config.load_root config_root in
+      check (config.path = legacy_path) "bsconfig.json is used as a fallback";
+      check
+        (List.exists
+           (fun message ->
+             Test_support.contains_text message "filename 'bsconfig.json'")
+           config.diagnostics)
+        "bsconfig.json emits a deprecation diagnostic";
+      write_file config_path {|{"name":"current-config"}|};
+      let config = Config.load_root config_root in
+      check
+        (config.path = config_path)
+        "rescript.json takes precedence over bsconfig.json";
+      write_file config_path
+        {|{
+          "name": "gentype-defaults",
+          "package-specs": {"module": "commonjs"},
+          "gentypeconfig": {}
+        }|};
+      let config = Config.load config_path in
+      check
+        (contains_adjacent "-bs-gentype-module" "commonjs" config.gentype_args)
+        "GenType inherits object package module";
+      check
+        (not (List.mem "-bs-gentype-suffix" config.gentype_args))
+        "GenType omits an unconfigured suffix";
+      write_file config_path
+        {|{"name":"gentype-suffix","suffix":".mjs","gentypeconfig":{}}|};
+      let config = Config.load config_path in
+      check
+        (contains_adjacent "-bs-gentype-suffix" ".mjs" config.gentype_args)
+        "GenType includes an explicitly configured suffix";
+      write_file config_path
+        {|{
+          "name": "gentype-shims",
+          "gentypeconfig": {
+            "shims": [" From = First ", "A=B"]
+          }
+        }|};
+      let config = Config.load config_path in
+      check
+        (contains_adjacent "-bs-gentype-shim" "From=First" config.gentype_args)
+        "legacy GenType shims are trimmed";
+      check
+        (List.length
+           (List.filter (( = ) "-bs-gentype-shim") config.gentype_args)
+        = 2)
+        "legacy GenType shims retain distinct entries";
+      write_file config_path {|{"name":"unsupported","generators":["legacy"]}|};
+      let config = Config.load config_path in
+      check
+        (List.exists
+           (fun message ->
+             Test_support.contains_text message "field 'generators'"
+             && Test_support.contains_text message "is not supported")
+           config.diagnostics)
+        "known unsupported config fields are distinguished from unknown fields")
+
+let dependency_validation_tests _context =
+  let test_executable = test_executable () in
+  let dependency_root =
+    Filename.temp_file "rewatch-ocaml-allowed-dependents-" ""
+  in
+  Sys.remove dependency_root;
+  Unix.mkdir dependency_root 0o755;
+  let dependency_root = Unix.realpath dependency_root in
+  Fun.protect
+    ~finally:(fun () -> File_util.remove_tree dependency_root)
+    (fun () ->
+      write_file
+        (Filename.concat dependency_root "rescript.json")
+        {|{"name":"app","dependencies":["restricted"]}|};
+      write_file
+        (List.fold_left Filename.concat dependency_root
+           ["node_modules"; "restricted"; "rescript.json"])
+        {|{"name":"restricted","allowed-dependents":["other"]}|};
+      let previous_bsc = Sys.getenv_opt "RESCRIPT_BSC_EXE" in
+      Unix.putenv "RESCRIPT_BSC_EXE" test_executable;
+      Fun.protect
+        ~finally:(fun () ->
+          match previous_bsc with
+          | Some value -> Unix.putenv "RESCRIPT_BSC_EXE" value
+          | None -> Test_support.unsetenv "RESCRIPT_BSC_EXE")
+        (fun () ->
+          let rejected =
+            try
+              Build.run
+                ~poll:(fun () -> ())
+                ~verbosity:0 ~folder:dependency_root ~prod:false ~features:None
+                ~warn_error:None ~after_build:None ~filter:None ~no_timing:false;
+              false
+            with Project_context.Error message ->
+              if
+                Test_support.contains_text message
+                  "app dependencies: restricted"
+              then true
+              else failwith ("unexpected allowed-dependents error: " ^ message)
+          in
+          check rejected "unallowed package dependency is rejected";
+          write_file
+            (Filename.concat dependency_root "rescript.json")
+            {|{"name":"app","dev-dependencies":["restricted"]}|};
+          let rejected =
+            try
+              Build.run
+                ~poll:(fun () -> ())
+                ~verbosity:0 ~folder:dependency_root ~prod:false ~features:None
+                ~warn_error:None ~after_build:None ~filter:None ~no_timing:false;
+              false
+            with Project_context.Error message ->
+              Test_support.contains_text message
+                "app dev-dependencies: restricted"
+          in
+          check rejected "unallowed development dependency is rejected"))
+
+let tests =
+  "unit_tests"
+  >::: [
+         "feature_requests" >:: feature_request_tests;
+         "string_util" >:: string_util_tests;
+         "process_parallel" >:: process_parallel_tests;
+         "process_cancellation" >:: process_cancellation_tests;
+         "process_dependency_graph" >:: process_dependency_graph_tests;
+         "platform" >:: platform_tests;
+         "scheduler" >:: scheduler_tests;
+         "graph_and_diagnostics" >:: graph_and_diagnostic_tests;
+         "locks" >:: lock_tests;
+         "configuration" >:: config_tests;
+         "dependency_validation" >:: dependency_validation_tests;
+       ]
diff --git a/tests/rewatch_ounit_tests/warning_state_tests.ml b/tests/rewatch_ounit_tests/warning_state_tests.ml
new file mode 100644
index 00000000000..50f76dbeda9
--- /dev/null
+++ b/tests/rewatch_ounit_tests/warning_state_tests.ml
@@ -0,0 +1,35 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let outputs state =
+  Warning_state.entries state
+  |> List.map (fun entry -> entry.Warning_state.output)
+
+let tests =
+  "warning_state_tests" >:: fun _context ->
+  let state = Warning_state.create () in
+  Warning_state.set state ~module_name:"Zed" ~package_root:"root"
+    ~path:"src/Zed.res" ~output:"warning: zed\n";
+  Warning_state.set state ~module_name:"Alpha" ~package_root:"root"
+    ~path:"src/Alpha.res" ~output:"warning: alpha\n";
+  check
+    (outputs state = ["warning: alpha\n"; "warning: zed\n"])
+    "stored warnings replay in module-name order";
+  Warning_state.set state ~module_name:"Middle" ~package_root:"root"
+    ~path:"src/Middle.res" ~output:"warning: fresh\n";
+  check
+    (outputs state = ["warning: alpha\n"; "warning: fresh\n"; "warning: zed\n"])
+    "fresh and stored warnings share one deterministic order";
+  Warning_state.remove state ~package_root:"root" ~path:"src/Alpha.res";
+  check
+    (outputs state = ["warning: fresh\n"; "warning: zed\n"])
+    "a successfully recompiled path loses its stored warning";
+  Warning_state.set state ~module_name:"Module" ~package_root:"root"
+    ~path:"src/Module.res" ~output:"warning: implementation\n";
+  Warning_state.set state ~module_name:"Module" ~package_root:"root"
+    ~path:"src/Module.resi" ~output:"warning: interface\n";
+  Warning_state.retain_paths state [Filename.concat "root" "src/Module.resi"];
+  check
+    (outputs state = ["warning: interface\n"])
+    "renamed implementations are discarded while matching interfaces remain"
diff --git a/tests/rewatch_ounit_tests/watcher_tests.ml b/tests/rewatch_ounit_tests/watcher_tests.ml
new file mode 100644
index 00000000000..11252d91ff3
--- /dev/null
+++ b/tests/rewatch_ounit_tests/watcher_tests.ml
@@ -0,0 +1,261 @@
+open OUnit2
+
+let check condition message = assert_bool message condition
+
+let signal_is_ignored signal =
+  match Sys.signal signal Sys.Signal_ignore with
+  | Sys.Signal_ignore -> true
+  | previous ->
+    ignore (Sys.signal signal previous);
+    false
+
+let tests =
+  "watcher_tests" >:: fun _context ->
+  check
+    (Watcher.For_test.is_control_file_name "rescript.json"
+    && Watcher.For_test.is_control_file_name "bsconfig.json"
+    && Watcher.For_test.is_control_file_name "package.json")
+    "configuration and package identity files trigger control-file rebuilds";
+  Test_support.with_temp_dir "rewatch-watch-scope-metadata-" (fun root ->
+      Test_support.write_file
+        (Filename.concat root "rescript.json")
+        {|{"name":"root","sources":"src"}|};
+      Test_support.write_file (Filename.concat root "package.json") "{invalid";
+      let scope =
+        Watch_scope.discover ~root ~prod:false ~features:None ~filter:None
+      in
+      check
+        (Watch_scope.path_in_scope scope (Filename.concat root "package.json"))
+        "malformed package metadata remains in recoverable watch scope");
+  Test_support.with_temp_dir "rewatch-missing-source-scope-" (fun root ->
+      Unix.mkdir (Filename.concat root "node_modules") 0o755;
+      Test_support.write_file
+        (Filename.concat root "rescript.json")
+        {|{"name":"root","sources":["generated/./nested","trailing/"],"dependencies":["@scope/dep"]}|};
+      let scope =
+        Watch_scope.discover ~root ~prod:false ~features:None ~filter:None
+      in
+      let generated = Filename.concat root "generated" in
+      let nested = Filename.concat generated "nested" in
+      check
+        (Watch_scope.path_is_source_ancestor scope generated)
+        "an existing ancestor of a missing source root triggers reconciliation";
+      check
+        (Watch_scope.path_is_in_source_tree scope nested)
+        "native separators match a slash-delimited configured source root";
+      check
+        (Watch_scope.path_in_scope scope (Filename.concat nested "Main.res"))
+        "source files use platform-normalized directory comparisons";
+      check
+        (Watch_scope.path_in_scope scope
+           (Filename.concat (Filename.concat root "trailing") "Main.res"))
+        "source comparisons ignore a redundant trailing separator";
+      let dependency_scope =
+        Filename.concat (Filename.concat root "node_modules") "@scope"
+      in
+      let dependency = Filename.concat dependency_scope "dep" in
+      check
+        (Watch_scope.path_is_unresolved_ancestor scope dependency_scope)
+        "a scoped dependency ancestor triggers watch reconciliation";
+      check
+        (Watch_scope.path_in_scope scope dependency)
+        "native separators match a slash-delimited unresolved dependency";
+      let rec filesystem_root path =
+        let parent = Filename.dirname path in
+        if parent = path then path else filesystem_root parent
+      in
+      let filesystem_root = filesystem_root root in
+      let root_scope =
+        {
+          scope with
+          sources =
+            [
+              Watch_scope.
+                {directory = filesystem_root; recursive = true; filter = None};
+            ];
+        }
+      in
+      check
+        (Watch_scope.path_is_in_source_tree root_scope
+           (Filename.concat filesystem_root "generated"))
+        "recursive filesystem-root sources include child directories";
+      if Sys.win32 then
+        let unc_scope =
+          {
+            scope with
+            sources =
+              [
+                Watch_scope.
+                  {
+                    directory = {|\\server\share\..\generated|};
+                    recursive = false;
+                    filter = None;
+                  };
+              ];
+          }
+        in
+        check
+          (Watch_scope.path_is_in_source_tree unc_scope
+             {|\\server\share\generated|})
+          "parent traversal cannot escape a UNC share root");
+  let snapshot path digest =
+    [Watch_snapshot.{path; state = File {modified = 1.; size = 1; digest}}]
+  in
+  let previous = snapshot "A.res" "old-a" @ snapshot "B.res" "old-b" in
+  let trigger = snapshot "A.res" "new-a" @ snapshot "B.res" "old-b" in
+  let before_build = snapshot "A.res" "new-a" @ snapshot "B.res" "new-b" in
+  let polling_changes =
+    Watch_snapshot.polling_build_changes ~previous ~trigger ~before_build
+  in
+  check
+    (List.map
+       (fun (change : Watch_snapshot.change) -> change.path)
+       polling_changes
+    = ["A.res"; "B.res"])
+    "polling includes edits that arrive after the triggering snapshot";
+  check
+    (Watch_snapshot.changes_are_incremental polling_changes)
+    "polling source modifications use incremental presentation";
+  check
+    (not
+       (Watch_snapshot.changes_are_incremental
+          [Watch_snapshot.{path = "rescript.json"; kind = Modified}]))
+    "polling control-file changes use full-rebuild presentation";
+  Test_support.with_temp_dir "rewatch-snapshot-update-" (fun root ->
+      let source = Filename.concat root "A.res" in
+      Test_support.write_file source "let value = 1\n";
+      let digest_cache = Hashtbl.create 1 in
+      let stat = Unix.stat source in
+      let previous =
+        Watch_snapshot.
+          [
+            {
+              path = source;
+              state =
+                File
+                  {
+                    modified = stat.Unix.st_mtime;
+                    size = stat.Unix.st_size;
+                    digest = Digest.file source |> Digest.to_hex;
+                  };
+            };
+          ]
+      in
+      Test_support.write_file source "let value = 2\n";
+      check
+        (Option.is_some
+           (Watch_snapshot.update_entries digest_cache previous
+              [Watch_snapshot.{path = source; kind = Modified}]))
+        "known content events update the retained snapshot";
+      check
+        (Option.is_none
+           (Watch_snapshot.update_entries digest_cache previous
+              [
+                Watch_snapshot.
+                  {path = Filename.concat root "B.res"; kind = Added};
+              ]))
+        "unknown content events request structural reconciliation";
+      Sys.remove source;
+      Unix.mkdir source 0o755;
+      check
+        (Option.is_some
+           (Watch_snapshot.update_entries digest_cache previous
+              [Watch_snapshot.{path = source; kind = Modified}]))
+        "a transient snapshot read failure preserves the rebuild baseline");
+  let root = Filename.temp_file "rewatch-watcher-lifecycle-" "" in
+  Sys.remove root;
+  Unix.mkdir root 0o700;
+  let root = Unix.realpath root in
+  let config_path = Filename.concat root "rescript.json" in
+  let channel = open_out config_path in
+  output_string channel {|{"name":"watcher-lifecycle","sources":[]}|};
+  close_out channel;
+  let original_sigint = Sys.signal Sys.sigint Sys.Signal_ignore in
+  let original_sigterm = Sys.signal Sys.sigterm Sys.Signal_ignore in
+  let restore () =
+    ignore (Sys.signal Sys.sigterm original_sigterm);
+    ignore (Sys.signal Sys.sigint original_sigint);
+    File_util.remove_tree root
+  in
+  Fun.protect ~finally:restore (fun () ->
+      let raised =
+        try
+          Watcher.run ~root ~prod:false ~features:None ~filter:None
+            ~clear_screen:false ~show_progress:true ~verbosity:0
+            ~build:(fun ~poll:_ ~changes:_ -> raise Exit);
+          false
+        with Exit -> true
+      in
+      check raised "build callback exception is preserved";
+      let lock = Filename.concat root "lib/watch.lock" in
+      check
+        (not (Sys.file_exists lock))
+        "watch lock is released when setup or the initial build fails";
+      check
+        (signal_is_ignored Sys.sigint)
+        "SIGINT handler is restored after failure";
+      check
+        (signal_is_ignored Sys.sigterm)
+        "SIGTERM handler is restored after failure";
+      Watcher.run ~root ~prod:false ~features:None ~filter:None
+        ~clear_screen:false ~show_progress:true ~verbosity:0
+        ~build:(fun ~poll ~changes:_ ->
+          poll ();
+          Sys.remove lock;
+          Watcher.Succeeded);
+      check
+        (signal_is_ignored Sys.sigint)
+        "SIGINT handler is restored after normal shutdown";
+      check
+        (signal_is_ignored Sys.sigterm)
+        "SIGTERM handler is restored after normal shutdown";
+      let source_dir = Filename.concat root "src" in
+      Unix.mkdir source_dir 0o700;
+      let source = Filename.concat source_dir "A.res" in
+      let channel = open_out source in
+      output_string channel "let value = 1\n";
+      close_out channel;
+      let channel = open_out config_path in
+      output_string channel
+        {|{"name":"watcher-lifecycle","sources":[{"dir":"src"}]}|};
+      close_out channel;
+      let builds = ref 0 in
+      let fallback_message = ref None in
+      let fallback_rebuilt =
+        try
+          Watcher.For_test.run_with_native_failure
+            ~message:"injected native setup failure"
+            ~on_fallback:(fun message -> fallback_message := Some message)
+            ~root ~prod:false ~features:None ~filter:None ~clear_screen:false
+            ~show_progress:false ~verbosity:0
+            ~build:(fun ~poll:_ ~changes ->
+              incr builds;
+              match (!builds, changes) with
+              | 1, None ->
+                let channel = open_out source in
+                output_string channel "let value = 2\n";
+                close_out channel;
+                Watcher.Succeeded
+              | 2, Some [Watcher.{path; kind = Modified}] when path = source ->
+                raise Exit
+              | _ ->
+                assert_failure
+                  "polling fallback requested an unexpected build transition");
+          false
+        with Exit -> true
+      in
+      check fallback_rebuilt
+        "native setup failure falls back to polling and observes source edits";
+      check
+        (!fallback_message = Some "injected native setup failure")
+        "polling fallback reports the native setup failure";
+      check (!builds = 2) "polling fallback rebuilds the source exactly once";
+      check
+        (not (Sys.file_exists lock))
+        "watch lock is released when the polling fallback exits";
+      check
+        (signal_is_ignored Sys.sigint)
+        "SIGINT handler is restored after polling fallback failure";
+      check
+        (signal_is_ignored Sys.sigterm)
+        "SIGTERM handler is restored after polling fallback failure")
diff --git a/yarn.config.cjs b/yarn.config.cjs
index 5282da9e7ad..4591f6848ce 100644
--- a/yarn.config.cjs
+++ b/yarn.config.cjs
@@ -51,28 +51,28 @@ async function enforceCompilerMeta({ Yarn }) {
     }
   }
 
-  const { compilerVersionFile } = await import("#dev/paths");
-  const versionFile = await fs.readFile(compilerVersionFile, "utf8");
   const versionPattern = /^let version = "(?<version>[^"]+)"$/m;
-
-  if (process.argv.includes("--fix")) {
-    await fs.writeFile(
-      compilerVersionFile,
-      versionFile.replace(
-        versionPattern,
-        `let version = "${EXPECTED_VERSION}"`,
-      ),
-    );
-  } else {
-    const versionMatch = versionFile.match(versionPattern);
-    const foundVersion = versionMatch?.groups?.version;
-    if (foundVersion !== EXPECTED_VERSION) {
-      Yarn.workspace().error(
-        `compiler/common/bs_version.ml file need to be fixed; expected ${EXPECTED_VERSION}, found ${foundVersion}.`,
+  async function enforceMlVersionFile(path) {
+    const content = await fs.readFile(path, "utf8");
+    if (process.argv.includes("--fix")) {
+      await fs.writeFile(
+        path,
+        content.replace(versionPattern, `let version = "${EXPECTED_VERSION}"`),
       );
+    } else {
+      const foundVersion = content.match(versionPattern)?.groups?.version;
+      if (foundVersion !== EXPECTED_VERSION) {
+        Yarn.workspace().error(
+          `${path} file need to be fixed; expected ${EXPECTED_VERSION}, found ${foundVersion}.`,
+        );
+      }
     }
   }
 
+  const { compilerVersionFile } = await import("#dev/paths");
+  await enforceMlVersionFile(compilerVersionFile);
+  await enforceMlVersionFile("rewatch-ocaml/rewatch_version.ml");
+
   const rewatchCargoFile = "rewatch/Cargo.toml";
   const rewatchCargoContent = await fs.readFile(rewatchCargoFile, "utf8");
   const rewatchVersionPattern = /^version = "(?<version>[^"]+)"$/m;
diff --git a/yarn.lock b/yarn.lock
index aa4b9d8d18c..a0f5097d6aa 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2833,6 +2833,7 @@ __metadata:
   bin:
     bsc: cli/bsc.js
     rescript: cli/rescript.js
+    rescript-rust: cli/rescript-rust.js
     rescript-tools: cli/rescript-tools.js
   languageName: unknown
   linkType: soft