diff --git a/CHANGELOG.md b/CHANGELOG.md
index fd79ed8..668b797 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,16 @@
 
 ### Fixed
 
+- **`mix mob.deploy --slim` is no longer a silent no-op** (MOB-73).
+  `NativeBuild.build_all/1` stored `slim` in the process dict
+  (`Process.put(:mob_slim, slim)`) but the actual gate at
+  `maybe_slim_otp_bundle/2` reads `System.get_env("MOB_SLIM")` — so
+  `--slim` from the CLI never reached the strip pass. Only
+  `mix mob.release` produced a slim OTP bundle, because that path
+  explicitly sets the env var. Fixed by publishing `MOB_SLIM=1` / `=0`
+  in `__apply_slim_env__/1`, which the gate already knows how to read.
+  Two revert-verified tests lock the env-var contract.
+
 - **Plugin signature verification now runs before `Code.eval_file`** on the
   manifest (MOB-74). The v1 signature covered the eval'd manifest map, so
   verifiers needed the eval to run first to rebuild the payload — letting
diff --git a/lib/mob_dev/native_build.ex b/lib/mob_dev/native_build.ex
index 4494a45..4eac5d0 100644
--- a/lib/mob_dev/native_build.ex
+++ b/lib/mob_dev/native_build.ex
@@ -40,7 +40,7 @@ defmodule MobDev.NativeBuild do
         do: narrow_platforms_for_device(platforms, device_id),
         else: platforms
 
-    Process.put(:mob_slim, slim)
+    __apply_slim_env__(slim)
 
     # Always regenerate the runtime plugin manifest from the CURRENT activated
     # plugins before bundling priv — like the driver_tab, it's derived state, not
@@ -5857,6 +5857,18 @@ defmodule MobDev.NativeBuild do
     :ok
   end
 
+  @doc false
+  # Publish the --slim flag into MOB_SLIM so `maybe_slim_otp_bundle/2` (which
+  # mirrors `release.ex`'s env-var gate) actually sees it. Before MOB-73 this
+  # went into `Process.put(:mob_slim, ...)` and nothing consulted it — so
+  # `mix mob.deploy --slim` was a silent no-op. Public-for-testing so the
+  # env-var contract is regression-guarded.
+  @spec __apply_slim_env__(boolean()) :: :ok
+  def __apply_slim_env__(slim) when is_boolean(slim) do
+    System.put_env("MOB_SLIM", if(slim, do: "1", else: "0"))
+    :ok
+  end
+
   defp maybe_slim_otp_bundle(app_path, cfg) do
     if System.get_env("MOB_SLIM") == "1" do
       otp_bundle = Path.join(app_path, "otp")
diff --git a/test/mob_dev/native_build_test.exs b/test/mob_dev/native_build_test.exs
index 7c22ac4..f2e6bd3 100644
--- a/test/mob_dev/native_build_test.exs
+++ b/test/mob_dev/native_build_test.exs
@@ -10,6 +10,39 @@ defmodule MobDev.NativeBuildTest do
 
   alias MobDev.NativeBuild
 
+  describe "__apply_slim_env__/1 (MOB-73 --slim gate)" do
+    setup do
+      # Save/restore MOB_SLIM across the test since it's process-global.
+      previous = System.get_env("MOB_SLIM")
+
+      on_exit(fn ->
+        case previous do
+          nil -> System.delete_env("MOB_SLIM")
+          val -> System.put_env("MOB_SLIM", val)
+        end
+      end)
+
+      :ok
+    end
+
+    test "true publishes MOB_SLIM=1 so the slim gate actually fires" do
+      # Before MOB-73 this went through `Process.put(:mob_slim, true)` and
+      # `maybe_slim_otp_bundle/2` never consulted the dict — so `mix mob.deploy
+      # --slim` was a silent no-op. The env-var read at native_build.ex:5861
+      # is what the strip pass gates on; the helper's job is to set it. Revert
+      # to Process.put and this fails.
+      System.put_env("MOB_SLIM", "0")
+      :ok = NativeBuild.__apply_slim_env__(true)
+      assert System.get_env("MOB_SLIM") == "1"
+    end
+
+    test "false publishes MOB_SLIM=0 so --no-slim is explicit and observable" do
+      System.put_env("MOB_SLIM", "1")
+      :ok = NativeBuild.__apply_slim_env__(false)
+      assert System.get_env("MOB_SLIM") == "0"
+    end
+  end
+
   describe "build_zig_supports_abi?/2" do
     test "true when the build.zig declares the ABI as a quoted string literal" do
       src = ~s|