diff --git a/CHANGELOG.md b/CHANGELOG.md
index 08c4d93..ff122a1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com), and this
 
 ## [Unreleased]
 
+### Fixed
+
+- `GTR_DEBUG=1` now reports the file, line and function of an unexpected failure. `bin/git-gtr` installed an `ERR` trap but ran under `set -e` alone, so the trap was never inherited by functions; since every command runs inside `main()` and a `cmd_*` handler, the variable had no observable effect. The script now uses `set -eE`, which changes nothing when the trap is not installed.
+
 ## [2.11.0] - 2026-08-19
 
 ### Added
diff --git a/bin/git-gtr b/bin/git-gtr
index 2b8e94b..24ae27d 100755
--- a/bin/git-gtr
+++ b/bin/git-gtr
@@ -3,7 +3,11 @@
 # Portable, cross-platform git worktree management
 # Invoked as: git gtr <command> (git subcommand via PATH discovery)
 
-set -e
+# -E propagates the ERR trap below into functions, command substitutions and
+# subshells. Without it the GTR_DEBUG trap never fires, because every command
+# runs inside main() and a cmd_* handler. With no ERR trap installed, -E has no
+# effect, so the default path behaves exactly as it did with plain -e.
+set -eE
 
 # Debug: show file:line:function on set -e failures
 if [ -n "${GTR_DEBUG:-}" ]; then
diff --git a/tests/debug_trap.bats b/tests/debug_trap.bats
new file mode 100644
index 0000000..1ec4405
--- /dev/null
+++ b/tests/debug_trap.bats
@@ -0,0 +1,84 @@
+#!/usr/bin/env bats
+# Tests for the GTR_DEBUG error trap in bin/git-gtr
+#
+# The trap is only useful when bin/git-gtr enables errtrace. An ERR trap is
+# inherited by functions, command substitutions and subshells only under
+# 'set -E'; with plain 'set -e' it never fires, because every command runs
+# inside main() and then a cmd_* handler. These tests run the real binary as a
+# subprocess so that the option line in bin/git-gtr is actually exercised.
+
+load test_helper
+
+setup() {
+  setup_integration_repo
+
+  # A git shim that fails one specific config write and passes everything else
+  # through, standing in for an unexpected git failure at an unguarded call
+  # site (cfg_set in lib/config.sh).
+  REAL_GIT=$(command -v git)
+  SHIM_DIR=$(mktemp -d)
+  cat > "$SHIM_DIR/git" <<SCRIPT
+#!/usr/bin/env bash
+if [ "\$1" = "config" ]; then
+  case "\$*" in
+    *--get*|*--list*|*--file*) : ;;
+    *gtr.editor.default*) exit 4 ;;
+  esac
+fi
+exec "$REAL_GIT" "\$@"
+SCRIPT
+  chmod +x "$SHIM_DIR/git"
+}
+
+teardown() {
+  [ -n "${SHIM_DIR:-}" ] && rm -rf "$SHIM_DIR"
+  teardown_integration_repo
+}
+
+@test "GTR_DEBUG reports file, line and function for an unguarded failure" {
+  run env PATH="$SHIM_DIR:$PATH" GTR_DEBUG=1 \
+    "$PROJECT_ROOT/bin/git-gtr" config set gtr.editor.default vim
+  [ "$status" -ne 0 ]
+  [[ "$output" == *"ERROR at "* ]]
+  [[ "$output" == *"lib/config.sh"* ]]
+  [[ "$output" == *"cfg_set()"* ]]
+}
+
+@test "the error trap stays silent when GTR_DEBUG is unset" {
+  run env PATH="$SHIM_DIR:$PATH" \
+    "$PROJECT_ROOT/bin/git-gtr" config set gtr.editor.default vim
+  [ "$status" -ne 0 ]
+  [[ "$output" != *"ERROR at "* ]]
+}
+
+@test "GTR_DEBUG does not report anything for a successful command" {
+  run env GTR_DEBUG=1 "$PROJECT_ROOT/bin/git-gtr" config set gtr.editor.default vim
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"ERROR at "* ]]
+}
+
+@test "GTR_DEBUG does not report handled errors" {
+  # cmd_go reports a missing worktree itself; that is a deliberate error path,
+  # not an unguarded failure, so the trap must not add noise to it.
+  run env GTR_DEBUG=1 "$PROJECT_ROOT/bin/git-gtr" go no-such-branch
+  [ "$status" -ne 0 ]
+  [[ "$output" != *"ERROR at "* ]]
+}
+
+@test "GTR_DEBUG reports a failure raised inside a subshell" {
+  # cmd_run executes the requested command in a subshell:
+  #   (cd "$worktree_path" && "${run_args[@]}")
+  # A failing command there is only reported when the ERR trap is inherited by
+  # subshells, which is the other half of what errtrace buys.
+  run env GTR_DEBUG=1 "$PROJECT_ROOT/bin/git-gtr" run 1 false
+  [ "$status" -ne 0 ]
+  [[ "$output" == *"ERROR at "* ]]
+  [[ "$output" == *"lib/commands/run.sh"* ]]
+  [[ "$output" == *"cmd_run()"* ]]
+}
+
+@test "a successful command under gtr run reports nothing" {
+  run env GTR_DEBUG=1 "$PROJECT_ROOT/bin/git-gtr" run 1 true
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"ERROR at "* ]]
+}