diff --git a/packages/mosaic/framework/tools/git/test-detect-platform-outside-repo.sh b/packages/mosaic/framework/tools/git/test-detect-platform-outside-repo.sh
index fa7978c3..13955f0e 100755
--- a/packages/mosaic/framework/tools/git/test-detect-platform-outside-repo.sh
+++ b/packages/mosaic/framework/tools/git/test-detect-platform-outside-repo.sh
@@ -27,8 +27,21 @@ run_outside() { # $1=function name -> "rc:sawmessage"
 }
 check() { if [ "$2" = "$3" ]; then echo "  PASS  $1 ($2)"; else echo "  FAIL  $1: got $2, want $3"; fail=1; fi; }
 
-# $TMP is deliberately not a git repo, and must not be inside one.
-( cd "$TMP" && git rev-parse --git-dir >/dev/null 2>&1 ) && { echo "  SKIP  scratch dir is inside a repo"; exit 0; }
+# $TMP must not be inside a git repo. Do not SKIP on failure: be-coder-07 showed the
+# original SKIP exited 0, so pointing TMPDIR beneath a git worktree made this test PASS
+# against unchanged main. A skip that exits 0 is indistinguishable from a pass.
+# GIT_CEILING_DIRECTORIES stops git walking above $TMP, making the condition hold
+# regardless of where TMPDIR lives, rather than merely detecting when it does not.
+# GIT_CEILING_DIRECTORIES is matched against the PHYSICAL path -- a symlinked TMPDIR
+# (/tmp is commonly one) makes the logical path never match, and the ceiling silently
+# does nothing. Resolve it before exporting.
+TMP="$(cd "$TMP" && pwd -P)"
+export GIT_CEILING_DIRECTORIES="$TMP"
+if ( cd "$TMP" && git rev-parse --git-dir >/dev/null 2>&1 ); then
+    echo "  FAIL  scratch dir is inside a git repo even with GIT_CEILING_DIRECTORIES set;"
+    echo "        the outside-a-repo precondition cannot be established -- refusing to report a result"
+    exit 1
+fi
 
 echo "== outside a git repo: rc=1 AND the diagnostic is emitted =="
 check "detect_platform" "$(run_outside detect_platform)" "1:yes"