diff --git a/simpler_setup/runtime_builder.py b/simpler_setup/runtime_builder.py
index 28d8d7fe8..bcadeb48d 100644
--- a/simpler_setup/runtime_builder.py
+++ b/simpler_setup/runtime_builder.py
@@ -48,8 +48,16 @@ def _invalidate_cache_if_stale(target_cache_dir: Path, current_commit: str) -> N
     cannot detect that source files changed. Comparing the HEAD commit stored
     at last build time against the current HEAD is a reliable signal that
     sources may have changed and a clean rebuild is needed.
+
+    When the current commit can't be determined (no git, transient failure),
+    fall through to a clean rebuild — a fresh compile is cheap relative to
+    the risk of linking against stale objects.
     """
     if not current_commit:
+        if target_cache_dir.is_dir():
+            logger.info("git HEAD unavailable, clearing cmake cache: %s", target_cache_dir)
+            shutil.rmtree(target_cache_dir)
+        target_cache_dir.mkdir(parents=True, exist_ok=True)
         return
     commit_file = target_cache_dir / _GIT_COMMIT_FILE
     if commit_file.is_file():
diff --git a/tests/ut/py/test_runtime_builder.py b/tests/ut/py/test_runtime_builder.py
index 122cf867b..a04281fa6 100644
--- a/tests/ut/py/test_runtime_builder.py
+++ b/tests/ut/py/test_runtime_builder.py
@@ -260,6 +260,54 @@ def test_propagates_compiler_error(self, MockCompiler, tmp_path, default_test_pl
             builder.get_binaries("test_rt", build=True)
 
 
+# --- _invalidate_cache_if_stale unit tests ---
+
+
+class TestInvalidateCacheIfStale:
+    """Test cmake cache invalidation behavior under varying git state."""
+
+    def test_clears_when_commit_mismatches(self, tmp_path):
+        from simpler_setup.runtime_builder import _GIT_COMMIT_FILE, _invalidate_cache_if_stale  # noqa: PLC0415
+
+        cache_dir = tmp_path / "cache"
+        cache_dir.mkdir()
+        (cache_dir / _GIT_COMMIT_FILE).write_text("old_sha\n")
+        (cache_dir / "stale_artifact.o").write_text("stale")
+
+        _invalidate_cache_if_stale(cache_dir, "new_sha")
+
+        assert not (cache_dir / "stale_artifact.o").exists()
+        assert (cache_dir / _GIT_COMMIT_FILE).read_text().strip() == "new_sha"
+
+    def test_keeps_when_commit_matches(self, tmp_path):
+        from simpler_setup.runtime_builder import _GIT_COMMIT_FILE, _invalidate_cache_if_stale  # noqa: PLC0415
+
+        cache_dir = tmp_path / "cache"
+        cache_dir.mkdir()
+        (cache_dir / _GIT_COMMIT_FILE).write_text("same_sha\n")
+        artifact = cache_dir / "kept_artifact.o"
+        artifact.write_text("fresh")
+
+        _invalidate_cache_if_stale(cache_dir, "same_sha")
+
+        assert artifact.exists()
+
+    def test_clears_when_commit_unavailable(self, tmp_path):
+        """No discoverable git HEAD should force a clean rebuild, not silently skip."""
+        from simpler_setup.runtime_builder import _GIT_COMMIT_FILE, _invalidate_cache_if_stale  # noqa: PLC0415
+
+        cache_dir = tmp_path / "cache"
+        cache_dir.mkdir()
+        (cache_dir / _GIT_COMMIT_FILE).write_text("old_sha\n")
+        (cache_dir / "stale_artifact.o").write_text("stale")
+
+        _invalidate_cache_if_stale(cache_dir, "")
+
+        assert not (cache_dir / "stale_artifact.o").exists()
+        assert not (cache_dir / _GIT_COMMIT_FILE).exists()
+        assert cache_dir.is_dir()
+
+
 # --- Full integration tests (real compilation) ---