diff --git a/archivebox/tests/test_test_harness.py b/archivebox/tests/test_test_harness.py
new file mode 100644
index 00000000..686c31a9
--- /dev/null
+++ b/archivebox/tests/test_test_harness.py
@@ -0,0 +1,29 @@
+import os
+from pathlib import Path
+
+import pytest
+
+from archivebox.tests import conftest as test_harness
+
+
+def test_session_data_dir_is_outside_repo_root():
+    assert test_harness.SESSION_DATA_DIR != test_harness.REPO_ROOT
+    assert test_harness.REPO_ROOT not in test_harness.SESSION_DATA_DIR.parents
+    assert test_harness.REPO_ROOT not in Path.cwd().parents
+    assert Path.cwd() != test_harness.REPO_ROOT
+
+
+def test_cli_helpers_reject_repo_root_runtime_paths():
+    with pytest.raises(AssertionError, match="repo root"):
+        test_harness.run_archivebox_cmd(["version"], data_dir=test_harness.REPO_ROOT)
+
+    with pytest.raises(AssertionError, match="repo root"):
+        test_harness.run_archivebox_cmd_cwd(["version"], cwd=test_harness.REPO_ROOT)
+
+    with pytest.raises(AssertionError, match="repo root"):
+        test_harness.run_python_cwd("print('hello')", cwd=test_harness.REPO_ROOT)
+
+
+def test_runtime_guard_rejects_chdir_into_repo_root():
+    with pytest.raises(AssertionError, match="repo root"):
+        os.chdir(test_harness.REPO_ROOT)