diff --git a/tests/conftest.py b/tests/conftest.py
index 138641a..d86b8f6 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -11,6 +11,28 @@ from vigilar.config import VigilarConfig, load_config
 from vigilar.storage.schema import metadata
 
 
+@pytest.fixture(autouse=True, scope="session")
+def _isolate_vigilar_config(tmp_path_factory):
+    """Prevent tests from writing to the real config/vigilar.toml.
+
+    Web endpoint handlers that call `_save_and_reload()` read the target
+    path from the VIGILAR_CONFIG env var, falling back to the relative
+    `"config/vigilar.toml"`. Without this fixture, any test that exercises
+    such an endpoint rewrites the repo's committed config file via a
+    Pydantic round-trip, stripping comments and non-default fields.
+    """
+    tmp_config = tmp_path_factory.mktemp("vigilar-config") / "vigilar.toml"
+    prev = os.environ.get("VIGILAR_CONFIG")
+    os.environ["VIGILAR_CONFIG"] = str(tmp_config)
+    try:
+        yield
+    finally:
+        if prev is None:
+            os.environ.pop("VIGILAR_CONFIG", None)
+        else:
+            os.environ["VIGILAR_CONFIG"] = prev
+
+
 def _create_test_engine(db_path: Path):
     """Create a fresh engine for testing (bypasses the global singleton)."""
     db_path.parent.mkdir(parents=True, exist_ok=True)