diff --git a/packages/server/tests/test_diagnostic_mode.py b/packages/server/tests/test_diagnostic_mode.py
index 39f9f7d..5ec26dd 100644
--- a/packages/server/tests/test_diagnostic_mode.py
+++ b/packages/server/tests/test_diagnostic_mode.py
@@ -23,11 +23,23 @@ from fastapi.testclient import TestClient
 
 
 def _reset_state() -> None:
-    """Clear the module-level ``_active`` dict between tests so prior
-    activations don't bleed across cases."""
+    """Clear the module-level ``_active`` dict and any pending fallback
+    tasks between tests so prior activations don't bleed across cases.
+
+    The ``_bg_tasks`` set retains tasks created via the asyncio-fallback
+    schedule path; without this, a test that schedules a 30-minute revert
+    leaves a pending task that inflates the ``len(_bg_tasks)`` invariant
+    checked by ``test_fallback_task_retained_until_fire``.
+    """
     from notify_bridge_server.services import diagnostic_mode as svc
 
     svc._active.clear()
+    # ``_bg_tasks`` from a previous test belong to that test's now-closed
+    # event loop — calling ``.cancel()`` here crosses loops and raises
+    # ``RuntimeError: Event loop is closed``. Dropping the references is
+    # enough: the tasks can't fire on a dead loop, and CPython will GC
+    # them once the prior loop releases them.
+    svc._bg_tasks.clear()
 
 
 @pytest.fixture(autouse=True)