diff --git a/tests/cron/test_cron_service.py b/tests/cron/test_cron_service.py
index 4c19580f8..8606e4f58 100644
--- a/tests/cron/test_cron_service.py
+++ b/tests/cron/test_cron_service.py
@@ -176,3 +176,35 @@ def test_reload_jobs(tmp_path):
         message="hello2",
     )
     assert len(service.list_jobs()) == 2
+
+
+@pytest.mark.asyncio
+async def test_running_service_picks_up_external_add(tmp_path):
+    """A running service should detect and execute a job added by another instance."""
+    store_path = tmp_path / "cron" / "jobs.json"
+    called: list[str] = []
+
+    async def on_job(job):
+        called.append(job.name)
+
+    service = CronService(store_path, on_job=on_job)
+    service.add_job(
+        name="heartbeat",
+        schedule=CronSchedule(kind="every", every_ms=150),
+        message="tick",
+    )
+    await service.start()
+    try:
+        await asyncio.sleep(0.05)
+
+        external = CronService(store_path)
+        external.add_job(
+            name="external",
+            schedule=CronSchedule(kind="every", every_ms=150),
+            message="ping",
+        )
+
+        await asyncio.sleep(0.6)
+        assert "external" in called
+    finally:
+        service.stop()