diff --git a/nanobot/heartbeat/service.py b/nanobot/heartbeat/service.py
index 7dbdc03..cb1a1c7 100644
--- a/nanobot/heartbeat/service.py
+++ b/nanobot/heartbeat/service.py
@@ -80,6 +80,9 @@ class HeartbeatService:
         if not self.enabled:
             logger.info("Heartbeat disabled")
             return
+        if self._running:
+            logger.warning("Heartbeat already running")
+            return
         
         self._running = True
         self._task = asyncio.create_task(self._run_loop())
diff --git a/tests/test_heartbeat_service.py b/tests/test_heartbeat_service.py
new file mode 100644
index 0000000..ec91c6b
--- /dev/null
+++ b/tests/test_heartbeat_service.py
@@ -0,0 +1,44 @@
+import asyncio
+
+import pytest
+
+from nanobot.heartbeat.service import (
+    HEARTBEAT_OK_TOKEN,
+    HeartbeatService,
+)
+
+
+def test_heartbeat_ok_detection() -> None:
+    def is_ok(response: str) -> bool:
+        return HEARTBEAT_OK_TOKEN in response.upper()
+
+    assert is_ok("HEARTBEAT_OK")
+    assert is_ok("`HEARTBEAT_OK`")
+    assert is_ok("**HEARTBEAT_OK**")
+    assert is_ok("heartbeat_ok")
+    assert is_ok("HEARTBEAT_OK.")
+
+    assert not is_ok("HEARTBEAT_NOT_OK")
+    assert not is_ok("all good")
+
+
+@pytest.mark.asyncio
+async def test_start_is_idempotent(tmp_path) -> None:
+    async def _on_heartbeat(_: str) -> str:
+        return "HEARTBEAT_OK"
+
+    service = HeartbeatService(
+        workspace=tmp_path,
+        on_heartbeat=_on_heartbeat,
+        interval_s=9999,
+        enabled=True,
+    )
+
+    await service.start()
+    first_task = service._task
+    await service.start()
+
+    assert service._task is first_task
+
+    service.stop()
+    await asyncio.sleep(0)