diff --git a/nanobot/heartbeat/service.py b/nanobot/heartbeat/service.py
index 3c1a6aa..ab2bfac 100644
--- a/nanobot/heartbeat/service.py
+++ b/nanobot/heartbeat/service.py
@@ -1,6 +1,7 @@
 """Heartbeat service - periodic agent wake-up to check for tasks."""
 
 import asyncio
+import re
 from pathlib import Path
 from typing import Any, Callable, Coroutine
 
@@ -35,6 +36,15 @@ def _is_heartbeat_empty(content: str | None) -> bool:
     return True
 
 
+def _is_heartbeat_ok_response(response: str | None) -> bool:
+    """Return True only for an exact HEARTBEAT_OK token (with simple markdown wrappers)."""
+    if not response:
+        return False
+
+    normalized = re.sub(r"[\s`*_>\-]", "", response).upper()
+    return normalized == HEARTBEAT_OK_TOKEN.replace("_", "")
+
+
 class HeartbeatService:
     """
     Periodic heartbeat service that wakes the agent to check for tasks.
@@ -75,6 +85,9 @@ 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())
@@ -115,7 +128,7 @@ class HeartbeatService:
                 response = await self.on_heartbeat(HEARTBEAT_PROMPT)
                 
                 # Check if agent said "nothing to do"
-                if HEARTBEAT_OK_TOKEN.replace("_", "") in response.upper().replace("_", ""):
+                if _is_heartbeat_ok_response(response):
                     logger.info("Heartbeat: OK (no action needed)")
                 else:
                     logger.info("Heartbeat: completed task")
diff --git a/tests/test_heartbeat_service.py b/tests/test_heartbeat_service.py
new file mode 100644
index 0000000..52d1b96
--- /dev/null
+++ b/tests/test_heartbeat_service.py
@@ -0,0 +1,40 @@
+import asyncio
+
+import pytest
+
+from nanobot.heartbeat.service import (
+    HeartbeatService,
+    _is_heartbeat_ok_response,
+)
+
+
+def test_heartbeat_ok_response_requires_exact_token() -> None:
+    assert _is_heartbeat_ok_response("HEARTBEAT_OK")
+    assert _is_heartbeat_ok_response("`HEARTBEAT_OK`")
+    assert _is_heartbeat_ok_response("**HEARTBEAT_OK**")
+
+    assert not _is_heartbeat_ok_response("HEARTBEAT_OK, done")
+    assert not _is_heartbeat_ok_response("done HEARTBEAT_OK")
+    assert not _is_heartbeat_ok_response("HEARTBEAT_NOT_OK")
+
+
+@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)