diff --git a/cron/jobs.py b/cron/jobs.py
index 4306fe8f5d877..b7796c1c51a79 100644
--- a/cron/jobs.py
+++ b/cron/jobs.py
@@ -3621,13 +3621,37 @@ def _sweep_completed_oneshots(
 
 
 def heartbeat_fire_claim(job_id: str, *, expected_owner: str) -> bool:
+    """Refresh this owner's ``fire_claim`` lease.
+
+    A heartbeat is not an external side effect: it only compare-and-swaps
+    ``fire_claim["at"]`` when ``fire_claim["by"]`` still equals
+    ``expected_owner``.  ``_jobs_lock()`` already serializes that CAS with both
+    an in-process lock and a cross-process flock, so it is safe on its own.
+
+    Taking the fire fence here is actively harmful.  The run thread holds the
+    per-job fence across its whole side effect (see ``_fire_job_lock``), and the
+    fence's reentrancy is thread-local, so it does not extend to the heartbeat
+    thread.  Any execution outliving ``_RUN_CLAIM_HEARTBEAT_SECONDS`` therefore
+    blocked here for ``_JOBS_LOCK_TIMEOUT_SECONDS`` and returned False, which
+    the scheduler reads as "ownership lost" and uses to interrupt a perfectly
+    healthy run -- reported to the operator as "Interrupted by shutdown before
+    terminal completion."
+
+    So: prefer the fence to keep the common path serialized with other owner
+    mutations, but when it is unavailable fall back to the CAS instead of
+    reporting a loss of ownership we have not actually observed.  A genuine
+    takeover still returns False, because ``by`` no longer matches.
+    """
     with _fire_job_lock(job_id) as acquired:
-        if not acquired:
-            return False
-        return _heartbeat_fire_claim_locked(
-            job_id,
-            expected_owner=expected_owner,
-        )
+        if acquired:
+            return _heartbeat_fire_claim_locked(
+                job_id,
+                expected_owner=expected_owner,
+            )
+
+    # Fence unavailable (held by this job's own run thread, or contended
+    # cross-process). The owner check below is authoritative either way.
+    return _heartbeat_fire_claim_locked(job_id, expected_owner=expected_owner)
 
 
 def _heartbeat_fire_claim_locked(job_id: str, *, expected_owner: str) -> bool:
diff --git a/tests/cron/test_claim_job_for_fire.py b/tests/cron/test_claim_job_for_fire.py
index 200199efd7e6b..64895ffa3f8b5 100644
--- a/tests/cron/test_claim_job_for_fire.py
+++ b/tests/cron/test_claim_job_for_fire.py
@@ -263,3 +263,47 @@ def reentrant_claimant():
     assert result == {"outer": True, "inner": True}
     thread.join(timeout=2)
     assert thread.is_alive() is False
+
+
+def test_heartbeat_survives_fence_held_by_its_own_run(temp_home, monkeypatch):
+    """A heartbeat must not report ownership lost merely because the fence is busy.
+
+    Regression for the false "Interrupted by shutdown before terminal
+    completion." that killed every cron job outliving the heartbeat interval:
+    the run thread holds the per-job fence across its side effect, the
+    heartbeat thread (a different thread, so the fence's thread-local
+    reentrancy does not apply) could not acquire it, and that timeout was
+    reported to the scheduler as a lost claim.
+    """
+    import cron.jobs as jobs
+
+    job = jobs.create_job(prompt="x", schedule="every 5m", name="long-run")
+    assert jobs.claim_job_for_fire(job["id"]) is True
+    owner = jobs.get_job(job["id"])["fire_claim"]["by"]
+
+    # Keep the failing path fast; the real value is 30s.
+    monkeypatch.setattr(jobs, "_JOBS_LOCK_TIMEOUT_SECONDS", 0.2)
+
+    fence_held = threading.Event()
+    release = threading.Event()
+
+    def hold_fence():
+        with jobs.fire_claim_fence(job["id"], expected_owner=owner) as owns:
+            assert owns is True
+            fence_held.set()
+            release.wait(timeout=5)
+
+    holder = threading.Thread(target=hold_fence, daemon=True)
+    holder.start()
+    try:
+        assert fence_held.wait(timeout=5)
+        # The run thread owns the fence; the heartbeat still confirms ownership.
+        assert jobs.heartbeat_fire_claim(job["id"], expected_owner=owner) is True
+        # A genuine takeover is still detected while the fence is busy.
+        assert (
+            jobs.heartbeat_fire_claim(job["id"], expected_owner="replacement-owner")
+            is False
+        )
+    finally:
+        release.set()
+        holder.join(timeout=5)