diff --git a/app-instance/backend/beaver/services/cron_service.py b/app-instance/backend/beaver/services/cron_service.py
index 21678a9..9072ec6 100644
--- a/app-instance/backend/beaver/services/cron_service.py
+++ b/app-instance/backend/beaver/services/cron_service.py
@@ -134,6 +134,7 @@ class CronService:
         return job
 
     def update_enabled(self, job_id: str, enabled: bool) -> CronJob | None:
+        updated_job: CronJob | None = None
         with self._lock:
             jobs = self._load_jobs_unlocked()
             for job in jobs:
@@ -143,9 +144,11 @@ class CronService:
                 job.updated_at_ms = _now_ms()
                 job.next_run_at_ms = compute_next_run(job.schedule) if job.enabled else None
                 self._save_jobs_unlocked()
-                self._arm_timer()
-                return job
-        return None
+                updated_job = job
+                break
+        if updated_job is not None:
+            self._arm_timer()
+        return updated_job
 
     def remove_job(self, job_id: str) -> bool:
         with self._lock:
diff --git a/app-instance/backend/tests/unit/test_cron_service.py b/app-instance/backend/tests/unit/test_cron_service.py
index 3bbe880..b6476a0 100644
--- a/app-instance/backend/tests/unit/test_cron_service.py
+++ b/app-instance/backend/tests/unit/test_cron_service.py
@@ -1,4 +1,5 @@
 import asyncio
+import threading
 
 from beaver.foundation.models import CronExecutionResult, CronRunRecord, CronSchedule
 from beaver.tools.base import ToolContext
@@ -108,6 +109,31 @@ def test_persisted_interval_job_keeps_schedule_and_next_run(tmp_path) -> None:
     assert reloaded.next_run_at_ms == job.next_run_at_ms
 
 
+def test_running_scheduler_can_disable_job_without_deadlock(tmp_path) -> None:
+    service = CronService(tmp_path / "jobs.json")
+    job = service.add_job(
+        name="Hydration reminder",
+        message="Drink water",
+        schedule=CronSchedule(kind="every", every_ms=30 * 60 * 1000),
+    )
+    service._running = True
+    completed = threading.Event()
+    enabled_values: list[bool] = []
+
+    def disable_job() -> None:
+        updated = service.update_enabled(job.id, False)
+        if updated is not None:
+            enabled_values.append(updated.enabled)
+        completed.set()
+
+    worker = threading.Thread(target=disable_job, daemon=True)
+    worker.start()
+
+    assert completed.wait(0.5), "disabling a running cron job should not deadlock"
+    assert enabled_values == [False]
+    assert service.get_job(job.id).enabled is False
+
+
 def test_cron_tool_uses_runtime_service(tmp_path) -> None:
     service = CronService(tmp_path / "jobs.json")
     tool = CronTool()