复核 R-b8（隔离时 asyncio 记一条回调异常）：断点写入的守卫改成协程包装、在协程里查 fence——LangGraph 在节点的完成回调里调 aput_writes 拿协程交给后台任务，以前同步包装在回调里就抛 Fenced，每次隔离都打一条「Exception in callback … FuturesDict.on_done」加 traceback；现在异常留在写断点的那个任务里，由 LangGraph 照常收走。补测试：卡住被接手那一条里 asyncio 不记回调异常、不记没人取的任务异常（改前的写法这条不过）

backend/app/engine/runner.py

@@ -1786,15 +1786,19 @@ run_manager = RunManager()
 
 
 def _fenced_saver(saver: BaseCheckpointSaver) -> BaseCheckpointSaver:
-    """断点库的写也要过 cluster/fence：本进程续不上租约之后，手上的执行不再写断点（接手的进程会从断点接着跑，两边都写就分叉了）。"""
+    """断点库的写也要过 cluster/fence：本进程续不上租约之后，手上的执行不再写断点（接手的进程会从断点接着跑，两边都写就分叉了）。
+
+    包成协程、在协程里查（复核 R-b8）：LangGraph 在节点的完成回调里调 aput_writes 拿协程交给后台任务——同步包装在回调里就抛了，
+    每次隔离 asyncio 都记一条「Exception in callback … FuturesDict.on_done」；在协程里抛，异常留在写断点的那个任务里，由 LangGraph
+    照常收走。"""
     for name in ("aput", "aput_writes"):
         original = getattr(saver, name, None)
         if original is None:
             continue
 
-        def guarded(*args: Any, __original: Any = original, **kwargs: Any) -> Any:
+        async def guarded(*args: Any, __original: Any = original, **kwargs: Any) -> Any:
             fence.check()
-            return __original(*args, **kwargs)
+            return await __original(*args, **kwargs)
 
         setattr(saver, name, guarded)
     return saver

backend/tests/test_cluster_fence.py

@@ -111,8 +111,17 @@ async def _stall_and_take_over(slow) -> str:
     return run.id
 
 
-async def test_a_stalled_executor_does_not_overwrite_the_takeover(slow):
+async def test_a_stalled_executor_does_not_overwrite_the_takeover(slow, caplog):
+    import gc
+    import logging
+
+    caplog.set_level(logging.ERROR, logger="asyncio")
     run_id = await _stall_and_take_over(slow)
+    await asyncio.sleep(0.1)
+    gc.collect()
+    await asyncio.sleep(0)
+    noisy = [r.getMessage() for r in caplog.records if r.name == "asyncio"]
+    assert noisy == [], f"隔离时 asyncio 不该记回调异常、没人取的任务异常（复核 R-b8）：{noisy}"
     async with SessionLocal() as session:
         row = await session.get(Run, run_id)
     assert row.status == "interrupted" and row.owner is None, "接手之后的样子没被盖掉"