backend/routers/slmm.py

@@ -269,10 +269,15 @@ async def proxy_websocket_monitor(websocket: WebSocket, unit_id: str):
         done, pending = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)
         for t in pending:
             t.cancel()
-        for t in pending:
+        # Await ALL tasks (the done one AND the cancelled one) and swallow both
+        # the expected WebSocketDisconnect and CancelledError. CancelledError is a
+        # BaseException, so a bare `except Exception` misses it — that's what leaked
+        # the traceback on stop; and awaiting only `pending` left the done task's
+        # exception unretrieved.
+        for t in tasks:
             try:
                 await t
-            except Exception:
+            except (asyncio.CancelledError, Exception):
                 pass
 
     except websockets.exceptions.WebSocketException as e: