Symptom

After a tool task is cancelled, the next model call fails with INVALID_CHAT_HISTORY.

Evidence what you'll see

An assistant message has tool_calls with no matching tool messages
The tool was stopped with task.cancel() or a timeout that raises CancelledError

Cause

asyncio.CancelledError subclasses BaseException, not Exception. ToolNode's error handler catches only Exception, so no error tool message is written.

Fix steps

0 of 3 done

Step 1Confirm the tool was cancelled rather than raising an error.
Step 2Inside the tool, turn timeouts into a normal Exception (for example, catch the timeout and raise a RuntimeError with a clear message) instead of cancelling the task.
Step 3Track issue #6726 for an upstream fix.

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Verify the fix worked

Force a timeout: the history now contains an error tool message and the next turn runs.

Did this fix it?

Sources and credit

Summarized in our own words from these public reports. Credit goes to their authors.

langgraph issue #6726 ↗

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