server: skip duplicate response on CancelledError #1153
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When a tool call is cancelled, RequestResponder.cancel() sends an error response. Previously, _handle_request would try to send another error response, causing "Request already responded to" assertion. This fix catches CancelledError and returns early, preventing the duplicate response. Fixes modelcontextprotocol#1152
ihrpr
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Jul 21, 2025
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| """Test that cancelled requests don't cause double responses.""" | |||
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There is a lot of mocking happening. I'd suggest to use test_integration or leverage create_connected_server_and_client_session and test with CancelledNotification
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Fixed, thanks for the directions!
- Use anyio.get_cancelled_exc_class() instead of asyncio.CancelledError for proper backend compatibility (supports both asyncio and trio) - Rewrote cancel handling tests to use integration testing pattern with create_connected_server_and_client_session instead of mocking - Tests now properly send CancelledNotification messages through real client-server communication - Added proper type annotations and fixed type issues in tests Addresses review feedback about using anyio idioms and integration testing. Reported-by: @ihrpr
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When clients cancel long-running tool calls, the server crashes with an assertion error and becomes unresponsive. This PR fixes the issue by preventing duplicate responses.
Problem
Solution
Catch CancelledError in _handle_request and return early without sending a second response.
Testing
Added test that cancels a tool call and verifies the server remains responsive.
Fixes #1152
Related: jlowin/fastmcp#508, jlowin/fastmcp#823