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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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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ihrpr
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Jul 22, 2025
tests/server/test_cancel_handling.py
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| @pytest.mark.anyio | ||
| async def test_cancelled_request_no_double_response(): |
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why do we need this test? it doesn't look like it's testing much and test_server_remains_functional_after_cancel looks complete to cover the fix?
ihrpr
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Jul 25, 2025
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…ol#1153) Co-authored-by: ihrpr <[email protected]>
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* Add regression test for stateless request memory cleanup (modelcontextprotocol#1140) * Implement RFC9728 - Support WWW-Authenticate header by MCP client (modelcontextprotocol#1071) * Add streamable HTTP starlette example to Python SDK docs (modelcontextprotocol#1111) * fix markdown error in README in main (modelcontextprotocol#1147) * README - replace code snippets with examples - add lowlevel to snippets (modelcontextprotocol#1150) * README - replace code snippets with examples - streamable http (modelcontextprotocol#1155) * chore: don't allow users to create issues outside the templates (modelcontextprotocol#1163) * Tests(cli): Add coverage for helper functions (modelcontextprotocol#635) * Docs: Update CallToolResult parsing in README (modelcontextprotocol#812) Co-authored-by: Felix Weinberger <[email protected]> * docs: add pre-commit install guide on CONTRIBUTING.md (modelcontextprotocol#995) Co-authored-by: Felix Weinberger <[email protected]> * fix flaky fix-test_streamablehttp_client_resumption test (modelcontextprotocol#1166) * README - replace code snippets with examples -- auth examples (modelcontextprotocol#1164) * Support falling back to OIDC metadata for auth (modelcontextprotocol#1061) * Add CODEOWNERS file for sdk (modelcontextprotocol#1169) * fix flaky test test_88_random_error (modelcontextprotocol#1171) * Make sure `RequestId` is not coerced as `int` (modelcontextprotocol#1178) * Fix: Replace threading.Lock with anyio.Lock for Ray deployment compatibility (modelcontextprotocol#1151) * fix: fix OAuth flow request object handling (modelcontextprotocol#1174) * update codeowners group (modelcontextprotocol#1191) * fix: perform auth server metadata discovery fallbacks on any 4xx (modelcontextprotocol#1193) * server: skip duplicate response on CancelledError (modelcontextprotocol#1153) Co-authored-by: ihrpr <[email protected]> * Unpack settings in FastMCP (modelcontextprotocol#1198) * chore: Remove unused prompt_manager.py file (modelcontextprotocol#1229) Co-authored-by: Tapan Chugh <[email protected]> * Improved supported for ProtectedResourceMetadata (modelcontextprotocol#1235) Co-authored-by: Paul Carleton <[email protected]> * chore: Remove unused variable notification_options (modelcontextprotocol#1238) * Improve README around the Context object (modelcontextprotocol#1203) * fix: allow to pass `list[str]` to `token_endpoint_auth_signing_alg_values_supported` (modelcontextprotocol#1226) * Remove strict validation on `response_modes_supported` member of `OAuthMetadata` (modelcontextprotocol#1243) * Add pyright strict mode on the whole project (modelcontextprotocol#1254) * Consistent casing for default headers Accept and Content-Type (modelcontextprotocol#1263) * Update dependencies and fix type issues (modelcontextprotocol#1268) Co-authored-by: Marcelo Trylesinski <[email protected]> * fix: prevent async generator cleanup errors in StreamableHTTP transport (modelcontextprotocol#1271) Co-authored-by: David Soria Parra <[email protected]> * chore: uncomment .idea/ in .gitignore (modelcontextprotocol#1287) Co-authored-by: Claude <[email protected]> * docs: clarify streamable_http_path configuration when mounting servers (modelcontextprotocol#1172) * feat: Add CORS configuration for browser-based MCP clients (modelcontextprotocol#1059) Co-authored-by: Marcelo Trylesinski <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> * Added Audio to FastMCP (modelcontextprotocol#1130) * fix: avoid uncessary retries in OAuth authenticated requests (modelcontextprotocol#1206) Co-authored-by: Felix Weinberger <[email protected]> * Add PATHEXT to default STDIO env vars in windows (modelcontextprotocol#1256) * fix: error too many values to unpack (expected 2) (modelcontextprotocol#1279) Signed-off-by: San Nguyen <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> * SDK Parity: Avoid Parsing Server Response for non-JsonRPCMessage Requests (modelcontextprotocol#1290) * types: Setting default value for method: Literal (modelcontextprotocol#1292) * changes structured temperature to not deadly (modelcontextprotocol#1328) * Update simple-resource example to use non-deprecated read_resource return type (modelcontextprotocol#1331) Co-authored-by: Claude <[email protected]> * docs: Update README to include link to API docs for modelcontextprotocol#1329 (modelcontextprotocol#1330) * Allow ping requests before initialization (modelcontextprotocol#1312) * Python lint: Ruff rules for pylint and code complexity (modelcontextprotocol#525) * Fix context injection for resources and prompts (modelcontextprotocol#1336) * fix(fastmcp): propagate mimeType in resource template list (modelcontextprotocol#1186) Co-authored-by: Felix Weinberger <[email protected]> * fix: allow elicitations accepted without content (modelcontextprotocol#1285) Co-authored-by: Olivier Schiavo <[email protected]> * Use --frozen in pre-commit config (modelcontextprotocol#1375) * Return HTTP 403 for invalid Origin headers (modelcontextprotocol#1353) * Add test for ProtectedResourceMetadataParsing (modelcontextprotocol#1236) Co-authored-by: Paul Carleton <[email protected]> Co-authored-by: Marcelo Trylesinski <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> * Fastmcp logging progress example (modelcontextprotocol#1270) Co-authored-by: Felix Weinberger <[email protected]> * feat: add paginated list decorators for prompts, resources, and tools (modelcontextprotocol#1286) Co-authored-by: Claude <[email protected]> * Remove "unconditionally" from conditional description (modelcontextprotocol#1289) * Use streamable-http consistently in examples (modelcontextprotocol#1389) * feat: Add SDK support for SEP-1034 default values in elicitation schemas (modelcontextprotocol#1337) Co-authored-by: Tapan Chugh <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> * Implementation of SEP 973 - Additional metadata + icons support (modelcontextprotocol#1357) * Merge upstream/main with custom filtering --------- Signed-off-by: San Nguyen <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> Co-authored-by: yurikunash <[email protected]> Co-authored-by: Pamela Fox <[email protected]> Co-authored-by: Inna Harper <[email protected]> Co-authored-by: Marcelo Trylesinski <[email protected]> Co-authored-by: Ian Davenport <[email protected]> Co-authored-by: Dagang Wei <[email protected]> Co-authored-by: Felix Weinberger <[email protected]> Co-authored-by: Stanley Law <[email protected]> Co-authored-by: Luca Chang <[email protected]> Co-authored-by: leweng <[email protected]> Co-authored-by: Clare Liguori <[email protected]> Co-authored-by: lukacf <[email protected]> Co-authored-by: ihrpr <[email protected]> Co-authored-by: Tapan Chugh <[email protected]> Co-authored-by: Tapan Chugh <[email protected]> Co-authored-by: Yann Jouanin <[email protected]> Co-authored-by: Paul Carleton <[email protected]> Co-authored-by: Sreenath Somarajapuram <[email protected]> Co-authored-by: Omer Korner <[email protected]> Co-authored-by: joesavage-silabs <[email protected]> Co-authored-by: Gregory L <[email protected]> Co-authored-by: David Soria Parra <[email protected]> Co-authored-by: Moustapha Ebnou <[email protected]> Co-authored-by: Max Isbey <[email protected]> Co-authored-by: Claude <[email protected]> Co-authored-by: Jerome <[email protected]> Co-authored-by: xavier <[email protected]> Co-authored-by: keurcien <[email protected]> Co-authored-by: Tim Esler <[email protected]> Co-authored-by: San Nguyen <[email protected]> Co-authored-by: Justin Wang <[email protected]> Co-authored-by: jess <[email protected]> Co-authored-by: Peter Alexander <[email protected]> Co-authored-by: Reid Geyer <[email protected]> Co-authored-by: Eleftheria Stein-Kousathana <[email protected]> Co-authored-by: Christian Clauss <[email protected]> Co-authored-by: pchoudhury22 <[email protected]> Co-authored-by: owengo <[email protected]> Co-authored-by: Olivier Schiavo <[email protected]> Co-authored-by: Steve Billings <[email protected]> Co-authored-by: Mike Salvatore <[email protected]>
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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