FazBrowse GitHub Viewer | Trending |
URL:
| Home
Tools: [Download Repo ZIP]   [Original HTTPS Page]

fix: stop sending JSON-RPC response on request cancellation by flobo3 · Pull Request #2481 · modelcontextprotocol/python-sdk · GitHub

/ python-sdk Public

fix: stop sending JSON-RPC response on request cancellation - #2481

Closed
flobo3 wants to merge 1 commit into
modelcontextprotocol:mainfrom
flobo3:fix/cancel-no-response
ConversationCommits (1)ChecksFiles changed
Closed

fix: stop sending JSON-RPC response on request cancellation#2481
flobo3 wants to merge 1 commit into
modelcontextprotocol:mainfrom
flobo3:fix/cancel-no-response

Conversation

flobo3 commented Apr 20, 2026

Copy link
Copy Markdown

Summary

RequestResponder.cancel() in mcp/shared/session.py sent an ErrorData(code=0, message="Request cancelled") JSON-RPC response back to the sender after a notifications/cancelled was received. The MCP cancellation spec explicitly states:

Receivers of cancellation notifications SHOULD NOT send a response for the cancelled request.

This unexpected response caused strict clients (e.g. Claude Code) to treat it as an unknown message ID, close the transport, and reconnect — making every cancellation of a long-running tool cost a full reconnect cycle.

Changes

  • src/mcp/shared/session.py: Removed _send_response() call from RequestResponder.cancel(). Added spec reference comment.
  • src/mcp/server/lowlevel/server.py: Updated log message and comment to reflect that no response is sent (was "duplicate response suppressed", now "no response sent per spec").
  • tests/shared/test_session.py: Rewrote test_request_cancellation to verify the server handler's cancel scope is cancelled, rather than expecting an MCPError response.
  • tests/server/test_cancel_handling.py: Added request_read_timeout_seconds to the cancelled request so it times out cleanly (no response is sent).

Fixes #2480

dwis-hacks commented Apr 20, 2026

Copy link
Copy Markdown

This is the right fix — sending an ErrorData response after notifications/cancelled violates JSON-RPC 2.0 which says you should send at most one response per request. Having both a notification and an error response for the same request ID is the kind of thing that makes conformant clients confused about whether to expect more data. Does this change affect any of the existing integration tests for the cancellation flow?

maxisbey commented May 18, 2026

Copy link
Copy Markdown
Contributor

Closing in favor of #2493, which also resolves the sender's local pending response (matching the TypeScript SDK), so the caller doesn't hang until timeout when the response is no longer sent. Thanks for the contribution.

AI Disclaimer

maxisbey closed this May 18, 2026
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Show hidden characters
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Reviewers

No reviews

Assignees

No one assigned

Labels

None yet

Projects

None yet

Milestone

No milestone

Development

Successfully merging this pull request may close these issues.

RequestResponder.cancel sends JSON-RPC response, violating cancellation spec

3 participants

Back | FazBrowse Home | New Git URL