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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"RequestResponder.cancel sends JSON-RPC response, violating cancellation spec","articleBody":"## Description\n\n`RequestResponder.cancel` in `mcp/shared/session.py` sends an `ErrorData(code=0, message=\"Request cancelled\")` JSON-RPC response back to the sender after a `notifications/cancelled` is received. The cancellation spec explicitly says receivers SHOULD NOT do this:\n\n\u003e Receivers of cancellation notifications **SHOULD**:\n\u003e - Stop processing the cancelled request\n\u003e - Free associated resources\n\u003e - **Not send a response for the cancelled request**\n\n— https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/docs/specification/draft/basic/utilities/cancellation.mdx#L33-L36\n\n## Reproduction\n\n`mcp` 1.27.0, Python 3.11. Driving the server over stdio:\n\n1. Send `initialize` + `notifications/initialized`.\n2. Send any `tools/call` whose handler enters an `await` (e.g. `await asyncio.sleep`).\n3. Send `notifications/cancelled` with the request id.\n4. Observe stdout: an unsolicited response is sent for the cancelled request.\n\n```json\n{\"jsonrpc\": \"2.0\", \"id\": 99, \"error\": {\"code\": 0, \"message\": \"Request cancelled\"}}\n```\n\nSource:\nhttps://github.com/modelcontextprotocol/python-sdk/blob/v1.27.0/src/mcp/shared/session.py#L137-L150\n\n```python\nasync def cancel(self) -\u003e None:\n    ...\n    self._cancel_scope.cancel()\n    self._completed = True\n    # Send an error response to indicate cancellation\n    await self._session._send_response(\n        request_id=self.request_id,\n        response=ErrorData(code=0, message=\"Request cancelled\", data=None),\n    )\n```\n\n## Impact\n\nStrict clients treat the unexpected response as an unknown message id and drop the transport. Claude Code, for example, logs `Received a response for an unknown message ID`, closes the stdio transport, and reconnects. With long-running tools (custom polling tools, long DB queries, etc.) every cancellation costs a reconnect cycle.\n\n## Suggested fix\n\nDrop the `_send_response` call from `RequestResponder.cancel`. The cancel scope is already cancelled and `_completed` is already set, so the request is correctly cleaned up server-side without violating the spec.\n\n## Workaround\n\nUntil this lands, downstream servers can monkey-patch `RequestResponder.cancel`:\n\n```python\nfrom mcp.shared.session import RequestResponder\n\nasync def _cancel_without_response(self) -\u003e None:\n    if not self._entered:\n        raise RuntimeError(\"RequestResponder must be used as a context manager\")\n    if not self._cancel_scope:\n        raise RuntimeError(\"No active cancel scope\")\n    self._cancel_scope.cancel()\n    self._completed = True\n\nRequestResponder.cancel = _cancel_without_response\n```\n\n## Environment\n- mcp 1.27.0\n- Python 3.11\n- stdio transport\n","author":{"url":"https://github.com/q-thomasdickson","@type":"Person","name":"q-thomasdickson"},"datePublished":"2026-04-20T09:26:36.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/2480/python-sdk/issues/2480"}