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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"MCP Python SDK Protocol Compliance Gap-1:Response Sent After Receiving Cancellation Notifications","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\n### MCP Python SDK Protocol Compliance Gap: Response Sent After Receiving Cancellation Notifications\n\n- Assessment: This is a protocol “SHOULD not” compliance gap. When acting as the receiver of a cancellation notification, the SDK still sends a JSON-RPC error response for the cancelled request, contrary to the spec’s guidance.\n\n### Description\nThe MCP specification states: “Receivers of cancellation notifications SHOULD not send a response for the cancelled request.”  \nIn the current implementation, after processing a `CancelledNotification`, the SDK sends an error response for the cancelled in-flight request.\n\n### Detailed Analysis\n- Dedicated handling branch for cancellation notifications(https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/shared/session.py#L382):\n```383:387:src/mcp/shared/session.py\n                            # Handle cancellation notifications\n                            if isinstance(notification.root, CancelledNotification):\n                                cancelled_id = notification.root.params.requestId\n                                if cancelled_id in self._in_flight:\n                                    await self._in_flight[cancelled_id].cancel()\n```\n- `cancel()` sends an error response instead of silently terminating( (https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/shared/session.py#L135)):\n```135:148:src/mcp/shared/session.py\n    async def cancel(self) -\u003e None:\n        \"\"\"Cancel this request and mark it as completed.\"\"\"\n        ...\n        self._cancel_scope.cancel()\n        self._completed = True  # Mark as completed so it's removed from in_flight\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- Result: Even when the cancellation is triggered by a `CancelledNotification`, the SDK responds with a JSON-RPC error for the cancelled request, diverging from the spec’s “SHOULD not” recommendation.\n\n### Potential Impact\n- Protocol consistency: Behavior deviates from the spec’s guidance, potentially surprising peers that expect silent termination.\n- Interoperability: Differences may surface when integrating with implementations that strictly follow the “SHOULD not” behavior.\n\n### Remediation\n- Implement silent cancellation for notification-triggered cancellations:\n  - Add a non-responding cancellation path on `RequestResponder` (e.g., `mark_cancelled_without_response()`), which cancels and marks complete without calling `_send_response`.\n  - In the cancellation notification handling branch, call the silent path instead of `cancel()`.\n\n### Suggested Implementation\n- Location: `src/mcp/shared/session.py` in `BaseSession._receive_loop()` cancellation branch and in `RequestResponder`.\n\nChange call site to silent cancellation:\n- From:\n  - `await self._in_flight[cancelled_id].cancel()`\n- To:\n  - `await self._in_flight[cancelled_id].mark_cancelled_without_response()`\n\n\n### Impact\n- Aligns behavior with the MCP spec’s “SHOULD not” guidance.\n- Reduces unnecessary response traffic and improves predictability and interoperability in cancellation flows.\n\n### Example Code\n\n```Python\n\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nLatest\n```","author":{"url":"https://github.com/younaman","@type":"Person","name":"younaman"},"datePublished":"2025-10-02T15:20:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/1419/python-sdk/issues/1419"}