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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"streamable_http client call_tool hangs when receiving invalid JSONRPCMessage","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\nIssue Description:\nDuring testing, we encountered a critical issue in the Server-Sent Events (SSE) handling mechanism:\n\n1. Problem Trigger:\nTo some reason, the `_handle_sse_event` method received a truncated `sse.data` payload, causing:\nA JSON parsing exception `message = JSONRPCMessage.model_validate_json(sse.data)`\nThen the exception handle sent the exc and returned false\n2. Current Behavior Gap:\nDespite the failed processing:\n·No error propagation to the client occurs\n·No cleanup/retry mechanism is triggered\n·The client remains stuck in a perpetual wait state for a tool_call result until timeout\n3. Critical Impact: The server has already completed the tool_call workflow and moved to the final /done state, creating a state desynchronization between client and server\n4. Root Cause Hypothesis:\nIt appears that the client failed to detect the reception of invalid responses. The `await read_stream_writer.send(exc)` did not work during exception handling\n\nOr maybe I missed some configs such as the retry mechanism or other error handling methods?\n\n### Example Code\n\n```Python\nasync def _handle_sse_event(\n    self,\n    sse: ServerSentEvent,\n    read_stream_writer: StreamWriter,\n    original_request_id: RequestId | None = None,\n    resumption_callback: Callable[[str], Awaitable[None]] | None = None,\n    is_initialization: bool = False,\n) -\u003e bool:\n    \"\"\"Handle an SSE event, returning True if the response is complete.\"\"\"\n    if sse.event == \"message\":\n        try:\n            message = JSONRPCMessage.model_validate_json(sse.data) ##### threw the exception\n            logger.debug(f\"SSE message: {message}\")\n\n            # Extract protocol version from initialization response\n            if is_initialization:\n                self._maybe_extract_protocol_version_from_message(message)\n\n            # If this is a response and we have original_request_id, replace it\n            if original_request_id is not None and isinstance(message.root, JSONRPCResponse | JSONRPCError):\n                message.root.id = original_request_id\n\n            session_message = SessionMessage(message)\n            await read_stream_writer.send(session_message)\n\n            # Call resumption token callback if we have an ID\n            if sse.id and resumption_callback:\n                await resumption_callback(sse.id)\n\n            # If this is a response or error return True indicating completion\n            # Otherwise, return False to continue listening\n            return isinstance(message.root, JSONRPCResponse | JSONRPCError)\n\n        except Exception as exc:\n            ##### handled exception but client did not receive ######\n            logger.exception(\"Error parsing SSE message\")\n            await read_stream_writer.send(exc)\n            return False\n    else:\n        logger.warning(f\"Unknown SSE event: {sse.event}\")\n        return False\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\npython == 3.12\nSDK == 1.11.0\n```","author":{"url":"https://github.com/Nuclear2","@type":"Person","name":"Nuclear2"},"datePublished":"2025-07-15T02:32:44.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":11},"url":"https://github.com/1144/python-sdk/issues/1144"}