Handle Abrupt Server Disconnect in Middle of Tool Call #1321
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fix: handle transport exceptions and ensure proper cleanup of in-flig…
felixweinberger
requested changes
Sep 23, 2025
| try: | ||
| await ctx.read_stream_writer.send(e) | ||
| finally: | ||
| await ctx.read_stream_writer.aclose() |
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Closing on any type of exception seems to broad - we should be more explicit about the failure cases that definitely mean the session is unusable, there might be recoverable errors.
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| # Transport-level exception. Forward it to the incoming | ||
| # handler for logging/observation, then fail all | ||
| # in-flight requests so callers don't hang forever. | ||
| await self._handle_incoming(message) | ||
| error = ErrorData(code=CONNECTION_CLOSED, message=str(message)) | ||
| # Send error to any pending request response streams immediately | ||
| for id, stream in list(self._response_streams.items()): | ||
| try: | ||
| await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error)) | ||
| await stream.aclose() | ||
| except Exception: | ||
| pass | ||
| self._response_streams.clear() | ||
| # Break out of the receive loop; connection is no longer usable. | ||
| break |
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This seems like a duplication of all the logic we have at the bottom of _receive_loop (L440-450) (except for the break)
The changes here also don't seem necessary to make the tests you added pass - do we need this?
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Propagates transport disconnects (e.g. server killed mid tool call) as CONNECTION_CLOSED errors instead of hanging indefinitely. Adds regression test.
Motivation and Context
A call_tool over the StreamableHTTP transport would hang forever if the server closed the SSE/HTTP connection mid-response (e.g. process crash/kill). The underlying RemoteProtocolError was logged but in‑flight requests never completed. This change converts that low-level failure into a deterministic JSON-RPC error for every pending request, matching expected resilience semantics.
How Has This Been Tested?
Breaking Changes
None. Behavior only changes for abnormal transport termination; normal flows unaffected. No public API changes.
Types of changes
Checklist
Additional context
Implementation details:
In _handle_sse_response (client transport) on exception: send the exception into the session read stream then close the writer to signal termination.
In BaseSession._receive_loop: on receiving an Exception, broadcast a synthesized JSONRPCError (code CONNECTION_CLOSED) to all pending request streams and clear them.
Integrated test placed alongside existing StreamableHTTP tests for consistency. Potential follow-up: introduce optional reconnection/resumption logic for recoverable disconnects.