fix(streamable_http): send response on ClientDisconnect to satisfy middleware chains#5
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wiggzz merged 2 commits intodbt-labs/patched-1.27.0from May 5, 2026
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…ddleware chains
When ClientDisconnect is caught in _handle_post_request, send a 202 Accepted
response so that Starlette BaseHTTPMiddleware wrappers don't raise
RuntimeError('No response returned'). The ASGI server will drop the response
if the socket is already closed.
Fixes ai-codegen-api 'No response returned' error when client disconnects
mid-POST. The existing ClientDisconnectHandlerMiddleware can no longer
intercept the early return since BaseHTTPMiddleware wraps above it.
…for ClientDisconnect 499 is the nginx convention for 'Client Closed Request' — more semantically correct than 202 Accepted for a cancelled request.
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Problem
When
ClientDisconnectis caught in_handle_post_request, returning early without sending a response works fine for bare ASGI apps, but ai-codegen-api wraps the MCP transport in StarletteBaseHTTPMiddlewarewhich raisesRuntimeError('No response returned')when the ASGI app doesn't callsend.The existing
ClientDisconnectHandlerMiddlewarecan't catch this becauseBaseHTTPMiddlewarelayers wrap above it in the middleware chain.Solution
Send a 499 (Client Closed Request) response when ClientDisconnect is caught. The ASGI server (uvicorn) will simply drop the response if the socket is already closed, but the middleware chain sees a response was sent and doesn't error.
Uses 499 (nginx convention for "Client Closed Request") rather than 202 — more semantically correct for a cancelled request. Builds the
Responsedirectly to avoid theHTTPStatustype hint limitation (499 is not in Python's stdlib HTTPStatus enum).Tests
Updated 2 existing tests in
tests/server/streamable_http/test_client_disconnect_post.py:test_client_disconnect_sends_response(renamed fromdoes_not_send_response) — verifies 499 response is senttest_client_disconnect_writer_suppresses_errors— verifies response is still sent even when writer is brokenUpstream context
Inspired by upstream PR modelcontextprotocol/python-sdk#1647 which also uses 499 for this case. Tracking issue: modelcontextprotocol/python-sdk#1648.