StreamableHTTPSessionManager is an ASGI app — it sends its own
http.response.start and http.response.body messages via the scope/receive/send
triple. Wrapping it inside a @app.api_route FastAPI handler causes FastAPI
to try to finalize the response after the handler returns, emitting a
second http.response.start. uvicorn rejects this with:
RuntimeError: Unexpected ASGI message 'http.response.start' sent,
after response already completed.
Every POST /mcp raises, making the server unreachable from any MCP client
(tested with Claude Code 2.x against mcp==1.27.0).
Fix: mount the session manager as a Starlette Mount so it owns the
response cycle directly. Auth middleware still applies because
add_middleware attaches at the app level, wrapping all mounted sub-apps.
Lucas Van Vonderen
d0a49d0a0d