Lucas Van Vonderen
d0a49d0a0d
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.
49 lines
1.4 KiB
Python
49 lines
1.4 KiB
Python
from contextlib import asynccontextmanager
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from fastapi import FastAPI
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from mcp.server import Server
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from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
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from starlette.routing import Mount
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from .auth import ApiKeyMiddleware
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from .backend_client import backend
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from .resources import register_resources
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from .tools import register_tools
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mcp_server = Server("homelable")
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register_resources(mcp_server)
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register_tools(mcp_server)
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session_manager = StreamableHTTPSessionManager(
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app=mcp_server,
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json_response=False,
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stateless=True,
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)
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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await backend.start()
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async with session_manager.run():
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yield
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await backend.stop()
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# Mount the session manager as an ASGI sub-app instead of wrapping it in a
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# FastAPI @app.api_route handler. Wrapping it in a route handler causes
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# FastAPI to send http.response.start after the session manager has already
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# started the response, raising `RuntimeError: Unexpected ASGI message
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# 'http.response.start' sent, after response already completed` on every
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# POST /mcp — which makes the server unreachable from any MCP client.
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app = FastAPI(
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title="Homelable MCP",
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lifespan=lifespan,
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routes=[Mount("/mcp", app=session_manager.handle_request)],
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)
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app.add_middleware(ApiKeyMiddleware)
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@app.get("/health")
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async def health():
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return {"status": "ok"}
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