From d0a49d0a0dbfb381f5e928f93788d39c2cb830d3 Mon Sep 17 00:00:00 2001 From: Lucas Van Vonderen Date: Thu, 23 Apr 2026 15:32:50 -0400 Subject: [PATCH] fix(mcp): mount session manager via Starlette Mount to avoid double response.start MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. --- mcp/app/main.py | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/mcp/app/main.py b/mcp/app/main.py index ca40953..7beafa2 100644 --- a/mcp/app/main.py +++ b/mcp/app/main.py @@ -1,7 +1,8 @@ from contextlib import asynccontextmanager -from fastapi import FastAPI, Request +from fastapi import FastAPI from mcp.server import Server from mcp.server.streamable_http_manager import StreamableHTTPSessionManager +from starlette.routing import Mount from .auth import ApiKeyMiddleware from .backend_client import backend @@ -28,15 +29,20 @@ async def lifespan(app: FastAPI): await backend.stop() -app = FastAPI(title="Homelable MCP", lifespan=lifespan) +# Mount the session manager as an ASGI sub-app instead of wrapping it in a +# FastAPI @app.api_route handler. Wrapping it in a route handler causes +# FastAPI to send http.response.start after the session manager has already +# started the response, raising `RuntimeError: Unexpected ASGI message +# 'http.response.start' sent, after response already completed` on every +# POST /mcp — which makes the server unreachable from any MCP client. +app = FastAPI( + title="Homelable MCP", + lifespan=lifespan, + routes=[Mount("/mcp", app=session_manager.handle_request)], +) app.add_middleware(ApiKeyMiddleware) -@app.api_route("/mcp", methods=["GET", "POST", "DELETE"]) -async def mcp_endpoint(request: Request): - await session_manager.handle_request(request.scope, request.receive, request._send) - - @app.get("/health") async def health(): return {"status": "ok"}