FastMCP: streamable_http_app() silently breaks when BaseHTTPMiddleware is added

Commencez par le point d’entrée FastMCP.streamable_http_app() et examinez comment le middleware est enregistré autour de l’application HTTP streamable. Reproduisez l’échec avec l’exemple no-op de BaseHTTPMiddleware, puis déterminez comment la détection au démarrage et le comportement des avertissements doivent fonctionner. Le travail est terminé lorsque l’échec est signalé clairement et que les utilisateurs sont orientés vers le Middleware de FastMCP ou le contournement ASGI brut.

Description

Adding a Starlette BaseHTTPMiddleware (e.g., for auth) to FastMCP.streamable_http_app() silently breaks the MCP server. Every request crashes with ClosedResourceError — the client sees "peer closed connection without sending complete message body."

Reproduction

from mcp.server.fastmcp import FastMCP
from starlette.middleware.base import BaseHTTPMiddleware

mcp = FastMCP("test")

# Any BaseHTTPMiddleware — even a no-op passthrough
class AuthMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        return await call_next(request)

mcp.streamable_http_app().add_middleware(BaseHTTPMiddleware, dispatch=AuthMiddleware)

Run the server, send any MCP initialize request → ClosedResourceError.

Root cause

BaseHTTPMiddleware wraps the ASGI receive/send channels in a way incompatible with SSE streaming. This is a known Starlette limitation (encode/starlette#919), but FastMCP users hit it naturally when they reach for the obvious auth pattern.

Expected behavior

At minimum: warn at startup when a BaseHTTPMiddleware subclass is detected on a streamable HTTP app. Ideally: document prominently that users should use FastMCP's own Middleware class (fastmcp.server.middleware.Middleware) instead of Starlette's BaseHTTPMiddleware.

Workaround (raw ASGI middleware)

from starlette.types import ASGIApp, Receive, Scope, Send

class RawAuthMiddleware:
    def __init__(self, app: ASGIApp):
        self.app = app

    async def __call__(self, scope: Scope, receive: Receive, send: Send):
        # auth check here — read headers from scope
        await self.app(scope, receive, send)

mcp.streamable_http_app().add_middleware(RawAuthMiddleware)

Environment

• mcp / FastMCP from modelcontextprotocol/python-sdk
• Starlette (any version — this is architectural, not a regression)
• Python 3.12