python runtime_error ai_generated true

RuntimeError: Unable to set headers after the response has started

ID: python/starlette-middleware-response-already-started

Also available as: JSON · Markdown · 中文
80%Fix Rate
84%Confidence
0Evidence
2024-10-05First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
0.30+ active — — —

Root Cause

A middleware tries to mutate headers after `await send(message)` has already sent `http.response.start`.

generic

中文

中间件在 `await send(message)` 已发送 `http.response.start` 后尝试修改头。

Workarounds

  1. 95% success 
    async def __call__(self, scope, receive, send):
    async def send_wrapper(message):
        if message['type'] == 'http.response.start':
            message['headers'].append((b'x-custom', b'1'))
        await send(message)
    await self.app(scope, receive, send_wrapper)
  2. 90% success 
    from starlette.middleware.base import BaseHTTPMiddleware
class M(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        resp = await call_next(request)
        resp.headers['x-custom'] = '1'
        return resp

Dead Ends

Common approaches that don't work:

  1. 85% fail

    Headers are silently missing; client sees wrong behavior.

  2. 95% fail

    State flag doesn't alter ASGI message ordering.