python network_error ai_generated true

Access to fetch at 'http://localhost:8000/api' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

ID: python/fastapi-cors-header-missing

Also available as: JSON · Markdown · 中文
80%Fix Rate
90%Confidence
0Evidence
2024-03-30First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.x active — — —

Root Cause

CORSMiddleware is not installed or does not include the requesting origin, so the browser blocks the response.

generic

中文

未安装 CORSMiddleware 或其允许的源不包含请求来源，因此浏览器阻止了响应。

Workarounds

  1. 95% success 
    from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(CORSMiddleware, allow_origins=['http://localhost:3000'], allow_credentials=True, allow_methods=['*'], allow_headers=['*'])
  2. 88% success 
    app.add_middleware(CORSMiddleware, allow_origin_regex=r'https://.*\.example\.com')

Dead Ends

Common approaches that don't work:

  1. 90% fail

    Browsers reject wildcard origin when credentials are included.

  2. 95% fail

    Not possible for end users; only a local dev flag.