{
  "id": "python/fastapi-cors-middleware-not-applied",
  "signature": "Response: CORS error: No 'Access-Control-Allow-Origin' header is present",
  "signature_zh": "响应：CORS 错误：缺少 'Access-Control-Allow-Origin' 头。",
  "regex": "Response:\\ CORS\\ error:\\ No\\ 'Access\\-Control\\-Allow\\-Origin'\\ header\\ is\\ present",
  "domain": "python",
  "category": "config_error",
  "subcategory": null,
  "root_cause": "CORS middleware not added to FastAPI app or misconfigured, causing browser to block cross-origin requests.",
  "root_cause_type": "generic",
  "root_cause_zh": "FastAPI 应用未添加 CORS 中间件或配置错误，导致浏览器阻止跨域请求。",
  "versions": [
    {
      "version": "3.x",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    }
  ],
  "os_specific": {},
  "dead_ends": [
    {
      "action": "",
      "why_fails": "Duplicated code and easy to miss; middleware is the standard way.",
      "fail_rate": 0.6,
      "condition": "",
      "sources": []
    },
    {
      "action": "",
      "why_fails": "Browsers reject wildcard with credentials; must specify exact origins.",
      "fail_rate": 0.8,
      "condition": "",
      "sources": []
    }
  ],
  "workarounds": [
    {
      "action": "",
      "success_rate": 0.95,
      "how": "from fastapi.middleware.cors import CORSMiddleware\napp.add_middleware(\n    CORSMiddleware,\n    allow_origins=[\"*\"],\n    allow_methods=[\"*\"],\n    allow_headers=[\"*\"],\n)",
      "condition": "",
      "sources": []
    },
    {
      "action": "",
      "success_rate": 0.9,
      "how": "For credentialed requests: allow_origins=[\"https://example.com\"], allow_credentials=True",
      "condition": "",
      "sources": []
    }
  ],
  "workarounds_zh": [],
  "transition_graph": {
    "leads_to": [],
    "preceded_by": [],
    "frequently_confused_with": []
  },
  "official_doc_url": null,
  "official_doc_section": null,
  "error_code": null,
  "verification_tier": "ai_generated",
  "confidence": 0.87,
  "fix_success_rate": 0.8,
  "resolvable": "true",
  "first_seen": "2024-04-18",
  "last_confirmed": "2025-01-01",
  "last_updated": "2025-01-01",
  "evidence_count": 0,
  "tags": [],
  "locale": "en",
  "aliases": []
}