# 响应：CORS 错误：缺少 'Access-Control-Allow-Origin' 头。

- **ID:** `python/fastapi-cors-middleware-not-applied`
- **领域:** python
- **类别:** config_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

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。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| 3.x | active | — | — |

## 解决方案

1. **** (95% 成功率)
   ```
   from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_methods=["*"],
    allow_headers=["*"],
)
   ```
2. **** (90% 成功率)
   ```
   For credentialed requests: allow_origins=["https://example.com"], allow_credentials=True
   ```

## 无效尝试

- **** — Duplicated code and easy to miss; middleware is the standard way. (60% 失败率)
- **** — Browsers reject wildcard with credentials; must specify exact origins. (80% 失败率)