# 跨域预检请求未通过：响应缺少 'Access-Control-Allow-Origin' 头

- **ID:** `python/fastapi-cors-preflight-failed`
- **领域:** python
- **类别:** network_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

缺少 CORSMiddleware，或 allow_origins 与浏览器来源不匹配，导致预检 OPTIONS 请求失败。

## 版本兼容性

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

## 解决方案

1. **** (93% 成功率)
   ```
   Add middleware before app start:
app.add_middleware(CORSMiddleware, allow_origins=['http://localhost:3000'], allow_methods=['*'], allow_headers=['*'])
   ```
2. **** (88% 成功率)
   ```
   List explicit origins when using credentials.
   ```

## 无效尝试

- **** — Browsers reject wildcard origin when credentials are included. (80% 失败率)
- **** — Preflight is answered before the route runs, so manual headers don't apply to OPTIONS. (70% 失败率)