# 运行时错误：中间件 'CORSMiddleware' 必须在 'TrustedHostMiddleware' 之前添加才能正常工作。

- **ID:** `python/fastapi-middleware-order`
- **领域:** python
- **类别:** config_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

FastAPI 中中间件的顺序很重要；某些中间件依赖于其他中间件先被应用。

## 版本兼容性

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

## 解决方案

1. **Add middleware in the correct order** (95% 成功率)
   ```
   app.add_middleware(CORSMiddleware, allow_origins=['*'])
app.add_middleware(TrustedHostMiddleware, allowed_hosts=['example.com'])
   ```
2. **Check FastAPI documentation for middleware order** (90% 成功率)
   ```
   Consult official docs for recommended order.
   ```

## 无效尝试

- **Ignoring the order and adding randomly** — May cause CORS or host validation issues. (90% 失败率)
- **Removing one middleware to avoid the error** — Loses functionality. (70% 失败率)