# RuntimeError: 应用启动后无法再添加中间件

- **ID:** `python/starlette-middleware-order-500`
- **领域:** python
- **类别:** runtime_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

在 ASGI 应用开始处理请求后（例如在 startup 事件或路由中）调用了 add_middleware。

## 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)
   ```
   Register middleware at module level before the app starts:
app.add_middleware(GZipMiddleware)
   ```
2. **** (85% 成功率)
   ```
   Use lifespan for startup logic, not middleware registration.
   ```

## 无效尝试

- **** — Startup runs after the middleware stack is built; the error is raised. (90% 失败率)
- **** — The middleware is never applied, so behavior silently differs. (75% 失败率)