FastAPI删除接口返回204时执行后台任务报内容长度错误

升级Starlette到0.27.0或FastAPI到0.96.0后，删除接口返回204状态码时触发“Response content longer than Content-Length”错误，核心原因是HTTP 204规范要求响应不能包含任何内容体，但你的代码里返回了JSONResponse({}, status_code=204)——空对象{}经过JSON序列化后仍是有长度的内容，新版本框架对HTTP规范的校验更严格，因此抛出错误。

解决方案 #

直接返回不带内容体的Response对象，替代JSONResponse：

核心修改点 #

将原返回JSONResponse的逻辑，根据状态码分支处理：当状态码为204时，返回无内容体的Response；其他状态码正常返回JSONResponse。

修改后代码示例 #

from fastapi import APIRouter, Depends, status, Response, BackgroundTasks, Request

def delete_assessment(response: Response,
                      request : Request,
                      background_tasks: BackgroundTasks,
                      header: Header = Depends(HeaderParams),
                      params: DeleteAssessmentArgs = Depends(DeleteAssessmentArgs)
                      ):
    response_obj = {
        'response_code': status.HTTP_204_NO_CONTENT,
        'response_obj': {}
    }
    try:
        dao = AssessmentRequestDao()
        response_obj = dao.delete_assessment(params.assessment_id, params.is_delete_with_submitted_Responses, background_tasks, request)
        
        # 添加后台任务
        deleted_assessment = response_obj['response_obj']['categories'][0]
        event_type = AssessmentEventTypes.Deleted
        endpoint = AssessmentEndPoint.Assessment
        if response_obj['response_code'] == 204:
            background_tasks.add_task(
                onestream_publish_service.send_business_event,
                request.headers.get('Correlation-Id'),
                params.assessment_id,
                endpoint, deleted_assessment, event_type
            )

        # 按状态码返回对应响应
        if response_obj['response_code'] == status.HTTP_204_NO_CONTENT:
            return Response(status_code=status.HTTP_204_NO_CONTENT)
        else:
            return JSONResponse(response_obj['response_obj'], status_code=response_obj['response_code'])
    except Exception as e:
        logger.exception(e)
        # 异常场景同样按状态码处理
        if response_obj['response_code'] == status.HTTP_204_NO_CONTENT:
            return Response(status_code=status.HTTP_204_NO_CONTENT)
        else:
            return JSONResponse(response_obj['response_obj'], status_code=response_obj['response_code'])

额外说明 #

• HTTP 204的定义就是“无内容”，任何包含响应体的返回都不符合规范，新版本框架收紧校验才暴露了这个潜在问题。
• 后台任务的执行不受影响，FastAPI会在返回响应后才运行已添加的后台任务，只要在返回响应前完成任务添加即可。

内容的提问来源于stack exchange，提问作者user15772986