---

• user/{id} but we don't have that specific id.
• abort(400) with some status code

examples/fastapi/send-400-error/main.py

from fastapi import FastAPI, Response, status

app = FastAPI()


@app.get("/user/{user_id}")
async def root(user_id: int, response: Response):
    if user_id > 40:
        response.status_code = status.HTTP_400_BAD_REQUEST
        return {'detail': 'User does not exist'}
    return {'user': user_id}