How to make a custom error message for Service Unavailable in FastAPI? [duplicate]

Question

I'm running my app like so:

uvicorn main:app --host 0.0.0.0 --port 8080 --workers 2 --limit-concurrency 10

Versions are:

fastapi==0.103.2
uvicorn==0.34.3

When I start slamming it, I get the expected 503 Service Unavailable error.

I want to have a custom error message when this happens.

I thought this code would catch this error, but that code never gets touched when I get a 503.

What am I doing wrong?

app = FastAPI()


class ServiceUnavailableException(Exception):
    def __init__(self, message: str):
        self.message = message


@app.exception_handler(ServiceUnavailableException)
async def service_unavailable_exception_handler(request: Request, exc: ServiceUnavailableException):
    return JSONResponse(
        status_code=status.HTTP_503_SERVICE_UNAVAILABLE, # Use 503 status code
        content={"message": exc.message}
    )

Hiroshi Ashikaga · Accepted Answer · 2025-06-05 21:11:30Z

If you want to use custom error message instead of Uvicorn's error message, you can use middle ware. So in this case, you can create a custom middleware that catches requests resulting in a 503 error and responds with a custom message.

Here is a code.

class ServiceUnavailableMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        response = await call_next(request)
        if response.status_code == status.HTTP_503_SERVICE_UNAVAILABLE:
            return JSONResponse(
                status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
                content={"message": "Service is temporarily unavailable. Please try again later."}
            )
        return response

app = FastAPI()

app.add_middleware(ServiceUnavailableMiddleware)

I think this middleware intercepts the responses with a 503 status code and modifies them in custom message.

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