Displaying of FastAPI validation errors to end users

FastAPI has a great Exception Handling, so you can customize your exceptions in many ways. You can raise an HTTPException, HTTPException is a normal Python exception with additional data relevant for APIs. But you can't return it you need to raise it because it's a Python exception from fastapi import HTTPException ... @app.get("/") async def hello(name: str): if not name: raise HTTPException(status_code=404, detail="Name field is required") return {"Hello": name} By adding name: str as a query parameter it automatically becomes required so you need to add Optional from typing import Optional ... @app.get("/") async def hello(name: Optional[str] = None): error = {"Error": "Name field is required"} if name: return {"Hello": name} return error $ curl 127.0.0.1:8000/?name=imbolc {"Hello":"imbolc"} ... $ curl 127.0.0.1:8000 {"Error":"Name field is required"} But in your case, and i think this is the best way to handling errors in FastAPI overriding the validation_exception_handler: from fastapi import FastAPI, Request, status from fastapi.encoders import jsonable_encoder from fastapi.exceptions import RequestValidationError from fastapi.responses import JSONResponse ... @app.exception_handler(RequestValidationError) async def validation_exception_handler(request: Request, exc: RequestValidationError): return JSONResponse( status_code=status.HTTP_422_UNPROCESSABLE_ENTITY, content=jsonable_encoder({"detail": exc.errors(), "Error": "Name field is missing"}), ) ... @app.get("/") async def hello(name: str): return {"hello": name} You will get a response like this: $ curl 127.0.0.1:8000 { "detail":[ { "loc":[ "query", "name" ], "msg":"field required", "type":"value_error.missing" } ], "Error":"Name field is missing" } You can customize your content however if you like: { "Error":"Name field is missing", "Customize":{ "This":"content", "Also you can":"make it simpler" } }