How does FastAPI handle validation errors and how can you customise the error response format?

When a request fails Pydantic validation, FastAPI automatically returns a 422 Unprocessable Entity with a structured JSON body listing every field error. You can override this default behaviour with a custom exception handler.

from fastapi import FastAPI, Request
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse
from pydantic import BaseModel, Field

app = FastAPI()

# Default 422 response body shape from FastAPI:
# {
#   "detail": [
#     {
#       "type": "missing",
#       "loc": ["body", "price"],
#       "msg": "Field required",
#       "input": {"name": "Widget"},
#       "url": "https://errors.pydantic.dev/..."
#     }
#   ]
# }

# Custom error handler — reshape the response
@app.exception_handler(RequestValidationError)
async def custom_validation_handler(
    request: Request, exc: RequestValidationError
) -> JSONResponse:
    errors = []
    for error in exc.errors():
        field = " -> ".join(str(loc) for loc in error["loc"])
        errors.append({"field": field, "message": error["msg"]})

    return JSONResponse(
        status_code=422,
        content={
            "success": False,
            "errors": errors,
        },
    )

class Product(BaseModel):
    name:  str = Field(min_length=1)
    price: float = Field(gt=0)
    stock: int   = Field(ge=0)

@app.post("/products")
def create_product(product: Product):
    return {"success": True, "product": product.model_dump()}

# Request: POST /products {}
# Custom response:
# {
#   "success": false,
#   "errors": [
#     {"field": "body -> name",  "message": "Field required"},
#     {"field": "body -> price", "message": "Field required"},
#     {"field": "body -> stock", "message": "Field required"}
#   ]
# }

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