[QUESTION] How to change response model for "422 Unprocessable Entity" in OpenAPI docs

Description I am trying to return my own response model for 422 Unprocessable Entity error.

I added a custom exception handler to my app, and add my response model type to responses parameter of router.post().

def MyValidationError(BaseModel):
  message: str

@app.exception_handler(RequestValidationError)
def handle_validation_error(request, exc) -> JSONResponse:
    return JSONResponse(
        status_code=HTTP_422_UNPROCESSABLE_ENTITY,
        content=MyValidationError('invalid parameter').dict(),
    )

@router.post("/user", responses={422: {"model": MyValidationError}})
def foo(user_id: str, user_name: str):
  ...

/user works as I expected.

But /openapi.json still refers the default HTTPValidationError model.

{
  ...
  "paths": {
    "/user": {
      "post": {
        "responses": {
          "422": {
            "description": "Validation Error",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/HTTPValidationError"
                }

I found the HTTPValidationError schema sesms to be hardcoded in fastapi/openapi/utils.py.

I expected that response models in openapi.json should reflect either custom exception handler for RequestValidationError or responses parameter for routes.

Is this a bug? If not, is there any way to set correct response models for status 422 in documents?

[phy25]

@rakesh4real https://fastapi.tiangolo.com/advanced/extending-openapi/

[kokumura]

OK. I got it. I’ll try to modify the OpenAPI schema. Thanks!