OpenAPI v 3.0.X invalid when model can be None

[panfilenok-epam]

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Example Code

from fastapi import FastAPI
from fastapi.encoders import jsonable_encoder
from fastapi.responses import JSONResponse
from fastapi.openapi.utils import get_openapi
from pydantic import BaseModel

class Item(BaseModel):
    name: str

class Model(BaseModel):
    item: Item | None

app = FastAPI()

def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="Test",
        version="1.0.0",
        openapi_version="3.0.2",
        routes=app.routes,
    )

    app.openapi_schema = openapi_schema
    return app.openapi_schema

app.openapi = custom_openapi

@app.get("/", response_model=Model)
async def get() -> JSONResponse:
    return JSONResponse(content=jsonable_encoder(Model(item=None)), status_code=200)

Description

The problem is when an OpenAPI document is generated it contains the following:

...{"properties":{"item":{"anyOf":[{"$ref":"#/components/schemas/Item"},{"type":"null"}]}}...

which appears to be invalid for the document schema v3.0.x
Structural error at components.schemas.Model.properties.item.anyOf.1.type should be equal to one of the allowed values allowedValues: array, boolean, integer, number, object, string

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.109.0

Pydantic Version

2.5.2

Python Version

3.11.7

Additional Context

No response

[JavierSanchezCastro]

It seems a bit weird, but if I run the code with your custom_openapi and a test endpoint, I get an error since you omitted the title parameter in get_openapi.

In another case, try to show and upload an MRE we can just run.

[panfilenok-epam]

thank you for the fast reply. I have updated the code and the error message

[panfilenok-epam]

Any update?