meows: int

    class Dog(BaseModel):
        pet_type: str = "dog"
        barks: float

    def get_pet_type(v):
        assert isinstance(v, dict)
        return v.get("pet_type", "")

    Pet = Annotated[
        Union[Annotated[Cat, Tag("cat")], Annotated[Dog, Tag("dog")]],
        Discriminator(get_pet_type),
    ]

    app = FastAPI()

    @app.post("/pet/assignment")