Item = Annotated[
        Union[Annotated[FirstItem, Tag("first")], Annotated[OtherItem, Tag("other")]],
        Field(discriminator="value"),
    ]

    @app.post("/items/")
    def save_union_body_discriminator(
        item: Item, q: Annotated[str, Field(description="Query string")]
    ) -> dict[str, Any]:
        return {"item": item}

    client = TestClient(app)