from pydantic import BaseModel

        class Item(BaseModel):
            name: str
            description: str | None = None

        app = FastAPI()
        router = APIRouter()

        @router.put("/items/{item_id}")
        def replace_item(item_id: str, item: Item):
            return {"message": "Item replaced", "id": item_id}

        app.include_router(router)
        ```