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Example Code

from typing import List, Optional
from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class MenuItem(BaseModel):
    name: str
    sub_items: Optional[List["MenuItem"]] = None

MenuItem.model_rebuild()  # Needed for recursive models in Pydantic v2

@app.get("/menu", response_model=MenuItem)
def get_menu():
    return MenuItem(
        name="Main Menu",
        sub_items=[
            MenuItem(name="File", sub_items=[MenuItem(name="New"), MenuItem(name="Open")]),
            MenuItem(name="Edit", sub_items=[MenuItem(name="Undo"), MenuItem(name="Redo")])
        ]
    )

Description

I want to create an API endpoint that returns nested JSON with recursive models using Pydantic.

Calling /menu returns JSON with nested items.

The OpenAPI docs currently don’t reflect the recursion properly in Swagger UI.

I am using FastAPI 0.121.3, Pydantic 2.11.1, Python 3.11.

How can I make the OpenAPI schema fully support recursive models and ensure Swagger UI displays the nested structure correctly?

Operating System

Windows

Operating System Details

No response

FastAPI Version

FastAPI Version: 0.121.3

Pydantic Version

Pydantic Version: 2.11.1

Python Version

Python Version: 3.11

Additional Context

I tried update_forward_refs() but Pydantic v2 recommends model_rebuild().

Would like an example that works with OpenAPI docs out of the box.