First Check

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• I already checked if it is not related to FastAPI but to Pydantic.
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Example Code

# routers/one.py
from fastapi import APIRouter
from pydantic import BaseModel

router = APIRouter(prefix="/one")

class Data(BaseModel):
    state: str

class Response(BaseModel):
    data: Data

@router.get("", response_model=Response)
def get_one():
    return {"data": {"state": "ok"}}


# routers/two.py
from fastapi import APIRouter
from pydantic import BaseModel

router = APIRouter(prefix="/two")

class Data(BaseModel):
    state: str

class Response(BaseModel):
    data: Data

@router.get("", response_model=Response)
def get_two():
    return {"data": {"state": "ok"}}


# main.py
from fastapi import FastAPI
from routers import one, two

app = FastAPI()
app.include_router(one.router)
app.include_router(two.router)

Description

When generating the OpenAPI schema via app.openapi(), models with the same name across different routers (like Data) sometimes get mapped to $ref names in a non-deterministic way. For example:

Operating System

Windows

Operating System Details

No response

FastAPI Version

python -c "import fastapi; print(fastapi.version)"

Pydantic Version

python -c "import pydantic; print(pydantic.version.VERSION)"

Python Version

python --version

Additional Context

No response