How to use root model for cookie parameter?

[Danstiv]

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Example Code

from typing import Annotated

from fastapi import Cookie, FastAPI
from pydantic import Field, RootModel

Key = RootModel[Annotated[str, Field(min_length=4)]]


app = FastAPI()

@app.post('/test/')
def test(key: Annotated[Key, Cookie()]):
    pass

Description

I'm trying to use RootModel as a Cookie parameter, but I'm getting the following error:
AssertionError: Param: key can only be a request body, using Body()

I'm guessing that it's not that easy to define what exactly a root model is, and it can't be used that way. Therefore, I will describe what I want to achieve.

Our project has a certain str root model with many constraints, a simplified version is used in the code example.

This root model is used as a nested model in several models, and now there is a need to use it in several fastapi endpoints.

So my question is: can I reuse such a root model in both pydantic models and fastapi endpoints?

And if this is not possible, is it possible to implement the same reusable entity without using the RootModel, or is the only solution to duplicate the code?

Operating System

Linux, Windows

Operating System Details

No response

FastAPI Version

0.109.2

Pydantic Version

2.5.1

Python Version

Python 3.12.0

Additional Context

No response

[jgould22]

Looks like this is currently an issue with all param types and root model

#11101