First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
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• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
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Example Code

from typing import Annotated
from fastapi import FastAPI, Query
from fastapi.testclient import TestClient
from pydantic import BaseModel
from pydantic_core import MISSING

app = FastAPI()

class Model(BaseModel):
    value: str | MISSING = MISSING

@app.get("/")
def get_item(args: Annotated[Model, Query()]):
    return args


with TestClient(app) as client:
    response = client.get("/")

    print(response.text)

Description

• FastAPI simply needs to ensure it doesn't try to copy a Sentinal. Very simple fix.

https://github.com/fastapi/fastapi/blob/master/fastapi/dependencies/utils.py#L739

from typing_extensions import Sentinel

...
return field.default if isinstance(field.default, Sentinel) else deepcopy(field.default)

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.128.0

Pydantic Version

2.12.5

Python Version

3.13.10

Additional Context

No response