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Example Code

from fastapi import FastAPI
from pydantic import BaseModel
from pydantic.json_schema import SkipJsonSchema


class Foo:
    def __init__(self, name: str):
        self.name = name


app = FastAPI()


class Item(BaseModel, arbitrary_types_allowed=True):
    foo: SkipJsonSchema[Foo | None] = None
    bar: str


@app.post('/')
async def create_item(item: Item) -> Item:
    return item

Description

After upgrading to 0.115.12 from 0.112.4 the API cannot serve normally with a custom object field in its request body. The error is:

pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'arbitrary_request_field.Foo'>. Set arbitrary_types_allowed=True in the model_config to ignore this error or implement __get_pydantic_core_schema__ on your type to fully support it.

If you got this error by calling handler() within __get_pydantic_core_schema__ then you likely need to call handler.generate_schema(<some type>) since we do not call __get_pydantic_core_schema__ on <some type> otherwise to avoid infinite recursion.

For further information visit https://errors.pydantic.dev/2.11/u/schema-for-unknown-type

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.115.12

Pydantic Version

2.11.4

Python Version

3.13.2

Additional Context

No response