First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• 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.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

# FastAPI
# https://github.com/fastapi/fastapi/blob/master/fastapi/types.py#L10
IncEx = Union[set[int], set[str], dict[int, Any], dict[str, Any]]

# Pydantic
# https://github.com/pydantic/pydantic/blob/main/pydantic/main.py#L77
IncEx: TypeAlias = Union[set[int], set[str], Mapping[int, Union['IncEx', bool]], Mapping[str, Union['IncEx', bool]]]

Description

• FastAPI is too strict. I'd like to pass a custom mapping, but this raises a type issue. Pydantic allows it just fine, but FastAPI is out of sync.
• Since FastAPI just passes this on to Pydantic, it should be updated to be in sync.
• I'd be happy to submit a PR

Operating System

Linux

Operating System Details

N/A

FastAPI Version

0.128.10

Pydantic Version

2.12.5

Python Version

3.13.11

Additional Context

No response