Issue with client generation for FastApi version > 118

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#1,340 opened on Oct 27, 2025 5 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 35/100

Issue type: Bug
Clarity: Needs clarification
Activity status: Stale
Tech stack: fastapi, python
Domain: api, tooling

Research direction

Start by reviewing the FastAPI pull request linked in the issue and the generator entry point that processes FastAPI's xxxx-Input and xxxx-Output schemas. Reproduce the client-generation failure with co-existing Pydantic V1 and V2 models, then add regression coverage showing that existing generated code remains compatible while the newer schemas are handled.

Written by the indexing model from the issue text.

Description

Describe the bug
Pydantic V1 and V2 models co-existing is creating an issue with backwards compatibility. Please check the latest changes from fast api viz https://github.com/fastapi/fastapi/pull/14168 which create the models as xxxx-Input and xxxx-Output which is breaking the existing code.

Dominant language: Python
Stars: 2k
Forks: 293
Avg merge: 34m
Merged PRs (30d): 1

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