FastAPI route validation fails with 422 after Starlette upgrade

critical

configurationUpdated Feb 27, 2026(via Exa)

Sources

How to Upgrade from Starlette 0.x to 1.0: Complete Migration Guide | BSWENdocs.bswen.com

Technologies:

FastAPISymptoms of this issue are visible in FastAPI metrics and logs

How to detect:

FastAPI routes return 422 status code instead of 200 after Starlette upgrade. FastAPI depends on Starlette; version incompatibility causes request validation failures.

Recommended action:

Check FastAPI version with `pip show fastapi`. Upgrade FastAPI to version 0.100 or higher for Starlette 1.0.0rc1 compatibility using `pip install fastapi --upgrade`. Verify request and response models. Test file uploads, custom middleware, GraphQL, and WebSocket endpoints.