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Summary

Add regression tests to verify OpenAPI schema generation when Pydantic v1 and v2 models are mixed in the same FastAPI application, both with and without separate_input_output_schemas.

Motivation

FastAPI supports applications that:

• Use Pydantic v2 for new models, and
• Still rely on Pydantic v1 models (via the compat layer) during migration.

OpenAPI generation must correctly merge schemas and references from both versions. This PR adds explicit tests to ensure:

• Schemas for both Pydantic v1 and v2 models appear in components.schemas.
• References to those schemas are valid and correctly scoped.
• Behavior is consistent when separate_input_output_schemas is enabled or disabled.

Changes

• New test module: tests/test_openapi_mixed_pydantic_versions.py

  • Defines Pydantic v1 models via may_v1.BaseModel: AddressV1, UserV1

  • Defines Pydantic v2 models via pydantic.BaseModel: AddressV2, UserV2 (nesting both v1 and v2 models)

  • Adds:

    • test_openapi_mixed_pydantic_models_with_separate_input_output_schemas

      • FastAPI(separate_input_output_schemas=True)
      • Ensures components.schemas contains entries for UserV1, UserV2, AddressV1, AddressV2
      • Asserts that references exist for each of those models somewhere in the OpenAPI document

    • test_openapi_mixed_pydantic_models_without_separate_input_output_schemas

      • FastAPI(separate_input_output_schemas=False)
      • Ensures the same schemas are present
      • Asserts that all reference values start with #/components/schemas/ (no broken references)

• Tests are guarded by:

  • @needs_pydanticv2 (only run with Pydantic v2)
  • @needs_py_lt_314 (skip on Python ≥ 3.14, where Pydantic v1 is not supported)

Testing

Executed from the repo root, in a virtual environment with requirements.txt installed:

python -m pytest tests/test_openapi_mixed_pydantic_versions.py -q

All tests pass.