🐛 Fix unhashable Annotated type in get_definitions OpenAPI schema generation

[Yuerchu]

Summary

• get_definitions() in fastapi/_compat/v2.py uses a set comprehension to collect field annotations for deduplication
• On Python 3.14 with Pydantic v2.13+, some Annotated types contain FieldInfoMetadata objects that are not hashable, causing a TypeError when generating the OpenAPI schema:

TypeError: cannot use 'typing._AnnotatedAlias' as a set element (unhashable type: 'FieldInfoMetadata')

• This is triggered when routes use Annotated parameters with metadata such as Query(max_length=..., description=...) or similar constructs that produce unhashable Pydantic field metadata

Fix

Use id() for identity-based deduplication instead of relying on __hash__ of annotation objects. Since annotations are interned type objects, id() comparison is semantically equivalent for this filtering purpose.

Changes

-    input_types = {f.field_info.annotation for f in fields}
+    input_type_ids = {id(f.field_info.annotation) for f in fields}
     unique_flat_model_fields = {
-        f for f in flat_model_fields if f.field_info.annotation not in input_types
+        f for f in flat_model_fields if id(f.field_info.annotation) not in input_type_ids
     }

Environment

• Python 3.14.3
• Pydantic 2.13.3
• FastAPI 0.136.1

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing Yuerchu:fix/unhashable-annotated-in-get-definitions (36ce51f) with master (f43cc1c)¹}

Footnotes

1. No successful run was found on master (b445855) during the generation of this report, so f43cc1c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[YuriiMotov]

SQLModel is already in test dependencies, so you can use it to make this test more realistic.
Ideally, it should be a FastAPI app, not just get_definitions

[YuriiMotov]

Please, review changes carefully before opening PR.
AI-generated PRs without proper human review will be closed. See https://fastapi.tiangolo.com/contributing/#automated-code-and-ai