# AN-2026-0794: FastAPI 0.126-0.128 (Dec 2025): Pydantic v1 and pydantic.v1 models dropped: "pydantic.v1 models are no longer supported by FastAPI."

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- id: AN-2026-0794 (version 1; cite https://breakwire.org/claim/AN-2026-0794/v/1)
- kind: breaking · status: active · review: editor
- source: <https://fastapi.tiangolo.com/release-notes/#:~:text=Drop%20support%20for%20pydantic.v1>
- more sources: <https://github.com/fastapi/fastapi/blob/0.162.0/fastapi/utils.py#L66-L70> <https://pypi.org/project/fastapi/0.128.0/>
- event date: 2025-12-27
- services: fastapi, pydantic, python
- affected: pypi:fastapi >=0.128.0
- severity: high
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. FastAPI release notes (opened 2026-11-04): 0.126.0 (2025-12-20) dropped support for Pydantic v1, keeping short temporary support for Pydantic v2's pydantic.v1, with minimum pydantic >=2.7.0; 0.127.0 (2025-12-21) added deprecation warnings when using pydantic.v1; 0.128.0 (2025-12-27) Breaking Changes: Drop support for pydantic.v1.  [✓0 ✗0]
1. FastAPI source (0.162.0 fastapi/utils.py) raises PydanticV1NotSupportedError "pydantic.v1 models are no longer supported by FastAPI. Please update the response model ..." when a route uses a pydantic.v1 model.  [✓0 ✗0]
2. Reproduced 2026-11-04 with fastapi 0.162.0 + pydantic 2.14.2: declaring a route whose body parameter or response_model is a `from pydantic.v1 import BaseModel` class raises fastapi.exceptions.PydanticV1NotSupportedError with that text at decoration time (the message says 'response model' for body parameters too).  [✓0 ✗0]

## Fix
```text
Migrate the models to Pydantic v2 (`from pydantic import BaseModel`; `.dict()` -> `.model_dump()`, `@validator` -> `@field_validator`, `class Config` -> `model_config`). As a stopgap pin `fastapi<0.128` (pydantic.v1 still works with a warning) or `fastapi<0.126` for real Pydantic v1.
```

## Error signatures
- substring: `pydantic.v1 models are no longer supported by FastAPI.` (fastapi/utils.py PydanticV1NotSupportedError (0.162.0 L66-70); reproduced on 0.162.0)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0794/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary sources opened 2026-11-04 (FastAPI release notes + 0.162.0 source, Starlette release notes + 1.10.0 source; messages reproduced on fastapi 0.162.0 / starlette 1.10.0)