tiangolo/fastapi · error · PydanticV1NotSupportedError

pydantic.v1 models are no longer supported by FastAPI. Pleas

Error message

pydantic.v1 models are no longer supported by FastAPI. Please update the model {obj!r}.

What it means

PydanticV1NotSupportedError raised at encoders.py:342 by jsonable_encoder when it encounters a pydantic.v1 model instance. FastAPI dropped support for pydantic.v1 models; the encoder checks is_pydantic_v1_model_instance(obj) as a fallback (after lists, registered types, custom encoders) and raises with the offending object's repr. It fires whenever a response model, response_model, or manual jsonable_encoder call receives a v1 BaseModel.

Source

Thrown at fastapi/encoders.py:342

                    include=include,
                    exclude=exclude,
                    by_alias=by_alias,
                    exclude_unset=exclude_unset,
                    exclude_defaults=exclude_defaults,
                    exclude_none=exclude_none,
                    custom_encoder=custom_encoder,
                    sqlalchemy_safe=sqlalchemy_safe,
                )
            )
        return encoded_list

    if type(obj) in ENCODERS_BY_TYPE:
        return ENCODERS_BY_TYPE[type(obj)](obj)
    for encoder, classes_tuple in encoders_by_class_tuples.items():
        if isinstance(obj, classes_tuple):
            return encoder(obj)
    if is_pydantic_v1_model_instance(obj):
        raise PydanticV1NotSupportedError(
            "pydantic.v1 models are no longer supported by FastAPI."
            f" Please update the model {obj!r}."
        )
    try:
        data = dict(obj)
    except Exception as e:
        errors: list[Exception] = []
        errors.append(e)
        try:
            data = vars(obj)
        except Exception as e:
            errors.append(e)
            raise ValueError(errors) from e
    return jsonable_encoder(
        data,
        include=include,
        exclude=exclude,
        by_alias=by_alias,

View on GitHub (pinned to 42a41db11f)

Solutions

1. Re-define the model inheriting from pydantic.BaseModel (v2) instead of pydantic.v1.BaseModel.
2. If a third-party returns v1 models, convert: MyV2Model.model_validate(v1_obj.dict()) before returning.
3. Run python -c 'import pydantic; print(pydantic.VERSION)' and grep the codebase for 'pydantic.v1' / 'from pydantic.v1' to find leftover v1 usage.
4. Pin/upgrade the third-party library to a version that emits v2 models.

Example fix

# before
from pydantic.v1 import BaseModel
class Item(BaseModel):
    name: str
# after
from pydantic import BaseModel
class Item(BaseModel):
    name: str

from fastapi._compat import is_pydantic_v1_model_instance
def ensure_v2(obj):
    if is_pydantic_v1_model_instance(obj):
        raise TypeError(f'{obj!r} is a pydantic.v1 model — migrate to pydantic.BaseModel')
    return obj

import typing
from pydantic import BaseModel
def is_pydantic_v2_model(v: typing.Any) -> typing.TypeGuard[BaseModel]:
    return isinstance(v, BaseModel)

from fastapi.exceptions import PydanticV1NotSupportedError
from fastapi.encoders import jsonable_encoder
try:
    jsonable_encoder(obj)
except PydanticV1NotSupportedError:
    obj = MyV2Model.model_validate(obj.dict())  # convert v1 -> v2
    jsonable_encoder(obj)

When it happens

Trigger: Returning a pydantic.v1.BaseModel subclass instance from a path operation; passing a v1 model to jsonable_encoder(); a nested field containing a v1 model that gets traversed during encoding; using a third-party library that still hands back v1 models.

Common situations: Migrating from FastAPI 0.99- to 0.100+ and forgetting to convert models from pydantic.v1.BaseModel to pydantic.BaseModel; a dependency (e.g. older ORM/SDK) returning v1 models; copy-pasted v1 model definitions still inheriting from pydantic.v1.

Related errors

• e.errors(include_url=False)
• {"errors": exc.errors(), "body": body.decode()}
• {"errors": exc.errors(), "body": body.decode()}
• Invalid ID format, it must start with "isbn-" or "imdb-"
• Expected UploadFile, received: {type(__input_value)}

AI-assisted analysis of tiangolo/fastapi@42a41db11f (2026-08-04). Data as JSON: /data/errors/bc673065736fdc29.json. Report an issue: GitHub.