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 response model {type_!r}.

What it means

`create_model_field` (utils.py:66-70) raises `PydanticV1NotSupportedError` (a `FastAPIError` subclass, exceptions.py:246) when `annotation_is_pydantic_v1(type_)` is true for a field annotation. This version of FastAPI only supports pydantic v2, so any `pydantic.v1.BaseModel` subclass used as a response_model, request body, query dependency, or embedded field type triggers it at app/route construction.

Source

Thrown at fastapi/utils.py:67

    "check that {type_} is a valid Pydantic field type. "
    "If you are using a return type annotation that is not a valid Pydantic "
    "field (e.g. Union[Response, dict, None]) you can disable generating the "
    "response model from the type annotation with the path operation decorator "
    "parameter response_model=None. Read more: "
    "https://fastapi.tiangolo.com/tutorial/response-model/"
)


def create_model_field(
    name: str,
    type_: Any,
    default: Any | None = Undefined,
    field_info: FieldInfo | None = None,
    alias: str | None = None,
    mode: Literal["validation", "serialization"] = "validation",
) -> ModelField:
    if annotation_is_pydantic_v1(type_):
        raise PydanticV1NotSupportedError(
            "pydantic.v1 models are no longer supported by FastAPI."
            f" Please update the response model {type_!r}."
        )
    field_info = field_info or FieldInfo(annotation=type_, default=default, alias=alias)
    try:
        return v2.ModelField(mode=mode, name=name, field_info=field_info)
    except PydanticSchemaGenerationError:
        raise fastapi.exceptions.FastAPIError(
            _invalid_args_message.format(type_=type_)
        ) from None


def generate_operation_id_for_path(
    *, name: str, path: str, method: str
) -> str:  # pragma: nocover
    warnings.warn(
        message="fastapi.utils.generate_operation_id_for_path() was deprecated, "
        "it is not used internally, and will be removed soon",

View on GitHub (pinned to 42a41db11f)

Solutions

1. Migrate the model to pydantic v2: `from pydantic import BaseModel` and update validators/`Config` to v2 idioms.
2. If you cannot migrate, disable response model generation for that endpoint with `response_model=None`.
3. Replace the v1 model annotation with a plain `dict` or a v2 model, or return a `Response` directly.
4. Upgrade or replace the third-party package that still exposes `pydantic.v1` models.

Example fix

# before
from pydantic.v1 import BaseModel

class Item(BaseModel):
    name: str

@app.get("/items/{i}", response_model=Item)
def read_item(i: int): ...

# after
from pydantic import BaseModel

class Item(BaseModel):
    name: str

@app.get("/items/{i}", response_model=Item)
def read_item(i: int): ...

import pydantic

def assert_pydantic_v2_model(model: type) -> None:
    """Reject pydantic.v1 models used as FastAPI field/response types."""
    if hasattr(pydantic, "v1") and issubclass(model, pydantic.v1.BaseModel):
        raise TypeError(
            f"{model!r} is a pydantic.v1 model, no longer supported. Migrate to pydantic v2."
        )

# usage, e.g. in a startup scan of declared response models
for m in (Item, Order, User):
    assert_pydantic_v2_model(m)

import pydantic

def is_pydantic_v2_model(obj: object) -> bool:
    return (
        isinstance(obj, type)
        and issubclass(obj, pydantic.BaseModel)
        and not (hasattr(pydantic, "v1") and issubclass(obj, pydantic.v1.BaseModel))
    )

# Construction-time failure: catch at bootstrap so the service logs a clear cause.
from fastapi.exceptions import PydanticV1NotSupportedError

try:
    app.include_router(legacy_router)  # router exposes a v1 response_model
except PydanticV1NotSupportedError as exc:
    raise SystemExit(
        f"Startup aborted: {exc}. Migrate the model to pydantic v2 or set response_model=None."
    ) from exc

When it happens

Trigger: Using `from pydantic.v1 import BaseModel` and annotating a path operation or `response_model=` with it; a third-party library that still ships v1 models and exposes them as types; embedding a v1 model inside a v2 model field; importing `BaseModel` from the wrong (compat) location after an upgrade.

Common situations: Upgrading FastAPI/pydantic without migrating models; mixing `pydantic.v1` compat shims left over from a 1.x->2.x migration; a dependency (SDK, ORM plugin) returning v1 models that you annotate as `response_model`.

Related errors

• pydantic.v1 models are no longer supported by FastAPI. Pleas
• {"errors": exc.errors(), "body": body.decode()}
• {"errors": exc.errors(), "body": body.decode()}
• e.errors(include_url=False)
• Invalid ID format, it must start with "isbn-" or "imdb-"

AI-assisted analysis of tiangolo/fastapi@42a41db11f (2026-08-04). Data as JSON: /data/errors/f1c2c44a6cfbe32f.json. Report an issue: GitHub.