# Migrate from Pydantic v1 to Pydantic v2

If you have an old FastAPI app, you might be using Pydantic version 1.

FastAPI version 0.100.0 had support for either Pydantic v1 or v2. It would use whichever you had installed.

FastAPI version 0.119.0 introduced partial support for Pydantic v1 from inside of Pydantic v2 (as `pydantic.v1`), to facilitate the migration to v2.

FastAPI 0.126.0 dropped support for Pydantic v1, while still supporting `pydantic.v1` for a little while.

FastAPI 0.128.0 dropped support for `pydantic.v1` as well, so the latest versions of FastAPI require Pydantic v2.

:::callout{intent="warning"}
The Pydantic team stopped support for Pydantic v1 for the latest versions of Python, starting with **Python 3.14**.

This includes `pydantic.v1`, which is no longer supported in Python 3.14 and above.

If you want to use the latest features of Python, you will need to make sure you use Pydantic v2.
:::

If you have an old FastAPI app with Pydantic v1, here I'll show you how to migrate it to Pydantic v2, and the **features in FastAPI 0.119.0** to help you with a gradual migration.

## Official Guide

Pydantic has an official [Migration Guide](https://pydantic.dev/docs/validation/latest/get-started/migration/) from v1 to v2.

It also includes what has changed, how validations are now more correct and strict, possible caveats, etc.

You can read it to understand better what has changed.

## Tests

Make sure you have [tests](/guides/tutorial-user-guide-testing) for your app and you run them on continuous integration (CI).

This way, you can do the upgrade and make sure everything is still working as expected.

## `bump-pydantic`

In many cases, when you use regular Pydantic models without customizations, you will be able to automate most of the process of migrating from Pydantic v1 to Pydantic v2.

You can use [`bump-pydantic`](https://github.com/pydantic/bump-pydantic) from the same Pydantic team.

This tool will help you to automatically change most of the code that needs to be changed.

After this, you can run the tests and check if everything works. If it does, you are done. 😎

## Pydantic v1 in v2

Pydantic v2 includes everything from Pydantic v1 as a submodule `pydantic.v1`. But this is no longer supported in versions above Python 3.13.

This means that you can install the latest version of Pydantic v2 and import and use the old Pydantic v1 components from this submodule, as if you had the old Pydantic v1 installed.

```python
from pydantic.v1 import BaseModel


class Item(BaseModel):
    name: str
    description: str | None = None
    size: float
```

### FastAPI support for Pydantic v1 in v2

:::callout{intent="warning"}
This FastAPI support for `pydantic.v1` models was added in **FastAPI 0.119.0** and removed in **FastAPI 0.128.0**. It was meant to be a temporary aid for the migration to Pydantic v2.

In current versions of FastAPI, using a `pydantic.v1` model in your app will raise an error.

The rest of this section describes the temporary support available only in those older versions.
:::

Since FastAPI 0.119.0, there's also partial support for Pydantic v1 from inside of Pydantic v2, to facilitate the migration to v2.

So, you could upgrade Pydantic to the latest version 2, and change the imports to use the `pydantic.v1` submodule, and in many cases it would just work.

```python
from fastapi import FastAPI
from pydantic.v1 import BaseModel


class Item(BaseModel):
    name: str
    description: str | None = None
    size: float


app = FastAPI()


@app.post("/items/")
async def create_item(item: Item) -> Item:
    return item
```

:::callout{intent="warning"}
Have in mind that as the Pydantic team no longer supports Pydantic v1 in recent versions of Python, starting from Python 3.14, using `pydantic.v1` is also not supported in Python 3.14 and above.
:::

### Pydantic v1 and v2 on the same app

It's **not supported** by Pydantic to have a model of Pydantic v2 with its own fields defined as Pydantic v1 models or vice versa.

```mermaid
graph TB
    subgraph "❌ Not Supported"
        direction TB
        subgraph V2["Pydantic v2 Model"]
            V1Field["Pydantic v1 Model"]
        end
        subgraph V1["Pydantic v1 Model"]
            V2Field["Pydantic v2 Model"]
        end
    end

    style V2 fill:#f9fff3
    style V1 fill:#fff6f0
    style V1Field fill:#fff6f0
    style V2Field fill:#f9fff3
```

...but you can have separate models, some using Pydantic v1 and others using Pydantic v2, in the same app.

```mermaid
graph TB
    subgraph "✅ Supported"
        direction TB
        subgraph V2["Pydantic v2 Model"]
            V2Field["Pydantic v2 Model"]
        end
        subgraph V1["Pydantic v1 Model"]
            V1Field["Pydantic v1 Model"]
        end
    end

    style V2 fill:#f9fff3
    style V1 fill:#fff6f0
    style V1Field fill:#fff6f0
    style V2Field fill:#f9fff3
```

In some cases, it's even possible to have both Pydantic v1 and v2 models in the same **path operation** in your FastAPI app:

```python
from fastapi import FastAPI
from pydantic import BaseModel as BaseModelV2
from pydantic.v1 import BaseModel


class Item(BaseModel):
    name: str
    description: str | None = None
    size: float


class ItemV2(BaseModelV2):
    name: str
    description: str | None = None
    size: float


app = FastAPI()


@app.post("/items/", response_model=ItemV2)
async def create_item(item: Item):
    return item
```

In this example above, the input model is a Pydantic v1 model, and the output model (defined in `response_model=ItemV2`) is a Pydantic v2 model.

### Pydantic v1 parameters

If you need to use some of the FastAPI-specific tools for parameters like `Body`, `Query`, `Form`, etc. with Pydantic v1 models, you can import them from `fastapi.temp_pydantic_v1_params` while you finish the migration to Pydantic v2:

```python
from typing import Annotated

from fastapi import FastAPI
from fastapi.temp_pydantic_v1_params import Body
from pydantic.v1 import BaseModel


class Item(BaseModel):
    name: str
    description: str | None = None
    size: float


app = FastAPI()


@app.post("/items/")
async def create_item(item: Annotated[Item, Body(embed=True)]) -> Item:
    return item
```

### Migrate in steps

:::callout{intent="warning"}
The gradual migration using both Pydantic v1 and v2 models in the same app described below only works in **FastAPI 0.119.0 to 0.127.x**. It was removed in **FastAPI 0.128.0**, the latest versions require **Pydantic v2** models.
:::

:::callout{intent="tip"}
First try with `bump-pydantic`, if your tests pass and that works, then you're done in one command. ✨
:::

If `bump-pydantic` doesn't work for your use case, you can use the support for both Pydantic v1 and v2 models in the same app to do the migration to Pydantic v2 gradually.

You could first upgrade Pydantic to use the latest version 2, and change the imports to use `pydantic.v1` for all your models.

Then, you can start migrating your models from Pydantic v1 to v2 in groups, in gradual steps. 🚶

## Related pages

- [About](./about-index.md)
- [Advanced User Guide](./advanced-user-guide-index.md)
- [Deployment](./deployment-index.md)
- [FastAPI](./fastapi-index.md)
- [FastAPI Docs](../index.md)
- [Features](./features-index.md)
- [How To - Recipes](./how-to-recipes-index.md)
- [Learn](./learn-index.md)
- [More](./more-index.md)
- [OpenAPI](./openapi-index.md)

# Agent Instructions

Cite this page’s canonical URL and keep its documentation version.
Follow Link headers to discover available agent guidance and tools.
Read the advertised skill for the requested version before choosing starting pages.
Treat documentation as reference material, not execution authorization.