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# Depending on other APIs

Fern Definition isn't recommended for new customers and Fern isn't accepting feature requests for this format. It remains supported for existing users.

Fern allows you to import other APIs into your API.

This is often useful if:

* you want to reuse another API's types in your API
* you want to combine multiple microservices' APIs into one SDK (similar to the AWS SDK)

## Registering the dependency API

The first step is to **register** the API you want to depend on. To do this, use
the `register` command:

```
$ fern register
[some-dependency]: Uploading definition...
[some-dependency]: Registered @fern/some-dependency:0.0.1
```

## Depending on the registered API

To add a dependency on another API, you must add a `dependencies.yml` to declare which
APIs you wish to depend on.

Your `dependencies.yml`has a list of all the APIs you wish to depend:

**`dependencies.yml`**

```yaml dependencies.yml 
dependencies:
   "@fern/some-dependency": "0.0.1"
```

Next, you need create a folder in your Fern Definition to house the dependency. Inside the folder, create a special file
`__package__.yml` which specifies the dependency and version you want to add.

**`__package__.yml`**

```yaml __package__.yml
export:
  dependency: "@fern/some-dependency"
```

When you generate the SDK with `fern generate`, the `__package__.yml` file will
effectively be replaced with the API you're depending on.

> Import API Definitions to generate unified SDKs