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Sep 19, 2025
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(api definitions) Add auth.yml example and fix client credentials syntax for Fern Definition #895

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51 changes: 45 additions & 6 deletions fern/products/api-def/ferndef-pages/auth.mdx
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Original file line number Diff line number Diff line change
Expand Up @@ -178,7 +178,7 @@ client = new Client({

## OAuth client credentials

If your API uses OAuth, you can specify an oauth scheme. Note that you'll need to define a token retrieval endpoint.
If your API uses OAuth, you can specify an oauth scheme in `api.yml` and define a token retrieval endpoint in a separate `auth.yml` file ([example](https://github.com/fern-api/fern/blob/3137938b70e058f3691ddef34d5c1cc29acc4b80/test-definitions/fern/apis/oauth-client-credentials/definition/api.yml)).

```yaml api.yml
name: api
Expand All @@ -194,15 +194,54 @@ auth-schemes:
client-id-env: YOUR_CLIENT_ID
client-secret-env: YOUR_CLIENT_SECRET
get-token:
endpoint: auth.getToken
endpoint: auth.getTokenWithClientCredentials
request-properties:
client-id: $request.client_id # Format: parameter-name: $request.property_name
client-secret: $request.client_secret # Format: parameter-name: $request.property_name
response-properties:
access-token: $response.access_token
expires-in: $response.expires_in
access-token: $response.access_token # Format: parameter-name: $response.property_name
expires-in: $response.expires_in # Format: parameter-name: $response.property_name

```

If the `expires-in` property is set, the generated OAuth token provider will automatically refresh the token when it expires.
Otherwise, it's assumed that the access token is valid indefinitely.
The `request-properties` and `response-properties` map OAuth standard parameters to your actual endpoint's request and response field names defined in `auth.yml`.

<Info>
If the `expires-in` property is set, the generated OAuth token provider will automatically refresh the token when it expires. Otherwise, it's assumed that the access token is valid indefinitely.
</Info>

The corresponding `auth.yml` file ([example](https://github.com/fern-api/fern/blob/3137938b70e058f3691ddef34d5c1cc29acc4b80/test-definitions/fern/apis/oauth-client-credentials/definition/auth.yml)) defines the token endpoint:

```yaml title="auth.yml"
types:
TokenResponse:
docs: |
An OAuth token response.
properties:
access_token: string
expires_in: integer
refresh_token: optional<string>

service:
auth: false
base-path: /
endpoints:
getTokenWithClientCredentials:
path: /token
method: POST
request:
name: GetTokenRequest
body:
properties:
client_id: string
client_secret: string
audience: literal<"https://api.example.com">
grant_type: literal<"client_credentials">
scope: optional<string>
response: TokenResponse
```

<Info>If your OAuth server is hosted at a different URL than your main API, you can use [multi-URL environments](/api-definitions/ferndef/api-yml/environments#multiple-urls-per-environment) to specify separate base URLs for authentication and API calls.</Info>

With this, all of the OAuth logic happens automatically in the generated SDKs. As long as you configure these settings, your
client will automatically retrieve an access token and refresh it as needed.
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