(api definitions) Document OpenAPI OAuth and clarify parameters

fern/products/api-def/openapi-pages/auth.mdx

@@ -1,6 +1,7 @@
 ---
 title: Authentication
-subtitle: Model auth schemes such as bearer, basic, and api key. 
+description: Model auth schemes such as bearer, basic, api key, and OAuth.
+max-toc-depth: 2
 ---
 
 Configuring authentication schemes happens in the `components.securitySchemes` section of OpenAPI. All Fern-generated SDKs support both direct configuration and environment variables for authentication credentials.
@@ -181,6 +182,60 @@ client = new Client({
 })
 ```
 
+## OAuth client credentials
+
+<Markdown src="/snippets/pro-plan.mdx" />
+
+Configure OAuth 2.0 client credentials in `generators.yml` rather than in the API specification:
+
+```yaml title="generators.yml" maxLines=10
+auth-schemes:
+  OAuth:
+    scheme: oauth
+    type: client-credentials
+    client-id-env: "OAUTH_CLIENT_ID"
+    client-secret-env: "OAUTH_CLIENT_SECRET"
+    get-token:
+      endpoint: "POST /oauth/token"
+      request-properties:
+        client-id: "client_id"
+        client-secret: "client_secret"
+      response-properties:
+        access-token: "access_token"
+        expires-in: "expires_in"
+        refresh-token: "refresh_token"
+    refresh-token:
+      endpoint: "POST /oauth/refresh"
+      request-properties:
+        refresh-token: "refresh_token"
+      response-properties:
+        access-token: "access_token"
+        expires-in: "expires_in"
+api:
+  auth: OAuth
+```
+
+<Info title="Endpoint configuration and token refresh">
+The `endpoint` values (e.g., `"POST /oauth/token"`) reference paths defined in your OpenAPI specification. When `expires-in` is returned, the SDK will automatically refresh tokens before they expire. For more details on OAuth configuration options, see the [Auth scheme reference](#oauth-authentication) below.
+</Info>
+
+The generated SDK would look like: 
+
+```ts index.ts
+
+// Uses process.env.OAUTH_CLIENT_ID and process.env.OAUTH_CLIENT_SECRET
+let client = new Client(); 
+
+// Or provide credentials explicitly
+client = new Client({
+  clientId: "your_client_id",
+  clientSecret: "your_client_secret"
+})
+
+// All token management happens automatically
+await client.users.list();
+```
+
 ## Multiple security schemes
 
 If you would like to define multiple security schemes, simply 
@@ -201,11 +256,11 @@ components:
 
 ## Override security scheme
 
-You can use `generators.yml` to define custom authentication schemes that will take precedence when generating SDKs. 
+You can use `generators.yml` to define custom authentication schemes that will take precedence when generating SDKs. This is also how OAuth authentication is configured for OpenAPI specs.
 
 First, use the `auth-schemes` property to define your authentication scheme. Then, specify your auth scheme in the `api` property to override your OpenAPI spec. 
 
-```yml title="generators.yml" {1-6, 8}
+```yml title="generators.yml"
 auth-schemes:                 # Define custom auth scheme
   Bearer:                     
     scheme: bearer
@@ -219,16 +274,16 @@ api:
 ### Auth scheme reference
 
 <AccordionGroup>
-<Accordion title="Header authentication">
+<Accordion title="Header">
 <Markdown src="/products/sdks/snippets/header-auth-params.mdx" />
 </Accordion>
-<Accordion title="Basic authentication">
+<Accordion title="Basic">
 <Markdown src="/products/sdks/snippets/basic-auth-params.mdx" />
 </Accordion>
-<Accordion title="Bearer token authentication">
+<Accordion title="Bearer token">
 <Markdown src="/products/sdks/snippets/bearer-auth-params.mdx" />
 </Accordion>
-<Accordion title="OAuth authentication">
+<Accordion title="OAuth client credentials">
 <Markdown src="/products/sdks/snippets/oauth-params.mdx" />
 
 #### get-token

fern/products/sdks/capabilities.mdx

@@ -93,7 +93,7 @@ layout: overview
 
   </Card>
 
-  <Card title="OAuth Token Refresh" icon="fa-duotone fa-arrows-rotate" href="/api-definitions/ferndef/authentication#oauth-client-credentials">
+  <Card title="OAuth Token Refresh" icon="fa-duotone fa-arrows-rotate" href="/sdks/reference/generators-yml#oauth">
     Fern supports OAuth as a first class citizen
 
   <p className="text-(color:--grayscale-a11) font-bold mt-2 flex items-center gap-1">

fern/products/sdks/reference/generators-yml-reference.mdx

@@ -57,9 +57,13 @@ groups:
 
 ## `auth-schemes`
 
-Define authentication methods for your API that your endpoints can reference. Choose from custom headers (API keys), HTTP Basic, Bearer token, or OAuth 2.0 authentication.
+Define authentication methods for your API that your endpoints can reference. Authentication schemes defined in `generators.yml` take precedence over authentication schemes defined in your spec. 
 
-Alternatively, you can [define authentication for individual SDKs](#override-api-authentication-settings). 
+Choose from custom headers (API keys), HTTP Basic, Bearer token, or OAuth 2.0 authentication.
+
+<Info>
+  Alternatively, you can [define authentication for individual SDKs](#override-api-authentication-settings). 
+</Info>
 
 ```yaml title="generators.yml" maxLines=10
 auth-schemes:
@@ -95,7 +99,7 @@ auth-schemes:
 <Accordion title="Bearer token">
 <Markdown src="/products/sdks/snippets/bearer-auth-params.mdx" />
 </Accordion>
-<Accordion title="OAuth">
+<Accordion title="OAuth client credentials">
 <Markdown src="/products/sdks/snippets/oauth-params.mdx" />
 
 #### `get-token`

fern/products/sdks/snippets/oauth-get-token.mdx

@@ -1,6 +1,6 @@
-Configuration for the token acquisition endpoint.
+Specifies the endpoint that exchanges client credentials for an access token. This endpoint is called automatically when the SDK client is initialized.
 
-```yaml
+```yaml title="generators.yml"
 get-token:
   endpoint: "auth.get_token"
   request-properties:
@@ -12,29 +12,29 @@ get-token:
 ```
 
 <ParamField path="endpoint" type="string" required={true}>
-  The endpoint to get the access token, such as `'auth.get_token'`.
+  The endpoint that issues access tokens, such as `'auth.get_token'`.
 </ParamField>
 <ParamField path="request-properties" type="object" required={false}>
-  Customizes the property names used in the token request.
+  Maps OAuth parameter names to your API's request field names. Use this when your token endpoint expects different field names than the OAuth standard (e.g., your API uses `clientId` instead of `client_id`).
 </ParamField>
-<ParamField path="client-id" type="string" required={false}>
-  The property name for the client ID in the request.
+<ParamField path="request-properties.client-id" type="string" required={false}>
+  The request field name for the client ID in your API (e.g., `"clientId"`, `"client_id"`).
 </ParamField>
-<ParamField path="client-secret" type="string" required={false}>
-  The property name for the client secret in the request.
+<ParamField path="request-properties.client-secret" type="string" required={false}>
+  The request field name for the client secret in your API (e.g., `"clientSecret"`, `"client_secret"`).
 </ParamField>
-<ParamField path="scopes" type="string" required={false}>
-  The property name for the scopes in the request.
+<ParamField path="request-properties.scopes" type="string" required={false}>
+  The request field name for scopes in your API (e.g., `"scope"`, `"scopes"`).
 </ParamField>
 <ParamField path="response-properties" type="object" required={false}>
-  Maps custom property names in your OAuth token response (e.g., if your API returns `accessToken` instead of `access_token`).
+  Maps your API's response field names to OAuth standard names. Use this when your API returns tokens with different field names (e.g., `accessToken` instead of `access_token`).
 </ParamField>
-<ParamField path="access-token" type="string" required={false}>
-  The property name for the access token in the response.
+<ParamField path="response-properties.access-token" type="string" required={false}>
+  The response field name for the access token in your API (e.g., `"accessToken"`, `"access_token"`).
 </ParamField>
-<ParamField path="expires-in" type="string" required={false}>
-  The property name for the expires in property in the response.
+<ParamField path="response-properties.expires-in" type="string" required={false}>
+  The response field name for token expiration time in seconds (e.g., `"expiresIn"`, `"expires_in"`). When present, the SDK automatically refreshes tokens before expiration.
 </ParamField>
 <ParamField path="refresh-token" type="string" required={false}>
-  The property name for the refresh token in the response.
+  The response field name for the refresh token in your API (e.g., `"refreshToken"`, `"refresh_token"`). Required if using the `refresh-token` flow.
 </ParamField>

fern/products/sdks/snippets/oauth-params.mdx

@@ -1,6 +1,10 @@
-Configure OAuth 2.0 client credentials authentication.
+<Note>
+  For Fern Definition, you can configure OAuth authentication either in `generators.yml` or [directly in your `api.yml` file](/api-definitions/ferndef/authentication#oauth-client-credentials). For OpenAPI, [OAuth must be configured in `generators.yml`](/api-definitions/openapi/authentication#oauth-client-credentials).
+</Note>
 
-```yaml
+Configure OAuth 2.0 client credentials authentication. Optionally configure a `refresh-token` endpoint for token renewal without re-authentication. 
+
+```yaml title="generators.yml" maxLines=10
 auth-schemes:
   my-oauth: # User-defined scheme name
     scheme: oauth
@@ -31,15 +35,14 @@ auth-schemes:
         expires-in: "expires_in"
         refresh-token: "refresh_token"
 ```
-
 <ParamField path="scheme" type="'oauth'" required={true}>
   Must be set to `"oauth"` for OAuth authentication schemes.
 </ParamField>
-<ParamField path="type" type="literal<'client-credentials'>" required={true}>
-  The OAuth flow type. Currently only `"client-credentials"` is supported.
+<ParamField path="type" type="'client-credentials'" required={true}>
+  The OAuth 2.0 grant type. Currently only `"client-credentials"` is supported.
 </ParamField>
-<ParamField path="scopes" type="list<string>" required={false}>
-  List of OAuth scopes to request during authentication.
+<ParamField path="scopes" type="list of strings" required={false}>
+  OAuth scopes to request when obtaining access tokens (e.g., `"read:users"`, `"write:orders"`).
 </ParamField>
 <ParamField path="client-id-env" type="string" required={false}>
   Environment variable name containing the OAuth client ID. When specified, the generated SDK will automatically scan for this environment variable at initialization.
@@ -48,8 +51,8 @@ auth-schemes:
   Environment variable name containing the OAuth client secret. When specified, the generated SDK will automatically scan for this environment variable at initialization.
 </ParamField>
 <ParamField path="token-prefix" type="string" default="Bearer">
-  Sets the token header value prefix.
+  Prefix added to the access token in the Authorization header (e.g., `"Bearer"` results in `"Authorization: Bearer <token>"`). Useful when your API expects a custom format.
 </ParamField>
 <ParamField path="token-header" type="string" default="Authorization">
-  Sets the token header key name.
+  HTTP header name used to send the access token. Defaults to `"Authorization"` but can be customized if your API uses a different header (e.g., `"X-API-Token"`).
 </ParamField>

fern/products/sdks/snippets/oauth-refresh-token.mdx

@@ -1,6 +1,6 @@
-Configuration for the token refresh endpoint.
+Specifies the endpoint that exchanges a refresh token for a new access token. When configured, the SDK automatically uses this endpoint to renew expired tokens without re-sending credentials. If not configured, the SDK will re-authenticate using `get-token` when tokens expire.
 
-```yaml
+```yaml title="generators.yml"
 refresh-token:
   endpoint: "auth.refresh_token"
   request-properties:
@@ -11,23 +11,23 @@ refresh-token:
 ```
 
 <ParamField path="endpoint" type="string" required={true}>
-  The endpoint to refresh the access token, such as `'auth.refresh_token'`.
+  The endpoint that refreshes access tokens (e.g., `"POST /oauth/refresh"` or `"auth.refreshToken"`).
 </ParamField>
 <ParamField path="request-properties" type="object" required={false}>
-  Maps custom property names in your refresh token request.
+  Maps OAuth parameter names to your API's request field names for the refresh flow.
 </ParamField>
-<ParamField path="refresh-token" type="string" required={true}>
-  The property name for the refresh token in the request.
+<ParamField path="request-properties.refresh-token" type="string" required={true}>
+  The request field name for the refresh token in your API (e.g., `"refreshToken"`, `"refresh_token"`).
 </ParamField>
 <ParamField path="response-properties" type="object" required={false}>
-  Maps custom property names in your refresh token response.
+  Maps your API's refresh response field names to OAuth standard names.
 </ParamField>
-<ParamField path="access-token" type="string" required={false}>
-  The property name for the access token in the response.
+<ParamField path="response-properties.access-token" type="string" required={false}>
+  The response field name for the new access token (e.g., `"accessToken"`, `"access_token"`).
 </ParamField>
-<ParamField path="expires-in" type="string" required={false}>
-  The property name for the expires in property in the response.
+<ParamField path="response-properties.expires-in" type="string" required={false}>
+  The response field name for the new token's expiration time in seconds (e.g., `"expiresIn"`, `"expires_in"`).
 </ParamField>
-<ParamField path="refresh-token" type="string" required={false}>
-  The property name for the refresh token in the response.
+<ParamField path="response-properties.refresh-token" type="string" required={false}>
+  The response field name if your API issues a new refresh token with each refresh (token rotation).
 </ParamField>