diff --git a/src/auth/credentials.ts b/src/auth/credentials.ts
index a9ee2e19..a50499b2 100644
--- a/src/auth/credentials.ts
+++ b/src/auth/credentials.ts
@@ -13,19 +13,57 @@
 // limitations under the License.
 
 export interface Credentials {
+  /**
+   * This field is only present if the access_type parameter was set to offline in the authentication request. For details, see Refresh tokens.
+   */
   refresh_token?: string | null;
+  /**
+   * The time in ms at which this token is thought to expire.
+   */
   expiry_date?: number | null;
+  /**
+   * A token that can be sent to a Google API.
+   */
   access_token?: string | null;
+  /**
+   * Identifies the type of token returned. At this time, this field always has the value Bearer.
+   */
   token_type?: string | null;
+  /**
+   * A JWT that contains identity information about the user that is digitally signed by Google.
+   */
   id_token?: string | null;
+  /**
+   * The scopes of access granted by the access_token expressed as a list of space-delimited, case-sensitive strings.
+   */
+  scope?: string;
 }
 
 export interface CredentialRequest {
+  /**
+   * This field is only present if the access_type parameter was set to offline in the authentication request. For details, see Refresh tokens.
+   */
   refresh_token?: string;
+  /**
+   * A token that can be sent to a Google API.
+   */
   access_token?: string;
+  /**
+   * Identifies the type of token returned. At this time, this field always has the value Bearer.
+   */
   token_type?: string;
+  /**
+   * The remaining lifetime of the access token in seconds.
+   */
   expires_in?: number;
+  /**
+   * A JWT that contains identity information about the user that is digitally signed by Google.
+   */
   id_token?: string;
+  /**
+   * The scopes of access granted by the access_token expressed as a list of space-delimited, case-sensitive strings.
+   */
+  scope?: string;
 }
 
 export interface JWTInput {