diff --git a/handwritten/storage/src/file.ts b/handwritten/storage/src/file.ts
index 27765d935a99..786998c5f4e4 100644
--- a/handwritten/storage/src/file.ts
+++ b/handwritten/storage/src/file.ts
@@ -148,21 +148,94 @@ export interface SignedPostPolicyV4Output {
   url: string;
   fields: PolicyFields;
 }
-
 export interface GetSignedUrlConfig
   extends Pick<SignerGetSignedUrlConfig, 'host' | 'signingEndpoint'> {
+  /**
+   * The action to permit with the signed URL.
+   * - `'read'`: Allows downloading/viewing the file (HTTP GET).
+   * - `'write'`: Allows uploading/overwriting the file (HTTP PUT).
+   * - `'delete'`: Allows removing the file (HTTP DELETE).
+   * - `'resumable'`: Allows resumable uploads (HTTP POST).
+   * Note: When using `'resumable'`, the header `X-Goog-Resumable: start` must be sent in the client request.
+   */
   action: 'read' | 'write' | 'delete' | 'resumable';
+
+  /**
+   * The signing version to use.
+   * @default 'v2'
+   */
   version?: 'v2' | 'v4';
+
+  /**
+   * Determines the URL structure for accessing bucket resources.
+   * - `true`: Uses virtual hosted-style URLs (e.g., `https://mybucket.storage.googleapis.com/...`)
+   * - `false`: Uses path-style URLs (e.g., `https://storage.googleapis.com/mybucket/...`).
+   * Virtual hosted-style URLs are generally preferred.
+   * @default false
+   */
   virtualHostedStyle?: boolean;
+
+  /**
+   * The custom domain name (CNAME) mapped to this bucket (e.g., `"https://cdn.example.com"`).
+   */
   cname?: string;
+
+  /**
+   * The MD5 digest value in base64. If provided, the client request **must**
+   * include an identical `Content-MD5` HTTP header.
+   * If omitted, the client request must not include this header.
+   */
   contentMd5?: string;
+
+  /**
+   * The expected Content-Type of the file. If provided, the client request **must**
+   * include an identical `Content-Type` HTTP header.
+   * If omitted, the client request must not include this header.
+   */
   contentType?: string;
+
+  /**
+   * The expiration timestamp for the link. Any provided value is passed directly to `new Date()`.
+   * @throws {Error} If an expiration timestamp from the past is given.
+   * Note: `'v4'` signing supports a maximum duration of 7 days (604,800 seconds) from the creation time.
+   */
   expires: string | number | Date;
+
+  /**
+   * The timestamp when this link becomes usable. Any provided value is passed directly to `new Date()`.
+   * @default Date.now()
+   * Note: Only supported/applicable when `version` is set to `'v4'`.
+   */
   accessibleAt?: string | number | Date;
+
+  /**
+   * Canonical extension headers that the server will validate against the client's request.
+   * Requirements:
+   * - Header names must be prefixed with `x-goog-` and must be entirely lowercase.
+   * - Multi-valued headers passed as an array are converted into a comma-separated string (no spaces).
+   * The client must format them identically to prevent signature mismatches.
+   */
   extensionHeaders?: http.OutgoingHttpHeaders;
+
+  /**
+   * The filename to prompt the browser/user to save the file as upon access.
+   * Note: This option is ignored if `responseDisposition` is explicitly set.
+   */
   promptSaveAs?: string;
+
+  /**
+   * Maps to the `response-content-disposition` query parameter in the signed URL.
+   */
   responseDisposition?: string;
+
+  /**
+   * Maps to the `response-content-type` query parameter in the signed URL.
+   */
   responseType?: string;
+
+  /**
+   * Additional query parameters to include natively in the generated signed URL.
+   */
   queryParams?: Query;
 }
 
@@ -3236,6 +3309,7 @@ class File extends ServiceObject<File, FileMetadata> {
       contentMd5: cfg.contentMd5,
       contentType: cfg.contentType,
       host: cfg.host,
+      signingEndpoint: cfg.signingEndpoint,
     };
 
     if (cfg.cname) {
diff --git a/handwritten/storage/src/signer.ts b/handwritten/storage/src/signer.ts
index a657cef6133d..ba5c17c04b75 100644
--- a/handwritten/storage/src/signer.ts
+++ b/handwritten/storage/src/signer.ts
@@ -25,7 +25,7 @@ type GoogleAuthLike = Pick<GoogleAuth, 'getCredentials' | 'sign'>;
  * @deprecated Use {@link GoogleAuth} instead
  */
 export interface AuthClient {
-  sign(blobToSign: string): Promise<string>;
+  sign(blobToSign: string, signingEndpoint?: string): Promise<string>;
   getCredentials(): Promise<{
     client_email?: string;
   }>;
diff --git a/handwritten/storage/test/file.ts b/handwritten/storage/test/file.ts
index 26823995b907..d9f9185a16e8 100644
--- a/handwritten/storage/test/file.ts
+++ b/handwritten/storage/test/file.ts
@@ -3794,11 +3794,30 @@ describe('File', () => {
           contentType: config.contentType,
           cname: CNAME,
           virtualHostedStyle: true,
+          signingEndpoint: undefined,
         });
         done();
       });
     });
 
+    it('should pass signingEndpoint to URLSigner', done => {
+      const signingEndpoint = 'https://my-endpoint.com';
+      const config = {
+        ...SIGNED_URL_CONFIG,
+        signingEndpoint,
+      };
+
+      file.getSignedUrl(config, (err: Error | null) => {
+        assert.ifError(err);
+        const getSignedUrlArgs = signerGetSignedUrlStub.getCall(0).args;
+        assert.strictEqual(
+          getSignedUrlArgs[0]['signingEndpoint'],
+          signingEndpoint
+        );
+        done();
+      });
+    });
+
     it('should add "x-goog-resumable: start" header if action is resumable', done => {
       SIGNED_URL_CONFIG.action = 'resumable';
       SIGNED_URL_CONFIG.extensionHeaders = {