@@ -1158,14 +1158,20 @@ export function encodeAbsoluteURI(uri: string): string {
  * Trims slashes, encodes path segments to prevent path traversal, and joins
  * URI components into a single relative path.
  *
+ * DEFAULT_PROJECT_ID_TOKEN is an internal constant set by the library, never
+ * user input, so bypassing encodeURIPath for it does not open a traversal
+ * path. Encoding it would produce '%7B%7BprojectId%7D%7D' in the URI, which
+ * replaceProjectIdToken() cannot match — causing 404s for clients that rely
+ * on lazy / ADC project-ID resolution (regression introduced by #9188).
+ *
  * @param {string[]} components - URI components to encode and join.
  * @return {string} The formatted and joined URI path.
  */
 export function joinURIComponents(components: string[]): string {
   return components
     .map(uriComponent => {
-      const trimSlashesRegex = /^\/*|\/*$/g;
-      const trimmed = uriComponent.replace(trimSlashesRegex, '');
+      const trimmed = uriComponent.replace(/^\/*|\/*$/g, '');
+      if (trimmed === DEFAULT_PROJECT_ID_TOKEN) return trimmed;
       return encodeURIPath(trimmed); // Encode and prevent path traversal.
     })
     .join('/');
 
@@ -567,6 +567,21 @@ describe('Service', () => {
           service.request_(reqOpts, assert.ifError);
         });
 
+        it('should pass DEFAULT_PROJECT_ID_TOKEN unencoded to makeAuthenticatedRequest', done => {
+          // Regression: #9188 caused {{projectId}} to be encoded as
+          // %7B%7BprojectId%7D%7D, so replaceProjectIdToken() could no longer
+          // find and substitute it, causing 404s for ADC clients.
+          const config = extend({}, CONFIG, {projectIdRequired: true});
+          const service = new Service(config, {}); // no projectId → DEFAULT_PROJECT_ID_TOKEN
+
+          service.makeAuthenticatedRequest = (reqOpts_: DecorateRequestOptions) => {
+            assert.match(reqOpts_.uri, /projects\/\{\{projectId\}\}\//);
+            done();
+          };
+
+          service.request_({uri: 'queries'}, assert.ifError);
+        });
+
         it('should use projectId override', done => {
           const config = extend({}, CONFIG, {projectIdRequired: true});
           const service = new Service(config, OPTIONS);
 
@@ -46,6 +46,7 @@ import {
   ParsedHttpRespMessage,
   ParsedHttpResponseBody,
   Util,
+  joinURIComponents,
 } from '../src/util';
 import {DEFAULT_PROJECT_ID_TOKEN} from '../src/service';
 
@@ -1922,4 +1923,40 @@ describe('common/util', () => {
       assert.strictEqual(cb, callback);
     });
   });
+
+  describe('joinURIComponents', () => {
+    it('should preserve DEFAULT_PROJECT_ID_TOKEN so replaceProjectIdToken can substitute it', () => {
+      // Regression: #9188 caused {{projectId}} to be encoded as %7B%7BprojectId%7D%7D,
+      // breaking ADC / lazy project-ID resolution. See #9256.
+      const joined = joinURIComponents([
+        'https://bigquery.googleapis.com/bigquery/v2',
+        'projects',
+        DEFAULT_PROJECT_ID_TOKEN,
+        'queries',
+      ]);
+      const substituted = replaceProjectIdToken(joined, 'my-project');
+      assert.strictEqual(
+        substituted,
+        'https://bigquery.googleapis.com/bigquery/v2/projects/my-project/queries',
+      );
+    });
+
+    it('should still reject path traversal in non-token components', () => {
+      assert.throws(() => {
+        joinURIComponents(['https://example.com/v1', '{{../../admin}}', 'child']);
+      });
+    });
+
+    it('should encode special characters in non-token path segments', () => {
+      const result = joinURIComponents([
+        'https://example.com/v1',
+        'resource with spaces',
+        'child',
+      ]);
+      assert.strictEqual(
+        result,
+        'https://example.com/v1/resource%20with%20spaces/child',
+      );
+    });
+  });
 });