diff --git a/packages/core/src/core/contentGenerator.test.ts b/packages/core/src/core/contentGenerator.test.ts
index 60cb4c563e..3fef648d87 100644
--- a/packages/core/src/core/contentGenerator.test.ts
+++ b/packages/core/src/core/contentGenerator.test.ts
@@ -508,13 +508,13 @@ describe('createContentGenerator', () => {
 
     expect(GoogleGenAI).toHaveBeenCalledWith(
       expect.objectContaining({
-        googleAuthOptions: {
-          clientOptions: {
-            transporterOptions: {
+        googleAuthOptions: expect.objectContaining({
+          clientOptions: expect.objectContaining({
+            transporterOptions: expect.objectContaining({
               agent: expect.any(HttpsProxyAgent),
-            },
-          },
-        },
+            }),
+          }),
+        }),
       }),
     );
   });
@@ -544,13 +544,13 @@ describe('createContentGenerator', () => {
 
     expect(GoogleGenAI).toHaveBeenCalledWith(
       expect.objectContaining({
-        googleAuthOptions: {
-          clientOptions: {
-            transporterOptions: {
+        googleAuthOptions: expect.objectContaining({
+          clientOptions: expect.objectContaining({
+            transporterOptions: expect.objectContaining({
               agent: expect.any(HttpsProxyAgent),
-            },
-          },
-        },
+            }),
+          }),
+        }),
       }),
     );
   });
@@ -582,13 +582,13 @@ describe('createContentGenerator', () => {
 
     expect(GoogleGenAI).toHaveBeenCalledWith(
       expect.objectContaining({
-        googleAuthOptions: {
-          clientOptions: {
-            transporterOptions: {
+        googleAuthOptions: expect.objectContaining({
+          clientOptions: expect.objectContaining({
+            transporterOptions: expect.objectContaining({
               agent: expect.any(HttpProxyAgent),
-            },
-          },
-        },
+            }),
+          }),
+        }),
       }),
     );
   });
@@ -618,13 +618,13 @@ describe('createContentGenerator', () => {
 
     expect(GoogleGenAI).toHaveBeenCalledWith(
       expect.objectContaining({
-        googleAuthOptions: {
-          clientOptions: {
-            transporterOptions: {
+        googleAuthOptions: expect.objectContaining({
+          clientOptions: expect.objectContaining({
+            transporterOptions: expect.objectContaining({
               agent: expect.any(HttpsProxyAgent),
-            },
-          },
-        },
+            }),
+          }),
+        }),
       }),
     );
   });
@@ -1003,6 +1003,38 @@ describe('createContentGenerator', () => {
     );
   });
 
+  it('should inject apiEndpoint into googleAuthOptions.clientOptions when GOOGLE_VERTEX_BASE_URL is set', async () => {
+    const mockConfig = {
+      getModel: vi.fn().mockReturnValue('gemini-pro'),
+      getProxy: vi.fn().mockReturnValue(undefined),
+      getUsageStatisticsEnabled: () => false,
+      getClientName: vi.fn().mockReturnValue(undefined),
+    } as unknown as Config;
+
+    const mockGenerator = {
+      models: {},
+    } as unknown as GoogleGenAI;
+    vi.mocked(GoogleGenAI).mockImplementation(() => mockGenerator as never);
+    vi.stubEnv('GOOGLE_VERTEX_BASE_URL', 'https://vertex.test.local');
+
+    await createContentGenerator(
+      {
+        authType: AuthType.USE_VERTEX_AI,
+      },
+      mockConfig,
+    );
+
+    expect(GoogleGenAI).toHaveBeenCalledWith(
+      expect.objectContaining({
+        googleAuthOptions: expect.objectContaining({
+          clientOptions: expect.objectContaining({
+            apiEndpoint: 'https://vertex.test.local',
+          }),
+        }),
+      }),
+    );
+  });
+
   it('should prefer an explicit baseUrl over GOOGLE_GEMINI_BASE_URL', async () => {
     const mockConfig = {
       getModel: vi.fn().mockReturnValue('gemini-pro'),
diff --git a/packages/core/src/core/contentGenerator.ts b/packages/core/src/core/contentGenerator.ts
index c893860d4c..cf7cdcd37f 100644
--- a/packages/core/src/core/contentGenerator.ts
+++ b/packages/core/src/core/contentGenerator.ts
@@ -361,7 +361,8 @@ export async function createContentGenerator(
           ? new HttpProxyAgent(proxyUrl)
           : new HttpsProxyAgent(proxyUrl)
         : undefined;
-
+      const useVertex =
+        config.vertexai ?? config.authType === AuthType.USE_VERTEX_AI;
       const googleGenAI = new GoogleGenAI({
         apiKey:
           config.authType === AuthType.GATEWAY
@@ -372,10 +373,17 @@ export async function createContentGenerator(
         vertexai: config.vertexai ?? config.authType === AuthType.USE_VERTEX_AI,
         httpOptions,
         ...(apiVersionEnv && { apiVersion: apiVersionEnv }),
-        ...(proxyAgent && {
+        // Merge proxy and GDCH endpoint into googleAuthOptions if either exists
+        ...((proxyAgent || (useVertex && baseUrl)) && {
           googleAuthOptions: {
             clientOptions: {
-              transporterOptions: { agent: proxyAgent },
+              ...(proxyAgent && {
+                transporterOptions: { agent: proxyAgent },
+              }),
+              ...(useVertex &&
+                baseUrl && {
+                  apiEndpoint: baseUrl,
+                }),
             },
           },
         }),