diff --git a/packages/core/src/config/config.test.ts b/packages/core/src/config/config.test.ts
index 5c49e50ec1..7bcf1ff941 100644
--- a/packages/core/src/config/config.test.ts
+++ b/packages/core/src/config/config.test.ts
@@ -556,29 +556,84 @@ describe('Server Config (config.ts)', () => {
     });
   });
 
-  describe('UseModelRouter Configuration', () => {
-    it('should default useModelRouter to false when not provided', () => {
-      const config = new Config(baseParams);
+  describe('Model Router with Auth', () => {
+    it('should disable model router by default for oauth-personal', async () => {
+      const config = new Config({
+        ...baseParams,
+        useModelRouter: true,
+      });
+      await config.refreshAuth(AuthType.LOGIN_WITH_GOOGLE);
       expect(config.getUseModelRouter()).toBe(false);
     });
 
-    it('should set useModelRouter to true when provided as true', () => {
-      const paramsWithModelRouter: ConfigParameters = {
+    it('should enable model router by default for other auth types', async () => {
+      const config = new Config({
         ...baseParams,
         useModelRouter: true,
-      };
-      const config = new Config(paramsWithModelRouter);
+      });
+      await config.refreshAuth(AuthType.USE_GEMINI);
       expect(config.getUseModelRouter()).toBe(true);
     });
 
-    it('should set useModelRouter to false when explicitly provided as false', () => {
-      const paramsWithModelRouter: ConfigParameters = {
+    it('should disable model router for specified auth type', async () => {
+      const config = new Config({
+        ...baseParams,
+        useModelRouter: true,
+        disableModelRouterForAuth: [AuthType.USE_GEMINI],
+      });
+      await config.refreshAuth(AuthType.USE_GEMINI);
+      expect(config.getUseModelRouter()).toBe(false);
+    });
+
+    it('should enable model router for other auth type', async () => {
+      const config = new Config({
+        ...baseParams,
+        useModelRouter: true,
+        disableModelRouterForAuth: [],
+      });
+      await config.refreshAuth(AuthType.LOGIN_WITH_GOOGLE);
+      expect(config.getUseModelRouter()).toBe(true);
+    });
+
+    it('should keep model router disabled when useModelRouter is false', async () => {
+      const config = new Config({
         ...baseParams,
         useModelRouter: false,
-      };
-      const config = new Config(paramsWithModelRouter);
+        disableModelRouterForAuth: [AuthType.USE_GEMINI],
+      });
+      await config.refreshAuth(AuthType.LOGIN_WITH_GOOGLE);
       expect(config.getUseModelRouter()).toBe(false);
     });
+
+    it('should keep the user-chosen model after refreshAuth, even when model router is disabled for the auth type', async () => {
+      const config = new Config({
+        ...baseParams,
+        useModelRouter: true,
+        disableModelRouterForAuth: [AuthType.USE_GEMINI],
+      });
+      const chosenModel = 'gemini-1.5-pro-latest';
+      config.setModel(chosenModel);
+
+      await config.refreshAuth(AuthType.USE_GEMINI);
+
+      expect(config.getUseModelRouter()).toBe(false);
+      expect(config.getModel()).toBe(chosenModel);
+    });
+
+    it('should keep the user-chosen model after refreshAuth, when model router is enabled for the auth type', async () => {
+      const config = new Config({
+        ...baseParams,
+        useModelRouter: true,
+        disableModelRouterForAuth: [AuthType.USE_GEMINI],
+      });
+      const chosenModel = 'gemini-1.5-pro-latest';
+      config.setModel(chosenModel);
+
+      await config.refreshAuth(AuthType.LOGIN_WITH_GOOGLE);
+
+      expect(config.getUseModelRouter()).toBe(true);
+      expect(config.getModel()).toBe(chosenModel);
+    });
   });
 
   describe('ContinueOnFailedApiCall Configuration', () => {
diff --git a/packages/core/src/config/config.ts b/packages/core/src/config/config.ts
index 6b683ac5ac..878c2fe782 100644
--- a/packages/core/src/config/config.ts
+++ b/packages/core/src/config/config.ts
@@ -46,6 +46,7 @@ import {
   DEFAULT_GEMINI_EMBEDDING_MODEL,
   DEFAULT_GEMINI_FLASH_MODEL,
   DEFAULT_GEMINI_MODEL,
+  DEFAULT_GEMINI_MODEL_AUTO,
   DEFAULT_THINKING_MODE,
 } from './models.js';
 import { shouldAttemptBrowserLaunch } from '../utils/browser.js';
@@ -279,6 +280,7 @@ export interface ConfigParameters {
   output?: OutputSettings;
   useModelRouter?: boolean;
   enableMessageBusIntegration?: boolean;
+  disableModelRouterForAuth?: AuthType[];
   codebaseInvestigatorSettings?: CodebaseInvestigatorSettings;
   continueOnFailedApiCall?: boolean;
   retryFetchErrors?: boolean;
@@ -377,7 +379,9 @@ export class Config {
   private readonly messageBus: MessageBus;
   private readonly policyEngine: PolicyEngine;
   private readonly outputSettings: OutputSettings;
-  private readonly useModelRouter: boolean;
+  private useModelRouter: boolean;
+  private readonly initialUseModelRouter: boolean;
+  private readonly disableModelRouterForAuth?: AuthType[];
   private readonly enableMessageBusIntegration: boolean;
   private readonly codebaseInvestigatorSettings: CodebaseInvestigatorSettings;
   private readonly continueOnFailedApiCall: boolean;
@@ -477,7 +481,11 @@ export class Config {
     this.enableToolOutputTruncation = params.enableToolOutputTruncation ?? true;
     this.useSmartEdit = params.useSmartEdit ?? true;
     this.useWriteTodos = params.useWriteTodos ?? false;
-    this.useModelRouter = params.useModelRouter ?? false;
+    this.initialUseModelRouter = params.useModelRouter ?? false;
+    this.useModelRouter = this.initialUseModelRouter;
+    this.disableModelRouterForAuth = params.disableModelRouterForAuth ?? [
+      AuthType.LOGIN_WITH_GOOGLE,
+    ];
     this.enableMessageBusIntegration =
       params.enableMessageBusIntegration ?? false;
     this.codebaseInvestigatorSettings = {
@@ -551,6 +559,16 @@ export class Config {
   }
 
   async refreshAuth(authMethod: AuthType) {
+    this.useModelRouter = this.initialUseModelRouter;
+    if (this.disableModelRouterForAuth?.includes(authMethod)) {
+      this.useModelRouter = false;
+      if (this.model === DEFAULT_GEMINI_MODEL_AUTO) {
+        this.model = DEFAULT_GEMINI_MODEL;
+      }
+    } else if (this.useModelRouter && this.model === DEFAULT_GEMINI_MODEL) {
+      this.model = DEFAULT_GEMINI_MODEL_AUTO;
+    }
+
     // Vertex and Genai have incompatible encryption and sending history with
     // thoughtSignature from Genai to Vertex will fail, we need to strip them
     if (