# feat(routing): Introduce useModelRouter feature flag (#8366)

packages/core/src/config/config.test.ts

@@ -523,6 +523,31 @@ describe('Server Config (config.ts)', () => {
     });
   });
 
+  describe('UseModelRouter Configuration', () => {
+    it('should default useModelRouter to false when not provided', () => {
+      const config = new Config(baseParams);
+      expect(config.getUseModelRouter()).toBe(false);
+    });
+
+    it('should set useModelRouter to true when provided as true', () => {
+      const paramsWithModelRouter: ConfigParameters = {
+        ...baseParams,
+        useModelRouter: true,
+      };
+      const config = new Config(paramsWithModelRouter);
+      expect(config.getUseModelRouter()).toBe(true);
+    });
+
+    it('should set useModelRouter to false when explicitly provided as false', () => {
+      const paramsWithModelRouter: ConfigParameters = {
+        ...baseParams,
+        useModelRouter: false,
+      };
+      const config = new Config(paramsWithModelRouter);
+      expect(config.getUseModelRouter()).toBe(false);
+    });
+  });
+
   describe('createToolRegistry', () => {
     it('should register a tool if coreTools contains an argument-specific pattern', async () => {
       const params: ConfigParameters = {

packages/core/src/config/config.ts

@@ -44,7 +44,6 @@ import { StartSessionEvent } from '../telemetry/index.js';
 import {
   DEFAULT_GEMINI_EMBEDDING_MODEL,
   DEFAULT_GEMINI_FLASH_MODEL,
-  DEFAULT_GEMINI_MODEL,
 } from './models.js';
 import { shouldAttemptBrowserLaunch } from '../utils/browser.js';
 import type { MCPOAuthConfig } from '../mcp/oauth-provider.js';
@@ -240,6 +239,7 @@ export interface ConfigParameters {
   useSmartEdit?: boolean;
   policyEngineConfig?: PolicyEngineConfig;
   output?: OutputSettings;
+  useModelRouter?: boolean;
 }
 
 export class Config {
@@ -327,6 +327,7 @@ export class Config {
   private readonly messageBus: MessageBus;
   private readonly policyEngine: PolicyEngine;
   private readonly outputSettings: OutputSettings;
+  private readonly useModelRouter: boolean;
 
   constructor(params: ConfigParameters) {
     this.sessionId = params.sessionId;
@@ -376,7 +377,7 @@ export class Config {
     this.cwd = params.cwd ?? process.cwd();
     this.fileDiscoveryService = params.fileDiscoveryService ?? null;
     this.bugCommand = params.bugCommand;
-    this.model = params.model || DEFAULT_GEMINI_MODEL;
+    this.model = params.model;
     this.extensionContextFilePaths = params.extensionContextFilePaths ?? [];
     this.maxSessionTurns = params.maxSessionTurns ?? -1;
     this.experimentalZedIntegration =
@@ -411,6 +412,7 @@ export class Config {
     this.enableToolOutputTruncation =
       params.enableToolOutputTruncation ?? false;
     this.useSmartEdit = params.useSmartEdit ?? true;
+    this.useModelRouter = params.useModelRouter ?? false;
     this.extensionManagement = params.extensionManagement ?? true;
     this.storage = new Storage(this.targetDir);
     this.enablePromptCompletion = params.enablePromptCompletion ?? false;
@@ -931,6 +933,10 @@ export class Config {
       : OutputFormat.TEXT;
   }
 
+  getUseModelRouter(): boolean {
+    return this.useModelRouter;
+  }
+
   async getGitService(): Promise<GitService> {
     if (!this.gitService) {
       this.gitService = new GitService(this.targetDir, this.storage);

packages/core/src/config/models.ts

@@ -8,6 +8,8 @@ export const DEFAULT_GEMINI_MODEL = 'gemini-2.5-pro';
 export const DEFAULT_GEMINI_FLASH_MODEL = 'gemini-2.5-flash';
 export const DEFAULT_GEMINI_FLASH_LITE_MODEL = 'gemini-2.5-flash-lite';
 
+export const DEFAULT_GEMINI_MODEL_AUTO = 'auto';
+
 export const DEFAULT_GEMINI_EMBEDDING_MODEL = 'gemini-embedding-001';
 
 // Some thinking models do not default to dynamic thinking which is done by a value of -1