Added validation for model on start

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

@@ -324,6 +324,38 @@ describe('Server Config (config.ts)', () => {
     });
   });
 
+  describe('model fallback', () => {
+    it('should fallback to default model when an obsolete model is provided', () => {
+      const config = new Config({
+        ...baseParams,
+        model: 'gemini-pro-latest',
+      });
+      expect(config.getModel()).toBe(DEFAULT_GEMINI_MODEL);
+    });
+
+    it('should fallback to default model when an invalid gemini model is provided', () => {
+      const config = new Config({
+        ...baseParams,
+        model: 'gemini-invalid-model',
+      });
+      expect(config.getModel()).toBe(DEFAULT_GEMINI_MODEL);
+    });
+
+    it('should fallback to default model when setModel is called with an invalid gemini model', () => {
+      const config = new Config(baseParams);
+      config.setModel('gemini-invalid-model');
+      expect(config.getModel()).toBe(DEFAULT_GEMINI_MODEL);
+    });
+
+    it('should allow custom models (non-gemini prefixed)', () => {
+      const config = new Config({
+        ...baseParams,
+        model: 'custom-model',
+      });
+      expect(config.getModel()).toBe('custom-model');
+    });
+  });
+
   describe('setShellExecutionConfig', () => {
     it('should preserve existing shell execution fields that are not being updated', () => {
       const config = new Config({
@@ -2613,7 +2645,7 @@ describe('Config getHooks', () => {
     targetDir: '/path/to/target',
     debugMode: false,
     sessionId: 'test-session-id',
-    model: 'gemini-pro',
+    model: 'gemini-2.5-flash',
     usageStatisticsEnabled: false,
   };
 
@@ -2862,7 +2894,7 @@ describe('Config getExperiments', () => {
     targetDir: '/path/to/target',
     debugMode: false,
     sessionId: 'test-session-id',
-    model: 'gemini-pro',
+    model: 'gemini-2.5-flash',
     usageStatisticsEnabled: false,
   };
 
@@ -2907,7 +2939,7 @@ describe('Config setExperiments logging', () => {
     targetDir: '/path/to/target',
     debugMode: false,
     sessionId: 'test-session-id',
-    model: 'gemini-pro',
+    model: 'gemini-2.5-flash',
     usageStatisticsEnabled: false,
   };
 

packages/core/src/config/config.ts

@@ -80,10 +80,12 @@ import { tokenLimit } from '../core/tokenLimits.js';
 import {
   DEFAULT_GEMINI_EMBEDDING_MODEL,
   DEFAULT_GEMINI_FLASH_MODEL,
+  DEFAULT_GEMINI_MODEL,
   DEFAULT_GEMINI_MODEL_AUTO,
   isAutoModel,
   isPreviewModel,
   isGemini2Model,
+  isValidModel,
   PREVIEW_GEMINI_FLASH_MODEL,
   resolveModel,
 } from './models.js';
@@ -1114,9 +1116,11 @@ export class Config implements McpContext, AgentLoopContext {
     this.cwd = params.cwd ?? process.cwd();
     this.fileDiscoveryService = params.fileDiscoveryService ?? null;
     this.bugCommand = params.bugCommand;
-    this.model = params.model;
+    this.model = isValidModel(params.model)
+      ? params.model
+      : DEFAULT_GEMINI_MODEL;
     this.disableLoopDetection = params.disableLoopDetection ?? false;
-    this._activeModel = params.model;
+    this._activeModel = this.model;
     this.enableAgents = params.enableAgents ?? true;
     this.agents = params.agents ?? {};
     this.disableLLMCorrection = params.disableLLMCorrection ?? true;
@@ -1902,17 +1906,20 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   setModel(newModel: string, isTemporary: boolean = true): void {
-    if (this.model !== newModel || this._activeModel !== newModel) {
-      this.model = newModel;
+    const validatedModel = isValidModel(newModel)
+      ? newModel
+      : DEFAULT_GEMINI_MODEL;
+    if (this.model !== validatedModel || this._activeModel !== validatedModel) {
+      this.model = validatedModel;
       // When the user explicitly sets a model, that becomes the active model.
-      this._activeModel = newModel;
-      coreEvents.emitModelChanged(newModel);
+      this._activeModel = validatedModel;
+      coreEvents.emitModelChanged(validatedModel);
       this.lastEmittedQuotaRemaining = undefined;
       this.lastEmittedQuotaLimit = undefined;
       this.emitQuotaChangedEvent();
     }
     if (this.onModelChange && !isTemporary) {
-      this.onModelChange(newModel);
+      this.onModelChange(validatedModel);
     }
     this.modelAvailabilityService.reset();
   }

packages/core/src/config/models.ts

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import { normalizeModelId } from '../utils/modelUtils.js';
+
 export interface ModelResolutionContext {
   useGemini3_1?: boolean;
   useGemini3_1FlashLite?: boolean;
@@ -110,6 +112,20 @@ export function getAutoModelDescription(
   return `Let Gemini CLI decide the best model for the task: ${proModel}, ${flashModel}`;
 }
 
+export function isValidModel(model: string): boolean {
+  const normalized = normalizeModelId(model);
+  return (
+    VALID_GEMINI_MODELS.has(normalized) ||
+    normalized === GEMINI_MODEL_ALIAS_AUTO ||
+    normalized === GEMINI_MODEL_ALIAS_PRO ||
+    normalized === GEMINI_MODEL_ALIAS_FLASH ||
+    normalized === GEMINI_MODEL_ALIAS_FLASH_LITE ||
+    normalized === PREVIEW_GEMINI_MODEL_AUTO ||
+    normalized === DEFAULT_GEMINI_MODEL_AUTO ||
+    !normalized.startsWith('gemini-')
+  );
+}
+
 /**
  * Resolves the requested model alias (e.g., 'auto', 'pro', 'flash', 'flash-lite')
  * to a concrete model name.
@@ -196,7 +212,12 @@ export function resolveModel(
       break;
     }
     default: {
-      resolved = normalizedModel;
+      if (!isValidModel(normalizedModel)) {
+        // Fallback to stable default for unknown/obsolete models.
+        resolved = DEFAULT_GEMINI_MODEL;
+      } else {
+        resolved = normalizedModel;
+      }
       break;
     }
   }