feat: ensure codebase investigator uses preview model when main agent does (#14412)

packages/core/src/agents/codebase-investigator.test.ts

@@ -12,7 +12,7 @@ import {
   LS_TOOL_NAME,
   READ_FILE_TOOL_NAME,
 } from '../tools/tool-names.js';
-import { DEFAULT_GEMINI_MODEL } from '../config/models.js';
+import { GEMINI_MODEL_ALIAS_PRO } from '../config/models.js';
 
 describe('CodebaseInvestigatorAgent', () => {
   it('should have the correct agent definition', () => {
@@ -26,7 +26,7 @@ describe('CodebaseInvestigatorAgent', () => {
     ).toBe(true);
     expect(CodebaseInvestigatorAgent.outputConfig?.outputName).toBe('report');
     expect(CodebaseInvestigatorAgent.modelConfig?.model).toBe(
-      DEFAULT_GEMINI_MODEL,
+      GEMINI_MODEL_ALIAS_PRO,
     );
     expect(CodebaseInvestigatorAgent.toolConfig?.tools).toEqual([
       LS_TOOL_NAME,

packages/core/src/agents/codebase-investigator.ts

@@ -11,7 +11,7 @@ import {
   LS_TOOL_NAME,
   READ_FILE_TOOL_NAME,
 } from '../tools/tool-names.js';
-import { DEFAULT_GEMINI_MODEL } from '../config/models.js';
+import { GEMINI_MODEL_ALIAS_PRO } from '../config/models.js';
 import { z } from 'zod';
 
 // Define a type that matches the outputConfig schema for type safety.
@@ -69,7 +69,7 @@ export const CodebaseInvestigatorAgent: AgentDefinition<
   processOutput: (output) => JSON.stringify(output, null, 2),
 
   modelConfig: {
-    model: DEFAULT_GEMINI_MODEL,
+    model: GEMINI_MODEL_ALIAS_PRO,
     temp: 0.1,
     top_p: 0.95,
     thinkingBudget: -1,

packages/core/src/agents/registry.test.ts

@@ -72,6 +72,25 @@ describe('AgentRegistry', () => {
         `[AgentRegistry] Initialized with ${agentCount} agents.`,
       );
     });
+
+    it('should use preview model for codebase investigator if main model is preview', async () => {
+      const previewConfig = makeFakeConfig({
+        model: 'gemini-3-pro-preview',
+        codebaseInvestigatorSettings: {
+          enabled: true,
+          model: 'pro',
+        },
+      });
+      const previewRegistry = new TestableAgentRegistry(previewConfig);
+
+      await previewRegistry.initialize();
+
+      const investigatorDef = previewRegistry.getDefinition(
+        'codebase_investigator',
+      );
+      expect(investigatorDef).toBeDefined();
+      expect(investigatorDef?.modelConfig.model).toBe('gemini-3-pro-preview');
+    });
   });
 
   describe('registration logic', () => {

packages/core/src/agents/registry.ts

@@ -9,6 +9,11 @@ import type { AgentDefinition } from './types.js';
 import { CodebaseInvestigatorAgent } from './codebase-investigator.js';
 import { type z } from 'zod';
 import { debugLogger } from '../utils/debugLogger.js';
+import {
+  DEFAULT_GEMINI_MODEL_AUTO,
+  GEMINI_MODEL_ALIAS_PRO,
+  PREVIEW_GEMINI_MODEL,
+} from '../config/models.js';
 import type { ModelConfigAlias } from '../services/modelConfigService.js';
 
 /**
@@ -48,13 +53,26 @@ export class AgentRegistry {
 
     // Only register the agent if it's enabled in the settings.
     if (investigatorSettings?.enabled) {
+      let model =
+        investigatorSettings.model ??
+        CodebaseInvestigatorAgent.modelConfig.model;
+
+      // If the user is using the preview model for the main agent, force the sub-agent to use it too
+      // if it's configured to use 'pro' or 'auto'.
+      if (this.config.getModel() === PREVIEW_GEMINI_MODEL) {
+        if (
+          model === GEMINI_MODEL_ALIAS_PRO ||
+          model === DEFAULT_GEMINI_MODEL_AUTO
+        ) {
+          model = PREVIEW_GEMINI_MODEL;
+        }
+      }
+
       const agentDef = {
         ...CodebaseInvestigatorAgent,
         modelConfig: {
           ...CodebaseInvestigatorAgent.modelConfig,
-          model:
-            investigatorSettings.model ??
-            CodebaseInvestigatorAgent.modelConfig.model,
+          model,
           thinkingBudget:
             investigatorSettings.thinkingBudget ??
             CodebaseInvestigatorAgent.modelConfig.thinkingBudget,

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

@@ -888,6 +888,11 @@ describe('Server Config (config.ts)', () => {
       expect(SubagentToolWrapperMock).not.toHaveBeenCalled();
     });
 
+    it('should not set default codebase investigator model in config (defaults in registry)', () => {
+      const config = new Config(baseParams);
+      expect(config.getCodebaseInvestigatorSettings()?.model).toBeUndefined();
+    });
+
     describe('with minified tool class names', () => {
       beforeEach(() => {
         Object.defineProperty(

packages/core/src/config/config.ts

@@ -48,7 +48,6 @@ import { tokenLimit } from '../core/tokenLimits.js';
 import {
   DEFAULT_GEMINI_EMBEDDING_MODEL,
   DEFAULT_GEMINI_FLASH_MODEL,
-  DEFAULT_GEMINI_MODEL,
   DEFAULT_THINKING_MODE,
 } from './models.js';
 import { shouldAttemptBrowserLaunch } from '../utils/browser.js';
@@ -573,7 +572,7 @@ export class Config {
       thinkingBudget:
         params.codebaseInvestigatorSettings?.thinkingBudget ??
         DEFAULT_THINKING_MODE,
-      model: params.codebaseInvestigatorSettings?.model ?? DEFAULT_GEMINI_MODEL,
+      model: params.codebaseInvestigatorSettings?.model,
     };
     this.continueOnFailedApiCall = params.continueOnFailedApiCall ?? true;
     this.enableShellOutputEfficiency =