fix(core): ensure sub-agent schema and prompt refresh during runtime (#16409)

Co-authored-by: Sehoon Shon <sshon@google.com>

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

@@ -167,13 +167,18 @@ const mockCoreEvents = vi.hoisted(() => ({
   emitFeedback: vi.fn(),
   emitModelChanged: vi.fn(),
   emitConsoleLog: vi.fn(),
+  on: vi.fn(),
 }));
 
 const mockSetGlobalProxy = vi.hoisted(() => vi.fn());
 
-vi.mock('../utils/events.js', () => ({
-  coreEvents: mockCoreEvents,
-}));
+vi.mock('../utils/events.js', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../utils/events.js')>();
+  return {
+    ...actual,
+    coreEvents: mockCoreEvents,
+  };
+});
 
 vi.mock('../utils/fetch.js', () => ({
   setGlobalProxy: mockSetGlobalProxy,

packages/core/src/config/config.ts

@@ -43,7 +43,7 @@ import {
   DEFAULT_OTLP_ENDPOINT,
   uiTelemetryService,
 } from '../telemetry/index.js';
-import { coreEvents } from '../utils/events.js';
+import { coreEvents, CoreEvent } from '../utils/events.js';
 import { tokenLimit } from '../core/tokenLimits.js';
 import {
   DEFAULT_GEMINI_EMBEDDING_MODEL,
@@ -735,6 +735,8 @@ export class Config {
     this.agentRegistry = new AgentRegistry(this);
     await this.agentRegistry.initialize();
 
+    coreEvents.on(CoreEvent.AgentsRefreshed, this.onAgentsRefreshed);
+
     this.toolRegistry = await this.createToolRegistry();
     discoverToolsHandle?.end();
     this.mcpClientManager = new McpClientManager(
@@ -1764,6 +1766,17 @@ export class Config {
 
     // Register Subagents as Tools
     // Register DelegateToAgentTool if agents are enabled
+    this.registerDelegateToAgentTool(registry);
+
+    await registry.discoverAllTools();
+    registry.sortTools();
+    return registry;
+  }
+
+  /**
+   * Registers the DelegateToAgentTool if agents or related features are enabled.
+   */
+  private registerDelegateToAgentTool(registry: ToolRegistry): void {
     if (
       this.isAgentsEnabled() ||
       this.getCodebaseInvestigatorSettings().enabled ||
@@ -1783,10 +1796,6 @@ export class Config {
         registry.registerTool(delegateTool);
       }
     }
-
-    await registry.discoverAllTools();
-    registry.sortTools();
-    return registry;
   }
 
   /**
@@ -1870,6 +1879,35 @@ export class Config {
     });
     debugLogger.debug('Experiments loaded', summaryString);
   }
+
+  private onAgentsRefreshed = async () => {
+    if (this.toolRegistry) {
+      this.registerDelegateToAgentTool(this.toolRegistry);
+    }
+    // Propagate updates to the active chat session
+    const client = this.getGeminiClient();
+    if (client?.isInitialized()) {
+      await client.setTools();
+      await client.updateSystemInstruction();
+    } else {
+      debugLogger.debug(
+        '[Config] GeminiClient not initialized; skipping live prompt/tool refresh.',
+      );
+    }
+  };
+
+  /**
+   * Disposes of resources and removes event listeners.
+   */
+  async dispose(): Promise<void> {
+    coreEvents.off(CoreEvent.AgentsRefreshed, this.onAgentsRefreshed);
+    if (this.agentRegistry) {
+      this.agentRegistry.dispose();
+    }
+    if (this.mcpClientManager) {
+      await this.mcpClientManager.stop();
+    }
+  }
 }
 // Export model constants for use in CLI
 export { DEFAULT_GEMINI_FLASH_MODEL };