fix: use getter for rootAgent to match Python SDK behavior

core/src/agents/base_agent.ts

@@ -89,8 +89,11 @@ export abstract class BaseAgent {
 
   /**
    * Root agent of this agent.
+   * Computed dynamically by traversing up the parent chain.
    */
-  readonly rootAgent: BaseAgent;
+  get rootAgent(): BaseAgent {
+    return getRootAgent(this);
+  }
 
   /**
    * The parent agent of this agent.
@@ -143,7 +146,6 @@ export abstract class BaseAgent {
     this.description = config.description;
     this.parentAgent = config.parentAgent;
     this.subAgents = config.subAgents || [];
-    this.rootAgent = getRootAgent(this);
     this.beforeAgentCallback = getCannonicalCallback(
       config.beforeAgentCallback,
     );

core/test/agents/base_agent_test.ts

@@ -0,0 +1,43 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {LlmAgent} from '@google/adk';
+
+describe('BaseAgent', () => {
+  describe('rootAgent', () => {
+    it('should return the actual root agent for sub-agents', () => {
+      const subAgent = new LlmAgent({
+        name: 'sub_agent',
+        description: 'A sub agent',
+      });
+
+      const rootAgent = new LlmAgent({
+        name: 'root_agent',
+        description: 'The root agent',
+        subAgents: [subAgent],
+      });
+
+      expect(subAgent.rootAgent).toBe(rootAgent);
+      expect(rootAgent.rootAgent).toBe(rootAgent);
+    });
+
+    it('should traverse multiple levels of nesting', () => {
+      const leafAgent = new LlmAgent({name: 'leaf_agent'});
+      const middleAgent = new LlmAgent({
+        name: 'middle_agent',
+        subAgents: [leafAgent],
+      });
+      const rootAgent = new LlmAgent({
+        name: 'root_agent',
+        subAgents: [middleAgent],
+      });
+
+      expect(leafAgent.rootAgent).toBe(rootAgent);
+      expect(middleAgent.rootAgent).toBe(rootAgent);
+      expect(rootAgent.rootAgent).toBe(rootAgent);
+    });
+  });
+});