test(core): add unit tests for subagent MCP tool isolation

Unit tests added:
1. Tool Registry Filtering: Verified that main registry hides all '__agent__' prefixed tools.
2. Subagent Tool Inheritance: Verified that agents correctly filter out other agents' MCP tools while retaining their own.
Verified with vitest in packages/core.

packages/core/src/agents/local-executor.test.ts

@@ -2510,5 +2510,49 @@ describe('LocalAgentExecutor', () => {
         mcpServers['test-server'],
       );
     });
+
+    it('should filter out other agents MCP tools when inheriting tools from parent registry', async () => {
+      const parentMcpTool1 = new DiscoveredMCPTool(
+        {} as unknown as CallableTool,
+        '__agent__OtherAgent__server1',
+        'tool1',
+        'desc1',
+        {},
+        mockConfig.getMessageBus(),
+      );
+      const parentMcpTool2 = new DiscoveredMCPTool(
+        {} as unknown as CallableTool,
+        '__agent__TestAgent__server2',
+        'tool2',
+        'desc2',
+        {},
+        mockConfig.getMessageBus(),
+      );
+
+      parentToolRegistry.registerTool(parentMcpTool1);
+      parentToolRegistry.registerTool(parentMcpTool2);
+
+      const definition = createTestDefinition();
+      definition.toolConfig = undefined; // trigger inheritance
+
+      vi.spyOn(mockConfig, 'getMcpClientManager').mockReturnValue({
+        maybeDiscoverMcpServer: vi.fn(),
+      } as unknown as ReturnType<typeof mockConfig.getMcpClientManager>);
+
+      const executor = await LocalAgentExecutor.create(
+        definition,
+        mockConfig,
+        onActivity,
+      );
+
+      const agentTools = (
+        executor as unknown as { toolRegistry: ToolRegistry }
+      ).toolRegistry.getAllToolNames();
+
+      expect(agentTools).toContain(parentMcpTool2.asFullyQualifiedTool().name);
+      expect(agentTools).not.toContain(
+        parentMcpTool1.asFullyQualifiedTool().name,
+      );
+    });
   });
 });

packages/core/src/tools/tool-registry.test.ts

@@ -284,6 +284,37 @@ describe('ToolRegistry', () => {
     });
   });
 
+  describe('subagent MCP tools filtering', () => {
+    it('should hide __agent__ prefixed tools when isMainRegistry is true', async () => {
+      const mainRegistry = new ToolRegistry(config, mockMessageBus, true);
+      const subagentRegistry = new ToolRegistry(config, mockMessageBus, false);
+
+      const mcpTool = createMCPTool(
+        '__agent__TestAgent__myServer',
+        'my-tool',
+        'description',
+      );
+      vi.spyOn(mcpTool, 'getSchema').mockReturnValue({
+        name: 'my_tool',
+        description: 'description',
+      } as unknown as FunctionDeclaration);
+
+      mainRegistry.registerTool(mcpTool);
+      subagentRegistry.registerTool(mcpTool);
+
+      const mainDeclarations =
+        mainRegistry.getFunctionDeclarations('test-model');
+      const subagentDeclarations =
+        subagentRegistry.getFunctionDeclarations('test-model');
+
+      expect(mainDeclarations.length).toBe(0);
+      expect(subagentDeclarations.length).toBe(1);
+      expect(subagentDeclarations[0].name).toBe(
+        'mcp___agent__TestAgent__myServer_my-tool',
+      );
+    });
+  });
+
   describe('excluded tools', () => {
     const simpleTool = new MockTool({
       name: 'tool-a',