fix: isolate concurrent browser agent instances

integration-tests/browser-agent.concurrent.responses

@@ -0,0 +1,8 @@
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"I'll launch two browser agents concurrently to check both repositories."},{"functionCall":{"name":"browser_agent","args":{"task":"Navigate to https://example.com and get the page title"}}},{"functionCall":{"name":"browser_agent","args":{"task":"Navigate to https://example.com and get the page title"}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":100,"candidatesTokenCount":50,"totalTokenCount":150}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"navigate_page","args":{"url":"https://example.com"}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":100,"candidatesTokenCount":20,"totalTokenCount":120}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"navigate_page","args":{"url":"https://example.com"}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":100,"candidatesTokenCount":20,"totalTokenCount":120}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"take_snapshot","args":{}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":150,"candidatesTokenCount":15,"totalTokenCount":165}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"take_snapshot","args":{}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":150,"candidatesTokenCount":15,"totalTokenCount":165}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"complete_task","args":{"result":{"success":true,"summary":"Page title is Example Domain."}}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":200,"candidatesTokenCount":30,"totalTokenCount":230}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"complete_task","args":{"result":{"success":true,"summary":"Page title is Example Domain."}}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":200,"candidatesTokenCount":30,"totalTokenCount":230}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"Both browser agents completed successfully. Agent 1 and Agent 2 both navigated to their respective pages and confirmed the page titles."}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":300,"candidatesTokenCount":40,"totalTokenCount":340}}]}

integration-tests/browser-agent.test.ts

@@ -307,4 +307,48 @@ describe.skipIf(!chromeAvailable)('browser-agent', () => {
 
     await run.expectText('successfully written', 15000);
   });
+
+  it('should handle concurrent browser agents with isolated session mode', async () => {
+    rig.setup('browser-concurrent', {
+      fakeResponsesPath: join(__dirname, 'browser-agent.concurrent.responses'),
+      settings: {
+        agents: {
+          overrides: {
+            browser_agent: {
+              enabled: true,
+            },
+          },
+          browser: {
+            headless: true,
+            // Isolated mode supports concurrent browser agents.
+            // Persistent/existing modes reject concurrent calls to prevent
+            // Chrome profile lock conflicts.
+            sessionMode: 'isolated',
+          },
+        },
+      },
+    });
+
+    const result = await rig.run({
+      args: 'Launch two browser agents concurrently to check example.com',
+    });
+
+    assertModelHasOutput(result);
+
+    const toolLogs = rig.readToolLogs();
+    const browserCalls = toolLogs.filter(
+      (t) => t.toolRequest.name === 'browser_agent',
+    );
+
+    // Both browser_agent invocations should have been called
+    expect(browserCalls.length).toBe(2);
+
+    // Both should complete successfully (no errors)
+    for (const call of browserCalls) {
+      expect(
+        call.toolRequest.success,
+        `browser_agent call failed: ${JSON.stringify(call.toolRequest)}`,
+      ).toBe(true);
+    }
+  });
 });

packages/core/src/agents/browser/browserAgentFactory.test.ts

@@ -38,6 +38,8 @@ const mockBrowserManager = {
   ]),
   callTool: vi.fn().mockResolvedValue({ content: [] }),
   close: vi.fn().mockResolvedValue(undefined),
+  acquire: vi.fn(),
+  release: vi.fn(),
 };
 
 // Mock dependencies