fix(mcp): serialize shared browser launch in --isolated mode

packages/playwright-core/src/tools/mcp/program.ts

@@ -95,7 +95,7 @@ export function decorateMCPCommand(command: Command) {
         const config = await resolveCLIConfigForMCP(options);
         const tools = filteredTools(config);
         const useSharedBrowser = config.sharedBrowserContext || config.browser.isolated;
-        let sharedBrowser: playwright.Browser | undefined;
+        let sharedBrowserPromise: Promise<playwright.Browser> | undefined;
         let clientCount = 0;
         const clientNameCounters = new Map<string, number>();
 
@@ -105,14 +105,19 @@ export function decorateMCPCommand(command: Command) {
           version,
           toolSchemas: tools.map(tool => tool.schema),
           create: async (clientInfo: ClientInfo) => {
-            if (useSharedBrowser && clientCount === 0) {
-              const { browser, canBind } = await createBrowserWithInfo(config, clientInfo, options);
-              sharedBrowser = browser;
-              if (canBind)
-                await browser.bind(clientInfo.clientName, { workspaceDir: clientInfo.cwd });
+            if (useSharedBrowser && !sharedBrowserPromise) {
+              sharedBrowserPromise = (async () => {
+                const { browser, canBind } = await createBrowserWithInfo(config, clientInfo, options);
+                if (canBind)
+                  await browser.bind(clientInfo.clientName, { workspaceDir: clientInfo.cwd });
+                return browser;
+              })().catch(error => {
+                sharedBrowserPromise = undefined;
+                throw error;
+              });
             }
             clientCount++;
-            const { browser, canBind } = sharedBrowser ? { browser: sharedBrowser, canBind: false } : await createBrowserWithInfo(config, clientInfo, options);
+            const { browser, canBind } = sharedBrowserPromise ? { browser: await sharedBrowserPromise, canBind: false } : await createBrowserWithInfo(config, clientInfo, options);
             if (canBind) {
               const count = (clientNameCounters.get(clientInfo.clientName) ?? 0) + 1;
               clientNameCounters.set(clientInfo.clientName, count);
@@ -124,11 +129,11 @@ export function decorateMCPCommand(command: Command) {
           },
           disposed: async backend => {
             clientCount--;
-            if (sharedBrowser && clientCount > 0)
+            if (sharedBrowserPromise && clientCount > 0)
               return;
 
             testDebug('close browser');
-            sharedBrowser = undefined;
+            sharedBrowserPromise = undefined;
             const browserContext = (backend as BrowserBackend).browserContext;
             await browserContext.close().catch(() => { });
             await browserContext.browser()!.close().catch(() => { });

tests/mcp/http.spec.ts

@@ -203,6 +203,35 @@ test('http transport browser lifecycle (isolated, multiclient)', async ({ server
   });
 });
 
+test('http transport browser lifecycle (isolated, concurrent clients)', { annotation: { type: 'issue', description: 'https://github.com/microsoft/playwright-mcp/issues/1607' } }, async ({ serverEndpoint, server }) => {
+  const { url, stderr } = await serverEndpoint({ args: ['--isolated'] });
+
+  const clients = await Promise.all([1, 2, 3].map(async () => {
+    const transport = new StreamableHTTPClientTransport(new URL('/mcp', url));
+    const client = new Client({ name: 'test', version: '1.0.0' });
+    await client.connect(transport);
+    return { transport, client };
+  }));
+
+  await Promise.all(clients.map(({ client }) => client.callTool({
+    name: 'browser_navigate',
+    arguments: { url: server.HELLO_WORLD },
+  })));
+
+  for (const { transport, client } of clients) {
+    await transport.terminateSession();
+    await client.close();
+  }
+
+  await expect.poll(() => formatLog(stderr())).toEqual({
+    'create http session': 3,
+    'delete http session': 3,
+    'create context': 3,
+    'create browser (isolated)': 1,
+    'close browser': 1,
+  });
+});
+
 test('http transport browser lifecycle (persistent)', async ({ serverEndpoint, server }) => {
   const { url, stderr } = await serverEndpoint();