fix(test-server): bind to 127.0.0.1 instead of 'localhost'

packages/playwright/src/program.ts

@@ -82,7 +82,7 @@ function addTestServerCommand(program: Command) {
   const command = program.command('test-server', { hidden: true });
   command.description('start test server');
   command.option('-c, --config <file>', `Configuration file, or a test directory with optional "playwright.config.{m,c}?{js,ts}"`);
-  command.option('--host <host>', 'Host to start the server on', 'localhost');
+  command.option('--host <host>', 'Host to start the server on', '127.0.0.1');
   command.option('--port <port>', 'Port to start the server on', '0');
   command.action(async opts => {
     await runTestServerAction(opts);

packages/utils/network.ts

@@ -177,7 +177,10 @@ export function createHttp2Server(...args: any[]): http2.Http2SecureServer {
 }
 
 export async function startHttpServer(server: http.Server, options: { host?: string, port?: number }) {
-  const { host = 'localhost', port = 0 } = options;
+  // Prefer IPv4 loopback over 'localhost' to avoid ambiguous DNS resolution
+  // on dual-stack systems (some environments resolve 'localhost' to '::1'
+  // first and then fail to connect over IPv6 loopback).
+  const { host = '127.0.0.1', port = 0 } = options;
   const errorPromise = new ManualPromise();
   const errorListener = (error: Error) => errorPromise.reject(error);
   server.on('error', errorListener);

tests/playwright-test/test-server.spec.ts

@@ -347,3 +347,17 @@ test('runGlobalSetup returns env', async ({ startTestServer, writeFiles }) => {
   expect(result2.env).toContainEqual(['MAGIC_BEFORE', null]);
   expect(result2.env).toContainEqual(['MAGIC_AFTER', '43']);
 });
+
+test('listens on 127.0.0.1 by default', async ({ startCLICommand, writeFiles }) => {
+  // 'localhost' resolution is ambiguous on dual-stack systems — some resolve
+  // to '::1' first, and IPv6 loopback has proven unreliable in certain
+  // environments (see https://github.com/microsoft/playwright/issues/40226).
+  // Default to IPv4 loopback so clients can always connect.
+  await writeFiles({ 'playwright.config.ts': `export default {};` });
+  const testServerProcess = await startCLICommand({}, 'test-server', []);
+  await testServerProcess.waitForOutput('Listening on');
+  const line = testServerProcess.output.split('\n').find(l => l.includes('Listening on'))!;
+  const wsEndpoint = line.split(' ')[2];
+  expect(wsEndpoint).toMatch(/^ws:\/\/127\.0\.0\.1:\d+\//);
+  await testServerProcess.kill();
+});