microsoft/playwright · error · Error

--mobile is not supported with the Firefox browser.

Error message

--mobile is not supported with the Firefox browser.

What it means

Thrown by configFromCLIOptions() when --mobile is set and the resolved browser is Firefox. The --mobile shorthand maps to a chromium/webkit device preset, and Firefox lacks equivalent mobile device descriptors in Playwright, so the combination is unsupported by design.

Solutions

1. Switch the browser to chromium or webkit if you need mobile emulation, e.g. `--browser chromium --mobile`.
2. If you must use Firefox, drop --mobile and pass equivalent context options manually (userAgent, viewport, isMobile, etc.).
3. Audit the config file and PLAYWRIGHT_MCP_BROWSER env var to ensure firefox is not being set implicitly.

Example fix

// before
playwright mcp --browser firefox --mobile
// after
playwright mcp --browser chromium --mobile

if (cliOptions.mobile && browserName === 'firefox') {
  throw new Error('--mobile is unsupported on Firefox; use chromium or webkit.');
}

function mobileSupportedFor(browserName?: string): boolean {
  return browserName === 'chromium' || browserName === 'webkit';
}

When it happens

Trigger: Running `playwright mcp --browser firefox --mobile`, or setting both the browser to firefox (via --browser, PLAYWRIGHT_MCP_BROWSER, or config) and --mobile.

Common situations: Defaulting to firefox in a config file then adding --mobile on the command line; assuming --mobile works uniformly across browsers; CI matrix that reuses the same flags for every browser.

Related errors

• Cannot use --mobile together with --device, pick one.
• Device emulation is not supported with cdpEndpoint.
• Invalid : . Valid values are
• Invalid resolution format: use
• Init page file does not exist

AI-assisted analysis of microsoft/playwright@f1d33b5029 (2026-08-12). Data as JSON: /api/errors/5354084fd8577ce5. Report an issue: GitHub.

Appendix: source

Thrown at packages/playwright-core/src/tools/mcp/config.ts:320

  // Launch options
  const launchOptions: playwrightTypes.LaunchOptions = {
    channel,
    executablePath: cliOptions.executablePath,
    headless: cliOptions.headless,
  };

  // --sandbox was passed, enable the sandbox
  // --no-sandbox was passed, disable the sandbox
  if (cliOptions.sandbox !== undefined)
    launchOptions.chromiumSandbox = cliOptions.sandbox;

  let device = cliOptions.device;
  if (cliOptions.mobile) {
    if (device)
      throw new Error('Cannot use --mobile together with --device, pick one.');
    if (browserName === 'firefox')
      throw new Error('--mobile is not supported with the Firefox browser.');
    device = browserName === 'webkit' ? 'iPhone 17' : 'Pixel 10';
  }

  if (device && cliOptions.cdpEndpoint)
    throw new Error('Device emulation is not supported with cdpEndpoint.');

  // Context options
  const contextOptions: playwrightTypes.BrowserContextOptions = device ? playwright.devices[device] : {};

  if (cliOptions.proxyServer) {
    const proxy: playwrightTypes.LaunchOptions['proxy'] = { server: cliOptions.proxyServer };
    if (cliOptions.proxyBypass)
      proxy.bypass = cliOptions.proxyBypass;
    // Set on both to ensure CLI takes precedence over any proxy set in the config file
    // (launchOptions.proxy applies at browser launch, contextOptions.proxy at context creation).
    launchOptions.proxy = proxy;
    contextOptions.proxy = proxy;
  }

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