feat: support initial viewport in the CLI

README.md

@@ -272,6 +272,10 @@ The Chrome DevTools MCP server supports the following configuration option:
   Path to a file to write debug logs to. Set the env variable `DEBUG` to `*` to enable verbose logs. Useful for submitting bug reports.
   - **Type:** string
 
+- **`--viewport`**
+  Initial viewport size for the Chromee instances started by the server. For example, `1280x720`
+  - **Type:** string
+
 <!-- END AUTO GENERATED OPTIONS -->
 
 Pass them via the `args` property in the JSON configuration. For example:

src/browser.ts

@@ -44,7 +44,7 @@ const connectOptions: ConnectOptions = {
   protocolTimeout: 10_000,
 };
 
-async function ensureBrowserConnected(browserURL: string) {
+export async function ensureBrowserConnected(browserURL: string) {
   if (browser?.connected) {
     return browser;
   }
@@ -64,6 +64,10 @@ interface McpLaunchOptions {
   headless: boolean;
   isolated: boolean;
   logFile?: fs.WriteStream;
+  viewport?: {
+    width: number;
+    height: number;
+  };
 }
 
 export async function launch(options: McpLaunchOptions): Promise<Browser> {
@@ -115,6 +119,14 @@ export async function launch(options: McpLaunchOptions): Promise<Browser> {
       browser.process()?.stderr?.pipe(options.logFile);
       browser.process()?.stdout?.pipe(options.logFile);
     }
+    if (options.viewport) {
+      const [page] = await browser.pages();
+      // @ts-expect-error internal API for now.
+      await page?.resize({
+        contentWidth: options.viewport.width,
+        contentHeight: options.viewport.height,
+      });
+    }
     return browser;
   } catch (error) {
     if (
@@ -134,7 +146,7 @@ export async function launch(options: McpLaunchOptions): Promise<Browser> {
   }
 }
 
-async function ensureBrowserLaunched(
+export async function ensureBrowserLaunched(
   options: McpLaunchOptions,
 ): Promise<Browser> {
   if (browser?.connected) {
@@ -144,20 +156,4 @@ async function ensureBrowserLaunched(
   return browser;
 }
 
-export async function resolveBrowser(options: {
-  browserUrl?: string;
-  executablePath?: string;
-  customDevTools?: string;
-  channel?: Channel;
-  headless: boolean;
-  isolated: boolean;
-  logFile?: fs.WriteStream;
-}) {
-  const browser = options.browserUrl
-    ? await ensureBrowserConnected(options.browserUrl)
-    : await ensureBrowserLaunched(options);
-
-  return browser;
-}
-
 export type Channel = 'stable' | 'canary' | 'beta' | 'dev';

src/cli.ts

@@ -54,6 +54,24 @@ export const cliOptions = {
     describe:
       'Path to a file to write debug logs to. Set the env variable `DEBUG` to `*` to enable verbose logs. Useful for submitting bug reports.',
   },
+  viewport: {
+    type: 'string' as const,
+    describe:
+      'Initial viewport size for the Chromee instances started by the server. For example, `1280x720`',
+    coerce: (arg: string | undefined) => {
+      if (arg === undefined) {
+        return;
+      }
+      const [width, height] = arg.split('x').map(Number);
+      if (!width || !height || Number.isNaN(width) || Number.isNaN(height)) {
+        throw new Error('Invalid viewport. Expected format is `1280x720`.');
+      }
+      return {
+        width,
+        height,
+      };
+    },
+  },
 };
 
 export function parseArguments(version: string, argv = process.argv) {
@@ -79,6 +97,10 @@ export function parseArguments(version: string, argv = process.argv) {
       ['$0 --channel stable', 'Use stable Chrome installed on this system'],
       ['$0 --logFile /tmp/log.txt', 'Save logs to a file'],
       ['$0 --help', 'Print CLI options'],
+      [
+        '$0 --viewport 1280x720',
+        'Launch Chrome with the initial viewport size of 1280x720px',
+      ],
     ]);
 
   return yargsInstance

src/main.ts

@@ -16,7 +16,7 @@ import type {CallToolResult} from '@modelcontextprotocol/sdk/types.js';
 import {SetLevelRequestSchema} from '@modelcontextprotocol/sdk/types.js';
 
 import type {Channel} from './browser.js';
-import {resolveBrowser} from './browser.js';
+import {ensureBrowserConnected, ensureBrowserLaunched} from './browser.js';
 import {parseArguments} from './cli.js';
 import {logger, saveLogsToFile} from './logger.js';
 import {McpContext} from './McpContext.js';
@@ -69,15 +69,18 @@ server.server.setRequestHandler(SetLevelRequestSchema, () => {
 
 let context: McpContext;
 async function getContext(): Promise<McpContext> {
-  const browser = await resolveBrowser({
-    browserUrl: args.browserUrl,
-    headless: args.headless,
-    executablePath: args.executablePath,
-    customDevTools: args.customDevtools,
-    channel: args.channel as Channel,
-    isolated: args.isolated,
-    logFile,
-  });
+  const browser = args.browserUrl
+    ? await ensureBrowserConnected(args.browserUrl)
+    : await ensureBrowserLaunched({
+        headless: args.headless,
+        executablePath: args.executablePath,
+        customDevTools: args.customDevtools,
+        channel: args.channel as Channel,
+        isolated: args.isolated,
+        logFile,
+        viewport: args.viewport,
+      });
+
   if (context?.browser !== browser) {
     context = await McpContext.from(browser, logger);
   }

src/tools/input.ts

@@ -197,7 +197,7 @@ export const uploadFile = defineTool({
         // a type=file element. In this case, we want to default to
         // Page.waitForFileChooser() and upload the file this way.
         try {
-          const page = await context.getSelectedPage();
+          const page = context.getSelectedPage();
           const [fileChooser] = await Promise.all([
             page.waitForFileChooser({timeout: 3000}),
             handle.asLocator().click(),

tests/browser.test.ts

@@ -42,4 +42,31 @@ describe('browser', () => {
       await browser1.close();
     }
   });
+
+  it('launches with the initial viewport', async () => {
+    const tmpDir = os.tmpdir();
+    const folderPath = path.join(tmpDir, `temp-folder-${crypto.randomUUID()}`);
+    const browser = await launch({
+      headless: true,
+      isolated: false,
+      userDataDir: folderPath,
+      executablePath: executablePath(),
+      viewport: {
+        width: 700,
+        height: 500,
+      },
+    });
+    try {
+      const [page] = await browser.pages();
+      const result = await page.evaluate(() => {
+        return {width: window.innerWidth, height: window.innerHeight};
+      });
+      assert.deepStrictEqual(result, {
+        width: 700,
+        height: 500,
+      });
+    } finally {
+      await browser.close();
+    }
+  });
 });

tests/cli.test.ts

@@ -55,4 +55,24 @@ describe('cli args parsing', () => {
       executablePath: '/tmp/test 123/chrome',
     });
   });
+
+  it('parses viewport', async () => {
+    const args = parseArguments('1.0.0', [
+      'node',
+      'main.js',
+      '--viewport',
+      '888x777',
+    ]);
+    assert.deepStrictEqual(args, {
+      _: [],
+      headless: false,
+      isolated: false,
+      $0: 'npx chrome-devtools-mcp@latest',
+      channel: 'stable',
+      viewport: {
+        width: 888,
+        height: 777,
+      },
+    });
+  });
 });