chore: implement a daemon process

src/daemon/daemon.ts

@@ -0,0 +1,214 @@
+#!/usr/bin/env node
+
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import fs from 'node:fs/promises';
+import {createServer, type Server} from 'node:net';
+import process from 'node:process';
+
+import {Client} from '@modelcontextprotocol/sdk/client/index.js';
+import {StdioClientTransport} from '@modelcontextprotocol/sdk/client/stdio.js';
+
+import {logger} from '../logger.js';
+import {PipeTransport} from '../third_party/index.js';
+
+import {
+  getSocketPath,
+  handlePidFile,
+  INDEX_SCRIPT_PATH,
+  IS_WINDOWS,
+} from './utils.js';
+
+const pidFile = handlePidFile();
+const socketPath = getSocketPath();
+
+let mcpClient: Client | null = null;
+let mcpTransport: StdioClientTransport | null = null;
+let server: Server | null = null;
+
+async function setupMCPClient() {
+  console.log('Setting up MCP client connection...');
+
+  const args = process.argv.slice(2);
+  // Create stdio transport for chrome-devtools-mcp
+  mcpTransport = new StdioClientTransport({
+    command: process.execPath,
+    args: [INDEX_SCRIPT_PATH, ...args],
+    env: process.env as Record<string, string>,
+  });
+  mcpClient = new Client(
+    {
+      name: 'chrome-devtools-cli-daemon',
+      // TODO: handle client version (optional).
+      version: '0.1.0',
+    },
+    {
+      capabilities: {},
+    },
+  );
+  await mcpClient.connect(mcpTransport);
+
+  console.log('MCP client connected');
+}
+
+interface McpContent {
+  type: string;
+  text?: string;
+}
+
+interface McpResult {
+  content?: McpContent[] | string;
+  text?: string;
+}
+
+type DaemonMessage =
+  | {
+      method: 'stop';
+    }
+  | {
+      method: 'invoke_tool';
+      tool: string;
+      args?: Record<string, unknown>;
+    };
+
+async function handleRequest(msg: DaemonMessage) {
+  try {
+    if (msg.method === 'invoke_tool') {
+      if (!mcpClient) {
+        throw new Error('MCP client not initialized');
+      }
+      const {tool, args} = msg;
+
+      const result = (await mcpClient.callTool({
+        name: tool,
+        arguments: args || {},
+      })) as McpResult | McpContent[];
+
+      return {
+        success: true,
+        result: JSON.stringify(result),
+      };
+    } else if (msg.method === 'stop') {
+      // Trigger cleanup asynchronously
+      setImmediate(() => {
+        void cleanup();
+      });
+      return {
+        success: true,
+        message: 'stopping',
+      };
+    } else {
+      return {
+        success: false,
+        error: `Unknown method: ${JSON.stringify(msg, null, 2)}`,
+      };
+    }
+  } catch (error: unknown) {
+    const errorMessage = error instanceof Error ? error.message : String(error);
+    return {
+      success: false,
+      error: errorMessage,
+    };
+  }
+}
+
+async function startSocketServer() {
+  // Remove existing socket file if it exists (only on non-Windows)
+  if (!IS_WINDOWS) {
+    try {
+      await fs.unlink(socketPath);
+    } catch {
+      // ignore errors.
+    }
+  }
+
+  return await new Promise<void>((resolve, reject) => {
+    server = createServer(socket => {
+      const transport = new PipeTransport(socket, socket);
+      transport.onmessage = async (message: string) => {
+        logger('onmessage', message);
+        const response = await handleRequest(JSON.parse(message));
+        transport.send(JSON.stringify(response));
+        socket.end();
+      };
+      socket.on('error', error => {
+        logger('Socket error:', error);
+      });
+    });
+
+    server.listen(
+      {
+        path: socketPath,
+        readableAll: false,
+        writableAll: false,
+      },
+      async () => {
+        console.log(`Daemon server listening on ${socketPath}`);
+
+        try {
+          // Setup MCP client
+          await setupMCPClient();
+          resolve();
+        } catch (err) {
+          reject(err);
+        }
+      },
+    );
+
+    server.on('error', error => {
+      logger('Server error:', error);
+      reject(error);
+    });
+  });
+}
+
+async function cleanup() {
+  console.log('Cleaning up daemon...');
+
+  try {
+    await mcpClient?.close();
+  } catch (error) {
+    logger('Error closing MCP client:', error);
+  }
+  try {
+    await mcpTransport?.close();
+  } catch (error) {
+    logger('Error closing MCP transport:', error);
+  }
+  server?.close(() => {
+    if (!IS_WINDOWS) {
+      void fs.unlink(socketPath).catch(() => undefined);
+    }
+  });
+  await fs.unlink(pidFile).catch(() => undefined);
+  process.exit(0);
+}
+
+// Handle shutdown signals
+process.on('SIGTERM', () => {
+  void cleanup();
+});
+process.on('SIGINT', () => {
+  void cleanup();
+});
+process.on('SIGHUP', () => {
+  void cleanup();
+});
+
+// Handle uncaught errors
+process.on('uncaughtException', error => {
+  logger('Uncaught exception:', error);
+});
+process.on('unhandledRejection', error => {
+  logger('Unhandled rejection:', error);
+});
+
+// Start the server
+startSocketServer().catch(error => {
+  logger('Failed to start daemon server:', error);
+  process.exit(1);
+});

src/daemon/utils.ts

@@ -0,0 +1,85 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import process from 'node:process';
+
+export const DAEMON_SCRIPT_PATH = path.join(import.meta.dirname, 'daemon.js');
+export const INDEX_SCRIPT_PATH = path.join(
+  import.meta.dirname,
+  '..',
+  'index.js',
+);
+
+const APP_NAME = 'chrome-devtools-mcp';
+
+// Using these paths due to strict limits on the POSIX socket path length.
+export function getSocketPath(): string {
+  const uid = os.userInfo().uid;
+
+  if (IS_WINDOWS) {
+    // Windows uses Named Pipes, not file paths.
+    // This format is required for server.listen()
+    return path.join('\\\\.\\pipe', APP_NAME, 'server.sock');
+  }
+
+  // 1. Try XDG_RUNTIME_DIR (Linux standard, sometimes macOS)
+  if (process.env.XDG_RUNTIME_DIR) {
+    return path.join(process.env.XDG_RUNTIME_DIR, APP_NAME, 'server.sock');
+  }
+
+  // 2. macOS/Unix Fallback: Use /tmp/
+  // We use /tmp/ because it is much shorter than ~/Library/Application Support/
+  // and keeps us well under the 104-character limit.
+  return path.join('/tmp', `${APP_NAME}-${uid}.sock`);
+}
+
+export function getRuntimeHome(): string {
+  const platform = os.platform();
+  const uid = os.userInfo().uid;
+
+  // 1. Check for the modern Unix standard
+  if (process.env.XDG_RUNTIME_DIR) {
+    return path.join(process.env.XDG_RUNTIME_DIR, APP_NAME);
+  }
+
+  // 2. Fallback for macOS and older Linux
+  if (platform === 'darwin' || platform === 'linux') {
+    // /tmp is cleared on boot, making it perfect for PIDs
+    return path.join('/tmp', `${APP_NAME}-${uid}`);
+  }
+
+  // 3. Windows Fallback
+  return path.join(os.tmpdir(), APP_NAME);
+}
+
+export const IS_WINDOWS = os.platform() === 'win32';
+
+export function handlePidFile() {
+  const runtimeDir = getRuntimeHome();
+  const pidPath = path.join(runtimeDir, 'daemon.pid');
+
+  if (fs.existsSync(pidPath)) {
+    const oldPid = parseInt(fs.readFileSync(pidPath, 'utf8'), 10);
+    try {
+      // Sending signal 0 checks if the process is still alive without killing it
+      process.kill(oldPid, 0);
+      console.error('Daemon is already running!');
+      process.exit(1);
+    } catch {
+      // Process is dead, we can safely overwrite the PID file
+      fs.unlinkSync(pidPath);
+    }
+  }
+
+  fs.mkdirSync(path.dirname(pidPath), {
+    recursive: true,
+  });
+  fs.writeFileSync(pidPath, process.pid.toString());
+  return pidPath;
+}

src/third_party/index.ts

@@ -30,6 +30,7 @@ export {
 } from 'puppeteer-core';
 export {default as puppeteer} from 'puppeteer-core';
 export type * from 'puppeteer-core';
+export {PipeTransport} from 'puppeteer-core/internal/node/PipeTransport.js';
 export type {CdpPage} from 'puppeteer-core/internal/cdp/Page.js';
 export {
   resolveDefaultUserDataDir,

tests/daemon/daemon.test.ts

@@ -0,0 +1,62 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import assert from 'node:assert';
+import {spawn} from 'node:child_process';
+import net from 'node:net';
+import path from 'node:path';
+import {describe, it} from 'node:test';
+
+import {getSocketPath} from '../../src/daemon/utils.js';
+
+const DAEMON_SCRIPT = path.join(
+  import.meta.dirname,
+  '..',
+  '..',
+  'src',
+  'daemon',
+  'daemon.js',
+);
+
+describe('Daemon', () => {
+  it('should terminate chrome instance when transport is closed', async () => {
+    const daemonProcess = spawn(process.execPath, [DAEMON_SCRIPT], {
+      env: {
+        ...process.env,
+      },
+      stdio: ['ignore', 'pipe', 'pipe'],
+    });
+
+    const socketPath = getSocketPath();
+    // Wait for daemon to be ready
+    await new Promise<void>((resolve, reject) => {
+      const onData = (data: Buffer) => {
+        const output = data.toString();
+        // Wait for MCP client to connect
+        if (output.includes('MCP client connected')) {
+          daemonProcess.stdout.off('data', onData);
+          resolve();
+        }
+      };
+      daemonProcess.stdout.on('data', onData);
+      daemonProcess.stderr.on('data', data => {
+        console.log('err', data.toString('utf8'));
+      });
+      daemonProcess.on('error', reject);
+      daemonProcess.on('exit', (code: number) => {
+        if (code !== 0 && code !== null) {
+          reject(new Error(`Daemon exited with code ${code}`));
+        }
+      });
+    });
+
+    const socket = net.createConnection(socketPath);
+    await new Promise<void>(resolve => socket.on('connect', resolve));
+
+    daemonProcess.kill();
+    assert.ok(daemonProcess.killed);
+  });
+});