Chrome DevTools CLI

Command-line interface for Chrome DevTools Protocol (CDP), optimized for LLM agents with NDJSON output format.

Overview

cdp-cli provides CLI access to all Chrome DevTools Protocol features, making it easy to automate browser interactions, debug web applications, and inspect network traffic - all from the command line with grep/tail-friendly output.

New Features

--inspect for Console Output

Fully expand nested objects and arrays in console messages instead of seeing Object or Array(12):

# Without --inspect
cdp-cli console "MyPage"
"Config: {options: Object, values: Array(5)}"

# With --inspect - see actual values
cdp-cli console "MyPage" --inspect
"Config: {options: {debug: true, level: 3}, values: [1, 2, 3, 4, 5]}"

--user-gesture for Click Command

Enable WebXR, fullscreen, and other activation-gated browser APIs that require user interaction:

# Standard click (uses Input.dispatchMouseEvent)
cdp-cli click "MyPage" "button#submit"

# User gesture click (uses Runtime.evaluate with userGesture: true)
# Required for WebXR session requests, fullscreen API, etc.
cdp-cli click "MyPage" "button#enter-vr" --user-gesture

Installation

npm install -g @myerscarpenter/cdp-cli

Quick Start

# Launch Chrome with remote debugging
cdp-cli launch

# List all open pages
cdp-cli tabs

# Navigate to a URL
cdp-cli new "https://example.com"

# Get page snapshot (accessibility tree)
cdp-cli snapshot "example"

# Click an element
cdp-cli click "example" "a"

# Take a screenshot
cdp-cli screenshot "example" screenshot.jpg

Or start Chrome manually:

# macOS
/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome \
  --remote-debugging-port=9222

# Linux
google-chrome --remote-debugging-port=9222

# Windows
chrome.exe --remote-debugging-port=9222

Output Format: NDJSON

All list commands output newline-delimited JSON (NDJSON) - one complete JSON object per line. This format is:

• LLM-friendly: Easy to parse programmatically
• Grep-compatible: Filter with standard Unix tools
• Streamable: Handle large datasets incrementally

Example NDJSON Output

$ cdp-cli tabs
{"id":"A1B2C3","title":"GitHub","url":"https://github.com","type":"page"}
{"id":"D4E5F6","title":"Google","url":"https://google.com","type":"page"}

$ cdp-cli console "example"
{"type":"log","timestamp":1698234567890,"text":"Page loaded","source":"console-api"}
{"type":"error","timestamp":1698234568123,"text":"TypeError: Cannot read...","source":"exception","line":42,"url":"https://example.com/app.js"}

$ cdp-cli network "example" | grep '"type":"fetch"'
{"url":"https://api.example.com/data","method":"GET","status":200,"type":"fetch","size":4567}

Commands

Page Management

tabs - List all open browser pages

cdp-cli tabs

new - Create a new page/tab

cdp-cli new "https://example.com"
cdp-cli new  # Empty page

go - Navigate page (URL, back, forward, reload)

cdp-cli go "example" "https://github.com"
cdp-cli go "example" back
cdp-cli go "example" forward
cdp-cli go "example" reload

close - Close a page

cdp-cli close "example"
cdp-cli close A1B2C3

Debugging

console - List console messages (minimal output by default, optimized for LLM token savings)

# Minimal output (bare strings, last 10 messages)
cdp-cli console "example"
"Page loaded"
"API call successful"

# Show more messages
cdp-cli console "example" --tail 50

# Show all messages (no limit)
cdp-cli console "example" --all
# or
cdp-cli console "example" --tail -1

# Note: When truncated, stderr shows: "(N messages skipped. Use --tail M or --all to see more)"
# This warning is visible to both humans and LLM agents

# Include message type (switches to object format)
cdp-cli console "example" --with-type
{"text":"Page loaded","type":"log","source":"console-api"}
{"text":"Error: failed","type":"error","source":"exception","line":42,"url":"app.js"}

# Include timestamp
cdp-cli console "example" --with-timestamp
{"text":"Page loaded","timestamp":1698234567890}

# Include source location (url, line number for exceptions)
cdp-cli console "example" --with-source
{"text":"Error: undefined","url":"https://example.com/app.js","line":42}

# Verbose mode - all fields (shortcut for all three flags)
cdp-cli console "example" --verbose
# or
cdp-cli console "example" -v
{"text":"Error: undefined","type":"error","source":"exception","timestamp":1698234567890,"url":"app.js","line":42}

# Expand nested objects/arrays (instead of "Object" or "Array(N)")
cdp-cli console "example" --inspect
# or
cdp-cli console "example" -i

# Filter by type (still outputs bare strings by default)
cdp-cli console "example" --type error

# Collect for longer duration (2 seconds)
cdp-cli console "example" --duration 2

snapshot - Get page content snapshot

# Accessibility tree (default) - great for LLM element identification!
cdp-cli snapshot "example"

# Text content only
cdp-cli snapshot "example" --format text

# DOM tree (JSON)
cdp-cli snapshot "example" --format dom

eval - Evaluate JavaScript expression

cdp-cli eval "example" "document.title"
cdp-cli eval "example" "window.location.href"
cdp-cli eval "example" "Array.from(document.querySelectorAll('h1')).map(h => h.textContent)"

screenshot - Take a screenshot

# Save to file
cdp-cli screenshot "example" --output screenshot.jpg

# Different formats
cdp-cli screenshot "example" --output screenshot.png --format png

# Output base64 (NDJSON)
cdp-cli screenshot "example"

Network Inspection

network - List network requests (collects for 0.1s by default)

# Collect requests (default 0.1 seconds)
cdp-cli network "example"

# Collect for longer duration (5 seconds)
cdp-cli network "example" --duration 5

# Filter by type
cdp-cli network "example" --type fetch
cdp-cli network "example" --type xhr

# Combine duration and filtering
cdp-cli network "example" --duration 5 --type fetch

Input Automation

click - Click an element by CSS selector

cdp-cli click "example" "button#submit"
cdp-cli click "example" "a.link" --double

# Use --user-gesture for WebXR, fullscreen, and other activation-gated APIs
cdp-cli click "example" "button#enter-vr" --user-gesture
cdp-cli click "example" "button#fullscreen" -g  # short flag

fill - Fill an input element

cdp-cli fill "example" "[email protected]" "input#email"
cdp-cli fill "example" "secret123" "input[name='password']"

key - Press a keyboard key

cdp-cli key "example" enter
cdp-cli key "example" tab
cdp-cli key "example" escape

LLM Usage Patterns

Pattern 1: Inspect and Interact

# 1. List pages to find target
cdp-cli tabs | grep "example"

# 2. Get accessibility tree to understand page structure
cdp-cli snapshot "example" --format ax > page-structure.json

# 3. Parse structure (LLM can identify element selectors)
# 4. Interact with elements
cdp-cli fill "example" "query" "input#search"
cdp-cli click "example" "button[type='submit']"

# 5. Capture result
cdp-cli screenshot "example" --output result.jpg

Pattern 2: Debug Web Application

# 1. Navigate to app
cdp-cli new "http://localhost:3000"

# 2. Monitor console for errors (increase duration for continuous monitoring)
cdp-cli console "localhost" --duration 10 --type error

# 3. Inspect failed network requests
cdp-cli network "localhost" --duration 5 | grep '"status":4'

Pattern 3: Automated Testing

# 1. Open test page
cdp-cli new "http://localhost:8080/test.html"

# 2. Fill form
cdp-cli fill "test" "testuser" "input#username"
cdp-cli fill "test" "testpass" "input#password"
cdp-cli click "test" "button#login"

# 3. Wait and verify
sleep 2
cdp-cli eval "test" "document.querySelector('.success-message')?.textContent"

# 4. Capture evidence
cdp-cli screenshot "test" --output test-result.jpg

Pattern 4: Data Extraction

# 1. Navigate to page
cdp-cli go "example" "https://example.com/data"

# 2. Extract data via JavaScript
cdp-cli eval "example" "Array.from(document.querySelectorAll('.item')).map(el => ({
  title: el.querySelector('.title').textContent,
  price: el.querySelector('.price').textContent
}))"

Global Options

• --cdp-url <url> - Chrome DevTools Protocol URL (default: http://localhost:9222)
• --help - Show help
• --version - Show version

Tips for LLM Agents

1. Use NDJSON parsing: Each line is a complete JSON object

   const lines = output.split('\n').filter(l => l.trim());
   const objects = lines.map(l => JSON.parse(l));

2. Leverage grep for filtering:

   cdp-cli network "example" | grep '"status":404'
   cdp-cli console "example" | grep error

3. Use accessibility tree for element discovery:

   cdp-cli snapshot "example" --format ax
   # Parse to find elements by role, name, etc.
   # Then construct CSS selectors for click/fill

4. Chain commands with Unix tools:

   cdp-cli tabs | jq -r '.title'
   cdp-cli console "example" | grep error | tail -5

5. Error handling: All errors output NDJSON with "error": true

   {"error":true,"message":"Page not found: example","code":"PAGE_NOT_FOUND"}

Architecture

Built with:

• TypeScript - Type-safe code
• yargs - CLI argument parsing
• ws - WebSocket for CDP communication
• NDJSON - LLM-friendly output format

Reuses battle-tested CDP logic from chrome-devtools-mcp.

Testing

This project includes a comprehensive test suite using Vitest.

Running Tests

# Run all tests
npm test

# Run tests in watch mode (auto-rerun on file changes)
npm run test:watch

# Run tests with coverage report
npm run test:coverage

# Run tests with interactive UI
npm run test:ui

Test Structure

tests/
├── fixtures/          # Sample CDP responses and test data
│   └── cdp-responses.ts
├── mocks/             # Mock implementations
│   ├── websocket.mock.ts   # WebSocket mock for CDP
│   └── fetch.mock.ts       # Fetch mock for REST API
├── helpers.ts         # Test utilities
├── setup.ts           # Test environment setup
└── unit/              # Unit tests
    ├── output.test.ts       # Output formatting tests
    ├── context.test.ts      # CDPContext tests
    └── commands/            # Command tests
        ├── pages.test.ts
        ├── debug.test.ts
        ├── network.test.ts
        └── input.test.ts

Test Coverage

Current coverage:

• Output formatting: 100% (10 tests)
• CDPContext: ~95% (23 tests)
• Pages commands: ~90% (11 tests)
• Overall: 80%+ lines, functions, and statements

Writing New Tests

Tests use mocked WebSocket and fetch, so no running Chrome instance is required:

import { describe, it, expect, beforeEach } from 'vitest';
import { CDPContext } from '../src/context.js';
import { installMockFetch } from './mocks/fetch.mock.js';

describe('My Test', () => {
  beforeEach(() => {
    installMockFetch(); // Mock CDP REST API
  });

  it('should test something', async () => {
    const context = new CDPContext();
    const pages = await context.getPages();
    expect(pages).toHaveLength(3);
  });
});

Continuous Integration

Tests run automatically on every commit and pull request (if CI is configured).

License

MIT

Related Projects

• chrome-devtools-mcp - MCP server for Chrome DevTools
• quest-devtools.mjs - Simple CDP CLI prototype

Built for LLM agents - Every command outputs structured, parseable, grep-friendly data.