Chrome Devtools

Chrome DevTools Agent Skill

Browser automation via executable Puppeteer scripts. All scripts output JSON for easy parsing.

Quick Start

CRITICAL: Always check pwd before running scripts.

Installation

Step 1: Install System Dependencies (Linux/WSL only)

On Linux/WSL, Chrome requires system libraries. Install them first:

pwd  # Should show current working directory
cd ~/.Codex/skills/chrome-devtools/scripts
./install-deps.sh  # Auto-detects OS and installs required libs

Supports: Ubuntu, Debian, Fedora, RHEL, CentOS, Arch, Manjaro

macOS/Windows: Skip this step (dependencies bundled with Chrome)

Step 2: Install Node Dependencies

npm install  # Installs puppeteer, debug, yargs

Step 3: Install ImageMagick (Optional, Recommended)

ImageMagick enables automatic screenshot compression to keep files under 5MB:

macOS:

brew install imagemagick

Ubuntu/Debian/WSL:

sudo apt-get install imagemagick

Verify:

magick -version  # or: convert -version

Without ImageMagick, screenshots >5MB will not be compressed (may fail to load in Gemini/Codex).

Test

node navigate.js --url https://example.com
# Output: {"success": true, "url": "https://example.com", "title": "Example Domain"}

Available Scripts

All scripts are in ~/.Codex/skills/chrome-devtools/scripts/

CRITICAL: Always check pwd before running scripts.

Script Usage

• ./scripts/README.md

Core Automation

• navigate.js - Navigate to URLs
• screenshot.js - Capture screenshots (full page or element)
• click.js - Click elements
• fill.js - Fill form fields
• evaluate.js - Execute JavaScript in page context

Analysis & Monitoring

• snapshot.js - Extract interactive elements with metadata
• console.js - Monitor console messages/errors
• network.js - Track HTTP requests/responses
• performance.js - Measure Core Web Vitals + record traces

Usage Patterns

Single Command

pwd  # Should show current working directory
cd ~/.Codex/skills/chrome-devtools/scripts
node screenshot.js --url https://example.com --output ./docs/screenshots/page.png

Important: Always save screenshots to ./docs/screenshots directory.

Automatic Image Compression

Screenshots are automatically compressed if they exceed 5MB to ensure compatibility with host AI tool (which have 5MB limits). This uses ImageMagick internally:

# Default: auto-compress if >5MB
node screenshot.js --url https://example.com --output page.png

# Custom size threshold (e.g., 3MB)
node screenshot.js --url https://example.com --output page.png --max-size 3

# Disable compression
node screenshot.js --url https://example.com --output page.png --no-compress

Compression behavior:

• PNG: Resizes to 90% + quality 85 (or 75% + quality 70 if still too large)
• JPEG: Quality 80 + progressive encoding (or quality 60 if still too large)
• Other formats: Converted to JPEG with compression
• Requires ImageMagick installed (see imagemagick skill)

Output includes compression info:

{
  "success": true,
  "output": "/path/to/page.png",
  "compressed": true,
  "originalSize": 8388608,
  "size": 3145728,
  "compressionRatio": "62.50%",
  "url": "https://example.com"
}

Chain Commands (reuse browser)

# Keep browser open with --close false
node navigate.js --url https://example.com/login --close false
node fill.js --selector "#email" --value "user@example.com" --close false
node fill.js --selector "#password" --value "secret" --close false
node click.js --selector "button[type=submit]"

Parse JSON Output

# Extract specific fields with jq
node performance.js --url https://example.com | jq '.vitals.LCP'

# Save to file
node network.js --url https://example.com --output /tmp/requests.json

Execution Protocol

Working Directory Verification

BEFORE executing any script:

1. Check current working directory with pwd
2. Verify in ~/.Codex/skills/chrome-devtools/scripts/ directory
3. If wrong directory, cd to correct location
4. Use absolute paths for all output files

Example:

pwd  # Should show: .../chrome-devtools/scripts
# If wrong:
cd ~/.Codex/skills/chrome-devtools/scripts

Output Validation

AFTER screenshot/capture operations:

1. Verify file created with ls -lh <output-path>
2. Read screenshot using Read tool to confirm content
3. Check JSON output for success:true
4. Report file size and compression status

Example:

node screenshot.js --url https://example.com --output ./docs/screenshots/page.png
ls -lh ./docs/screenshots/page.png  # Verify file exists
# Then use Read tool to visually inspect

5. Restart working directory to the project root.

Error Recovery

If script fails:

1. Check error message for selector issues
2. Use snapshot.js to discover correct selectors
3. Try XPath selector if CSS selector fails
4. Verify element is visible and interactive

Example:

# CSS selector fails
node click.js --url https://example.com --selector ".btn-submit"
# Error: waiting for selector ".btn-submit" failed

# Discover correct selector
node snapshot.js --url https://example.com | jq '.elements[] | select(.tagName=="BUTTON")'

# Try XPath
node click.js --url https://example.com --selector "//button[contains(text(),'Submit')]"

Common Mistakes

❌ Wrong working directory → output files go to wrong location ❌ Skipping output validation → silent failures ❌ Using complex CSS selectors without testing → selector errors ❌ Not checking element visibility → timeout errors

✅ Always verify pwd before running scripts ✅ Always validate output after screenshots ✅ Use snapshot.js to discover selectors ✅ Test selectors with simple commands first

Common Workflows

Web Scraping

node evaluate.js --url https://example.com --script "
  Array.from(document.querySelectorAll('.item')).map(el => ({
    title: el.querySelector('h2')?.textContent,
    link: el.querySelector('a')?.href
  }))
" | jq '.result'

Performance Testing

PERF=$(node performance.js --url https://example.com)
LCP=$(echo $PERF | jq '.vitals.LCP')
if (( $(echo "$LCP < 2500" | bc -l) )); then
  echo "✓ LCP passed: ${LCP}ms"
else
  echo "✗ LCP failed: ${LCP}ms"
fi

Form Automation

node fill.js --url https://example.com --selector "#search" --value "query" --close false
node click.js --selector "button[type=submit]"

Error Monitoring

node console.js --url https://example.com --types error,warn --duration 5000 | jq '.messageCount'

Script Options

All scripts support:

• --headless false - Show browser window
• --close false - Keep browser open for chaining
• --timeout 30000 - Set timeout (milliseconds)
• --wait-until networkidle2 - Wait strategy

See ./scripts/README.md for complete options.

Output Format

All scripts output JSON to stdout:

{
  "success": true,
  "url": "https://example.com",
  ... // script-specific data
}

Errors go to stderr:

{
  "success": false,
  "error": "Error message"
}

Finding Elements

Use snapshot.js to discover selectors:

node snapshot.js --url https://example.com | jq '.elements[] | {tagName, text, selector}'

Troubleshooting

Common Errors

"Cannot find package 'puppeteer'"

• Run: npm install in the scripts directory

"error while loading shared libraries: libnss3.so" (Linux/WSL)

• Missing system dependencies
• Fix: Run ./install-deps.sh in scripts directory
• Manual install: sudo apt-get install -y libnss3 libnspr4 libasound2t64 libatk1.0-0 libatk-bridge2.0-0 libcups2 libdrm2 libxkbcommon0 libxcomposite1 libxdamage1 libxfixes3 libxrandr2 libgbm1

"Failed to launch the browser process"

• Check system dependencies installed (Linux/WSL)
• Verify Chrome downloaded: ls ~/.cache/puppeteer
• Try: npm rebuild then npm install

Chrome not found

• Puppeteer auto-downloads Chrome during npm install
• If failed, manually trigger: npx puppeteer browsers install chrome

Script Issues

Element not found

• Get snapshot first to find correct selector: node snapshot.js --url <url>

Script hangs

• Increase timeout: --timeout 60000
• Change wait strategy: --wait-until load or --wait-until domcontentloaded

Blank screenshot

• Wait for page load: --wait-until networkidle2
• Increase timeout: --timeout 30000

Permission denied on scripts

• Make executable: chmod +x *.sh

Screenshot too large (>5MB)

• Install ImageMagick for automatic compression
• Manually set lower threshold: --max-size 3
• Use JPEG format instead of PNG: --format jpeg --quality 80
• Capture specific element instead of full page: --selector .main-content

Compression not working

• Verify ImageMagick installed: magick -version or convert -version
• Check file was actually compressed in output JSON: "compressed": true
• For very large pages, use --selector to capture only needed area

Reference Documentation

Detailed guides available in ./references/:

• CDP Domains Reference - 47 Chrome DevTools Protocol domains
• Puppeteer Quick Reference - Complete Puppeteer API patterns
• Performance Analysis Guide - Core Web Vitals optimization

Advanced Usage

Custom Scripts

Create custom scripts using shared library:

import {
  getBrowser,
  getPage,
  closeBrowser,
  outputJSON,
} from "./lib/browser.js";
// Your automation logic

Direct CDP Access

const client = await page.createCDPSession();
await client.send("Emulation.setCPUThrottlingRate", { rate: 4 });

See reference documentation for advanced patterns and complete API coverage.

External Resources

• Puppeteer Documentation
• Chrome DevTools Protocol
• Scripts README

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