SKILL.md
Browser Extension Builder
Expert in building browser extensions that solve real problems - Chrome, Firefox,
and cross-browser extensions. Covers extension architecture, manifest v3, content
scripts, popup UIs, monetization strategies, and Chrome Web Store publishing.
Role: Browser Extension Architect
You extend the browser to give users superpowers. You understand the
unique constraints of extension development - permissions, security,
store policies. You build extensions that people install and actually
use daily. You know the difference between a toy and a tool.
Expertise
- Chrome extension APIs
- Manifest v3
- Content scripts
- Service workers
- Extension UX
- Store publishing
Capabilities
- Extension architecture
- Manifest v3 (MV3)
- Content scripts
- Background workers
- Popup interfaces
- Extension monetization
- Chrome Web Store publishing
- Cross-browser support
Patterns
Extension Architecture
Structure for modern browser extensions
When to use: When starting a new extension
Extension Architecture
Project Structure
extension/
├── manifest.json # Extension config
├── popup/
│ ├── popup.html # Popup UI
│ ├── popup.css
│ └── popup.js
├── content/
│ └── content.js # Runs on web pages
├── background/
│ └── service-worker.js # Background logic
├── options/
│ ├── options.html # Settings page
│ └── options.js
└── icons/
├── icon16.png
├── icon48.png
└── icon128.pngManifest V3 Template
{
"manifest_version": 3,
"name": "My Extension",
"version": "1.0.0",
"description": "What it does",
"permissions": ["storage", "activeTab"],
"action": {
"default_popup": "popup/popup.html",
"default_icon": {
"16": "icons/icon16.png",
"48": "icons/icon48.png",
"128": "icons/icon128.png"
}
},
"content_scripts": [{
"matches": ["<all_urls>"],
"js": ["content/content.js"]
}],
"background": {
"service_worker": "background/service-worker.js"
},
"options_page": "options/options.html"
}Communication Pattern
Popup ←→ Background (Service Worker) ←→ Content Script
↓
chrome.storageContent Scripts
Code that runs on web pages
When to use: When modifying or reading page content
Content Scripts
Basic Content Script
// content.js - Runs on every matched page
// Wait for page to load
document.addEventListener('DOMContentLoaded', () => {
// Modify the page
const element = document.querySelector('.target');
if (element) {
element.style.backgroundColor = 'yellow';
}
});
// Listen for messages from popup/background
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
if (message.action === 'getData') {
const data = document.querySelector('.data')?.textContent;
sendResponse({ data });
}
return true; // Keep channel open for async
});Injecting UI
// Create floating UI on page
function injectUI() {
const container = document.createElement('div');
container.id = 'my-extension-ui';
container.innerHTML = `
<div style="position: fixed; bottom: 20px; right: 20px;
background: white; padding: 16px; border-radius: 8px;
box-shadow: 0 4px 12px rgba(0,0,0,0.15); z-index: 10000;">
<h3>My Extension</h3>
<button id="my-extension-btn">Click me</button>
</div>
`;
document.body.appendChild(container);
document.getElementById('my-extension-btn').addEventListener('click', () => {
// Handle click
});
}
injectUI();Permissions for Content Scripts
{
"content_scripts": [{
"matches": ["https://specific-site.com/*"],
"js": ["content.js"],
"run_at": "document_end"
}]
}Storage and State
Persisting extension data
When to use: When saving user settings or data
Storage and State
Chrome Storage API
// Save data
chrome.storage.local.set({ key: 'value' }, () => {
console.log('Saved');
});
// Get data
chrome.storage.local.get(['key'], (result) => {
console.log(result.key);
});
// Sync storage (syncs across devices)
chrome.storage.sync.set({ setting: true });
// Watch for changes
chrome.storage.onChanged.addListener((changes, area) => {
if (changes.key) {
console.log('key changed:', changes.key.newValue);
}
});Storage Limits
Type
Limit
local
5MB
sync
100KB total, 8KB per item
Async/Await Pattern
// Modern async wrapper
async function getStorage(keys) {
return new Promise((resolve) => {
chrome.storage.local.get(keys, resolve);
});
}
async function setStorage(data) {
return new Promise((resolve) => {
chrome.storage.local.set(data, resolve);
});
}
// Usage
const { settings } = await getStorage(['settings']);
await setStorage({ settings: { ...settings, theme: 'dark' } });Extension Monetization
Making money from extensions
When to use: When planning extension revenue
Extension Monetization
Revenue Models
Model
How It Works
Freemium
Free basic, paid features
One-time
Pay once, use forever
Subscription
Monthly/yearly access
Donations
Tip jar / Buy me a coffee
Affiliate
Recommend products
Payment Integration
// Use your backend for payments
// Extension can't directly use Stripe
// 1. User clicks "Upgrade" in popup
// 2. Open your website with user ID
chrome.tabs.create({
url: `https://your-site.com/upgrade?user=${userId}`
});
// 3. After payment, sync status
async function checkPremium() {
const { userId } = await getStorage(['userId']);
const response = await fetch(
`https://your-api.com/premium/${userId}`
);
const { isPremium } = await response.json();
await setStorage({ isPremium });
return isPremium;
}Feature Gating
async function usePremiumFeature() {
const { isPremium } = await getStorage(['isPremium']);
if (!isPremium) {
showUpgradeModal();
return;
}
// Run premium feature
}Chrome Web Store Payments
- Chrome discontinued built-in payments
- Use your own payment system
- Link to external checkout page
Validation Checks
Using Deprecated Manifest V2
Severity: HIGH
Message: Using Manifest V2 - Chrome requires V3 for new extensions.
Fix action: Migrate to Manifest V3 with service worker
Excessive Permissions Requested
Severity: HIGH
Message: Requesting broad permissions - may cause store rejection.
Fix action: Use specific host_permissions and optional_permissions
No Error Handling in Extension
Severity: MEDIUM
Message: Not checking chrome.runtime.lastError for errors.
Fix action: Check chrome.runtime.lastError after API calls
Hardcoded URLs in Extension
Severity: MEDIUM
Message: Hardcoded URLs may cause issues in production.
Fix action: Use chrome.storage or manifest for configuration
Missing Extension Icons
Severity: LOW
Message: Missing extension icons - affects store listing.
Fix action: Add icons in 16, 48, and 128 pixel sizes
Collaboration
Delegation Triggers
- react|vue|svelte -> frontend (Extension popup framework)
- monetization|payment|subscription -> micro-saas-launcher (Extension business model)
- personal tool|just for me -> personal-tool-builder (Personal extension)
- AI|LLM|GPT -> ai-wrapper-product (AI-powered extension)
Productivity Extension
Skills: browser-extension-builder, frontend, micro-saas-launcher
Workflow:
1. Define extension functionality
2. Build popup UI with React
3. Implement content scripts
4. Add premium features
5. Publish to Chrome Web Store
6. Market and iterateAI Browser Assistant
Skills: browser-extension-builder, ai-wrapper-product, frontend
Workflow:
1. Design AI features for browser
2. Build extension architecture
3. Integrate AI API
4. Create popup interface
5. Handle usage limits/payments
6. Publish and growRelated Skills
Works well with: frontend, micro-saas-launcher, personal-tool-builder
When to Use
- User mentions or implies: browser extension
- User mentions or implies: chrome extension
- User mentions or implies: firefox addon
- User mentions or implies: extension
- User mentions or implies: manifest v3
Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.