browser-extension-builder – Agents & Automation skill for AI agents

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

Architecture Patterns

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.png

Manifest 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": ["https://specific-site.com/*"],
    "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.storage

Content 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
function modifyPage() {
  // Modify the page
  const element = document.querySelector('.target');
  if (element) {
    element.style.backgroundColor = 'yellow';
  }
}
if (document.readyState === 'loading') {
  document.addEventListener('DOMContentLoaded', modifyPage, { once: true });
} else {
  modifyPage();
}

// 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 | 10MB (5MB on Chrome 113 and earlier) |

| sync | 100KB total, 8KB per item |

Async/Await Pattern

// Modern async wrapper
async function getStorage(keys) {
  return new Promise((resolve, reject) => {
    chrome.storage.local.get(keys, (value) => {
      if (chrome.runtime.lastError) reject(new Error(chrome.runtime.lastError.message));
      else resolve(value);
    });
  });
}

async function setStorage(data) {
  return new Promise((resolve, reject) => {
    chrome.storage.local.set(data, () => {
      if (chrome.runtime.lastError) reject(new Error(chrome.runtime.lastError.message));
      else resolve();
    });
  });
}

// Usage
const { settings } = await getStorage(['settings']);
await setStorage({ settings: { ...settings, theme: 'dark' } });

Security boundaries

Use minimal host access, validate message sender/shape and never interpolate page/user text into innerHTML. Browser extension storage is not a secret vault; sync may transmit data across devices. [Chrome storage quotas](https://developer.chrome.com/docs/extensions/reference/api/storage) vary by version. Client-side isPremium flags and user IDs in URLs are UI state, not entitlement authorization: authenticate and verify payment/ownership on the server. MV3 workers can suspend; persist necessary state and test actual Chrome/Firefox APIs separately.

Extension Monetization

Making money from extensions

When to use: When planning extension revenue

Security boundaries

Use minimal host access, validate message sender/shape and never interpolate page/user text into innerHTML. Browser extension storage is not a secret vault; sync may transmit data across devices. [Chrome storage quotas](https://developer.chrome.com/docs/extensions/reference/api/storage) vary by version. Client-side isPremium flags and user IDs in URLs are UI state, not entitlement authorization: authenticate and verify payment/ownership on the server. MV3 workers can suspend; persist necessary state and test actual Chrome/Firefox APIs separately.

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)

… (full content is in the download package)