Chrome Extension Manifest V3 basics

When building a Chrome extension with Manifest V3, you need these key parts:

The manifest.json

{
  "manifest_version": 3,
  "name": "My Extension",
  "version": "1.0.0",
  "permissions": ["activeTab", "storage"],
  "action": {
    "default_popup": "popup.html"
  },
  "content_scripts": [{
    "matches": ["<all_urls>"],
    "js": ["content.js"]
  }],
  "background": {
    "service_worker": "background.js"
  }
}

Key differences from MV2

• Service workers replace background pages (no persistent background)
• action replaces browser_action and page_action
• host_permissions is separate from permissions
• More restrictive CSP by default

Loading locally: chrome://extensions → Developer mode → Load unpacked