Manifest V3

Manifest V3

When to Use

• Starting a new browser extension project
• Migrating an existing MV2 extension to MV3
• Configuring permissions, background service workers, or declarative net request rules
• Debugging "service worker terminated" issues

Core Jobs

1. Manifest Structure

Required fields:

{
  "manifest_version": 3,
  "name": "My Extension",
  "version": "1.0.0",
  "description": "...",
  "permissions": [],
  "host_permissions": [],
  "background": {
    "service_worker": "background.js",
    "type": "module"
  },
  "action": {
    "default_popup": "popup.html",
    "default_icon": "icons/icon48.png"
  },
  "icons": { "16": "icons/icon16.png", "48": "icons/icon48.png", "128": "icons/icon128.png" },
  "content_scripts": [],
  "web_accessible_resources": []
}

2. Permissions Design

• Declare ONLY what you need (Chrome Web Store rejects over-permissioned extensions)
• permissions = extension APIs (storage, tabs, contextMenus, alarms, notifications)
• host_permissions = website access (*://*.example.com/* or <all_urls>)
• Use optional_permissions for features users might not need
• Dangerous permissions requiring justification: <all_urls>, webNavigation, history, bookmarks

3. Service Worker (MV3 Background)

Key differences from MV2 background pages:

• Service worker TERMINATES when idle (no persistent state in memory)
• Use chrome.storage (not global variables) to persist data
• Register event listeners at top level (not inside callbacks)
• Keep-alive pattern for long-running tasks: chrome.alarms API

// ✅ Correct — top-level listener
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  // handle message
  return true; // keep channel open for async response
});

// ❌ Wrong — listener inside async callback
chrome.tabs.query({}, (tabs) => {
  chrome.runtime.onMessage.addListener(...); // never registered reliably
});

4. declarativeNetRequest (replaces webRequest)

• MV3 cannot block requests dynamically with webRequest
• Use declarativeNetRequest for URL blocking/redirecting
• Rules defined in JSON files and declared in manifest

{
  "declarative_net_request": {
    "rule_resources": [{ "id": "ruleset_1", "enabled": true, "path": "rules.json" }]
  }
}

5. Content Security Policy

• MV3 enforces strict CSP: no inline scripts, no eval()
• Remote code execution prohibited (no loading scripts from CDN at runtime)
• All scripts must be bundled in the extension package
• Use web_accessible_resources for resources injected into pages

Key Concepts

• Manifest V3 — current extension standard; MV2 deprecated Jan 2025 in Chrome
• Service worker — event-driven background script that terminates when idle
• host_permissions — controls which websites the extension can access
• optional_permissions — permissions requested at runtime (better UX)
• declarativeNetRequest — static rules for network request modification (replaces dynamic webRequest)
• web_accessible_resources — extension files accessible from web pages

Checklist

• manifest_version: 3 (not 2)?
• No inline scripts (CSP compliant)?
• No remote code execution (all scripts bundled)?
• Service worker uses chrome.storage not in-memory state?
• Event listeners registered at top level (not in callbacks)?
• Minimal permissions — only what's needed?
• host_permissions scoped as narrowly as possible?
• Icons at 16, 48, 128px?

Output Format

• 🔴 Critical — MV2 manifest, remote code execution, missing required fields
• 🟡 Warning — overly broad host_permissions (<all_urls>), persistent state in service worker
• 🟢 Suggestion — use optional_permissions for non-core features

Common Pitfalls

• Service worker termination: store state in chrome.storage.session or chrome.storage.local, not global vars
• return true in onMessage listener is required to keep the message channel open for async responses
• MV3 blocks all inline scripts — use external .js files even for tiny scripts
• web_accessible_resources must explicitly list files injected into pages