chrome-extensions

Chrome Extensions

Build production-quality Chrome extensions using Manifest V3 and publish them to the Chrome Web Store.

Part 1 — Building Extensions

Mandatory Rules

These address the most common causes of broken extensions. Violating any produces a non-functional build.

1. Icons: only reference files you create — or omit icons entirely

❌ BROKEN — referencing files that don't exist or reusing one file for all sizes:
   "icons": { "16": "icon.png", "48": "icon.png", "128": "icon.png" }

✅ CORRECT — each size is a separate file at the correct pixel dimensions:
   "icons": { "16": "icons/icon-16.png", "48": "icons/icon-48.png", "128": "icons/icon-128.png" }
   (where icon-16.png is 16×16px, icon-48.png is 48×48px, icon-128.png is 128×128px)

✅ ALSO CORRECT — omit icons from manifest if you cannot generate real PNG files:
   (just remove the "icons" and "default_icon" fields — Chrome uses a default icon)

If you include icon references, you MUST create the actual image files. Generate them with a script (see references/extensions/icons.md) or leave them out. Never reference non-existent files.

2. Side panel: you MUST provide a way to open it

Defining "side_panel": {"default_path": "..."} does NOT make it openable. Add a trigger:

// In service-worker.js — open side panel on extension icon click
// IMPORTANT: chrome.action.onClicked ONLY fires when there is NO default_popup
chrome.action.onClicked.addListener(async (tab) => {
  await chrome.sidePanel.open({ windowId: tab.windowId });
});

If the extension has both a popup AND side panel, add a button in the popup that calls chrome.sidePanel.open(). Alternatively, use chrome.sidePanel.setPanelBehavior({ openPanelOnActionClick: true }) — but the property is openPanelOnActionClick, NOT openPanelOnActionIconClick; the "Icon" variant causes a synchronous TypeError that silently aborts the service worker. Do NOT also define default_popup when using setPanelBehavior. See references/extensions/side-panel.md.

3. Code execution: sandboxed iframes ONLY

Extension CSP blocks eval(), new Function(), inline <script> in all extension pages.

// ❌ BROKEN — direct iframe DOM access throws SecurityError
iframe.contentDocument.write(html);

// ❌ BROKEN — eval in extension page
eval(userCode); // CSP blocks this

// ✅ OPTION A: Sandbox in manifest + postMessage
// manifest.json: { "sandbox": { "pages": ["sandbox.html"] } }
iframe.contentWindow.postMessage({ html, css, js }, '*');
// sandbox.html receives and runs:
window.addEventListener('message', (e) => { eval(e.data.js); /* allowed in sandbox */ });

// ✅ OPTION B: Blob URL (creates separate origin, bypasses extension CSP)
iframe.src = URL.createObjectURL(new Blob([doc], { type: 'text/html' }));

// ✅ OPTION C: srcdoc
iframe.srcdoc = `<style>${css}</style>${html}<script>${js}<\/script>`;

See references/extensions/csp-sandbox.md for full details.

4. tab.url requires the tabs permission

Without it, tab.url silently returns undefined — no error thrown.

// manifest.json — REQUIRED if you read tab.url or tab.title anywhere:
{ "permissions": ["tabs"] }

See references/extensions/tab-management.md.

5. Always use async/await — never .then() chains

// ❌ BAD
chrome.tabs.query({active: true, currentWindow: true}).then(tabs => {
  chrome.scripting.executeScript({target: {tabId: tabs[0].id}, files: ['content.js']}).then(() => {});
});

// ✅ GOOD
const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
await chrome.scripting.executeScript({ target: { tabId: tab.id }, files: ['content.js'] });

For runtime.onMessage listeners that do async work:

chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  (async () => {
    const data = await chrome.storage.local.get('key');
    sendResponse({ data });
  })();
  return true; // keeps channel open
});

6. Content scripts: don't block the main thread

When modifying many DOM elements, batch with requestAnimationFrame and yield between batches:

async function highlightAll(elements) {
  const BATCH = 20;
  for (let i = 0; i < elements.length; i += BATCH) {
    await new Promise(r => requestAnimationFrame(() => {
      elements.slice(i, i + BATCH).forEach(el => el.style.backgroundColor = 'yellow');
      r();
    }));
    if (globalThis.scheduler?.yield) await scheduler.yield();
  }
}

See references/extensions/content-scripts.md.

7. Service workers are ephemeral — never store state in variables

// ❌ BROKEN — state lost when SW terminates (~30s of inactivity)
let count = 0;
chrome.tabs.onUpdated.addListener(() => { count++; });

// ✅ CORRECT — persist in chrome.storage, read on every event
chrome.tabs.onUpdated.addListener(async (tabId, changeInfo) => {
  if (changeInfo.status !== 'complete') return;
  const { count = 0 } = await chrome.storage.local.get('count');
  await chrome.storage.local.set({ count: count + 1 });
  await chrome.action.setBadgeText({ text: String(count + 1) });
});

Use chrome.alarms instead of setTimeout/setInterval. See references/extensions/service-worker.md.

8. chrome.identity: extension ID differs between dev and production

When using Google sign-in, the OAuth client_id is tied to a specific extension ID. The ID changes between unpacked development and the Chrome Web Store.

To stabilize the ID during development, add a "key" field to manifest.json:

1. Pack the extension once (chrome://extensions → Pack)
2. Extract the public key from the .crx
3. Add "key": "MIIBIjANBgkqh..." to manifest.json

Always document: "After publishing to the Chrome Web Store, update the OAuth client with the store-assigned extension ID." See references/extensions/auth-identity.md.

9. Context menus: show user feedback after action

When a context menu item performs an action (save, copy, etc.), confirm it to the user. Use a notification, badge flash, or injected toast — don't let actions happen silently. See references/extensions/context-menus.md for a complete toast implementation.

10. Prompt API: available in service workers, popup, and side panel

The LanguageModel API works in all extension contexts — service worker, popup, and side panel — with no additional manifest permissions required. Extensions also get LanguageModel.params(), which is unavailable on the web:

const params = await LanguageModel.params();
// { defaultTopK: 3, maxTopK: 128, defaultTemperature: 1, maxTemperature: 2 }

For general Prompt API patterns (availability checks, session creation, streaming), use the modern-web-guidance skill. See references/extensions/prompt-api.md for the extension-specific wiring example.

11. chrome.action API requires action in manifest

Using chrome.action.setBadgeText, chrome.action.setIcon, or chrome.action.onClicked requires an "action" key in manifest.json — even if it's empty. Without it, chrome.action is undefined.

// ❌ BROKEN — manifest has no "action" key
await chrome.action.setBadgeText({ text: '5' });
// TypeError: Cannot read properties of undefined (reading 'setBadgeText')

// ✅ FIX — add "action" to manifest.json (at minimum an empty object)
{ "action": {} }
// or with a popup:
{ "action": { "default_popup": "popup/popup.html" } }

12. activeTab only works on direct user gestures — not from side panels

activeTab grants temporary access to the current tab ONLY when triggered by:

• Clicking the extension action icon
• A context menu item
• A keyboard shortcut from the commands API
• Accepting an omnibox suggestion

It does NOT grant access when clicking a button in a side panel, popup button that opens later, or any programmatic trigger.

// ❌ BROKEN — activeTab does NOT work from a side panel button click
document.getElementById('summarize').addEventListener('click', async () => {
  const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
  await chrome.scripting.executeScript({ target: { tabId: tab.id }, func: () => document.body.innerText });
});

// ✅ FIX — use "tabs" permission + specific host_permissions instead
// manifest.json: { "permissions": ["tabs", "scripting"], "host_permissions": ["<all_urls>"] }

See references/extensions/side-panel.md.

13. DevTools panel URLs are relative to the extension root

When creating a DevTools panel, the panel HTML path is relative to the extension root, NOT relative to the devtools page that calls chrome.devtools.panels.create().

// ❌ BROKEN — path relative to devtools/ directory
chrome.devtools.panels.create("My Panel", "", "panel/panel.html");

// ✅ CORRECT — full path from extension root
chrome.devtools.panels.create("My Panel", "", "devtools/panel/panel.html");

See references/extensions/devtools.md.

14. Offscreen documents have NO access to most chrome.* APIs

Offscreen documents (chrome.offscreen) are severely restricted. Most chrome.* APIs are unavailable, including chrome.downloads, chrome.tabs, chrome.action, and others.

// ❌ BROKEN — chrome.downloads is undefined in offscreen documents
chrome.downloads.download({ url, filename: 'recording.webm' }); // TypeError

// ❌ BROKEN — chrome.action is undefined in offscreen documents
chrome.action.setBadgeText({ text: 'REC' }); // TypeError

The only APIs available in offscreen documents are:

• chrome.runtime.sendMessage / chrome.runtime.onMessage
• chrome.runtime.getURL
• Standard Web APIs (DOM, fetch, MediaRecorder, Canvas, Web Audio, etc.)

Rule of thumb: Offscreen documents do the Web API work (recording, parsing, audio). The service worker does all chrome.* API work (downloads, badge updates, notifications). Use chrome.runtime.sendMessage to bridge between them. See references/extensions/message-passing.md.

15. Notifications and badge icons must referenc

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