Cursor Plugin Modern Web 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 reference real image files

chrome.notifications.create() requires a valid iconUrl pointing to an actual image file. If the file doesn't exist or the path is wrong, the call fails with "Unable to download all specified images."

// ❌ BROKEN — icon file doesn't exist
chrome.notifications.create('reminder', {
  type: 'basic',
  iconUrl: 'icons/icon-128.png', // File not in extension!
  title: 'Reminder',
  message: 'Time is up!'
});

// ✅ Generate a data URL at runtime via OffscreenCanvas — no file needed.
// See `references/extensions/icons.md` for a reusable implementation.
const iconUrl = await getIconDataUrl();
chrome.notifications.create('reminder', { type: 'basic', iconUrl, title: 'Reminder', message: 'Time is up!' });

This applies to ALL image references in chrome.* APIs — notifications, chrome.action.setIcon, context menu icons, etc. If you reference a file, it must exist.

16. Tab capture: guard against double-start with state locking

chrome.tabCapture.getMediaStreamId() fails with "Cannot capture a tab with an active stream" if called while a previous capture is still active. Fast double-clicks on the extension icon easily trigger this. Use explicit state locking:

// ❌ BROKEN — no guard against rapid clicks
let isRecording = false;
chrome.action.onClicked.addListener(async (tab) => {
  if (isRecording) { stopRecording(); isRecording = false; }
  else { isRecording = true; startRecording(tab); } // Second click = "active stream" error
});

// ✅ CORRECT — use transitional states to lock out concurrent operations
// State machine: 'idle' → 'starting' → 'recording' → 'stopping' → 'idle'
// Store state in chrome.storage.session (survives SW restart, cleared on browser close)
chrome.action.onClicked.addListener(async (tab) => {
  const { recordingState = 'idle' } = await chrome.storage.session.get('recordingState');

  if (recordingState === 'starting' || recordingState === 'stopping') return;

  if (recordingState === 'idle') {
    await chrome.storage.session.set({ recordingState: 'starting' });
    try {
      await startRecording(tab);
      await chrome.storage.session.set({ recordingState: 'recording' });
      await chrome.action.setBadgeText({ text: 'REC' });
      await chrome.action.setBadgeBackgroundColor({ color: '#FF0000' });
    } catch (err) {
      console.error('Failed to start recording:', err);
      await chrome.storage.session.set({ recordingState: 'idle' });
    }
  } else if (recordingState === 'recording') {
    await chrome.storage.session.set({ recordingState: 'stopping' });
    try { await stopRecording(); }
    finally {
      await chrome.storage.session.set({ recordingState: 'idle' });
      await chrome.action.setBadgeText({ text: '' });
    }
  }
});

This pattern applies to any chrome API that manages exclusive resources: chrome.tabCapture, chrome.desktopCapture, chrome.offscreen.createDocument (only one offscreen document allowed at a time). See references/extensions/media-capture.md.

17. chrome.desktopCapture requires a target tab with URL access

When calling chrome.desktopCapture.chooseDesktopMedia() from a service worker, you must pass the active tab as the targetTab parameter. The tab object must have its url field populated, which requires the "tabs" permission.

// ❌ BROKEN — called without targetTab from service worker
chrome.desktopCapture.chooseDesktopMedia(['screen', 'window'], (streamId) => { ... });
// Error: "A target tab is required when called from a service worker context."

// ❌ BROKEN — tab doesn't have url field (missing "tabs" permission)
const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
chrome.desktopCapture.chooseDesktopMedia(['screen', 'window'], tab, (streamId) => { ... });
// Error: "targetTab doesn't have URL field set."

// ✅ CORRECT — "tabs" permission in manifest + pass tab object
// manifest.json: { "permissions": ["tabs", "desktopCapture"] }
const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
chrome.desktopCapture.chooseDesktopMedia(['screen', 'window'], tab, (streamId) => {
  if (!streamId) return; // User cancelled
});

Note: Prefer chrome.tabCapture.getMediaStreamId() for tab-only recording. Use chrome.desktopCapture only when the user should choose which screen/window to capture. See references/extensions/media-capture.md.

18. chrome.windows has NO .query() method — use getAll, getLastFocused, or getCurrent

Unlike chrome.tabs.query(), the chrome.windows API does NOT have a .query() method.

// ❌ BROKEN — chrome.windows.query does not exist
const windows = await chrome.windows.query({ focused: true });
// TypeError: chrome.windows.query is not a function

// ✅ CORRECT — use the right method for your need
const focused = await chrome.windows.getLastFocused({ populate: true });
const current = await chrome.windows.getCurrent({ populate: true });
const all     = await chrome.windows.getAll({ populate: true });

chrome.windows methods: getAll, getLastFocused, getCurrent, get(windowId), create, update, remove. See references/extensions/tab-management.md.

Always Manifest V3

Never generate Manifest V2 code.

• background.service_worker not background.scripts
• chrome.action not chrome.browserAction
• chrome.scripting.executeScript not chrome.tabs.executeScript
• host_permissions is separate from permissions
• No inline scripts in HTML — use <script src="file.js">
• No inline event handlers — use addEventListener

Part 2 — Publishing to the Chrome Web Store

Manage CHROMEWEBSTORE.md — the single source of truth for all Chrome Web Store listing metadata, permissions justifications, privacy disclosures, version history, and publishing readiness for a Chrome extension project.

Core Workflow

Every time you touch a Chrome extension project in a way that affects its store presence, update (or create) CHROMEWEBSTORE.md in the project root. The file tracks everything the developer needs to fill out in the Chrome Developer Dashboard, so they can copy-paste from a single doc instead of scrambling at publish time.

When to create CHROMEWEBSTORE.md

Create it the moment any of these happen:

• The user says they want to publish an extension
• The user asks to "prepare for the store" or "get ready to publish"
• You're building a new extension that will clearly end up on the store
• The user asks about store listing requirements

Use the template in references/webstore/chromewebstore-template.md as your starting point. Read it before generating the file.

When to update CHROMEWEBSTORE.md

Update it whenever:

• User-facing changes: Bump the "Last Updated" date, update the feature list in descriptions, and add an entry to Version History
• manifest.json changes: If permissions, host_permissions, or content_scripts changed, update the Permissions Justification section — every permission needs a plain-English reason the review team can understand
• New release: Add a Version History entry with version number, date, and summary
• Privacy-relevant changes: If data collection, storage, or transmission changed, update the Privacy & Data Use section and the privacy policy
• Asset changes: If icons or UI changed, note which screenshots need refreshing
• Rejection response: If the user reports a CWS rejection, update the file with the fix and add a note to Version History

How to fill it out

For each section, pull information from the actual project files:

1. Read manifest.json to extract name, version, description, permissions, host_permissions
2. Scan the codebase for data collection (storage, fetch calls, analytics)
3. Check for icon files and their dimensions
4. Look at the extension's UI to understand features for the description

Write store-facing copy in a tone that is specific, honest, and benefit-oriented. The Chrome Web Store review team rejects vague descriptions. "Makes your life easier" will be rejected. "Highlights search results on any webpage and lets you save highlights to a local list" will pass.

CHROMEWEBSTORE.md Sections

Read references/webstore/chromewebstore-template.md before generating the file — it defines what each section covers and how to fill it out. The highest-risk section is Permissions Justification: write a specific plain-English reason per permission and per host_permission. "Needed for the extension to work" will be rejected. Read references/webstore/privacy-policy.md for guidance on generating a privacy policy.

Pre-Publish Checklist

Before submission, run through references/webstore/review-checklist.md. The most common first-submission failures:

• Every permission and host_permission must have a specific justification (not "needed to work")
• Privacy policy URL must be live and match the data use disclosure form
• At least 1 screenshot at 1280×800 or 640×400
• ZIP must exclude .git/, node_modules/, .env, CHROMEWEBSTORE.md

Store Listing Copy Guidelines

For copy guidelines and common rejection reasons, see references/webstore/store-listing.md. Key rule: lead with function ("Highlights search terms on any webpage"), not feeling ("Enjoy searching again").

Reference Files

For detailed API patterns and publishing guidance, read the relevant file BEFORE writing code or content:

Output Checklist

Verify EVERY item before delivering:

• manifest_version: 3 — no V2 APIs anywhere
• All icon files referenced in manifest exist as real files with correct dimensions — or icons are omitted
• Side panel has an explicit open trigger (not just a manifest declaration)
• Code execution uses sandbox/blob/srcdoc — no eval() in extension pages
• tabs permission declared if tab.url or tab.title is accessed
• All code uses async/await — no .then() chains
• Content scripts batch DOM updates with requestAnimationFrame
• Service worker stores NO state in global variables — uses chrome.storage
• No inline scripts or event handlers in HTML
• Context menu actions show user confirmation
• "action": {} (or more) present in manifest if using chrome.action.* APIs
• If reading/scripting tabs from a side panel: use tabs + host_permissions (NOT activeTab)
• DevTools panel paths in chrome.devtools.panels.create() are relative to extension root
• Offscreen documents use ONLY chrome.runtime messaging — no chrome.downloads, chrome.action, etc.
• All image refs in chrome.notifications, chrome.action.setIcon, etc. point to real files (or use data URLs)
• Tab/desktop capture uses state locking to prevent double-start errors
• chrome.desktopCapture.chooseDesktopMedia passes targetTab with tabs permission
• chrome.windows calls use getAll/getLastFocused/getCurrent — NOT .query() (it doesn't exist)
• sidePanel.setPanelBehavior uses openPanelOnActionClick — NOT openPanelOnActionIconClick
• Error handling on all async operations
• host_permissions scoped to specific domains (not <all_urls> unless needed)
• return true in onMessage listeners with async responses