Chrome Extension Agent Skill

Chrome Extension Development (Manifest V3)

This skill covers everything needed to build, debug, and publish Chrome extensions with MV3. It is organized as a routing document: read this file first to understand the architecture and decision points, then load the relevant reference file for implementation details.

Reference files

Read only the reference files relevant to the current task. Each file is self-contained.

| File | When to read |

| --- | --- |

| references/manifest-v3.md | Setting up or modifying manifest.json, configuring icons, versioning |

| references/service-worker.md | Background logic, lifecycle, state persistence, alarms, events |

| references/content-scripts.md | Injecting code into pages, isolated/main world, dynamic injection, SPA handling, orphaning |

| references/messaging-rpc.md | Communication between any contexts, typed protocols, RPC layer, async handler patterns |

| references/ui-surfaces.md | Popup, options page, side panel, context menus, commands, notifications, omnibox, devtools panel |

| references/storage.md | chrome.storage (local/sync/session), quotas, reactive patterns, framework hooks |

| references/network-csp.md | HTTP requests from content scripts, CSP bypass relay, declarativeNetRequest, offscreen docs, CORS |

| references/permissions.md | Required/optional permissions, host permissions, activeTab, runtime request flow |

| references/web-accessible-resources.md | Exposing extension files to web pages, security implications |

| references/typescript-build.md | TypeScript setup, project structure, build tools comparison, bundling |

| references/publishing.md | Chrome Web Store submission, review process, rejection reasons, updates, privacy policy |

| references/execution-contexts.md | Communication flow diagrams, per-context capabilities/limits, choosing the right messaging method |

| references/debugging-mistakes.md | DevTools for extensions, testing SW termination, common gotchas, error patterns |

Architecture overview

A Chrome extension has up to 5 execution contexts that communicate via message passing:

┌──────────────────────────────────────────────────────────┐
│ Extension Process                                        │
│  ┌─────────────────┐  ┌───────┐  ┌─────────┐  ┌──────┐ │
│  │ Service Worker   │  │ Popup │  │ Options │  │ Side │ │
│  │ (background)     │  │       │  │  Page   │  │Panel │ │
│  │ - No DOM         │  │ Full  │  │  Full   │  │ Full │ │
│  │ - Ephemeral      │  │ DOM   │  │  DOM    │  │ DOM  │ │
│  │ - All chrome.*   │  │ All   │  │  All    │  │ All  │ │
│  │   APIs           │  │ APIs  │  │  APIs   │  │ APIs │ │
│  └────────┬─────────┘  └───┬───┘  └────┬────┘  └──┬───┘ │
│           │ chrome.runtime.sendMessage / connect   │     │
└───────────┼────────────────┼───────────┼──────────┼──────┘
            │                │           │          │
    chrome.tabs.sendMessage  │           │          │
            │                │           │          │
┌───────────┼────────────────┼───────────┼──────────┼──────┐
│ Web Page  ▼                                              │
│  ┌──────────────────┐    ┌──────────────────┐            │
│  │ Content Script    │    │ Main World Script │            │
│  │ (isolated world)  │◄──►│ (page context)    │            │
│  │ - Shared DOM      │    │ - Shared DOM      │            │
│  │ - Own JS scope    │    │ - Page JS scope   │            │
│  │ - chrome.runtime  │    │ - No chrome.* API │            │
│  │ - chrome.storage  │    │ - Full page access│            │
│  │ - Subject to CSP  │    │ - Subject to CSP  │            │
│  │   (network only)  │    │   (fully)         │            │
│  └──────────────────┘    └──────────────────┘            │
│           ▲ window.postMessage                           │
│           │ (through shared DOM)                         │
└──────────────────────────────────────────────────────────┘

Communication flows (labeled channels)

┌───────────────────────────────────────────────────────────────────────────┐
│ Extension Process                                                         │
│                                                                           │
│  ┌─────────────────┐  chrome.runtime   ┌───────┐  ┌─────────┐  ┌──────┐ │
│  │ Service Worker   │◄─.sendMessage()──│ Popup │  │ Options │  │ Side │ │
│  │ (background)     │◄─.connect()──────│       │  │  Page   │  │Panel │ │
│  │                  │                  └───────┘  └─────────┘  └──────┘ │
│  │ - No DOM         │  ┌────────────────────────────────────────────┐   │
│  │ - Ephemeral 30s  │  │ SW cannot push to these pages.             │   │
│  │ - All chrome.*   │  │ Use: ports (.connect) or storage.onChanged │   │
│  └────────┬─────────┘  └────────────────────────────────────────────┘   │
│           │                                                              │
│  chrome.storage.onChanged ◄── fires across ALL contexts simultaneously  │
│                                                                           │
└───────────┼──────────────────────────────────────────────────────────────┘
            │ chrome.tabs.sendMessage(tabId, ...) [SW must know tabId]
            │
┌───────────┼──────────────────────────────────────────────────────────────┐
│ Web Page  ▼                                                              │
│  ┌──────────────────┐  window.postMessage  ┌──────────────────┐         │
│  │ Content Script    │◄───────────────────►│ Main World Script │         │
│  │ (isolated world)  │  Custom DOM events  │ (page context)    │         │
│  │                   │                     │                   │         │
│  │ chrome.runtime ───┼── to/from SW        │ No chrome.* APIs  │         │
│  │ chrome.storage    │                     │ Full page JS      │         │
│  │ Shared DOM        │                     │ Shared DOM        │         │
│  │ Page CSP (network)│                     │ Page CSP (full)   │         │
│  └──────────────────┘                     └──────────────────┘         │
└──────────────────────────────────────────────────────────────────────────┘

For detailed flow diagrams (three-layer bridge, cross-extension, storage broadcast) and a per-context breakdown of permissions, limits, and workarounds: → Read references/execution-contexts.md

Communication methods at a glance

| Method | Direction | Best for |

| --- | --- | --- |

| chrome.runtime.sendMessage | Any ext context → SW | One-shot request/response (90% of cases) |

| chrome.tabs.sendMessage | SW → content script (by tabId) | Pushing data to a specific tab |

| chrome.runtime.connect (Port) | Bidirectional | Streaming, progress, SW ↔ popup |

| window.postMessage | Between worlds on same page | Page JS ↔ content script bridge |

| chrome.storage.onChanged | Broadcast to all contexts | Settings sync, no messaging needed |

→ Full matrix with limits and edge cases: references/execution-contexts.md → Implementation patterns, typed protocols, RPC layer: references/messaging-rpc.md

Key architectural rules

• Service worker is ephemeral. It terminates after 30s of inactivity. All state must be persisted to chrome.storage. All event listeners must be registered synchronously at the top level. Never use setTimeout/setInterval for anything beyond a few seconds. → Read references/service-worker.md

• Content scripts run in the page's origin. Network requests from content scripts are subject to the page's CSP and CORS. To bypass, relay through the service worker. → Read references/network-csp.md

• Messaging is the backbone. Every cross-context interaction uses chrome.runtime messaging. The #1 bug: forgetting to return true from async message listeners. → Read references/messaging-rpc.md

• Permissions determine CWS review speed. Broad host_permissions trigger manual review (weeks). activeTab + optional permissions = fast automated review. → Read references/permissions.md

• Popup is destroyed on blur. Side panel persists. Choose based on interaction duration. → Read references/ui-surfaces.md

Decision tree: which context handles what?

"I need to run code when the user visits a page"

→ Content script. Static (manifest) for known URL patterns, dynamic (chrome.scripting) for user-triggered injection. Default to isolated world unless you need page JS access. → Read references/content-scripts.md

"I need to make an HTTP request to my API"

• From popup/options/side panel: direct fetch() works (extension origin, no CSP issues)
• From content script on a page with restrictive CSP: relay through service worker
• From service worker: direct fetch() works (requires host_permissions for the target domain) → Read references/network-csp.md

"I need to store user settings"

• Settings that sync across devices: chrome.storage.sync (100KB limit)
• Large data or caches: chrome.storage.local (10MB, or unlimited with permission)
• Ephemeral state surviving SW restarts: chrome.storage.session → Read references/storage.md

"I need to modify HTTP headers or block requests"

→ declarativeNetRequest (NOT webRequest, which lost blocking in MV3) → Read references/network-csp.md

"I need the page's JavaScript to talk to my extension"

→ Three-layer bridge: page (window.postMessage) → content script → service worker → Read references/messaging-rpc.md

"I need to understand what each context can and cannot do"

→ Read references/execution-contexts.md — per-context cards listing chrome.\* access, DOM, network, storage, lifetime, hard limits, and practical workarounds.

"I need periodic background tasks"

→ chrome.alarms (minimum 30s interval). NOT setTimeout. → Read references/service-worker.md

"I need DOM APIs in the background" (DOMParser, Canvas, Audio)

→ Offscreen document. One per extension, only chrome.runtime available. → Read references/network-csp.md

"I need to authenticate with OAuth"

→ chrome.identity.launchWebAuthFlow() or chrome.identity.getAuthToken() (Google only) → Read references/service-worker.md (identity section)

Workflow: new extension from scratch

• Define the manifest with minimum permissions. Start with activeTab + scripting. → Read references/manifest-v3.md

• Set up TypeScript and build tooling (or use CRXJS for Vite-based dev). → Read references/typescript-build.md

• Implement the service worker with all event listeners at the top level. → Read references/service-worker.md

• Add content scripts if you need page interaction. → Read references/content-scripts.md

• Build UI surfaces (popup, options, side panel) as needed. → Read references/ui-surfaces.md

• Wire up messaging between all contexts. → Read references/messaging-rpc.md

• Test with DevTools, specifically test service worker termination. → Read references/debugging-mistakes.md

• Publish to Chrome Web Store. → Read references/publishing.md

Workflow: adding a feature to an existing extension

• Identify which context the feature belongs to (see decision tree above).
• Read the relevant reference file(s) for that context.
• Check if new permissions are needed. Prefer optional_permissions for new capabilities. → Read references/permissions.md
• Update the manifest if adding new content scripts, UI surfaces, or permissions.
• Handle extension updates gracefully (content script orphaning). → Read references/content-scripts.md (orphaning section)

Minimal manifest.json template

{
  "manifest_version": 3,
  "name": "My Extension",
  "version": "1.0.0",
  "description": "What it does in one sentence",
  "permissions": ["storage", "activeTab", "scripting"],
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "background": {
    "service_worker": "background.js",
    "type": "module"
  },
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  }
}

→ For the full manifest reference with all fields: references/manifest-v3.md

Code patterns quick reference

Async message handler (the safe pattern)

// Wrap async handlers to avoid the return-true trap
function asyncHandler(
  fn: (msg: any, sender: chrome.runtime.MessageSender) => Promise<any>,
) {
  return (
    message: any,
    sender: chrome.runtime.MessageSender,
    sendResponse: (r: any) => void,
  ) => {
    fn(message, sender)
      .then(sendResponse)
      .catch((e) => sendResponse({ __error: true, message: e.message }));
    return true; // literal true, not Promise<true>
  };
}

chrome.runtime.onMessage.addListener(
  asyncHandler(async (msg, sender) => {
    if (msg.type === "FETCH") {
      const res = await fetch(msg.url);
      return { ok: res.ok, data: await res.text() };
    }
  }),
);

CSP bypass relay (content script → service worker → API)

// content-script.ts
async function apiCall(endpoint: string, options?: RequestInit) {
  return chrome.runtime.sendMessage({ type: "API_RELAY", endpoint, options });
}

// background.ts
const ALLOWED_ENDPOINTS = ["https://api.example.com"];
chrome.runtime.onMessage.addListener(
  asyncHandler(async (msg) => {
    if (msg.type !== "API_RELAY") return;
    if (!ALLOWED_ENDPOINTS.some((e) => msg.endpoint.startsWith(e))) {
      throw new Error("Blocked endpoint");
    }
    const res = await fetch(msg.endpoint, msg.options);
    return { ok: res.ok, status: res.status, data: await res.text() };
  }),
);

Persist state across SW restarts

// Use chrome.storage.session for ephemeral state
chrome.storage.session.setAccessLevel({
  accessLevel: "TRUSTED_AND_UNTRUSTED_CONTEXTS",
});

async function getState<T>(key: string, fallback: T): Promise<T> {
  const result = await chrome.storage.session.get(key);
  return result[key] ?? fallback;
}
async function setState<T>(key: string, value: T): Promise<void> {
  await chrome.storage.session.set({ [key]: value });
}

Orphaned content script detection

function isExtensionContextValid(): boolean {
  try {
    return !!chrome.runtime?.id;
  } catch {
    return false;
  }
}

// Before any chrome.runtime call
if (!isExtensionContextValid()) {
  showRefreshBanner();
  return;
}

What NOT to do

• Do NOT use eval(), new Function(), or load remote scripts. MV3 forbids it.
• Do NOT use setTimeout/setInterval for anything > 5s in service workers.
• Do NOT register event listeners inside callbacks or async functions.
• Do NOT use <all_urls> host permission unless absolutely necessary.
• Do NOT rely on DevTools keeping the service worker alive during testing.
• Do NOT forget return true in async message listeners.
• Do NOT use localStorage or sessionStorage in service workers (they don't exist there).
• Do NOT assume content scripts survive extension updates.
• Do NOT use webRequest blocking (removed in MV3). Use declarativeNetRequest.
• Do NOT use chrome.extension.getBackgroundPage() (removed in MV3).