build-chrome-extension

name	build-chrome-extension
description	Use if building or debugging a Chrome MV3 extension — manifest v3, service_worker, content_scripts.

Build Chrome Extension

Build, debug, package, and ship Chrome Manifest V3 extensions. Optimize for restart-safe service workers, isolated content scripts, least-privilege permissions, and Web Store review readiness.

When To Use

Trigger when the request matches any of:

• creating a new Chrome MV3 extension or scaffolding from a framework (WXT, Plasmo, CRXJS, Vite)
• editing or generating manifest.json with manifest_version: 3, service_worker, content_scripts, host_permissions, action, or side_panel fields
• implementing or debugging chrome.runtime, chrome.storage, chrome.alarms, chrome.scripting, chrome.tabs, chrome.declarativeNetRequest, chrome.sidePanel, or chrome.offscreen APIs
• fixing MV3 service-worker lifecycle issues: idle termination, top-level listener registration, async onMessage returns, chrome.alarms vs setInterval
• bridging content scripts between ISOLATED and MAIN worlds, or routing fetches through the service worker for cross-origin host permissions
• migrating an MV2 extension to MV3 (background page → service worker, blocking webRequest → declarativeNetRequest, browser_action → action)
• validating a built dist/ or .output/chrome-mv3/ folder, packaging the Web Store zip, writing privacy/permission justifications, or preparing review notes
• choosing or comparing Chrome MV3 frameworks for a new project

Do NOT use this skill when:

• the target is Firefox, Safari, or a cross-browser polyfill (browser.* namespace, web-ext tooling, Safari App Extensions)
• the request is generic browser automation outside an extension context — route to run-agent-browser
• publishing a regular npm package or library — route to publish-npm-package
• building a different platform's extension surface (Raycast script command, VS Code extension, Edge enterprise-policy package) — route to that platform's skill

Pinned Defaults — Apply Before Generating Code

Key	Default
Manifest target	manifest_version: 3 only; never generate MV2
Greenfield framework	WXT unless the repo already chose another MV3 tool
Persistent state	chrome.storage.local; chrome.storage.session for ephemeral restart-safe state
Background model	Event-driven service worker, listeners registered synchronously at top level
Periodic work	chrome.alarms — never setTimeout/setInterval for background scheduling
Network modification	chrome.declarativeNetRequest — never blocking webRequest
Permissions	Least privilege; prefer optional_permissions and optional_host_permissions granted via chrome.permissions.request
Loaded folder	Built output only: WXT .output/chrome-mv3-dev/ or .output/chrome-mv3/; Plasmo build/chrome-mv3-*; CRXJS/Vite dist/
Package preflight	Run scripts/check-mv3-manifest.sh and scripts/preflight-extension.sh against the production build before zipping

MV3 Footguns — Keep In Working Memory

These are the failures that recur across every MV3 build. Internalize before writing code:

• Service workers idle out (~30s default). Global module-scope state disappears between events. Persist before each await and rehydrate at event entry.
• Register chrome.runtime.onMessage, onInstalled, onStartup, alarm, and tab listeners synchronously at top level. Late-registered listeners miss wake-up events.
• setTimeout/setInterval cannot keep a service worker alive and will not fire reliably across idle cycles. Use chrome.alarms.create with periodInMinutes >= 0.5.
• chrome.runtime.onMessage async handlers must return true synchronously to keep the message channel open; otherwise sendResponse throws.
• Content scripts run in an isolated world by default. The page's JS, frameworks, and window.* globals are invisible. Use world: "MAIN" only for page-JS access, then bridge with postMessage plus a same-origin token.
• Extension-origin fetch requires matching host_permissions. Content-script fetch is bound by the page's origin and CORS rules — route privileged requests through the service worker via chrome.runtime.sendMessage.
• MV3 CSP forbids inline <script>, eval(), new Function(), and remote executable code. Bundle everything; no CDN-loaded scripts.
• Hand-written manifest.json paths must point at built artifacts (e.g. background.js, content.js, popup.html), never src/*.ts or unbuilt source.
• Requesting <all_urls> or broad host permissions at install time triggers Web Store review friction. Prefer activeTab plus optional host grants.

Decision Rules

Decision	Default	Escalate when
Background state	chrome.storage.session/local	Transactions, large indexes, binary blobs → IndexedDB
Periodic work	chrome.alarms	Real-time stream → design reconnect + queue
Network modification	declarativeNetRequest	Read-only observation → non-blocking webRequest
Page data fetch	Service worker fetch with host permissions	Page origin is sufficient → content-script fetch
Page JS access	world: "MAIN" bridge with payload validation	DOM-only access → stay in ISOLATED world
UI state persistence	chrome.storage.session	Must survive browser restart → local or sync
Host access	activeTab or optional host grants	Extension is non-functional without install-time host access
Side panel vs popup	Side panel for persistent companion UI	Quick action or short form → popup
Offscreen document	DOM/canvas/clipboard/audio/Worker from service worker	Popup/options/content script can own it → skip

Routing Boundary

Request shape	Action
"Build a Chrome extension"	Use this skill; target MV3
"Build for Chrome and Firefox"	Keep Chrome MV3 here; cross-browser layer is out of scope
"Automate a website in a browser"	Route to run-agent-browser
"Write Playwright tests for an extension"	Extension launch/load notes here; broader Playwright authoring is out of scope for this pack
"Publish to npm"	Route to publish-npm-package
"Submit to Chrome Web Store"	Use this skill; read references/publishing/web-store.md
"Deploy via enterprise policy"	Out of scope unless a dedicated enterprise skill exists

Workflow

1. Scope The Extension

Ask: "Which extension surfaces exist, which Chrome APIs are required, which permissions can be deferred to runtime?"

1. List required surfaces: popup, options, side panel, content script, service worker, offscreen document, devtools.
2. List required Chrome APIs and resulting permissions.
3. Split install-time vs optional_permissions/optional_host_permissions.
4. Choose the framework — see references/frameworks/comparison.md.
5. Record the built-output folder Chrome will load.

2. Scaffold Or Adapt

Ask: "Greenfield, existing framework build, or hand-written manifest?"

• Greenfield default: WXT.
• Existing Vite app becoming an extension: CRXJS.
• Custom build pipeline / explicit control: vanilla + Vite.
• Existing repo: follow its framework and output conventions.

Chrome must load the built output folder, never src/ or unbundled TypeScript.

3. Implement Extension Contexts

Ask: "Which context owns the state, which owns the DOM, which messages cross the boundary?"

Read the matching reference before writing code:

• Service worker lifecycle, persistence, alarms, offscreen → references/patterns/service-worker.md
• Content scripts, world isolation, idempotency, cross-origin routing → references/patterns/content-scripts.md
• Popup, options, side panel, devtools, new-tab surfaces → references/patterns/ui-surfaces.md
• One-time messages, ports, external/native messaging, page bridges → references/apis/messaging.md
• chrome.storage areas, quotas, typed wrappers, migrations → references/apis/storage.md
• Tabs, scripting, alarms, DNR, side panel, offscreen, runtime → references/apis/core-apis.md
• Manifest, permission, and MV2 migration decisions are covered by the pinned defaults and guardrails in this SKILL.md.

Validate every cross-boundary payload: messages, storage reads, external API responses, MAIN-world bridge data.

4. Validate Built Output

Ask: "Can Chrome load this exact folder, and do all manifest paths exist there?"

Run from the skill directory against your build folder:

scripts/check-mv3-manifest.sh dist
scripts/preflight-extension.sh dist

Adjust the path per framework:

Framework	Dev output	Production output
WXT	.output/chrome-mv3-dev/	.output/chrome-mv3/
Plasmo	build/chrome-mv3-dev/	build/chrome-mv3-prod/
CRXJS	dist/	dist/
Vanilla Vite	dist/	dist/

Read scripts/check-mv3-manifest.md and scripts/preflight-extension.md before modifying either script.

5. Test Extension Behavior

Ask: "What failure only appears once Chrome loads the extension?"

• Unit-test pure logic and message/storage helpers; mock chrome.* only at boundaries.
• Manually load the built output via chrome://extensions → "Load unpacked".
• Inspect the service worker from the extensions page; trigger an idle/wake cycle.
• Test content-script injection on allowed and disallowed URLs.
• Verify the runtime permission request UX (chrome.permissions.request) before the host call.

Read references/testing/testing-guide.md for extension-specific tests and references/testing/debugging.md for service-worker, popup, content-script, permission, and storage debugging.

6. Package For Web Store Review

Ask: "Could a reviewer state the single purpose, each permission's need, the data use, and the remote-code posture in one sentence each?"

1. Build production output.
2. Run both bundled scripts against that production build.
3. Strip junk before zipping: source maps (unless intentionally shipped), tests, .DS_Store, __MACOSX, framework caches.
4. Create the zip from inside the built-output folder (so manifest.json sits at the zip root).
5. Prepare: privacy practices, per-permission justification, remote-code disclosure, data-use certification, reviewer test instructions.
6. Read references/publishing/web-store.md before submission.

Minimal Build Evidence

Match evidence to the task before stopping:

Task	Evidence
New scaffold	Built-output path + check-mv3-manifest.sh result
Feature change	Relevant unit/integration test + manual-load note when Chrome behavior changed
Manifest/permission change	check-mv3-manifest.sh result + permission justification
Content-script change	Allowed-URL injection result + disallowed-URL injection result
Service-worker change	Restart-resilience note OR explicit "not exercised" caveat
Package/Web Store work	preflight-extension.sh result + zip path + reviewer notes

Output Contract

Final reports include:

• loaded build directory
• zip path (when packaged)
• Chrome version used for manual load (when manually tested)
• summary of permissions and host-permissions justification
• Web Store policy posture: single purpose, data use, remote code, MV3 compliance
• scripts/tests run, including failures
• reviewer notes for remaining manual checks

Guardrails

• Never generate or preserve Manifest V2 for new work.
• Never load src/ in Chrome unless the framework explicitly emits loadable code there.
• Never hold durable state in module-scope globals inside the service worker.
• Never rely on setTimeout/setInterval to keep the service worker alive.
• Never use blocking webRequest for normal MV3 network modification.
• Never request <all_urls> without a feature-level justification and a documented narrower alternative.
• Never expose arbitrary background fetch through unauthenticated content-script messages.
• Never ship remote executable code or undeclared remote-code behavior.
• Always validate built-output paths against manifest.json before claiming done.
• Always make each permission explainable in one sentence.

Bottom Line

MV3 punishes the same mistakes repeatedly: lost service-worker state, late-registered listeners, timer-based scheduling, isolated-world surprises, over-broad permissions, and shipping src/ instead of built output. Apply the pinned defaults, route to references for depth, run the bundled scripts against the built folder, and produce review-ready evidence.