Mv3 Doctor

Building Chrome Manifest V3 Extensions

Manifest V3 broke assumptions that a decade of extension tutorials — and most training data — still take for granted. Code that looks correct, passes review by eye, and even runs during a quick manual test will fail in production because the service worker was still warm when you tested it.

Follow the rules below while writing, then run the validator before you claim the work is done.

The one idea that explains most MV3 bugs

The background service worker is ephemeral. Chrome starts it to deliver an event and terminates it after roughly 30 seconds of inactivity. It is not a long-running process. Every mistake below follows from forgetting this.

MV2 mental model (wrong): a background page that stays alive, holds state in memory, keeps sockets open, and runs timers.

MV3 mental model (right): a collection of stateless event handlers. Each one starts from nothing, reads what it needs from storage, does its work, and may be killed the moment it goes idle.

Rules

1. Never keep state in module-level variables

// WRONG — resets to 0 without warning when the worker restarts
let requestCount = 0;
chrome.runtime.onMessage.addListener(() => { requestCount += 1; });

// RIGHT — state outlives the worker
chrome.runtime.onMessage.addListener(async () => {
  const { requestCount = 0 } = await chrome.storage.session.get("requestCount");
  await chrome.storage.session.set({ requestCount: requestCount + 1 });
});

chrome.storage.session is in-memory and cleared when the browser closes — good for tokens. chrome.storage.local persists to disk. Both are async.

Module-level const for configuration is fine. The problem is mutable state.

2. Never use timers for anything beyond a few seconds

setInterval stops when the worker dies. A setTimeout longer than the ~30s idle timeout never fires.

// WRONG
setInterval(poll, 60_000);

// RIGHT — alarms wake the worker back up
chrome.alarms.create("poll", { periodInMinutes: 1 });
chrome.alarms.onAlarm.addListener((alarm) => {
  if (alarm.name === "poll") poll();
});

Note periodInMinutes has a 30-second floor. If you need finer granularity than that, reconsider the design — an extension probably should not be polling that aggressively.

3. There is no DOM in a service worker

document, window, localStorage, sessionStorage and alert do not exist and will throw. Use self for the global scope. When you genuinely need DOM APIs (parsing HTML, canvas, audio), create an offscreen document:

await chrome.offscreen.createDocument({
  url: "offscreen.html",
  reasons: ["DOM_PARSER"],
  justification: "Parse HTML returned by the API",
});

4. All code must ship inside the package

MV3 forbids remote code. No importScripts("https://…"), no import("https://…"), no injecting a `, no eval(), no new Function(). Bundle dependencies at build time. A CSP with a remote script-src or 'unsafe-eval'` is rejected at review.

Remote data (JSON from your API) is fine. Remote code is not.

5. Blocking webRequest is gone

Use declarativeNetRequest. Rules are declared up front so Chrome can apply them without waking your worker:

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

You can still observe requests with non-blocking webRequest, but you cannot modify or cancel them that way.

6. Request the narrowest permissions you can justify

The most common Web Store rejections are permissions the extension does not visibly use, and ` without justification. Prefer activeTab, or request hosts at runtime with chrome.permissions.request()`. Write the justification text as if a reviewer who has never seen your extension is reading it — because that is what happens.

7. Register listeners synchronously at the top level

A listener registered inside an async callback may not exist yet when the worker is woken to handle that event.

// WRONG — the listener may be registered too late
(async () => {
  await init();
  chrome.runtime.onMessage.addListener(handler);
})();

// RIGHT — register immediately, do async work inside the handler
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  handleAsync(message).then(sendResponse);
  return true; // keeps the message channel open for the async response
});

Returning true from an onMessage listener is required whenever you respond asynchronously. Forgetting it is a very common, very confusing bug.

Verify before finishing

Run the validator on the extension directory and fix every error:

npx mv3-doctor path/to/extension

Use --json when you need to parse the findings programmatically. Exit code is 1 if any error was found, 0 otherwise (warnings do not fail the run).

Do not report the work as complete until the validator is clean, or until you can explain why a specific finding is a false positive. To knowingly accept one:

// mv3-doctor-disable-next-line
setInterval(cheapUiTick, 1000);

Deeper references

Read these only when the task actually touches the area:

• references/service-worker-lifecycle.md — termination, wake-up, keep-alive
• references/storage-and-state.md — choosing between the storage areas
• references/network-and-dnr.md — declarativeNetRequest patterns
• references/web-store-review.md — permission justifications and rejections

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: n0liu
• Source: n0liu/mv3-doctor
• License: MIT

Install and usage instructions live in the source repository linked above.