background

Use it for extensions whose value depends on running continuously, even when the user isn't actively using Chrome.

Examples:

• A chat or email client extension that needs to receive notifications when no Chrome window is open.
• A monitoring tool that polls a remote service on a schedule.
• A native-messaging bridge that brokers data between Chrome and a native helper application.

The permission goes in the permissions array. It is independent of the "background" manifest key, which still has to be set if the extension actually needs a service worker.

{
  "name": "My Always-On Extension",
  "version": "1.0",
  "manifest_version": 3,
  "permissions": [
    "background",
    "alarms"
  ],
  "background": {
    "service_worker": "background.js",
    "type": "module"
  }
}

Why is it medium risk?

By itself this permission grants no access to user data — its only effect is keeping Chrome (and therefore the extension's service worker) alive. The risk it adds is that any other permissions the extension holds remain effective even when the user has closed every Chrome window and might believe Chrome is no longer running.

Users can override it from chrome://settings/system ("Continue running background apps when Google Chrome is closed") regardless of what the extension declares.

There is no API tied to this permission — declaring it just changes Chrome's lifecycle. The service worker code itself is the same as for any other extension:

// background.js — runs whenever the service worker is awake.

chrome.runtime.onInstalled.addListener(() => {
  // Wake the service worker every minute, even if no Chrome window is open.
  chrome.alarms.create('heartbeat', { periodInMinutes: 1 });
});

chrome.alarms.onAlarm.addListener((alarm) => {
  if (alarm.name === 'heartbeat') {
    console.log('Background heartbeat fired.');
  }
});