Extension sandbox

Extension UI code runs in a dedicated Web Worker inside a sandboxed runtime iframe. Every extension shares the runtime's origin, so any browser API scoped to that origin would let one extension read or overwrite another extension's data. The platform therefore removes these APIs before your bundle is evaluated.

Blocked APIs

Behaviour change

indexedDB, caches, BroadcastChannel and the other Worker-global APIs listed below are blocked in the extension Worker. Code or third-party libraries that use them throw a SecurityError on first access. Move that state to useExtensionStorage.

Worker global

Reading or assigning any of these throws a SecurityError ("<name> is not available in the Lumio extension sandbox"). Even typeof indexedDB throws, because it reads the property.

API	Why it is blocked
indexedDB	Origin-scoped database, shared by all extensions
caches (CacheStorage)	Origin-scoped cache, shared by all extensions
BroadcastChannel	Messages reach every context on the shared origin
navigator.storage	Quota, persistence and the origin-private file system
navigator.locks	Web Locks are shared by every context on the origin
navigator.serviceWorker	Service workers are registered per origin
navigator.storageBuckets	Each Storage Bucket opens its own indexedDB, caches and file system, shared across the origin
Worker, SharedWorker	A nested worker would start with an unrestricted global
webkitRequestFileSystem*, webkitResolveLocalFileSystem*URL	Legacy origin-scoped file systems

The blocks cannot be undone: the properties are non-configurable, and the original accessors on the prototypes are replaced too.

Network

The Worker's Content Security Policy blocks network requests from extension UI code: fetch, XMLHttpRequest, WebSocket, EventSource and importScripts from other origins all fail. Read data through server functions, which can call declared egress hosts with ctx.fetch().

Runtime frame

The runtime iframe that hosts the Worker also blocks localStorage, sessionStorage, indexedDB, caches and BroadcastChannel, and document.cookie always reads as "" (writes are dropped). Extension code does not run in that frame, but the restriction is listed here for completeness.

What to use instead

You need	Use
State that survives a reload, shared by all surfaces of your extension	useExtensionStorage
Data your server functions own	Server functions (query() / mutation())
Values configured by the streamer	useLumioConfig
Short-lived state in one surface	React state (useState, useReducer) or module variables

Detecting the sandbox in libraries

A library that probes for storage with typeof indexedDB !== "undefined" throws instead of falling back. Wrap the probe:

function hasIndexedDb(): boolean {
  try {
    return typeof indexedDB !== "undefined";
  } catch {
    return false;
  }
}

If you cannot change the library, configure it for in-memory storage or pick one without a persistence layer.