Build Apps

A Judt App is one Worker in the style of Cloudflare Workers. celld, the Judt runtime, runs the App on V8 isolates. It runs the App in the App’s sandbox as a dev server while you build, and in production after you deploy. The pages in this tab explain how to write the code that runs inside an App. To create, deploy, and publish Apps, and to manage their sandboxes, Secrets, and access, see Manage Apps.

The runtime

celld embeds V8 and implements the Cloudflare Workers runtime APIs. It’s a WinterCG web runtime, not workerd, Node.js, or a browser. Write standard web-platform JavaScript, such as fetch, Request, Response, streams, URL, and Web Crypto. Reach storage and background work through the bindings that you declare in the wrangler.json file. Most of what you know about Cloudflare Workers applies. Where celld differs, the page for each feature says so and says what to do instead. If this documentation and your knowledge of Cloudflare disagree, follow this documentation. For a summary of the differences, see Differences from Cloudflare Workers.

A minimal App

The smallest App is a wrangler.json file and an entry module that exports a fetch handler:

{
  "name": "app",
  "main": "index.js",
  "compatibility_date": "2026-09-17"
}

export default {
  async fetch(request, env, ctx) {
    return new Response("Hello from Judt.", {
      headers: { "content-type": "text/plain; charset=utf-8" },
    });
  },
};

For the full list of files that an App can contain, see The App contract.

Choose storage and background work

Module-level variables aren’t state. Each node runs its own isolates, and celld recycles isolates, so celld doesn’t keep a value in a module variable. Keep all durable state in a binding. An App can use many bindings together. The following table lists what to use for each kind of need:

Need	Use
State for one thing, such as a room, document, cart, game, rate limit, or agent session, that needs order, coordination, or WebSockets	Durable Objects with SQLite
Relational data that you query across many entities	D1
Values that you read by key, such as sessions, flags, and cached API responses with a TTL	KV. KV is strongly consistent on celld, and it has no atomic increment.
Files and blobs	R2. There are no presigned URLs, so all bytes go through the Worker.
HTML, CSS, browser JavaScript, icons, and fonts that ship with the code	Static assets
Background work, fan-out, and retries	Queues
Work on a schedule	Cron Triggers
A timer for one object	A Durable Object alarm. For more information, see Durable Objects.
A process with many steps, sleeps, or waits for events	Workflows
Code that you didn’t write at build time	Dynamic Workers
A private database for each tenant or plugin	Durable Object facets
An AI agent that calls a model and tools in a loop	A Durable Object with alarms. For more information, see Build an AI agent loop.

Explore the docs

The App contract

The files of an App, the source limits, and the rules that keep data attached to your code.

Test on the dev server

Run and debug the App in its sandbox before you deploy.

Configure wrangler.json

The configuration keys and bindings that Judt accepts.

npm packages and builds

Add dependencies, TypeScript, JSX, or a frontend bundler.

Durable Objects

Per-entity state with SQLite, alarms, and WebSockets.

Runtime reference

Handlers, globals, Node.js built-in modules, crypto, and limits.

Get started

Project

Storage

Background work

Advanced

Reference