loop.js

Loop.js is the framework for loop engineering.

You state a goal and what "done" means. The engine runs an agent at it, Round after Round — fresh context every Round, memory read back from disk — until a separate, skeptical judge rules the bar met. Run it from a terminal, schedule it — on your machine or deployed to Modal's cloud — or embed it in a product: same engine, same guarantees.

import { Loop } from "@loop.js/core"

const loop = Loop.define({
  goal: "Build a playable 2D platformer",
  verify: "It builds clean, `bun test` passes, and the game boots to a controllable character",
  limits: { rounds: 20, usd: 10 },
})

const exit = await loop.run().done()
// { settled: true, verdict: { ok: true, reason: "…" } } — the judge said done, not the worker

Loop engineering is designing the system that prompts an agent instead of prompting it turn by turn: a goal, a way to work, a way to verify, a stop discipline — the loop does the iterating. loop.js is that system as a framework: what is loop engineering →

Why loops need a framework

Running an agent in while (true) is one line. Whether that loop ever converges is everything around it — and that is the part loop.js owns: