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loop.js

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A loop engineering framework — state a Goal; Rounds run until a skeptical, read-only Verify agent settles it.

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# Installation Guide

1. Get the code

git clone https://github.com/loop-js/loop.js

Downloads the entire project code from GitHub to your computer.

cd loop.js

Moves into the project folder you just downloaded.

2. Node.js

Easy Recommended

Prerequisites

Git Needed to download the project code from GitHub.
Node.js Node.js must be installed to use npm. The LTS version is recommended.

npm create @loop.js@latest my-loop

Type this command into your terminal and run it.

cd my-loop && npm install

Moves into the project folder you just downloaded.

✅ After running the command, open the address shown in the terminal (usually something like http://localhost:3000) in your browser.

Pulled directly from this repo's README.

// repository documentation

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# 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.

[![npm](https://img.shields.io/npm/v/@loop.js/core.svg)](https://www.npmjs.com/package/@loop.js/core)
[![license](https://img.shields.io/badge/license-Apache--2.0-blue.svg)](https://raw.githubusercontent.com/loop-js/loop.js/master/LICENSE)
[![docs](https://img.shields.io/badge/docs-loop--js.mintlify.site-0f172a.svg)](https://loop-js.mintlify.site/)

```ts
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 →](https://loop-js.mintlify.site/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:

| The pain | The machinery |
| --- | --- |
| The agent says "done" when it isn't | A separate **Verify** agent — own session, own (cheaper, if you like) model, **read-only by default** — judges every Round. Only its verdict settles the Loop; the worker never grades its own homework. |
| Long sessions bloat and drift | Every **Round** starts with fresh context and reads its memory back from disk — the worker's own handoff notes. Round 40 starts as fresh as Round 1. |
| Retries that don't converge | A "not yet" verdict carries a **mandatory reason**, fed to the next Round. A goal that can never pass settles as an explicit give-up instead of burning the budget. |
| Money runs away | Declared guards: total **`usd`** (step-granular ledger), **`rounds`**, per-Round **`timeout`** — and they default tight ($1, 3 Rounds, 5m). Every ending is a typed Exit with its own process exit code. |
| Crashes, restarts, double triggers | All state lives on disk; `loop run` is idempotent and resumes from the cursor. The **Lock** (compare-and-set + heartbeat) refuses a live owner and takes over a dead one — any trigger cadence is overlap-safe. |
| Babysitting | [`loop cron`](https://loop-js.mintlify.site/cli/cron) installs into a real scheduler — crontab, launchd, Task Scheduler, or **Modal** in the cloud. No daemon, ever. |

## What do you loop?

**Build something until it's actually done.** The verdict — not vibes — decides when to stop:

```ts
// loop.config.ts
export default Loop.define({
  goal: "Build a playable 2D platformer — arrow keys, jump physics, win and lose states",
  verify: "`bun test` passes and `bun run build` emits a bundle that boots without console errors",
  limits: { rounds: 30, usd: 15, timeout: "20m" },
})
```

```sh
npx loop run   # Rounds stream by until the judge says ok — or a guard fires, with its own exit code
```

**Keep something true, on a schedule.** A time-dependent goal goes stale by itself — each
morning the settled Loop is *re-judged*: yesterday's brief no longer satisfies "today's brief
exists", so the Loop re-opens and writes a new one. On days the bar still holds, the trigger
costs one judge turn and re-settles.

```ts
export default Loop.define({
  goal: "Today's brief on my watchlist (NVDA, TSLA, BTC) exists in ./briefs, named by date — overnight moves, headlines, one paragraph of context each",
  verify: { model: "claude-haiku-4-5" }, // bar = the goal itself; judged by a cheaper model
  permissions: "bypass",                 // gating off: headlines need the network — run this loop inside your own container
})
```

```sh
npx loop cron add "0 8 * * 1-5" --until forever   # weekday mornings, until you remove it
```

**Trade a strategy, audited every day.** The worker trades through your broker's API;
the judge audits every order against the strategy — and a breach settles as a give-up, so
the loop never retries its way into more orders:

```ts
export default Loop.define({
  goal: "Today's trades are executed and logged in ./trades, dated — follow ./strategy.md: check its signals, size positions within its caps, attach a stop-loss to every order",
  verify: "Every order in today's log matches a strategy.md rule, respects its caps, carries a stop-loss, and reconciles with the broker's confirmations — any breach is a give-up, not a retry",
  permissions: "bypass", // broker API + market data need the network — run contained
  limits: { rounds: 3, usd: 2 },
})
```

```sh
npx loop cron add "30 9 * * 1-5" --until forever --backend modal   # weekday mornings — deployed to Modal, no machine of yours stays on
```

The `usd` guard caps what the loop spends on the model, never what the strategy trades —
position caps live in `strategy.md` and in your broker account's own limits.

**Chores that should stay done.** Same shape, pointed at upkeep — schedule with
`--until settled` (the entry removes itself at the first settle) or `--until forever`:

- "Dependencies are current, `bun test` is green, and the changelog has an entry" — weekly
- "Every new issue has a triage label and a first response" — nightly
- "Every TODO in ./src links an issue or is deleted"
- "Test coverage ≥ 80%, no skipped tests"
- "Yesterday's ETL output exists in ./data and passes its sanity checks"
- "Every post in ./posts has an up-to-date Chinese translation"

**One pass, no judging.** `Agent.define` is the same Execute phase run bare — one ungraded
pass, no verdict, no convergence machinery:

```ts
import { Agent } from "@loop.js/core"

await Agent.define({ goal: "Summarize yesterday's git log into ./standup.md" }).run().done()
```

## Quickstart

```sh
npm create @loop.js@latest my-loop
cd my-loop && npm install
export ANTHROPIC_API_KEY=sk-ant-…   # agents run on the Claude Agent SDK

npx loop run      # drives Rounds until the Loop settles or a guard fires
npx loop status   # check on it any time, from any shell — human or --json
```

The scaffold is goal-only: `goal` is the field you edit; the `limits` block spells out
the tight engine defaults — 3 Rounds, $1, 5 minutes per Round — and every other knob is a
commented line carrying its default. First runs stop cheap by design; raising the guards
is the deliberate act.
[Full quickstart →](https://loop-js.mintlify.site/quickstart)

## Schedule it — local, or deployed to Modal

`loop cron` installs an Entry into a **real scheduler** and never runs one itself. A fired
Entry simply runs `loop run` — the Lock makes any cadence overlap-safe — and every Entry
declares its own lifetime at `add`:

```sh
npx loop cron add "*/30 * * * *" --until settled                # a watchdog: gone at the first settle (capped)
npx loop cron add "0 8 * * *"    --until forever                # standing: each tick re-judges through the Verify gate
npx loop cron add "0 8 * * *"    --until forever --backend modal   # the same Entry, deployed to Modal's cloud
npx loop cron list
npx loop cron remove <id>
```

| Backend | Where it runs | Where State lives |
| --- | --- | --- |
| `local` (default) | crontab (Linux), launchd (macOS), Task Scheduler (Windows) | the project directory |
| `modal` | a `modal.Cron` fires an ephemeral Runner per tick — no machine of yours stays on | a Modal Volume — `remove` deletes the Entry, **never** its Volume |

With `--backend modal`, `add` deploys with your own Modal token and stores your
`ANTHROPIC_API_KEY` once as a shared `modal.Secret` named `loop-js` — created only when
absent, rotated with one `modal secret create --force`, no redeploy.
[Scheduling →](https://loop-js.mintlify.site/cli/cron)

## One Round

```
               Goal — what "done" means; required, judged every Round
                 │
   ┌─────────────┼─────────────────── one Round ───────────────────────┐
   │             ▼                                                     │
   │  Execute   the worker agent builds in the work tree               │
   │     │      (fresh context; memory read back from disk)            │
   │     ▼                                                             │
   │  Handoff   the worker writes a note to its successor              │
   │     ▼                                                             │
   │  Verify    a separate, skeptical agent judges against the bar     │
   │     │      (read-only; escalates to the tree, the build, the      │
   │     ▼       transcript when a claim needs ground truth)           │
   │  Persist   record + journal — resumable after any crash           │
   └─────────────┬─────────────────────────────────────────────────────┘
                 ▼
        ok          → settled: the Loop succeeds
        not yet     → the reason lands on disk and feeds the next Round
        impossible  → settled: explicit give-up, budget preserved
```

Only a verdict settles a Loop — `rounds`, `usd`, and `timeout` are runaway guards, never a
definition of done.

## Goal · Execute · Verify

Everything you author is a **prompt** — one shape, three homes. A prompt is a string, a
`{ file: "./verify.md" }` (re-read fresh every Round, so you can move the bar mid-loop), or
a per-round function.

```
         goal          what "done" means — the one required thing
        /    \
  execute    verify    the mirrored pair — optional, each falls back to the goal
  how to work it       how to judge it
```

```ts
Loop.define({
  goal: "…",
  execute: { file: "./execute.md" },   // what to work on each Round
  verify: "the checks to run and the end-state that must hold",
})

// the object form binds a phase's model or permissions:
Loop.define({
  goal: "…",
  verify: { prompt: { file: "./verify.md" }, model: "claude-haiku-4-5" },
})
```

Goal-only is first-class: omit both and the engine works toward — and judges against — the
goal itself. [Prompts →](https://loop-js.mintlify.site/concepts/prompts)

## Embed it

The same engine is a typed library. `loop.run()` self-drives and returns an async-iterable
handle — iterating observes, it doesn't control; breaking out unsubscribes without
cancelling:

```ts
const run = loop.run()
for await (const e of run) {
  if (e.type === "text-delta") ui.type(e.text)                // live typewriter
  if (e.type === "verdict")    ui.badge(e.round, e.ok, e.reason)
  if (e.type === "exit")       ui.done(e.exit)                // terminal — stream completes
}
```

Every event is typed, and all but the stream-only `text-delta` are journaled; startup failures throw, everything after resolves to a
final `exit` — iterating never throws. [Events →](https://loop-js.mintlify.site/api/events)

## Neighbors

**Shell loops (the ralph tradition)** proved the core insight: re-prompt with fresh context,
keep state on disk. loop.js keeps that and adds what a bare loop can't give you — an
independent verdict with mandatory reasons, typed guards and exits, crash-safe resume, real
schedulers. [Where loop.js stands →](https://loop-js.mintlify.site/loop-engineering)

**Claude Code's `/goal`, `/loop`, `/schedule`** are the right tool for driving your own
session. Reach for loop.js when the loop outlives your terminal: someone else (or cron)
triggers it, money is on the line, or "done" must be judged by an agent the worker can't
influence. [Full comparison →](https://loop-js.mintlify.site/why-loop-js)

## CLI

| Command       | What it does                                                                     |
| ------------- | -------------------------------------------------------------------------------- |
| `loop run`    | drives Rounds until the Loop settles or a guard fires; the exit code says which  |
| `loop status` | one snapshot of what has happened, from disk — human-readable or `--json`        |
| `loop cron`   | install / list / remove schedule entries — local OS scheduler or Modal           |

## Status

`v0.2` beta. Shipping today: the engine (Rounds, verdicts, budgets, Lock, journal), the CLI
(`run` / `status` / `cron` with local and Modal backends), and the Claude Agent SDK
executor — tested at every boundary (Lock CAS, event ordering, crash-partial folding; 800+
tests). Ahead of 1.0: sandbox-contained runs, a public executor interface, remote
observation. Pre-1.0 the API may still move.

## Packages

| Package                                              | What it is                                  |
| ---------------------------------------------------- | ------------------------------------------- |
| [`@loop.js/core`](packages/loop-js)                  | engine + `loop` CLI                         |
| [`@loop.js/create`](packages/create-loop-js)         | the scaffolder behind `npm create @loop.js` |

## License

[Apache-2.0](LICENSE)