javascript-sandbox

javascript-sandbox is a skill for Claude Code, Codex from stagewise-io/stagewise. It costs 44 tokens per session (1,190 once invoked), scanned A, original, AGPL-3.0.

A guide to using a built-in JavaScript workspace for running small programs and working with browser tools, files, and outside packages.

In plain words
What is it for?
Use it for browser debugging, browser interaction, file access, experiments, and small applications.
Why use it?
It explains how to use the workspace correctly instead of having to work out its available tools and limits by trial and error.

Skill for Claude CodeCodex

About the project

stagewise is an open-source agentic IDE that combines a coding agent, browser-based app previews, debugging tools, and git workflows in one development environment. Developers use it to build and inspect applications with models from different providers. Catalogue add-ons extend the IDE's agent workflows.

stagewise-io/stagewise · 6,801 stars · on GitHub

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/stagewise-io/stagewise/javascript-sandbox
Any agent
npx skills add stagewise-io/stagewise --skill javascript-sandbox
Clone the repo
git clone --depth 1 https://github.com/stagewise-io/stagewise

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for javascript-sandbox

README.md
[![agentmods](https://agentmods.dev/badge/skills/stagewise-io/stagewise/javascript-sandbox.svg)](https://agentmods.dev/skills/stagewise-io/stagewise/javascript-sandbox)
Your own site
<a href="https://agentmods.dev/skills/stagewise-io/stagewise/javascript-sandbox"><img src="https://agentmods.dev/badge/skills/stagewise-io/stagewise/javascript-sandbox.svg" alt="Measured on agentmods" height="20"></a>
Per session 44 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,190 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. Scan, not verified.
Origin unknown No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00044 $0.01190
Opus 5 $0.00022 $0.00595
Sonnet 5 $0.00009 $0.00238
Haiku 4.5 $0.00004 $0.00119

Measured 5d ago against content hash ddc6502a48d1, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

javascript-sandbox scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 5d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Runs shell commandslowCapability

Expected in a hook, worth knowing in a rule or an instructions file.

**Node.js built-ins** (via `require()`): `buffer`, `crypto`, `events`, `path`, `querystring`, `stream`, `string_decoder`, `url`, `util`, `zlib`, `assert`. Blocked: `net`, `http`, `https`, `child_process`, `worker_threads
apps/browser/bundled/plugins/javascript-sandbox/SKILL.md · 98 lines

The source is not reproduced here

Licensed AGPL-3.0

The repository is licensed AGPL-3.0, which this catalogue does not treat as permission to reproduce the file. Read it at the source.

Read it on GitHub

Files

What ships with it

2 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 5d ago First seen · 98 lines · 44 tokens per session scan A ddc6502a48d1

Subscribe to this mod's changes

javascript-sandbox is a skill published in the GitHub repository stagewise-io/stagewise (6,801 stars, last pushed 2d ago), licensed AGPL-3.0. It adds 44 tokens to every session and 1,190 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 1 finding (runs shell commands). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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