MCP server · Coding

JavaScript Sandbox MCP server

Let your AI run small JavaScript programs safely on your own computer and keep results between steps.

Coding Developer tools For it and development Code Execution

This is a helper that lets your AI write and run JavaScript code in a safe, locked-down space on your own machine. Instead of your AI only talking, it can actually calculate, sort, and reshape data for you. It is handy if you often ask your AI to work with lists, numbers, or small bits of code.

What is an MCP server? The 30-second version

On its own, your AI can only chat. An MCP server is a small helper program that gives your AI a new skill or a connection to something else. This one gives your AI a safe place to run JavaScript code. So when you ask it to do a calculation or tidy up some data, it can actually run the code and show you the result.

What this MCP server does

You ask your AI to run a piece of JavaScript, or to solve a small problem by writing code. Your AI sends that code to this helper program running on your computer. The helper runs the code inside a sealed-off JavaScript engine, so it cannot touch your files or the internet unless you allow it. Then it sends the printed output back to your AI. Your AI shows you the result in the chat, and in the stateful mode it can remember variables from one request to the next.

Flow diagram: you ask your AI “Run this JavaScript: console.log([1,2,3].map(x => x*2))”, on your own computer the JavaScript Sandbox MCP server works with your computer, and you get back printed result in chat.

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What you can do with it

Run a short JavaScript snippet and see what it prints
Do math or transform a list of numbers without doing it by hand
Keep variables and results between several requests in one session
Cancel a long-running piece of code if it takes too long
Look back at the output of earlier runs
Run TypeScript code too (the types are simply removed before running)

Try asking your AI

“Run this JavaScript: console.log([1,2,3].map(x => x*2))”
“Write and run code that sorts this list of names alphabetically: Tom, Ana, Ben”
“Run a script that calculates the total of these numbers: 12, 45, 7, 33”
“Remember this list in a variable called items, then next time add up its values”

What it gives back to you

You get back whatever the code printed, shown as text in your chat. In the stateful mode you also get an execution ID, which your AI can use to check progress, read more output, or cancel the run. If the code has an error, the error message comes back so your AI can fix it and try again.

Before you start

What you need

The mcp-v8 program installed on your computer (the README gives a one-line install command for Linux and Mac)
An MCP client such as Claude Desktop, Claude Code, or Cursor
A folder path if you want to keep sessions between runs, for example /tmp/mcp-v8-heaps

Good to know

By default the code cannot reach the internet, your files, or other programs, but if you turn those permissions on, the code your AI writes can then read or change things on your computer, so only allow what you trust.

Install it with your AI

Add JavaScript Sandbox MCP server to your AI, no technical skills needed

You don't install anything by hand. You copy one prompt, paste it into an AI that can work on your computer, and it checks, installs and connects the server for you, asking you when it needs something.

Sign in to get the install prompt

Members get a ready-made prompt that lets the Claude desktop app check JavaScript Sandbox MCP server, install it and connect it for them, step by step. You don't need any technical skills: you copy, paste and answer a few questions. Your connected AI can also find and install any of the 4,066 MCP servers here for you.

Who it's for

People who ask their AI to do small calculations, data cleanup, or coding tasks and want the AI to actually run the code instead of just describing it.