JavaScript Sandbox MCP server for AI agents

A Model Context Protocol server that provides a secure JavaScript execution environment for executing code within a controlled sandbox. It allows for isolated execution with configurable time and memory limits to protect against malicious code execution.

Installation

Installing via Smithery

You can install the JavaScript Sandbox Server automatically using Smithery:

npx -y @smithery/cli install @garc33/js-sandbox-mcp-server --client claude

Manual Configuration

To use with Claude Desktop, add the server configuration to your Claude configuration file:

MacOS:

~/Library/Application Support/Claude/claude_desktop_config.json

Windows:

%APPDATA%/Claude/claude_desktop_config.json

Add the following configuration to the file:

{
  "mcpServers": {
    "js-sandbox": {
      "command": "/path/to/js-sandbox/build/index.js"
    }
  }
}

Replace /path/to/js-sandbox/build/index.js with the actual path to the built server on your system.

Usage

Using the JavaScript Execution Tool

The server provides a tool called execute_js that allows you to run JavaScript code in an isolated environment.

Parameters

code (required): The JavaScript code you want to execute
timeout (optional): Maximum execution time in milliseconds (range: 100-30000ms)
memory (optional): Memory limit in bytes (range: 1MB-100MB)

Examples

Basic JavaScript Execution:

// Execute a simple calculation
execute_js({ code: "2 + 2" })

Setting Timeout Limit:

// Execute code with a 5-second timeout
execute_js({ 
  code: "const result = Array(1000000).fill(0).reduce((a, b) => a + b, 0); result;",
  timeout: 5000 
})

Setting Memory Limit:

// Execute code with a 10MB memory limit
execute_js({ 
  code: "const largeArray = new Array(1000000).fill('data');",
  memory: 10485760  // 10MB in bytes
})

Debugging

Since MCP servers communicate over stdio, you can use MCP Inspector for debugging:

npm run inspector

The Inspector will provide a URL to access debugging tools in your browser, allowing you to monitor the communication between your application and the MCP server.

How to install this MCP server

For Claude Code

To add this MCP server to Claude Code, run this command in your terminal:

claude mcp add-json "js-sandbox" '{"command":"/path/to/js-sandbox/build/index.js"}'

See the official Claude Code MCP documentation for more details.

For Cursor

There are two ways to add an MCP server to Cursor. The most common way is to add the server globally in the ~/.cursor/mcp.json file so that it is available in all of your projects.

If you only need the server in a single project, you can add it to the project instead by creating or adding it to the .cursor/mcp.json file.

Adding an MCP server to Cursor globally

To add a global MCP server go to Cursor Settings > Tools & Integrations and click "New MCP Server".

When you click that button the ~/.cursor/mcp.json file will be opened and you can add your server like this:

{
    "mcpServers": {
        "js-sandbox": {
            "command": "/path/to/js-sandbox/build/index.js"
        }
    }
}

Adding an MCP server to a project

To add an MCP server to a project you can create a new .cursor/mcp.json file or add it to the existing one. This will look exactly the same as the global MCP server example above.

How to use the MCP server

Once the server is installed, you might need to head back to Settings > MCP and click the refresh button.

The Cursor agent will then be able to see the available tools the added MCP server has available and will call them when it needs to.

You can also explicitly ask the agent to use the tool by mentioning the tool name and describing what the function does.

For Claude Desktop

To add this MCP server to Claude Desktop:

1. Find your configuration file:

macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
Windows: %APPDATA%\Claude\claude_desktop_config.json
Linux: ~/.config/Claude/claude_desktop_config.json

2. Add this to your configuration file:

{
    "mcpServers": {
        "js-sandbox": {
            "command": "/path/to/js-sandbox/build/index.js"
        }
    }
}

3. Restart Claude Desktop for the changes to take effect