QuickJS Sandbox MCP server

The MCP QuickJS Runner server is an implementation of the Model Context Protocol (MCP) that allows you to securely execute JavaScript code within a sandboxed QuickJS WebAssembly environment. This tool enables language models or other MCP clients to run JavaScript code snippets safely without risking the host system's security.

Prerequisites

• Node.js (v23.x or later recommended)
• npm or yarn package manager
• QuickJS WASM file (qjs-wasi.wasm) - must be available in the same directory as the compiled server script

Installation

To install the MCP QuickJS Runner server:

# Clone the repository (if applicable)
git clone <repository-url>
cd mcp-quickjs-runner

# Install dependencies
npm install

Running the Server

To start the server:

node server.ts

How It Works

The server exposes a single tool named run_javascript_code through the Model Context Protocol. When a client sends JavaScript code to this tool:

1. The code is executed within a sandboxed QuickJS WASM environment
2. Standard output (stdout) and standard error (stderr) streams are captured
3. Any execution errors are recorded
4. All results are returned to the MCP client according to protocol standards

Security Features

The JavaScript execution environment is completely isolated thanks to:

• WebAssembly sandbox using QuickJS
• Node.js WASI implementation for safe system interface access
• Capture of I/O through temporary files that are properly cleaned up after execution

Technical Implementation

The server implementation:

1. Uses a pre-compiled QuickJS engine targeting WebAssembly
2. Leverages Node.js's built-in WASI module
3. Redirects stdio through temporary files to capture output
4. Communicates with clients via the MCP protocol over standard I/O

This approach ensures that potentially untrusted code can be executed without compromising the host system, making it ideal for language models or other automated systems that need to run arbitrary JavaScript safely.

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 "quickjs-sandbox" '{"command":"node","args":["server.ts"]}'

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": {
        "quickjs-sandbox": {
            "command": "node",
            "args": [
                "server.ts"
            ]
        }
    }
}

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": {
        "quickjs-sandbox": {
            "command": "node",
            "args": [
                "server.ts"
            ]
        }
    }
}

3. Restart Claude Desktop for the changes to take effect