What is Mcp Js

mcp-js is a Rust-based Model Context Protocol (MCP) server that provides a V8 JavaScript runtime for AI agents like Claude and Cursor. It enables the execution of JavaScript code in a secure environment and supports persistent heap snapshots.

Use cases

Use cases for mcp-js include developing AI agents that utilize JavaScript for processing, integrating with other MCP clients, and maintaining stateful interactions in AI applications.

How to use

To use mcp-js, install it using the provided install script. After installation, run the server with either S3 or a local directory for heap storage by specifying the appropriate command line arguments.

Key features

Key features of mcp-js include V8 JavaScript execution, support for persistent heap snapshots via S3 or local filesystem, implementation of the MCP protocol for integration with AI tools, and configurable storage options.

Where to use

mcp-js can be used in modern AI development environments, particularly for applications that require JavaScript execution and state persistence across sessions.

mcp-v8: V8 JavaScript MCP Server

A Rust-based Model Context Protocol (MCP) server that exposes a V8 JavaScript runtime as a tool for AI agents like Claude and Cursor. Supports persistent heap snapshots via S3 or local filesystem, and is ready for integration with modern AI development environments.

Features

• V8 JavaScript Execution: Run arbitrary JavaScript code in a secure, isolated V8 engine.
• Heap Snapshots: Persist and restore V8 heap state between runs, supporting both S3 and local file storage.
• MCP Protocol: Implements the Model Context Protocol for seamless tool integration with Claude, Cursor, and other MCP clients.
• Configurable Storage: Choose between S3 or local directory for heap storage at runtime.

Installation

Install mcp-v8 using the provided install script:

curl -fsSL https://raw.githubusercontent.com/r33drichards/mcp-js/main/install.sh | sudo bash

This will automatically download and install the latest release for your platform to /usr/local/bin/mcp-v8 (you may be prompted for your password).

Advanced users: If you prefer to build from source, see the Build from Source section at the end of this document.

Command Line Arguments

mcp-v8 supports the following command line arguments:

• --s3-bucket <bucket>: Use AWS S3 for heap snapshots. Specify the S3 bucket name. (Conflicts with --directory-path)
• --directory-path <path>: Use a local directory for heap snapshots. Specify the directory path. (Conflicts with --s3-bucket)

Note: You must specify either --s3-bucket or --directory-path. If neither is provided, the server defaults to S3 with the bucket name test-mcp-js-bucket.

Quick Start

After installation, you can run the server directly. Choose one of the following options:

# Use S3 for heap storage (recommended for cloud/persistent use)
mcp-v8 --s3-bucket my-bucket-name

# Use local filesystem directory for heap storage (recommended for local development)
mcp-v8 --directory-path /tmp/mcp-v8-heaps

Integration

Claude for Desktop

1. Install the server as above.
2. Open Claude Desktop → Settings → Developer → Edit Config.
3. Add your server to claude_desktop_config.json:

{
  "mcpServers": {
    "js": {
      "command": "/usr/local/bin/mcp-v8 --s3-bucket my-bucket-name"
    }
  }
}

4. Restart Claude Desktop. The new tools will appear under the hammer icon.

Cursor

1. Install the server as above.
2. Create or edit .cursor/mcp.json in your project root:

{
  "mcpServers": {
    "js": {
      "command": "/usr/local/bin/mcp-v8 --directory-path /tmp/mcp-v8-heaps"
    }
  }
}

3. Restart Cursor. The MCP tools will be available in the UI.

Example Usage

• Ask Claude or Cursor: “Run this JavaScript: 1 + 2”
• Use heap snapshots to persist state between runs.

Heap Storage Options

You can configure heap storage using the following command line arguments:

• S3: --s3-bucket <bucket>
  • Example: mcp-v8 --s3-bucket my-bucket-name
  • Requires AWS credentials in your environment.
• Filesystem: --directory-path <path>
  • Example: mcp-v8 --directory-path /tmp/mcp-v8-heaps

Note: Only one storage backend can be used at a time. If both are provided, the server will return an error.

Limitations

While mcp-v8 provides a powerful and persistent JavaScript execution environment, there are limitations to its runtime.

• No async/await or Promises: Asynchronous JavaScript is not supported. All code must be synchronous.
• No fetch or network access: There is no built-in way to make HTTP requests or access the network.
• No console.log or standard output: Output from console.log or similar functions will not appear. To return results, ensure the value you want is the last line of your code.
• No file system access: The runtime does not provide access to the local file system or environment variables.
• No npm install or external packages: You cannot install or import npm packages. Only standard JavaScript (ECMAScript) built-ins are available.
• No timers: Functions like setTimeout and setInterval are not available.
• No DOM or browser APIs: This is not a browser environment; there is no access to window, document, or other browser-specific objects.

Build from Source (Advanced)

If you prefer to build from source instead of using the install script:

Prerequisites

• Rust (nightly toolchain recommended)
• (Optional) AWS credentials for S3 storage

Build the Server

cd server
cargo build --release

The built binary will be located at server/target/release/server. You can use this path in the integration steps above instead of /usr/local/bin/mcp-v8 if desired.