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.
Stateless Mode: Optional mode for fresh executions without heap persistence, ideal for serverless environments.
MCP Protocol: Implements the Model Context Protocol for seamless tool integration with Claude, Cursor, and other MCP clients.
Configurable Storage: Choose between S3, local directory, or stateless mode at runtime.
Multiple Transports: Supports stdio, HTTP, and SSE (Server-Sent Events) transport protocols.

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 --stateless)
--directory-path <path>: Use a local directory for heap snapshots. Specify the directory path. (Conflicts with --stateless)
--stateless: Run in stateless mode - no heap snapshots are saved or loaded. Each JavaScript execution starts with a fresh V8 isolate. (Conflicts with --s3-bucket and --directory-path)
--http-port <port>: Enable HTTP transport on the specified port. If not provided, the server uses stdio transport (default).
--sse-port <port>: Enable SSE (Server-Sent Events) transport on the specified port. (Conflicts with --http-port)

Note: For heap storage, if neither --s3-bucket, --directory-path, nor --stateless is provided, the server defaults to using /tmp/mcp-v8-heaps as the local directory.

Quick Start

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

Stdio Transport (Default)

# 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

# Use stateless mode - no heap persistence (recommended for one-off computations)
mcp-v8 --stateless

HTTP Transport

The HTTP transport uses the HTTP/1.1 upgrade mechanism to switch from HTTP to the MCP protocol:

# Start HTTP server on port 8080 with local filesystem storage
mcp-v8 --directory-path /tmp/mcp-v8-heaps --http-port 8080

# Start HTTP server on port 8080 with S3 storage
mcp-v8 --s3-bucket my-bucket-name --http-port 8080

# Start HTTP server on port 8080 in stateless mode
mcp-v8 --stateless --http-port 8080

The HTTP transport is useful for:

Network-based MCP clients
Testing and debugging with tools like the MCP Inspector
Containerized deployments
Remote MCP server access

SSE Transport

Server-Sent Events (SSE) transport for streaming responses:

# Start SSE server on port 8081 with local filesystem storage
mcp-v8 --directory-path /tmp/mcp-v8-heaps --sse-port 8081

# Start SSE server on port 8081 in stateless mode
mcp-v8 --stateless --sse-port 8081

Stateless vs Stateful Mode

Stateless Mode (`--stateless`)

Stateless mode runs each JavaScript execution in a fresh V8 isolate without any heap persistence.

Benefits:

Faster execution: No snapshot creation/serialization overhead
No storage I/O: Doesn't read or write heap files
Fresh isolates: Every JS execution starts clean
Perfect for: One-off computations, stateless functions, serverless environments

Example use case: Simple calculations, data transformations, or any scenario where you don't need to persist state between executions.

Stateful Mode (default)

Stateful mode persists the V8 heap state between executions using either S3 or local filesystem storage.

Benefits:

State persistence: Variables and objects persist between runs
Faster subsequent runs: Preloaded context and data
Perfect for: Interactive sessions, building up complex state over time

Example use case: Building a data structure incrementally, maintaining session state, or reusing expensive computations.

Integration

Claude for Desktop

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

Stateful mode with S3:

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

Stateless mode:

{
  "mcpServers": {
    "js": {
      "command": "/usr/local/bin/mcp-v8 --stateless"
    }
  }
}

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

Cursor

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

Stateful mode with local filesystem:

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

Stateless mode:

{
  "mcpServers": {
    "js": {
      "command": "/usr/local/bin/mcp-v8 --stateless"
    }
  }
}

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

Claude (Web/Cloud) via Railway

You can also use the hosted version on Railway without installing anything locally:

Option 1: Using Claude Settings

Go to Claude's connectors settings page
Add a new custom connector:
- Name: "mcp-v8"
- URL: https://mcp-js-production.up.railway.app/sse

Option 2: Using Claude Code CLI

claude mcp add mcp-v8 -t sse https://mcp-js-production.up.railway.app/sse

Then test by running claude and asking: "Run this JavaScript: [1,2,3].map(x => x * 2)"

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.
- Ideal for cloud deployments and sharing state across instances.
Filesystem: --directory-path <path>
- Example: mcp-v8 --directory-path /tmp/mcp-v8-heaps
- Stores heap snapshots locally on disk.
- Ideal for local development and testing.
Stateless: --stateless
- Example: mcp-v8 --stateless
- No heap persistence - each execution starts fresh.
- Ideal for one-off computations and serverless environments.

Note: Only one storage option can be used at a time. If multiple 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.