mcp-v8 MCP server

mcp-v8 — a JavaScript/TypeScript runtime for AI agents

mcp-v8 is a Model Context Protocol server, written in Rust, that lets an AI agent run JavaScript and TypeScript in a sandboxed V8 isolate. Instead of wiring up dozens of narrow tools, you give the agent one tool — run_js — and it writes code: looping, branching, transforming data, and calling other tools, often with far fewer tokens than equivalent tool-call chains.

In its default stateful mode the V8 heap is saved as a content-addressed snapshot, so an agent can build up state across many turns. Host capabilities (network, filesystem, subprocess, WebAssembly, module imports, and calls to other MCP servers) are all off by default and unlocked only by explicit OPA/Rego policies.

Why mcp-v8

• One tool, unbounded capability. The agent runs a program, not a fixed menu of tools.
• Durable state. Heap snapshots persist variables and objects across calls.
• Secure by default. fetch, filesystem, subprocess, and external imports are denied until you grant them via policy.
• Kernel-enforced confinement. Opt-in --sandbox-manifest confines the whole process with a nono capability manifest (Landlock on Linux, Seatbelt on macOS) as defense in depth beneath the policy layer.
• Production-ready. stdio / Streamable HTTP / SSE transports, a REST sidecar, async execution with pagination, JWKS auth, and Raft-replicated clustering.

Documentation

Maintainers: see Publishing to the MCP Registry for the Docker-backed registry manifest and automated release process.

Full documentation lives at **** (built from site-docs/) — tutorials, how-to guides, concept explanations, and complete reference for the CLI flags, HTTP API, and MCP tools.

Quick start

Install

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

# Optional CLI client
curl -fsSL https://raw.githubusercontent.com/r33drichards/mcp-js/main/install-cli.sh | sudo bash

Installs to /usr/local/bin. Supported platforms: Linux x8664/arm64 and macOS Apple Silicon. You can also nix run github:r33drichards/mcp-js, use Docker (see the docker-compose..yml stacks), or build from source.

Prefer a hosted server? Deploy on Railway — the repo's railway.json configures the build, healthcheck, and restart policy, and RAILWAY.md walks through the volume, variables, and one-click template setup.

Connect an MCP client

# Claude Code (stdio)
claude mcp add mcp-v8 -- mcp-v8 --directory-path /tmp/mcp-v8-heaps   # stateful
claude mcp add mcp-v8 -- mcp-v8 --stateless                          # stateless

For Claude Desktop / Cursor, add to the client's mcpServers config:

{ "mcpServers": { "js": { "command": "mcp-v8", "args": ["--stateless"] } } }

Then ask the agent: "Run this JavaScript: console.log([1,2,3].map(x => x2))"*.

Run over HTTP

mcp-v8 --stateless --http-port 8080
# MCP endpoint: POST http://localhost:8080/mcp

Shortened. The full README is on GitHub.