mcp-jsA 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

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Reviewed by Daniel Nikulshyn·Updated July 2026

Overview

mcp-js is a Rust-based Model Context Protocol (MCP) server that exposes a V8 JavaScript runtime for AI agents like Claude and Cursor. It allows agents to run JavaScript and TypeScript in a sandboxed V8 isolate, enabling them to build up state across multiple turns and perform complex tasks with fewer tokens. The server supports persistent heap snapshots via S3 or local filesystem and is secure by default, with host capabilities disabled until explicitly granted via OPA/Rego policies.

Key features

Sandboxed V8 JavaScript runtime
Persistent heap snapshots via S3 or local filesystem
OPA/Rego policy control for host capabilities
Support for stdio, Streamable HTTP, and SSE transports
REST sidecar and async execution with pagination
JWKS auth and Raft-replicated clustering

Pricing

Model: Free
Category: MCP Servers
Rating: No reviews yet

Use cases

AI Agent Development

mcp-js enables developers to build AI agents that can run complex JavaScript and TypeScript tasks with durable state management and secure policy control.

Automation and Integration

The MCP server can be used to automate tasks and integrate with other tools and services, providing a flexible and scalable solution for AI agent development.

Pros & Cons

Pros

One tool with unbounded capability for AI agents
Durable state management through persistent heap snapshots
Secure by default with explicit policy control over host capabilities

Cons

Requires installation and configuration of the MCP server and client
May require additional setup for specific use cases or integrations

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Q&A

What security features does mcp-js have?

mcp-js is secure by default, with host capabilities disabled until explicitly granted via OPA/Rego policies.

Asked by Wei Chen · Nov 27, 2025

What types of transports are supported?

mcp-js supports stdio, Streamable HTTP, and SSE transports, as well as REST sidecar and async execution with pagination.

Asked by Gabriel Duarte · Nov 28, 2025

How are heap snapshots stored?

Heap snapshots are stored via S3 or local filesystem, allowing for persistent state management.

Asked by Priya Nair · Oct 8, 2025

What runtime does mcp-js expose?

mcp-js exposes a V8 JavaScript runtime for AI agents like Claude and Cursor.

Asked by Liam O’Connor · Oct 6, 2025

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