ZeroMCP — Ruby

Drop a .rb file in a folder, get a sandboxed MCP server. Stdio out of the box, zero dependencies.

Getting started

# tools/hello.rb — this is a complete MCP server
tool description: "Say hello to someone",
     input: { name: "string" }

execute do |args, ctx|
  "Hello, #{args['name']}!"
end

ruby -I lib bin/zeromcp serve ./tools

That's it. Stdio works immediately. Drop another .rb file to add another tool. Delete a file to remove one.

vs. the official SDK

The official Ruby SDK requires server setup, transport configuration, and explicit tool registration. ZeroMCP is file-based — each tool is its own file, discovered automatically. Zero external dependencies.

In benchmarks, ZeroMCP Ruby handles 15,327 requests/second over stdio versus the official SDK's 12,935 — 1.2x faster with 50% less memory (12 MB vs 24 MB). Over HTTP (Rack+Puma), ZeroMCP serves 3,217 rps at 26 MB versus the official SDK's 2,163 rps at 49–56 MB. The official SDK crashed on binary garbage input and corrupted responses under slow tools in chaos testing. ZeroMCP survived 22/22 attacks.

The official SDK has no sandbox. ZeroMCP lets tools declare network, filesystem, and exec permissions.

Ruby passes all 10 conformance suites.

HTTP / Streamable HTTP

ZeroMCP doesn't own the HTTP layer. You bring your own framework; ZeroMCP gives you a handle_request method that takes a Hash and returns a Hash (or nil for notifications).

# response = server.handle_request(request)

Sinatra

require 'sinatra'
require 'json'

post '/mcp' do
  request_body = JSON.parse(request.body.read)
  response = server.handle_request(request_body)

  if response.nil?
    status 204
  else
    content_type :json
    response.to_json
  end
end

Requirements

• Ruby 3.0+
• No external dependencies

Install

gem build zeromcp.gemspec
gem install zeromcp-0.1.0.gem

Sandbox

tool description: "Fetch from our API",
     input: { url: "string" },
     permissions: {
       network: ["api.example.com", "*.internal.dev"],
       fs: false,
       exec: false
     }

execute do |args, ctx|
  # ...
end

Directory structure

Tools are discovered recursively. Subdirectory names become namespace prefixes:

tools/
  hello.rb          -> tool "hello"
  math/
    add.rb          -> tool "math_add"

Testing

ruby -I lib -I test -e 'Dir["test/**/*_test.rb"].each { |f| require_relative f }'