Expose Ruby Commands as MCP Tools for AI Assistants

Foobara::McpConnector

Exposes Foobara commands according to the Model Context Protocol (MCP) specification

• Foobara::McpConnector
  • Installation
  • Usage
    • Code demo video!
    • Code examples
    • Super basic example
    • An example with entities
    • A destructive example
    • Moar examples
  • Contributing
  • License

Installation

Typical stuff: add gem "foobara-mcp-connector to your Gemfile or .gemspec file. Or even just
gem install foobara-mcp-connector if just playing with it directly in scripts.

Usage

Code demo video!

You can watch a code demo here: https://youtu.be/_w3ZHdiJEGU

Code examples

You can find examples in examples/

Super basic example

Let's create a simple Foobara command:

class BuildSuperDuperSecret < Foobara::Command
  inputs do
    seed :integer, :required
  end
  result :integer

  def execute
    seed * seed * seed
  end
end

This just cubes the integer we pass to it. You can run it with BuildSuperDuperSecret.run!(seed: 3) which
would give 27. See the foobara gem for more info about Foobara commands.

Now, let's connect it to an McpConnector:

require "foobara/mcp_connector"

mcp_connector = Foobara::McpConnector.new
mcp_connector.connect(BuildSuperDuperSecret)

And we can start a stdio server like so:

mcp_connector.run_stdio_server

Putting it all together in a single script called simple-mcp-server-example we get:

#!/usr/bin/env ruby

require "foobara/mcp_connector"

class BuildSuperDuperSecret < Foobara::Command
  inputs do
    seed :integer, :required
  end
  result :integer

  def execute
    seed * seed * seed
  end
end

mcp_connector = Foobara::McpConnector.new
mcp_connector.connect(BuildSuperDuperSecret)
mcp_connector.run_stdio_server

We can now add it to programs that can consume MCP servers. For example, with claude code, we can
tell claude code about it by running claude mcp add and following the instructions or we
can create a .mcp.json file like this:

{
  "mcpServers": {
    "mcp-test": {
      "type": "stdio",
      "command": "simple-mcp-server-example",
      "args": [],
      "env": {}
    }
  }
}

You need to set "command" to the path of your script.

Now when we run claude, we can ask it a question that would result in it running our command:

$ claude
> Hi! Could you please build me a super duper secret using a seed of 5?
● mcp-test:BuildSuperDuperSecret (MCP)(seed: 5)…
  ⎿  125
● 125
> Thanks!
● You're welcome!

An example with entities

Let's say we have a model (see examples/capybaras.rb):

class Capybara < Foobara::Entity
  attributes do
    id :integer
    name :string, :required
    year_of_birth :integer, :required
  end

  primary_key :id
end

As well as some commands like FindAllCapybaras, CreateCapybara, and UpdateCapybara
(see examples/capybara_commands.rb)

We can write an MCP connector to expose those commands so we can ask questions that require
running those commands to answer:

require "foobara/mcp_connector"
require_relative "capybara_commands"

CreateCapybara.run!(name: "Fumiko", year_of_birth: 2020)
CreateCapybara.run!(name: "Barbara", year_of_birth: 2019)
CreateCapybara.run!(name: "Basil", year_of_birth: 2021)

mcp_connector = Foobara::McpConnector.new
mcp_connector.connect(FindAllCapybaras)

mcp_connector.run_stdio_server

We can now ask a tool like claude a relevant question:

$ claude
> Which Capybara is the oldest?
● mcp-test:FindAllCapybaras (MCP)()…
  ⎿  [
       {
         "name": "Fumiko",
         "year_of_birth": 2020,
         "id": 1

     … +7 lines (ctrl+r to expand)

         "name": "Basil",
         "year_of_birth": 2021,
         "id": 3
       }
     ]

● Barbara (born in 2019)
> Thanks!
● You're welcome!

A destructive example

Let's say we would like to mutate data. Let's pretend we've accidentally entered a year-of-birth
in a 2-digit format where a 4-digit format was expected. Let's do that and also expose our UpdateCapybara
command:

# We will simulate a year accidentally being entered with a 2-digit format where a 4-digit format was expected
UpdateCapybara.run!(id: barbara.id, year_of_birth: 19)

mcp_connector = Foobara::McpConnector.new

mcp_connector.connect(FindAllCapybaras)
mcp_connector.connect(UpdateCapybara)

mcp_connector.run_stdio_server

Now we can ask a tool like claude to find and fix our data:

$ claude
> Hi! There's a Capybara whose birth year was entered incorrectly. Can you find which one
  and fix it? Thanks!
● I'll help find and fix the capybara with the incorrect birth year. Let me search for the
  capybaras first.
● mcp-test:FindAllCapybaras (MCP)()…
  ⎿  [
       {
         "name": "Fumiko",
         "year_of_birth": 2020,
         "id": 1

     … +7 lines (ctrl+r to expand)

         "name": "Basil",
         "year_of_birth": 2021,
         "id": 3
       }
     ]
● It looks like Barbara (id 2) has an incorrect birth year of 19, which is too low. Let me
  fix that to 2019.
● mcp-test:UpdateCapybara (MCP)(id: 2, year_of_birth: 2019)…
  ⎿  {
       "name": "Barbara",
       "year_of_birth": 2019,
       "id": 2
     }
● Fixed! Barbara's birth year has been updated from 19 to 2019.
> Great! Thanks!
● You're welcome!

Moar examples

Please see the examples/ directory for executable scripts of these examples.