ActionMCP is a Ruby gem focused on providing Model Context Protocol (MCP) capability to Ruby on Rails applications, specifically as a server.
ActionMCP is designed for production Rails environments and does not support STDIO transport. STDIO is not included because it is not production-ready and is only suitable for desktop or script-based use cases. Instead, ActionMCP is built for robust, network-based deployments.
The client functionality in ActionMCP is intended to connect to remote MCP servers, not to local processes via STDIO.
It offers base classes and helpers for creating MCP applications, making it easier to integrate your Ruby/Rails application with the MCP standard.
With ActionMCP, you can focus on your app's logic while it handles the boilerplate for MCP compliance.
Model Context Protocol (MCP) is an open protocol that standardizes how applications provide context to large language models (LLMs).
Think of it as a universal interface for connecting AI assistants to external data sources and tools.
MCP allows AI systems to plug into various resources in a consistent, secure way, enabling two-way integration between your data and AI-powered applications.
This means an AI (like an LLM) can request information or actions from your application through a well-defined protocol, and your app can provide context or perform tasks for the AI in return.
ActionMCP is targeted at developers building MCP-enabled Rails applications. It simplifies the process of integrating Ruby and Rails apps with the MCP standard by providing a set of base classes and an easy-to-use server interface.
ActionMCP supports MCP 2025-06-18 (current) with backward compatibility for MCP 2025-03-26. For a detailed (and entertaining) breakdown of protocol versions, features, and our design decisions, see The Hitchhiker's Guide to MCP.
Don't Panic: The guide contains everything you need to know about surviving MCP protocol versions.
Note: STDIO transport is not supported in ActionMCP. This gem is focused on production-ready, network-based deployments. STDIO is only suitable for desktop or script-based experimentation and is intentionally excluded.
Instead of implementing MCP support from scratch, you can subclass and configure the provided Prompt, Tool, and ResourceTemplate classes to expose your app's functionality to LLMs.
ActionMCP handles the underlying MCP message format and routing, so you can adhere to the open standard with minimal effort.
In short, ActionMCP helps you build an MCP server (the component that exposes capabilities to AI) more quickly and with fewer mistakes.
Client connections: The client part of ActionMCP is meant to connect to remote MCP servers only. Connecting to local processes (such as via STDIO) is not supported.
To start using ActionMCP, add it to your project:
-
Using Bundler (Rails or Ruby projects): Add the gem to your Gemfile and run bundle install:
$ bundle add actionmcp
This will load the ActionMCP library so you can start defining MCP prompts, tools, and resources in your application.
ActionMCP provides three core abstractions to streamline MCP server development:
ActionMCP::Prompt enables you to create reusable prompt templates that can be discovered and used by LLMs. Each prompt is defined as a Ruby class that inherits from ApplicationMCPPrompt.
Key features:
- Define expected arguments with descriptions and validation rules
- Build multi-step conversations with mixed content types
- Support for text, images, audio, and resource attachments
- Add messages with different roles (user/assistant)
Example:
class AnalyzeCodePrompt < ApplicationMCPPrompt
prompt_name "analyze_code"
description "Analyze code for potential improvements"
argument :language, description: "Programming language", default: "Ruby"
argument :code, description: "Code to explain", required: true
validates :language, inclusion: { in: %w[Ruby Python JavaScript] }
def perform
render(text: "Please analyze this #{language} code for improvements:")
render(text: code)
# You can add assistant messages too
render(text: "Here are some things to focus on in your analysis:", role: :assistant)
# Even add resources if needed
render(resource: "file://documentation/#{language.downcase}_style_guide.pdf",
mime_type: "application/pdf",
blob: get_style_guide_pdf(language))
end
private
def get_style_guide_pdf(language)
# Implementation to retrieve style guide as base64
end
endPrompts can be executed by instantiating them and calling the call method:
analyze_prompt = AnalyzeCodePrompt.new(language: "Ruby", code: "def hello; puts 'Hello, world!'; end")
result = analyze_prompt.callActionMCP::Tool allows you to create interactive functions that LLMs can call with arguments to perform specific tasks. Each tool is a Ruby class that inherits from ApplicationMCPTool.
Key features:
- Define input properties with types, descriptions, and validation
- Return multiple response types (text, images, errors)
- Progressive responses with multiple render calls
- Automatic input validation based on property definitions
Example:
class CalculateSumTool < ApplicationMCPTool
tool_name "calculate_sum"
description "Calculate the sum of two numbers"
property :a, type: "number", description: "First number", required: true
property :b, type: "number", description: "Second number", required: true
def perform
sum = a + b
render(text: "Calculating #{a} + #{b}...")
render(text: "The sum is #{sum}")
# You can render errors if needed
if sum > 1000
render(error: ["Warning: Sum exceeds recommended limit"])
end
# Or even images
render(image: generate_visualization(a, b), mime_type: "image/png")
end
private
def generate_visualization(a, b)
# Implementation to create a visualization as base64
end
endTools can be executed by instantiating them and calling the call method:
sum_tool = CalculateSumTool.new(a: 5, b: 10)
result = sum_tool.callActionMCP::ResourceTemplate facilitates the creation of URI templates for dynamic resources that LLMs can access.
This allows models to request specific data using parameterized URIs.
Example:
class ProductResourceTemplate < ApplicationMCPResTemplate
uri_template "product/{id}"
description "Access product information by ID"
parameter :id, description: "Product identifier", required: true
validates :id, format: { with: /\A\d+\z/, message: "must be numeric" }
def resolve
product = Product.find_by(id: id)
return unless product
ActionMCP::Resource.new(
uri: "ecommerce://products/#{product_id}",
name: "Product #{product_id}",
description: "Product information for product #{product_id}",
mime_type: "application/json",
size: product.to_json.length
)
end
endbefore_resolve do |template|
logger.tagged("ProductsTemplate") { logger.info("Starting to resolve product: #{template.product_id}") }
end
after_resolve do |template|
logger.tagged("ProductsTemplate") { logger.info("Finished resolving product resource for product: #{template.product_id}") }
end
around_resolve do |template, block|
start_time = Time.current
logger.tagged("ProductsTemplate") { logger.info("Starting resolution for product: #{template.product_id}") }
resource = block.call
if resource
logger.tagged("ProductsTemplate") { logger.info("Product #{template.product_id} resolved successfully in #{Time.current - start_time}s") }
else
logger.tagged("ProductsTemplate") { logger.info("Product #{template.product_id} not found") }
end
resource
endResource templates are automatically registered and used when LLMs request resources matching their patterns.
ActionMCP is configured via config.action_mcp in your Rails application.
By default, the name is set to your application's name and the version defaults to "0.0.1" unless your app has a version file.
You can override these settings in your configuration (e.g., in config/application.rb):
module Tron
class Application < Rails::Application
config.action_mcp.name = "Friendly MCP (Master Control Program)" # defaults to Rails.application.name
config.action_mcp.version = "1.2.3" # defaults to "0.0.1"
config.action_mcp.logging_enabled = true # defaults to true
config.action_mcp.logging_level = :info # defaults to :info, can be :debug, :info, :warn, :error, :fatal
config.action_mcp.vibed_ignore_version = false # defaults to false, set to true to ignore client protocol version mismatches
end
endFor dynamic versioning, consider adding the rails_app_version gem.
By default, ActionMCP requires clients to use the exact protocol version supported by the server (currently "2025-03-26"). If the client specifies a different version during initialization, the request will be rejected with an error.
To support clients with incompatible protocol versions, you can enable the vibed_ignore_version option:
# In config/application.rb or an initializer
Rails.application.config.action_mcp.vibed_ignore_version = trueWhen enabled, the server will ignore protocol version mismatches from clients and always use the latest supported version. This is useful for:
- Development environments with older client libraries
- Supporting clients that cannot be easily updated
- Situations where protocol differences are minor and known to be compatible
Note: Using
vibed_ignore_version = truein production is not recommended as it may lead to unexpected behavior if clients rely on specific protocol features that differ between versions.
ActionMCP uses a pub/sub system for real-time communication. You can choose between several adapters:
- SolidCable - Database-backed pub/sub (no Redis required)
- Simple - In-memory pub/sub for development and testing
- Redis - Redis-backed pub/sub (if you prefer Redis)
If you were previously using ActionCable with ActionMCP, you will need to migrate to the new PubSub system. Here's how:
- Remove the ActionCable dependency from your Gemfile (if you don't need it for other purposes)
- Install one of the PubSub adapters (SolidCable recommended)
- Create a configuration file at
config/mcp.yml(you can use the generator:bin/rails g action_mcp:config) - Run your tests to ensure everything works correctly
The new PubSub system maintains the same API as the previous ActionCable-based implementation, so your existing code should continue to work without changes.
Configure your adapter in config/mcp.yml:
development:
adapter: solid_cable
polling_interval: 0.1.seconds
# Thread pool configuration (optional)
# min_threads: 5 # Minimum number of threads in the pool
# max_threads: 10 # Maximum number of threads in the pool
# max_queue: 100 # Maximum number of tasks that can be queued
test:
adapter: test # Uses the simple in-memory adapter
production:
adapter: solid_cable
polling_interval: 0.5.seconds
# Optional: connects_to: cable # If using a separate database
# Thread pool configuration for high-traffic environments
min_threads: 10 # Minimum number of threads in the pool
max_threads: 20 # Maximum number of threads in the pool
max_queue: 500 # Maximum number of tasks that can be queuedFor SolidMCP, add it to your Gemfile:
gem "solid_mcp" # Database-backed adapter optimized for MCPThen install it:
bundle install
bin/rails solid_mcp:installThe installer will create the necessary database migration for message storage. Configure it in your config/mcp.yml.
If you prefer Redis, add it to your Gemfile:
gem "redis", "~> 5.0"Then configure the Redis adapter in your config/mcp.yml:
production:
adapter: redis
url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
channel_prefix: your_app_production
# Thread pool configuration for high-traffic environments
min_threads: 10 # Minimum number of threads in the pool
max_threads: 20 # Maximum number of threads in the pool
max_queue: 500 # Maximum number of tasks that can be queuedActionMCP provides a pluggable session storage system that allows you to choose how sessions are persisted based on your environment and requirements.
ActionMCP includes three session store implementations:
-
:volatile- In-memory storage using Concurrent::Hash- Default for development and test environments
- Sessions are lost on server restart
- Fast and lightweight for local development
- No external dependencies
-
:active_record- Database-backed storage- Default for production environment
- Sessions persist across server restarts
- Supports session resumability
- Requires database migrations
-
:test- Special store for testing- Tracks notifications and method calls
- Provides assertion helpers
- Automatically used in test environment when using TestHelper
You can configure the session store type in your Rails configuration or config/mcp.yml:
# config/application.rb or environment files
Rails.application.configure do
config.action_mcp.session_store_type = :active_record # or :volatile
endOr in config/mcp.yml:
# Global session store type (used by both client and server)
session_store_type: volatile
# Client-specific session store type (falls back to session_store_type if not specified)
client_session_store_type: volatile
# Server-specific session store type (falls back to session_store_type if not specified)
server_session_store_type: active_recordThe defaults are:
- Production:
:active_record - Development:
:volatile - Test:
:volatile(or:testwhen using TestHelper)
You can configure different session store types for client and server operations:
session_store_type: Global setting used by both client and server when specific types aren't setclient_session_store_type: Session store used by ActionMCP client connections (falls back to global setting)server_session_store_type: Session store used by ActionMCP server sessions (falls back to global setting)
This allows you to optimize each component separately. For example, you might use volatile storage for client sessions (faster, temporary) while using persistent storage for server sessions (maintains state across restarts).
# The session store is automatically selected based on configuration
# You can access it directly if needed:
session_store = ActionMCP::Server.session_store
# Create a session
session = session_store.create_session(session_id, {
status: "initialized",
protocol_version: "2025-03-26",
# ... other session attributes
})
# Load a session
session = session_store.load_session(session_id)
# Update a session
session_store.update_session(session_id, { status: "active" })
# Delete a session
session_store.delete_session(session_id)With the :active_record store, clients can resume sessions after disconnection:
# Client includes session ID in request headers
# Server automatically resumes the existing session
headers["Mcp-Session-Id"] = "existing-session-id"
# If the session exists, it will be resumed
# If not, a new session will be createdYou can create custom session stores by inheriting from ActionMCP::Server::SessionStore::Base:
class MyCustomSessionStore < ActionMCP::Server::SessionStore::Base
def create_session(session_id, payload = {})
# Implementation
end
def load_session(session_id)
# Implementation
end
def update_session(session_id, updates)
# Implementation
end
def delete_session(session_id)
# Implementation
end
def exists?(session_id)
# Implementation
end
end
# Register your custom store
ActionMCP::Server.session_store = MyCustomSessionStore.newActionMCP uses thread pools to efficiently handle message callbacks. This prevents the system from being overwhelmed by too many threads under high load.
You can configure the thread pool in your config/mcp.yml:
production:
adapter: solid_cable
# Thread pool configuration
min_threads: 10 # Minimum number of threads to keep in the pool
max_threads: 20 # Maximum number of threads the pool can grow to
max_queue: 500 # Maximum number of tasks that can be queuedThe thread pool will automatically:
- Start with
min_threadsthreads - Scale up to
max_threadsas needed - Queue tasks up to
max_queuelimit - Use caller's thread if queue is full (fallback policy)
When your application is shutting down, you should call:
ActionMCP::Server.shutdownThis ensures all thread pools are properly terminated and tasks are completed.
ActionMCP runs as a standalone Rack application. Do not attempt to mount it in your application's routes.rb—it is not designed to be mounted as an engine at a custom path. When you use run ActionMCP::Engine in your mcp.ru, the MCP endpoint is always available at the root path (/).
ActionMCP includes generators to help you set up your project quickly. The install generator creates all necessary base classes and configuration files:
# Install ActionMCP with base classes and configuration
bin/rails generate action_mcp:installThis will create:
app/mcp/prompts/application_mcp_prompt.rb- Base prompt classapp/mcp/tools/application_mcp_tool.rb- Base tool classapp/mcp/resource_templates/application_mcp_res_template.rb- Base resource template classapp/mcp/application_gateway.rb- Gateway for authenticationconfig/mcp.yml- Configuration file with example settings for all environments
Note: Authentication and authorization are not included. You are responsible for securing the endpoint.
ActionMCP provides a Gateway system similar to ActionCable's Connection for handling authentication. The Gateway allows you to authenticate users and make them available throughout your MCP components.
ActionMCP supports multiple authentication methods including OAuth 2.1, JWT tokens, and no authentication for development. For detailed OAuth 2.1 configuration and usage, see the OAuth Authentication Guide.
When you run the install generator, it creates an ApplicationGateway class:
# app/mcp/application_gateway.rb
class ApplicationGateway < ActionMCP::Gateway
# Specify what attributes identify a connection
identified_by :user
protected
def authenticate!
token = extract_bearer_token
raise ActionMCP::UnauthorizedError, "Missing token" unless token
payload = ActionMCP::JwtDecoder.decode(token)
user = resolve_user(payload)
raise ActionMCP::UnauthorizedError, "Unauthorized" unless user
# Return a hash with all identified_by attributes
{ user: user }
end
private
def resolve_user(payload)
user_id = payload["user_id"] || payload["sub"]
User.find_by(id: user_id) if user_id
end
endYou can identify connections by multiple attributes:
class ApplicationGateway < ActionMCP::Gateway
identified_by :user, :organization
protected
def authenticate!
# ... authentication logic ...
{
user: user,
organization: user.organization
}
end
endOnce authenticated, the current user (and other identifiers) are available in your tools, prompts, and resource templates:
class MyTool < ApplicationMCPTool
def perform
# Access the authenticated user
if current_user
render text: "Hello, #{current_user.name}!"
else
render text: "Hi Stranger! It's been a while "
end
end
endActionMCP uses Rails' CurrentAttributes to store the authenticated context. The ActionMCP::Current class provides:
ActionMCP::Current.user- The authenticated userActionMCP::Current.gateway- The gateway instance- Any other attributes you define with
identified_by
# Load the full Rails environment to access models, DB, Redis, etc.
require_relative "config/environment"
# No need to set a custom endpoint path. The MCP endpoint is always served at root ("/")
# when using ActionMCP::Engine directly.
run ActionMCP::Enginebin/rails s -c mcp.ru -p 62770 -P tmp/pids/mcps0.pidIf your Rails application uses middleware that interferes with MCP server operation (like Devise, Warden, Ahoy, Rack::Cors, etc.), use mcp_vanilla.ru instead:
# mcp_vanilla.ru - A minimal Rack app with only essential middleware
# This avoids conflicts with authentication, tracking, and other web-specific middleware
# See the file for detailed documentation on when and why to use it
bundle exec rails s -c mcp_vanilla.ru -p 62770
# Or with Falcon:
bundle exec falcon serve --bind http://0.0.0.0:62770 mcp_vanilla.ruCommon middleware that can cause issues:
- Devise/Warden - Expects cookies and sessions, throws
Devise::MissingWardenerrors - Ahoy - Analytics tracking that intercepts requests
- Rack::Attack - Rate limiting designed for web traffic
- Rack::Cors - CORS headers meant for browsers
- Any middleware assuming HTML responses or cookie-based authentication
An example of a minimal mcp_vanilla.ru file is located in the dummy app : test/dummy/mcp_vanilla.ru.
This file is a minimal Rack application that only includes the essential middleware needed for MCP server operation, avoiding conflicts with web-specific middleware.
But remember to add any instrumentation or logging middleware you need, as the minimal setup will not include them by default.
## Production Deployment of MCPS0
In production, **MCPS0** (the MCP server) is a standard Rack application. You can run it using any Rack-compatible server (such as Puma, Unicorn, or Passenger).
> **For best performance and concurrency, it is highly recommended to use a modern, synchronous server like [Falcon](https://github.com/socketry/falcon)**. Falcon is optimized for streaming and concurrent workloads, making it ideal for MCP servers. You can still use Puma, Unicorn, or Passenger, but Falcon will generally provide superior throughput and responsiveness for real-time and streaming use cases.
You have two main options for exposing the server:
### 1. Dedicated Port
Run MCPS0 on its own TCP port (commonly `62770`):
**With Falcon:**
```bash
bundle exec falcon serve --bind http://0.0.0.0:62770 mcp.ruWith Puma:
bundle exec rails s -c mcp.ru -p 62770Then, use your web server (Nginx, Apache, etc.) to reverse proxy requests to this port.
Alternatively, you can run MCPS0 on a Unix socket for improved performance and security (especially when the web server and app server are on the same machine):
With Falcon:
bundle exec falcon serve --bind unix:/tmp/mcps0.sock mcp.ruWith Puma:
bundle exec puma -C config/puma.rb -b unix:///tmp/mcps0.sock -c mcp.ruAnd configure your web server to proxy to the socket:
location /mcp/ {
proxy_pass http://unix:/tmp/mcps0.sock:;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}Key Points:
- MCPS0 is a standalone Rack app—run it separately from your main Rails server.
- You can expose it via a TCP port (e.g., 62770) or a Unix socket.
- Use a reverse proxy (Nginx, Apache, etc.) to route requests to MCPS0 as needed.
- This separation ensures reliability and scalability for both your main app and MCP services.
ActionMCP includes Rails generators to help you quickly set up your MCP server components.
First, install ActionMCP to create base classes and configuration:
bin/rails action_mcp:install:migrations # to copy the migrations
bin/rails generate action_mcp:install This will create the base application classes, configuration file, and authentication gateway in your app directory.
bin/rails generate action_mcp:prompt AnalyzeCodebin/rails generate action_mcp:tool CalculateSumActionMCP provides a TestHelper module to simplify testing of tools and prompts:
require "test_helper"
require "action_mcp/test_helper"
class ToolTest < ActiveSupport::TestCase
include ActionMCP::TestHelper
test "CalculateSumTool returns the correct sum" do
assert_tool_findable("calculate_sum")
result = execute_tool("calculate_sum", a: 5, b: 10)
assert_tool_output(result, "15.0")
end
test "AnalyzeCodePrompt returns the correct analysis" do
assert_prompt_findable("analyze_code")
result = execute_prompt("analyze_code", language: "Ruby", code: "def hello; puts 'Hello, world!'; end")
assert_equal "Analyzing Ruby code: def hello; puts 'Hello, world!'; end", assert_prompt_output(result)
end
endYou can use the MCP Inspector to test your server implementation:
npx @modelcontextprotocol/inspectorThe default path will be http://localhost:3000/action_mcp
Here's a section you can add to explain the profile system in ActionMCP:
ActionMCP supports a flexible profile system that allows you to selectively expose tools, prompts, and resources based on different usage scenarios. This is particularly useful for applications that need different MCP capabilities for different contexts (e.g., public API vs. admin interface).
Profiles are named configurations that define:
- Which tools are available
- Which prompts are accessible
- Which resources can be accessed
- Configuration options like logging level and change notifications
By default, ActionMCP includes two profiles:
primary: Exposes all tools, prompts, and resourcesminimal: Exposes no tools, prompts, or resources by default
Profiles are configured via a config/mcp.yml file in your Rails application. If this file doesn't exist, ActionMCP will use default settings from the gem.
Example configuration:
default:
tools:
- all # Include all tools
prompts:
- all # Include all prompts
resources:
- all # Include all resources
options:
list_changed: false
logging_enabled: true
logging_level: info
resources_subscribe: false
api_only:
tools:
- calculator
- weather
prompts: [] # No prompts for API
resources:
- user_profile
options:
list_changed: false
logging_level: warn
admin:
tools:
- all
options:
logging_level: debug
list_changed: true
resources_subscribe: trueEach profile can specify:
tools: Array of tool names to include (useallto include all tools)prompts: Array of prompt names to include (useallto include all prompts)resources: Array of resource names to include (useallto include all resources)options: Additional configuration options:list_changed: Whether to send change notificationslogging_enabled: Whether to enable logginglogging_level: The logging level to useresources_subscribe: Whether to enable resource subscriptions
You can switch between profiles programmatically in your code:
# Permanently switch to a different profile
ActionMCP.configuration.use_profile(:only_tools) # Switch to a profile named "only_tools"
# Temporarily use a profile for a specific operation
ActionMCP.with_profile(:minimal) do
# Code here uses the minimal profile
# After the block, reverts to the previous profile
endThis makes it easy to control which MCP capabilities are available in different contexts of your application.
ActionMCP includes rake tasks to help you manage and inspect your profiles:
# List all available profiles with their configurations
bin/rails action_mcp:list_profiles
# Show detailed information about a specific profile
bin/rails action_mcp:show_profile[admin]
# List all tools, prompts, resources, and profiles
bin/rails action_mcp:listThe profile inspection tasks will highlight any issues, such as configured tools, prompts, or resources that don't actually exist in your application.
Profiles are particularly useful for:
- Multi-tenant applications: Use different profiles for different customer tiers with Dorp or other gems
- Access control: Create profiles for different user roles (admin, staff, public)
- Performance optimization: Use a minimal profile for high-traffic endpoints
- Testing environments: Use specific test profiles in your test environment
- Progressive enhancement: Start with a minimal profile and gradually add capabilities
By leveraging profiles, you can maintain a single ActionMCP codebase while providing tailored MCP capabilities for different contexts.