ActionMCP

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.

Introduction

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.

Protocol Support

ActionMCP supports MCP 2025-06-18 (current) with backward compatibility for MCP 2025-03-26. The protocol implementation is fully compliant with the MCP specification, including:

• JSON-RPC 2.0 transport layer
• Capability negotiation during initialization
• Error handling with proper error codes (-32601 for method not found, -32002 for consent required)
• Session management with resumable sessions
• Change notifications for dynamic capability updates

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.

Requirements

• Ruby: 3.4.8+ or 4.0.0+
• Rails: 8.1.1+
• Database: PostgreSQL, MySQL, or SQLite3

ActionMCP is tested against Ruby 3.4.8 and 4.0.0 with Rails 8.1.1+.

Installation

To start using ActionMCP, add it to your project:

# Add gem to your Gemfile
$ bundle add actionmcp

# Install dependencies
bundle install

# Copy migrations from the engine
bin/rails action_mcp:install:migrations

# Generate base classes and configuration
bin/rails generate action_mcp:install

# Create necessary database tables
bin/rails db:migrate

The action_mcp:install generator will:

• Create base application classes (ApplicationGateway, ApplicationMCPTool, etc.)
• Generate the MCP configuration file (config/mcp.yml)
• Set up the basic directory structure for MCP components (app/mcp/)

Database migrations are copied separately using bin/rails action_mcp:install:migrations.

Core Components

ActionMCP provides three core abstractions to streamline MCP server development:

ActionMCP::Prompt

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
end

Prompts 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.call

ActionMCP::Tool

ActionMCP::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
• Consent management for sensitive operations

Example:

class CalculateSumTool < ApplicationMCPTool
  tool_name "calculate_sum"
  description "Calculate the sum of two numbers"

  property :a, type: "number", description: "The first number", required: true
  property :b, type: "number", description: "The second number", required: true

  def perform
    sum = a + b
    render(text: "Calculating #{a} + #{b}...")
    render(text: "The sum is #{sum}")

    # You can report errors if needed
    if sum > 1000
      report_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
end

Consent Management

For tools that perform sensitive operations (file system access, database modifications, external API calls), you can require explicit user consent:

class FileSystemTool < ApplicationMCPTool
  tool_name "read_file"
  description "Read contents of a file"

  # Require explicit consent before execution
  requires_consent!

  property :file_path, type: "string", description: "Path to file", required: true

  def perform
    # This code only runs after user grants consent
    content = File.read(file_path)
    render(text: "File contents: #{content}")
  end
end

Consent Flow:

1. When a consent-required tool is called without consent, it returns a JSON-RPC error with code -32002
2. The client must explicitly grant consent for the specific tool
3. Once granted, the tool can execute normally for that session
4. Consent is session-scoped and can be revoked at any time

Managing Consent:

# Check if consent is granted
session.consent_granted_for?("read_file")

# Grant consent for a tool
session.grant_consent("read_file")

# Revoke consent
session.revoke_consent("read_file")

Tools can be executed by instantiating them and calling the call method:

sum_tool = CalculateSumTool.new(a: 5, b: 10)
result = sum_tool.call

Structured output (output_schema)

Advertise a JSON Schema for your tool's structuredContent and return machine-validated results alongside any text output.

class PriceQuoteTool < ApplicationMCPTool
  tool_name "price_quote"
  description "Return a structured price quote"

  property :sku, type: "string", description: "SKU to price", required: true

  output_schema do
    string :sku, required: true, description: "SKU that was priced"
    number :price_cents, required: true, description: "Total price in cents"
    object :meta do
      string :currency, required: true, enum: %w[USD EUR GBP]
      boolean :cached, default: false
    end
  end

  def perform
    price_cents = lookup_price_cents(sku) # Implement your lookup

    render structured: { sku: sku,
                         price_cents: price_cents,
                         meta: { currency: "USD", cached: false } }
  end
end

The schema is included in the tool definition, and the structured payload is emitted as structuredContent in the response while remaining compatible with text/audio/image renders.

Returning resource links from a tool

When you want to hand back a URI instead of embedding the payload, use the built-in render_resource_link, which produces the MCP resource_link content type.

class ReportLinkTool < ApplicationMCPTool
  tool_name "report_link"
  description "Return a downloadable report link"

  property :report_id, type: "string", required: true

  def perform
    render_resource_link(
      uri: "reports://#{report_id}.json",
      name: "Report #{report_id}",
      description: "Downloadable JSON for report #{report_id}",
      mime_type: "application/json"
    )
  end
end

Clients can resolve the URI with a separate resources/read call, keeping tool responses lightweight while still discoverable.

Task-augmented tools (async execution with progress)

Use MCP Tasks when work might take seconds/minutes. Advertise task support with task_required! (or task_optional!) and let callers opt in by sending _meta.task on tools/call. While running as a task, you can emit progress updates with report_progress!.

class BatchIndexTool < ApplicationMCPTool
  tool_name "batch_index"
  description "Index many items asynchronously with progress updates"

  task_required! # advertise that this tool is intended to run as a task
  property :items, type: "array_string", description: "Items to index", required: true

  def perform
    total = items.length
    items.each_with_index do |item, idx|
      index_item(item) # your indexing logic

      percent = ((idx + 1) * 100.0 / total).round
      report_progress!(percent: percent, message: "Indexed #{idx + 1}/#{total}")
    end

    render(text: "Indexed #{total} items")
  end

  private

  def index_item(item)
    # Implement your indexing logic here
  end
end

Call it as a task from a client by adding _meta.task (creates a Task record and runs the tool via ToolExecutionJob):

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "batch_index",
    "arguments": { "items": ["a", "b", "c"] },
    "_meta": { "task": { "ttl": 120000, "pollInterval": 2000 } }
  }
}

Poll task status with tasks/get or fetch the result when finished with tasks/result. Use tasks/cancel to stop non-terminal tasks. tasks/list returns tasks in recent-first order and always paginates (default 50 per page, or pagination_page_size if configured). The response includes an opaque nextCursor when more results are available — treat cursors as opaque tokens.

ActionMCP::ResourceTemplate

ActionMCP::ResourceTemplate facilitates the creation of URI templates for dynamic resources that LLMs can access. This allows models to request specific data using parameterized URIs.

Templates support two MCP resource surfaces:

• resources/templates/list — parameterized URI patterns (existing behavior)
• resources/list — concrete resources with known URIs (via self.list)

Note: Not all MCP clients support both resource endpoints. Claude Code (as of v2.1.50) only calls resources/list, and Codex stubs resource methods entirely. Implement self.list on your templates to ensure resources are visible to all clients. Crush and VS Code support both endpoints.

Example:

class ProductResourceTemplate < ApplicationMCPResTemplate
  uri_template "product/{id}"
  description "Access product information by ID"
  mime_type "application/json"

  parameter :id, description: "Product identifier", required: true

  validates :id, format: { with: /\A\d+\z/, message: "must be numeric" }

  # Optional: enumerate concrete resources for resources/list
  def self.list(session: nil)
    Product.limit(50).map do |product|
      build_resource(
        uri: "product/#{product.id}",
        name: product.name,
        title: "Product ##{product.id}"
      )
    end
  end

  # Resolve a specific resource for resources/read
  def resolve
    product = Product.find_by(id: id)
    return unless product

    ActionMCP::Content::Resource.new(
      "product/#{id}",
      mime_type,
      text: product.to_json
    )
  end
end

Callbacks:

before_resolve do |template|
  # Starting to resolve product: #{template.product_id}
end

after_resolve do |template|
  # Finished resolving product resource for product: #{template.product_id}
end

around_resolve do |template, block|
  start_time = Time.current
  resource = block.call
  resource
end

Resource templates are automatically registered and used when LLMs request resources matching their patterns. See RESOURCE_TEMPLATES.md for the full API reference including pagination, deduplication, and read contract details.

📚 Documentation

ActionMCP provides comprehensive documentation across multiple specialized guides. Each guide focuses on specific aspects to keep information organized and prevent context overload:

Getting Started & Setup

• Installation & Configuration - Initial setup, database migrations, and basic configuration
• Authentication with Gateway - User authentication and authorization patterns

Component Development

• 📋 TOOLS.MD - Complete guide to developing MCP tools

  • Generator usage and best practices
  • Property definitions, validation, and consent management
  • Output schemas for structured responses
  • Error handling, testing, and security considerations
  • Advanced features like additional properties and authentication context

• 📝 PROMPTS.MD - Prompt template development guide

  • Creating reusable prompt templates
  • Multi-step conversations and mixed content types
  • Argument validation and prompt chaining

• 🔗 RESOURCE_TEMPLATES.md - Resource template implementation

  • URI template patterns and parameter extraction
  • Dynamic resource resolution and collections
  • Callbacks and validation patterns

Client & Integration

• 🔌 CLIENTUSAGE.MD - Complete client implementation guide
  • Session management and resumability
  • Transport configuration and connection handling
  • Tool, prompt, and resource collections
  • Production deployment patterns
• 🔐 GATEWAY.md - Authentication gateway guide
  • Implementing ApplicationGateway
  • Identifier handling via ActionMCP::Current
  • Auth patterns, error handling, and hardening tips

Protocol & Technical Details

• 🚀 The Hitchhiker's Guide to MCP - Protocol versions and migration
  • Comprehensive comparison of MCP protocol versions (2024-11-05, 2025-03-26, 2025-06-18)
  • Design decisions and architectural rationale
  • Migration paths and compatibility considerations
  • Feature evolution and technical specifications (Don't Panic!)

Advanced Configuration

• Session Storage - Volatile vs ActiveRecord vs custom session stores
• Thread Pool Management - Performance tuning and graceful shutdown
• Profiles System - Multi-tenant capability filtering
• Production Deployment - Falcon, Unix sockets, and reverse proxy setup

Development & Testing

• Generators - Rails generators for scaffolding components
• Testing with TestHelper - Comprehensive testing strategies
• Development Commands - Rake tasks for debugging and inspection
• MCP Inspector Integration - Interactive testing and validation

Troubleshooting & Production

• Error Handling - JSON-RPC error codes and debugging
• Production Considerations - Security, performance, and monitoring
• Middleware Conflicts - Using mcp_vanilla.ru for production

💡 Pro Tip: Start with the component-specific guides (TOOLS.MD, PROMPTS.MD, RESOURCE_TEMPLATES.md) for hands-on development, then reference the Hitchhiker's Guide for protocol details and CLIENTUSAGE.MD for integration patterns.

Configuration

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

    # Server instructions - helps LLMs understand the server's purpose
    config.action_mcp.server_instructions = [
      "Use this server to access and control Tron system programs",
      "Helpful for managing system processes and user data"
    ]
  end
end

For dynamic versioning, consider adding the rails_app_version gem.

Pagination

All list endpoints (tools/list, prompts/list, resources/list, resources/templates/list) support cursor-based pagination per the MCP specification. Pagination is off by default — set pagination_page_size to enable:

config.action_mcp.pagination_page_size = 10

Or in config/mcp.yml:

shared:
  pagination_page_size: 10

When set, responses include an opaque nextCursor when more items are available. When nil (default), all items are returned in a single response. Enable only when your clients support cursor-based pagination.

tasks/list always paginates regardless of this setting (defaults to 50 per page, or pagination_page_size if configured).

Server Instructions

Server instructions help LLMs understand what your server is for and when to use it. They describe the server's purpose and goal, not technical details like rate limits or authentication (tools are self-documented via their own descriptions).

Instructions are returned at the top level of the MCP initialization response.

You can configure server instructions in your config/mcp.yml file:

shared:
  # Describe the server's purpose - helps LLMs know when to use this server
  server_instructions:
    - "Use this server to manage Fizzy project tickets and workflows"
    - "Helpful for tracking bugs, features, and sprint planning"

development:
  # Development-specific purpose description
  server_instructions:
    - "Development server for testing Fizzy integration"
    - "Use for prototyping ticket management workflows"

production:
  # Production-specific purpose description
  server_instructions:
    - "Production Fizzy server for managing live project data"

Instructions are sent as a single string (joined by newlines) at the top level of the initialization response, helping LLMs understand your server's purpose.

Session Storage

ActionMCP provides a pluggable session storage system that allows you to choose how sessions are persisted based on your environment and requirements.

Session Store Types

ActionMCP includes three session store implementations:

1. :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

2. :active_record - Database-backed storage

   • Default for production environment
   • Sessions persist across server restarts
   • Supports session resumability
   • Requires database migrations

3. :test - Special store for testing

   • Tracks notifications and method calls
   • Provides assertion helpers
   • Automatically used in test environment when using TestHelper

Configuration

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
end

Or 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_record

The defaults are:

• Production: :active_record
• Development: :volatile
• Test: :volatile (or :test when using TestHelper)

Separate Client and Server Session Stores

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 set
• client_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).

Using Different Session Stores

# 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)

Session Resumability

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 created

Custom Session Stores

You 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.new

Thread Pool Management

ActionMCP uses thread pools to efficiently handle message callbacks. This prevents the system from being overwhelmed by too many threads under high load.

Thread Pool Configuration

You can configure the thread pool in your config/mcp.yml:

production:
  # 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 queued

The thread pool will automatically:

• Start with min_threads threads
• Scale up to max_threads as needed
• Queue tasks up to max_queue limit
• Use caller's thread if queue is full (fallback policy)

Graceful Shutdown

When your application is shutting down, you should call:

ActionMCP::Server.shutdown

This ensures all thread pools are properly terminated and tasks are completed.

Engine and Mounting

WARNING: Do NOT mount ActionMCP::Engine in your routes.rb. ActionMCP is a standalone Rack application that runs on its own port via mcp/config.ru. Mounting it as a Rails engine route will not work correctly.

When you use run ActionMCP.server in your mcp/config.ru, the MCP endpoint is available at the root path (/) by default and can be configured via config.action_mcp.base_path. Always use ActionMCP.server (not ActionMCP::Engine directly) — it initializes required subsystems.

Installing ActionMCP

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:install

This will create:

• app/mcp/prompts/application_mcp_prompt.rb - Base prompt class
• app/mcp/tools/application_mcp_tool.rb - Base tool class
• app/mcp/resource_templates/application_mcp_res_template.rb - Base resource template class
• app/mcp/application_gateway.rb - Gateway for authentication
• config/mcp.yml - Configuration file with example settings for all environments
• mcp/config.ru - Standalone Rack server configuration
• bin/mcp - Server binstub (prefers Falcon, falls back to Puma)

Note: Authentication and authorization are not included. You are responsible for securing the endpoint.

Authentication with Gateway

ActionMCP provides a Gateway system for handling authentication. The Gateway allows you to authenticate users and make them available throughout your MCP components. For the full gateway reference including identifier classes, session persistence, profile switching, and production hardening tips, see GATEWAY.md.

ActionMCP uses a Gateway pattern with pluggable identifiers for authentication. You can implement custom authentication strategies using session-based auth, API keys, bearer tokens, or integrate with existing authentication systems like Warden, Devise, or external OAuth providers.

Note: Auth errors return HTTP 200 with a JSON-RPC error payload (not HTTP 401). This is correct per the MCP specification — all MCP communication uses JSON-RPC over HTTP, and protocol-level errors are expressed within the JSON-RPC envelope. The initialize request bypasses authentication per MCP spec.

Creating an ApplicationGateway

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
end

Using Multiple Identifiers

You 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
end

Accessing Current User in Components

Once 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
end

Current Attributes

ActionMCP uses Rails' CurrentAttributes to store the authenticated context. The ActionMCP::Current class provides:

• ActionMCP::Current.user - The authenticated user
• ActionMCP::Current.gateway - The gateway instance
• Any other attributes you define with identified_by

Persisting Context on Sessions

Override configure_session in your gateway to persist auth context (user_id, tenant_id, roles) on the session for use in audit trails, background jobs, and cross-request scenarios:

class ApplicationGateway < ActionMCP::Gateway
  identified_by UserIdentifier

  def configure_session(session)
    session.session_data = { "user_id" => user.id, "tenant_id" => user.tenant_id }
  end
end

Tools access it via session_data["user_id"]. See GATEWAY.md for details.

1. Create mcp/config.ru

The install generator (rails generate action_mcp:install) creates this automatically. If you need to create it manually:

# Load the full Rails environment to access models, DB, Redis, etc.
require_relative "../config/environment"

$stdout.sync = true

# Eager load so all tools, prompts, and resources are registered.
Rails.application.eager_load!

# IMPORTANT: Use ActionMCP.server — it initializes required subsystems.
# Do NOT use ActionMCP::Engine directly.
run ActionMCP.server

2. Start the server

bin/mcp                                          # Uses Falcon (recommended)
bundle exec rails s -c mcp/config.ru -p 62770   # Uses Puma (fallback)

Dealing with Middleware Conflicts

If 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 --config mcp_vanilla.ru

Common middleware that can cause issues:

• Devise/Warden - Expects cookies and sessions, throws Devise::MissingWarden errors
• 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 several 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 --config mcp/config.ru

With Puma:

bundle exec rails s -c mcp/config.ru -p 62770

With Passenger:

passenger start --rackup mcp/config.ru --port 62770

Then, use your web server (Nginx, Apache, etc.) to reverse proxy requests to this port.

2. Unix Socket

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/config.ru

With Puma:

bundle exec puma -C config/puma.rb -b unix:///tmp/mcps0.sock -c mcp/config.ru

With Passenger:

passenger start --rackup mcp/config.ru --socket /tmp/mcps0.sock

And 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;
}

3. Nginx With Passenger

You can run both the main app and the MCP app using Passenger processes within Nginx.

location / {
  root /path/to/current/public;
  passenger_app_root /path/to/current;
  passenger_enabled on;

   # ... additional configuration for the main Rails app
}

location ~* ^/mcp {
  root /path/to/current/public;
  passenger_app_root /path/to/current;
  passenger_enabled on;
  passenger_startup_file mcp/config.ru;
  passenger_app_group_name mcp;
}

You must set the config.action_mcp.base_path to match the above Nginx configuration, i.e. config.action_mcp.base_path = '/mcp'.

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.

Generators

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, authentication gateway, mcp/config.ru rackup file, and bin/mcp binstub in your app directory.

Generate a New Prompt

bin/rails generate action_mcp:prompt AnalyzeCode

Generate a New Tool

bin/rails generate action_mcp:tool CalculateSum

Testing with TestHelper

ActionMCP 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_mcp_tool_findable("calculate_sum")
    result = execute_mcp_tool("calculate_sum", a: 5, b: 10)
    assert_mcp_tool_output("15.0", result)
  end

  test "AnalyzeCodePrompt returns the correct analysis" do
    assert_mcp_prompt_findable("analyze_code")
    result = execute_mcp_prompt("analyze_code", language: "Ruby", code: "def hello; puts 'Hello, world!'; end")
    assert_mcp_prompt_output("Analyzing Ruby code: def hello; puts 'Hello, world!'; end", result)
  end
end

The TestHelper provides several assertion and execution methods:

Tools:

• assert_mcp_tool_findable(name) - Verifies a tool exists and is registered
• execute_mcp_tool(name, **args) - Executes a tool with arguments and asserts success
• execute_mcp_tool_with_error(name, **args) - Executes a tool without asserting success (for testing error cases)
• assert_mcp_tool_output(expected, response) - Asserts tool output matches expected content

Prompts:

• assert_mcp_prompt_findable(name) - Verifies a prompt exists and is registered
• execute_mcp_prompt(name, **args) - Executes a prompt with arguments
• assert_mcp_prompt_output(expected, response) - Asserts prompt output matches expected content

Resource Templates:

• assert_mcp_resource_template_findable(name) - Verifies a resource template exists and is registered
• resolve_mcp_resource(uri) - Resolves a resource URI and asserts success
• resolve_mcp_resource_with_error(uri) - Resolves a resource URI without asserting success (for testing error cases)

General:

• assert_mcp_error_code(code, response) - Asserts a specific JSON-RPC error code

Testing Resource Templates

require "test_helper"
require "action_mcp/test_helper"

class ProductResourceTest < ActiveSupport::TestCase
  include ActionMCP::TestHelper

  test "product template is registered" do
    assert_mcp_resource_template_findable("products")
  end

  test "resolves a product resource by URI" do
    resp = resolve_mcp_resource("ecommerce://products/1")

    assert resp.success?
    assert_not_empty resp.contents
    assert_equal "application/json", resp.contents.first.mime_type
  end

  test "returns error for nonexistent product" do
    resp = resolve_mcp_resource_with_error("ecommerce://products/0")

    assert resp.is_error
  end
end

Inspecting Your MCP Server

You can use the MCP Inspector to test your server implementation:

# Start your MCP server
bundle exec rails s -c mcp/config.ru -p 62770

# In another terminal, run the inspector
npx @modelcontextprotocol/inspector --url http://localhost:62770

The MCP Inspector provides an interactive interface to:

• Test tool executions with custom arguments
• Validate prompt responses
• Inspect resource templates and their outputs
• Debug protocol compliance and error handling

Development Commands

ActionMCP includes several rake tasks for development and debugging:

# List all MCP components
bundle exec rails action_mcp:list

# List specific component types
bundle exec rails action_mcp:list_tools
bundle exec rails action_mcp:list_prompts
bundle exec rails action_mcp:list_resources
bundle exec rails action_mcp:list_profiles

# Show configuration and statistics
bundle exec rails action_mcp:info
bundle exec rails action_mcp:stats

# Show profile configuration
bundle exec rails action_mcp:show_profile[profile_name]

Error Handling and Troubleshooting

ActionMCP provides comprehensive error handling following the JSON-RPC 2.0 specification:

Error Codes

• -32601: Method not found - The requested method doesn't exist
• -32002: Consent required - Tool requires user consent to execute
• -32603: Internal error - Server encountered an unexpected error
• -32600: Invalid request - The request is malformed

Context-Aware Error Messages

Tools should return clear error messages to the LLM using the render method:

class MyTool < ApplicationMCPTool
  def perform
    # Check for error conditions and return clear messages
    if some_error_condition?
      report_error("Clear error message for the LLM")
      return
    end

    # Normal processing
    render(text: "Success message")
  end
end

Common Issues

1. Session not found: Ensure sessions are properly created and saved in the session store
2. Tool not registered: Verify tools are properly defined and inherit from ApplicationMCPTool
3. Consent required: Grant consent using session.grant_consent(tool_name)
4. Middleware conflicts: Use mcp_vanilla.ru to avoid web-specific middleware

Debugging Tips

• Check server logs for detailed error information
• Use bundle exec rails action_mcp:info to verify configuration
• Test with MCP Inspector to isolate protocol issues
• Ensure proper session management in production environments

Profiles

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).

Understanding Profiles

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 resources
• minimal: Exposes no tools, prompts, or resources by default

Configuring Profiles

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: true

Each profile can specify:

• tools: Array of tool names to include (use all to include all tools)
• prompts: Array of prompt names to include (use all to include all prompts)
• resources: Array of resource names to include (use all to include all resources)
• options: Additional configuration options:
  • list_changed: Whether to send change notifications
  • logging_enabled: Whether to enable logging
  • logging_level: The logging level to use
  • resources_subscribe: Whether to enable resource subscriptions

Switching Profiles

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
end

This makes it easy to control which MCP capabilities are available in different contexts of your application.

Inspecting Profiles

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:list

The profile inspection tasks will highlight any issues, such as configured tools, prompts, or resources that don't actually exist in your application.

Use Cases

Profiles are particularly useful for:

1. Multi-tenant applications: Use different profiles for different customer tiers with Dorp or other gems
2. Access control: Create profiles for different user roles (admin, staff, public)
3. Performance optimization: Use a minimal profile for high-traffic endpoints
4. Testing environments: Use specific test profiles in your test environment
5. 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.

Client Usage

ActionMCP includes a client for connecting to remote MCP servers. The client handles session management, protocol negotiation, and provides a simple API for interacting with MCP servers.

For comprehensive client documentation, including examples, session management, transport configuration, and API usage, see CLIENTUSAGE.md.

Production Considerations

Security

• Never expose sensitive data through MCP components

• Use authentication via Gateway for production deployments

• Implement proper authorization in your tools and prompts

• Validate all inputs using property definitions and Rails validations

• Use consent management for sensitive operations

• Protect against DNS rebinding: ActionMCP validates the Origin header on every request. If Origin is present and its host doesn't match the server's own host, the server returns HTTP 403 with a JSON-RPC error body (no id), as required by the MCP spec. Non-browser clients (Claude Desktop, CLI tools) don't send Origin and are unaffected. To allow additional trusted origins, configure allowed_origins:

  # config/initializers/action_mcp.rb
  ActionMCP.configure do |config|
    config.allowed_origins = ["app.example.com", "api.example.com"]
  end
  

  For defence-in-depth, also configure Rails' ActionDispatch::HostAuthorization to restrict which Host headers are accepted (a separate check against host-header injection):

  # config/environments/production.rb
  config.hosts = ["api.example.com"]
  

Performance

• Configure appropriate thread pools for high-traffic scenarios
• Use Redis or SolidMCP for production pub/sub
• Choose ActiveRecord session store for session persistence
• Monitor session cleanup to prevent memory leaks
• Use profiles to limit exposed capabilities

Monitoring

• Enable logging and configure appropriate log levels
• Monitor session statistics using action_mcp:stats
• Track tool usage and performance metrics
• Set up alerts for error rates and response times

Deployment

• Use Falcon for optimal performance with streaming workloads
• Deploy on dedicated ports or Unix sockets
• Use reverse proxies (Nginx, Apache) for SSL termination
• Implement health checks for your MCP endpoints
• Use mcp_vanilla.ru to avoid middleware conflicts