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 targets the released MCP 2025-11-25 protocol. Older protocol versions and unreleased draft versions are not accepted. Core handling includes:

• 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)
• Stateful session management with explicit termination
• Tasks, tools, prompts, resources, completion, logging, sampling, roots, and elicitation message handling

The built-in Streamable HTTP endpoint returns one application/json message for requests and intentionally returns HTTP 405 for GET because it does not provide SSE streams. That behavior is allowed by the transport specification. Outbound server requests and notifications are retained in session message storage, but an application needs an SSE or custom transport to push those messages to a client outside the response to an active request.

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.