Add Model Context Protocol support with fast-mcp

[jjmata]

Summary

• integrate fast-mcp gem and mount MCP endpoints
• add base classes for MCP tools and resources
• document how to interact with MCP

Testing

• bundle exec rails test (fails: ActiveRecord::DatabaseConnectionError: There is an issue connecting with your hostname: 127.0.0.1)

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https://chatgpt.com/codex/tasks/task_e_6891a4e9a218833284588e9f072ae5db

[gemini-code-assist]

Summary of Changes

Hello @jjmata, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates the fast-mcp gem to add Model Context Protocol (MCP) support to the application. This enables the application to expose JSON-RPC and Server-Sent Events (SSE) endpoints for interacting with defined tools and resources, facilitating external communication and real-time updates.

Highlights

• MCP Integration: I've added the fast-mcp gem to the project, which provides a Ruby implementation of the Model Context Protocol.
• Endpoint Exposure: The PR configures and mounts the MCP middleware in the Rails application, exposing /mcp/messages for JSON-RPC requests and /mcp/sse for Server-Sent Events.
• Base Classes for MCP Components: I've introduced ApplicationTool and ApplicationResource as base classes for defining custom MCP tools and resources, respectively, ensuring automatic registration with the MCP server.
• Documentation: Comprehensive documentation (docs/MCP.md) has been added to explain how to define tools and resources, and how to interact with the new MCP endpoints.

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[gemini-code-assist]

Code Review

This pull request successfully integrates the fast-mcp gem to add Model Context Protocol (MCP) support. The changes include adding the gem, creating base classes for resources and tools, providing documentation, and setting up an initializer. The implementation is solid, but I have a couple of suggestions for the initializer file to improve maintainability and ensure correct behavior in the development environment.

[gemini-code-assist]

In the development environment, Rails lazy-loads classes, so ApplicationTool.descendants and ApplicationResource.descendants will likely be empty when this initializer runs. This will prevent your tools and resources from being registered. You should explicitly eager-load these directories to ensure all tool and resource classes are loaded before registration.

    # Eager-load tools and resources to ensure they are discovered, especially in development.
    Rails.autoloaders.main.eager_load_dir(Rails.root.join("app/tools"))
    Rails.autoloaders.main.eager_load_dir(Rails.root.join("app/resources"))

    server.register_tools(*ApplicationTool.descendants)
    server.register_resources(*ApplicationResource.descendants)

[gemini-code-assist]

To improve maintainability and avoid using a 'magic string', it's a good practice to define the version as a constant. This makes it easier to manage and update the version number, especially if it's referenced elsewhere in the application.

MCP_VERSION = '1.0.0'

FastMcp.mount_in_rails(
  Rails.application,
  name: Rails.application.class.module_parent_name.underscore.dasherize,
  version: MCP_VERSION,