Add Model Context Protocol support with fast-mcp

Gemfile

@@ -80,6 +80,7 @@ gem "after_commit_everywhere", "~> 1.0"
 
 # AI
 gem "ruby-openai"
+gem "fast-mcp"
 
 group :development, :test do
   gem "debug", platforms: %i[mri windows]

Gemfile.lock

@@ -182,6 +182,35 @@ GEM
       dotenv (= 3.1.8)
       railties (>= 6.1)
     drb (2.2.3)
+    dry-configurable (1.3.0)
+      dry-core (~> 1.1)
+      zeitwerk (~> 2.6)
+    dry-core (1.1.0)
+      concurrent-ruby (~> 1.0)
+      logger
+      zeitwerk (~> 2.6)
+    dry-inflector (1.2.0)
+    dry-initializer (3.2.0)
+    dry-logic (1.6.0)
+      bigdecimal
+      concurrent-ruby (~> 1.0)
+      dry-core (~> 1.1)
+      zeitwerk (~> 2.6)
+    dry-schema (1.14.1)
+      concurrent-ruby (~> 1.0)
+      dry-configurable (~> 1.0, >= 1.0.1)
+      dry-core (~> 1.1)
+      dry-initializer (~> 3.2)
+      dry-logic (~> 1.5)
+      dry-types (~> 1.8)
+      zeitwerk (~> 2.6)
+    dry-types (1.8.3)
+      bigdecimal (~> 3.0)
+      concurrent-ruby (~> 1.0)
+      dry-core (~> 1.0)
+      dry-inflector (~> 1.0)
+      dry-logic (~> 1.4)
+      zeitwerk (~> 2.6)
     erb (5.0.1)
     erb_lint (0.9.0)
       activesupport
@@ -206,6 +235,13 @@ GEM
       net-http (>= 0.5.0)
     faraday-retry (2.3.2)
       faraday (~> 2.0)
+    fast-mcp (1.5.0)
+      addressable (~> 2.8)
+      base64
+      dry-schema (~> 1.14)
+      json (~> 2.0)
+      mime-types (~> 3.4)
+      rack (~> 3.1)
     ffi (1.17.2-aarch64-linux-gnu)
     ffi (1.17.2-aarch64-linux-musl)
     ffi (1.17.2-arm-linux-gnu)
@@ -321,6 +357,10 @@ GEM
     matrix (0.4.2)
     memory_profiler (1.1.0)
     method_source (1.1.0)
+    mime-types (3.7.0)
+      logger
+      mime-types-data (~> 3.2025, >= 3.2025.0507)
+    mime-types-data (3.2025.0729)
     mini_histogram (0.3.1)
     mini_magick (5.2.0)
       benchmark
@@ -640,6 +680,7 @@ DEPENDENCIES
   faraday
   faraday-multipart
   faraday-retry
+  fast-mcp
   foreman
   hotwire-livereload
   hotwire_combobox

app/resources/application_resource.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class ApplicationResource < ActionResource::Base
+  # write your custom logic to be shared across all resources here
+end

app/tools/application_tool.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class ApplicationTool < ActionTool::Base
+  # write your custom logic to be shared across all tools here
+end

config/initializers/fast_mcp.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+# FastMcp - Model Context Protocol for Rails
+# This initializer sets up the MCP middleware in your Rails application.
+#
+# In Rails applications, you can use:
+# - ActionTool::Base as an alias for FastMcp::Tool
+# - ActionResource::Base as an alias for FastMcp::Resource
+#
+# All your tools should inherit from ApplicationTool which already uses ActionTool::Base,
+# and all your resources should inherit from ApplicationResource which uses ActionResource::Base.
+
+# Mount the MCP middleware in your Rails application
+# You can customize the options below to fit your needs.
+require 'fast_mcp'
+
+FastMcp.mount_in_rails(
+  Rails.application,
+  name: Rails.application.class.module_parent_name.underscore.dasherize,
+  version: '1.0.0',
+  path_prefix: '/mcp', # This is the default path prefix
+  messages_route: 'messages', # This is the default route for the messages endpoint
+  sse_route: 'sse' # This is the default route for the SSE endpoint
+  # Add allowed origins below, it defaults to Rails.application.config.hosts
+  # allowed_origins: ['localhost', '127.0.0.1', '[::1]', 'example.com', /.*\.example\.com/],
+  # localhost_only: true, # Set to false to allow connections from other hosts
+  # whitelist specific ips to if you want to run on localhost and allow connections from other IPs
+  # allowed_ips: ['127.0.0.1', '::1']
+  # authenticate: true,       # Uncomment to enable authentication
+  # auth_token: 'your-token', # Required if authenticate: true
+) do |server|
+  Rails.application.config.after_initialize do
+    # FastMcp will automatically discover and register:
+    # - All classes that inherit from ApplicationTool (which uses ActionTool::Base)
+    # - All classes that inherit from ApplicationResource (which uses ActionResource::Base)
+    server.register_tools(*ApplicationTool.descendants)
+    server.register_resources(*ApplicationResource.descendants)
+    # alternatively, you can register tools and resources manually:
+    # server.register_tool(MyTool)
+    # server.register_resource(MyResource)
+  end
+end

docs/MCP.md

@@ -0,0 +1,32 @@
+# Model Context Protocol (MCP)
+
+This application ships with [fast-mcp](https://github.com/yjacquin/fast-mcp), a Ruby implementation of the Model Context Protocol. The middleware is mounted at `/mcp` and exposes two endpoints:
+
+- `POST /mcp/messages` – JSON‑RPC endpoint for MCP requests
+- `GET  /mcp/sse` – Server‑sent events stream
+
+## Defining tools and resources
+
+Add tools under `app/tools/` and resources under `app/resources/`. Tools inherit from `ApplicationTool` and resources from `ApplicationResource`.
+
+Any subclasses of these base classes are automatically registered with the MCP server on boot.
+
+## Example request
+
+Once the Rails server is running, an MCP client can send a JSON‑RPC message to invoke a tool:
+
+```bash
+curl -X POST http://localhost:3000/mcp/messages \
+  -H "Content-Type: application/json" \
+  -d '{"jsonrpc":"2.0","id":"1","method":"tools/execute","params":{"name":"your_tool","arguments":{}}}'
+```
+
+Replace `your_tool` with the name of a tool you defined.
+
+Use the SSE endpoint to subscribe to resource updates or other MCP events:
+
+```bash
+curl http://localhost:3000/mcp/sse
+```
+
+Consult the [fast-mcp documentation](https://github.com/yjacquin/fast-mcp) for full protocol details.