MCP - Model Context Protocol Server

An Elixir implementation of the Model Context Protocol (MCP) server that communicates via Server-Sent Events over HTTP.

Installation

Add mcp to your list of dependencies in mix.exs:

def deps do
  [
    {:mcp, "~> 0.1.0"}
  ]
end

Phoenix Integration

Mount the MCP router in your Phoenix application:

# In your Phoenix router (lib/my_app_web/router.ex)
defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  # Your existing routes...
  
  # Mount MCP router under /mcp prefix
  forward "/mcp", MCP.Router, init_callback: &MyApp.MCP.Tools.init_callback/2
end

Define your tools:

# lib/my_app/mcp/tools.ex
defmodule MyApp.MCP.Tools do
  def init_callback(_session_id, _init_params) do
    tools = [
      %{
        spec: %{
          "name" => "echo",
          "description" => "Echoes back the input text",
          "inputSchema" => %{
            "type" => "object",
            "properties" => %{
              "text" => %{"type" => "string", "description" => "Text to echo"}
            },
            "required" => ["text"]
          }
        },
        callback: fn %{"text" => text} ->
          {:ok, %{content: [%{type: "text", text: "Echo: #{text}"}]}}
        end
      }
    ]

    {:ok, %{
      server_info: %{name: "My App MCP Server", version: "1.0.0"},
      tools: tools
    }}
  end
end

Your MCP server will be available at:

• SSE endpoint: GET /mcp/
• JSON-RPC messages: POST /mcp/message

Protocol Implementation

This implementation follows the MCP specification version 2024-11-05 with:

Core Features:

• Server-Sent Events (SSE) transport
• JSON-RPC 2.0 message protocol
• Session management with automatic cleanup
• Tool discovery via tools/list
• Tool execution via tools/call
• Protocol version negotiation

Tool Support:

• MCP-compliant tool specifications
• JSON Schema validation for input parameters
• Text content responses
• Error handling with isError flag
• Tool callback execution

Not Implemented:

• Image/resource content types
• listChanged notifications
• Advanced pagination features

License

MIT License - see LICENSE file for details.