NexusMCP (NexusMCP v0.3.0)

MCP (Model Context Protocol) server library for Elixir.

Implements the 2025-11-25 spec over the Streamable HTTP transport, with a GenServer-per-session architecture. Each client gets its own process, tool calls execute concurrently in supervised Tasks, and SSE connections are monitored for cleanup.

Supports all three MCP server primitives:

• Tools — model-controlled functions (NexusMCP.Server.Tool.deftool/4)
• Prompts — user-controlled message templates (NexusMCP.Server.Prompt.defprompt/4)
• Resources — application-controlled context (NexusMCP.Server.Resource.defresource/3, NexusMCP.Server.Resource.defresource_template/3)

Quick start

defmodule MyApp.MCP do
  use NexusMCP.Server,
    name: "my-app",
    version: "1.0.0"

  deftool "hello", "Say hello",
    params: [name: {:string!, "Person's name"}] do
    {:ok, "Hello, #{params["name"]}!"}
  end

  defprompt "greet", "Greet someone formally",
    arguments: [name: {:string!, "Name"}] do
    {:ok, [%{role: "user",
             content: %{type: "text",
                        text: "Compose a formal greeting for " <> params["name"]}}]}
  end

  defresource "config://app",
    name: "app_config",
    mime_type: "application/json" do
    {:ok, Jason.encode!(MyApp.config())}
  end
end

Add the supervisor to your application:

children = [
  {NexusMCP.Supervisor, []},
  # ...
]

Route requests to the transport:

forward "/mcp", NexusMCP.Transport, server: MyApp.MCP

See NexusMCP.Server for the full behaviour reference.

Spec coverage

This release implements the MCP 2025-11-25 server spec for:

• initialize + notifications/initialized
• ping
• tools/list, tools/call (with annotations)
• prompts/list, prompts/get
• resources/list, resources/templates/list, resources/read

Not yet implemented: resource subscriptions, list_changed notifications, completion/complete, and pagination cursors. The library advertises subscribe: false and listChanged: false for the relevant capabilities.