MCP Server Development Guide (Elixir Phoenix Edition)

Leverage Elixir's high concurrency advantages to build efficient and stable MCP Servers.

Why You Need This Elixir Guide?

MCP Servers require high concurrency and low latency. Elixir and Phoenix, with their lightweight processes and real-time communication capabilities, become the ideal choice. However, many developers are confused about how to build MCP Servers with Phoenix. As a long-time practitioner in the Elixir ecosystem, I'll guide you from scratch to implement intelligent context management and plugin mechanisms using the most idiomatic Phoenix code.

If you want to build high-performance intelligent services, this guide is not to be missed.

🧱 Project Initialization: Phoenix API-only Mode

First, install Elixir and Phoenix, then create an API-only project:

mix archive.install hex phx_new
mix phx.new mcp_server --no-html --no-webpack
cd mcp_server

Configure mix.exs and install dependencies:

mix deps.get

🧠 Create Core Interface: Implement /api/invoke Route

Edit lib/mcp_server_web/controllers/invoke_controller.ex:

defmodule McpServerWeb.InvokeController do
  use McpServerWeb, :controller

  @context_store :ets.new(:context_store, [:named_table, :public, read_concurrency: true])

  def invoke(conn, %{"session_id" => session_id, "message" => message}) do
    history = case :ets.lookup(:context_store, session_id) do
      [{^session_id, msgs}] -> msgs ++ [message]
      [] -> [message]
    end

    :ets.insert(:context_store, {session_id, history})

    reply = if String.starts_with?(message, "weather") do
      city = String.trim_leading(message, "weather")
      weather_plugin(city)
    else
      "You said: #{message}"
    end

    json(conn, %{reply: reply, history: history})
  end

  defp weather_plugin(city), do: "#{city} is sunny today, temperature 26°C."
end

🧩 Configure Routes

Add routes in lib/mcp_server_web/router.ex:

scope "/api", McpServerWeb do
  post "/invoke", InvokeController, :invoke
end

🔐 Add Security Authentication

You can use Plug to add API Key verification:

defmodule McpServerWeb.Plugs.ApiKeyAuth do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_req_header(conn, "x-api-key") do
      ["supersecret"] -> conn
      _ -> conn |> send_resp(401, "Unauthorized") |> halt()
    end
  end
end

Then use it in the Router:

pipeline :api_auth do
  plug McpServerWeb.Plugs.ApiKeyAuth
end

scope "/api", McpServerWeb do
  pipe_through [:api, :api_auth]

  post "/invoke", InvokeController, :invoke
end

🚀 Run and Deploy

Start locally:

mix phx.server

Default listening on http://localhost:4000.

For production deployment, recommend using Distillery or Gigalixir.

🧪 Debugging Tips

• Use Logger to print context state
• Utilize Phoenix's built-in LiveDashboard to monitor requests
• Ensure request content is JSON and contains correct fields

🧱 Extension Ideas

• Use GenServer or Agent to persist context
• Support multiple plugins for parallel calls, integrate async task queues
• Combine with Phoenix Channels for real-time context synchronization

💬 Summary and Call to Action

The advantage of Elixir and Phoenix lies in easily supporting high concurrency and real-time features, perfectly matching MCP Server requirements. I hope this guide helps you build highly scalable intelligent control services.

📌 Start trying now! Bookmark this article, share it with developers who care about performance, and explore the charm of the Elixir ecosystem together!

Think: What innovations do you think Elixir's concurrency model will bring to MCP Servers? Welcome to discuss in the comments below 👇