Any standalone working server example (stdio or SSE or Streamable HTTP)? How to start it properly so that it doesn't exit by itself?

[plugnburn]

Hello,

First off, I'd like to thank you for such a cool MCP library. I figured out how to dynamically instantiate MCP clients, but having some trouble with the server part.

Maybe I misunderstand something in the docs but I can't get an MCP server working as expected. Regardless of the transport, the subprocess just exits and the main process too. Here's my code:

defmodule Exai.RefServer.App do
  use Application

  def main(args \\ []) do
    configpath =
      case args do
        [fname] -> fname
        _ -> Path.join(File.cwd!(), "config.yaml")
      end

    {:ok, config} = YamlElixir.read_from_file(configpath)

    IO.puts("Starting the EXAI reference MCP server...")

    tr_obj =
      case config["mcp_server"] do
        %{"transport" => "stdio"} ->
          {Exai.RefServer, transport: :stdio}

        %{"transport" => "sse", "port" => port} ->
          {Exai.RefServer,
           transport: :sse, sse: [port: port, sse_path: "/sse", post_path: "/messages"]}
        %{"transport" => "http", "port" => port} ->
          {Exai.RefServer, transport: :streamable_http, streamable_http: [port: port]}

        _ ->
          nil
      end

    children = [Anubis.Server.Registry, tr_obj]

    {:ok, _pid} =
      Supervisor.start_link(children, strategy: :one_for_one, name: Exai.RefServer.Supervisor)
  end

  def start(_type, args) do
    main(args)
  end
end

where the Exai.RefServer contains the init and tool definition according to the docs. However, after starting it as a normal application with mix run, I'm observing the following behavior:

Starting the EXAI reference MCP server...

21:54:57.883 [debug] MCP server event: starting

21:54:57.883 [debug] MCP event details: %{module: Exai.RefServer, capabilities: %{"tools" => %{}}, server_info: %{"name" => "exai-mcp-server", "version" => "1.0.0"}}

21:54:57.883 [debug] MCP transport event: starting

21:54:57.883 [debug] MCP transport details: %{server: Exai.RefServer, transport: :streamable_http}

And then the process just exits with no errors or warnings whatsoever.

Could you please provide a complete example of a server started as an application or point out what I'm missing in terms of starting it?

[zoedsoupe]

sorry for the late response, but:

1. which client you're integrating with? (cursor, claude code)
2. which MCP protocol version are you supporting on the server implementation?
3. which version of the library are you using?
4. which elixir/otp version are you using?

exiting without warning, error or exit code (even from application) is kinda unusual...

also, for the question itself, on https://github.com/zoedsoupe/anubis-mcp/tree/main/priv/dev there are 3 server implementation examples, using SSE/streamable http and plug/phoenix and live view, yo could give it a try and take a look on their application.ex setup

for STDIO transport, we indeed have a bug on the server part because of the library logs and wrong implementation, which i didn't track as an issue here since i got lost on the priorities for server architecture refactor as stated on #24 , so ill open a new issue to track that and correctly implement this kind of transport layer

[plugnburn]

Thanks for the response.

1. The server itself is exiting even before I try consuming it by anything.
2. I'm trying to implement the earliest one, also testing with SSE, but this behavior doesn't depend on it.
3. The latest one available (0.14.0). Tried with 0.13.1, tried multiple ways of instantiating the server supervisors, same result.
4. The latest stable versions: Elixir 1.18.4 with Erlang/OTP 28.

Is plug a required dependency for SSE/StreamableHTTP to work correctly? If so, is it stated anywhere in the docs? Is there a complete application.ex example to use it?

[plugnburn]

Kinda figured out the issue but had to use Plug+Bandit:

#...

defmodule Exai.SHRouter do
  use Plug.Router

  alias Anubis.Server.Transport.StreamableHTTP
  alias Anubis.Server.Transport.SSE

  plug(Plug.Logger)
  plug(:match)
  plug(:dispatch)

  forward("/mcp", to: StreamableHTTP.Plug, init_opts: [server: Exai.RefServer])
  forward("/sse", to: SSE.Plug, init_opts: [server: Exai.RefServer, mode: :sse])
  forward("/messages", to: SSE.Plug, init_opts: [server: Exai.RefServer, mode: :post])

  match _ do
    send_resp(conn, 404, "not found")
  end
end

defmodule Exai.RefServer.App do
  use Application

  def main(args \\ []) do
    configpath =
      case args do
        [fname] -> fname
        _ -> Path.join(File.cwd!(), "config.yaml")
      end

    {:ok, config} = YamlElixir.read_from_file(configpath)

    tr_obj =
      case config["mcp_server"] do
        %{"transport" => "stdio"} ->
          {Exai.RefServer, transport: :stdio}

        %{"transport" => "sse", "port" => port} ->
          {Exai.RefServer,
           transport: :sse, sse: [port: port, sse_path: "/sse", post_path: "/messages"]}

        %{"transport" => "http", "port" => port} ->
          {Exai.RefServer, transport: :streamable_http, streamable_http: [port: port]}

        _ ->
          nil
      end

    # start everything
    IO.puts("Starting the EXAI toolsets and the reference MCP server...")

    children =
      [
        {Exai.Ref, config},
        Anubis.Server.Registry,
        tr_obj
      ] ++
        case config["mcp_server"] do
          %{"transport" => "stdio"} -> []
          %{"port" => port} -> [{Bandit, plug: Exai.SHRouter, port: port}]
          _ -> nil
        end

    Supervisor.start_link(
      children,
      strategy: :one_for_one,
      name: Exai.RefServer.Supervisor
    )
    |> IO.inspect()
  end

  def start(_type, args) do
    main(args)
  end
end

Miraculously, stdio transport started working by itself at this point. Didn't test it explicitly but it started responding back. I did have to use mix run --no-halt but that option hadn't been working before and started now.

It didn't respect the port settings for the transports themselves though. Is there a way to just omit them and only supply them to Bandit?