gen_mcp

defmodule GenMCP do
  @moduledoc """
  The main behaviour for MCP servers.

  Implement this behaviour to create a custom MCP server. If you are looking for a high-level framework to build tools and resources, see `GenMCP.Suite`.

  ## Example

      defmodule MyServer do
        @behaviour GenMCP

        alias GenMCP.MCP

        @impl true
        def init(_session_id, _opts) do
          {:ok, %{}}
        end

        @impl true
        def handle_request(%MCP.InitializeRequest{} = req, _chan_info, state) do
          # Protocol version check omitted for brevity
          result = MCP.intialize_result(
            capabilities: MCP.capabilities(tools: true),
            server_info: MCP.server_info(name: "My Server", version: "1.0.0")
          )
          {:reply, {:result, result}, state}
        end

        def handle_request(%MCP.ListToolsRequest{}, _chan_info, state) do
          result = MCP.list_tools_result([
            %MCP.Tool{
              name: "hello",
              description: "Say hello",
              inputSchema: %{
                type: "object",
                properties: %{
                  name: %{type: "string"}
                }
              }
            }
          ])
          {:reply, {:result, result}, state}
        end

        @impl true
        def handle_notification(%MCP.InitializedNotification{}, state) do
          {:noreply, state}
        end

        def handle_notification(_notif, state) do
          {:noreply, state}
        end

        @impl true
        def handle_info(_msg, state) do
          {:noreply, state}
        end
      end
  """

  alias GenMCP.MCP
  alias GenMCP.Mux.Channel

  require(Elixir.GenMCP.MCP.ModMap).require_all()

  @doc """
  Returns the supported protocol versions.
  """
  def supported_protocol_versions do
    ["2025-06-18"]
  end

  @type state :: term

  @doc """
  Initializes the server state.

  Called when a new MCP session is established.
  """
  @callback init(session_id :: String.t(), init_arg :: term) :: {:ok, state} | {:stop, term}

  @type request ::
          MCP.InitializeRequest.t()
          | MCP.ListToolsRequest.t()
          | MCP.CallToolRequest.t()
          | MCP.ListResourcesRequest.t()
          | MCP.ReadResourceRequest.t()
          | MCP.ListResourceTemplatesRequest.t()
          | MCP.ListPromptsRequest.t()
          | MCP.GetPromptRequest.t()
          | MCP.PingRequest.t()

  @type result ::
          MCP.InitializeResult.t()
          | MCP.ListToolsResult.t()
          | MCP.CallToolResult.t()
          | MCP.ListResourcesResult.t()
          | MCP.ReadResourceResult.t()
          | MCP.ListResourceTemplatesResult.t()
          | MCP.ListPromptsResult.t()
          | MCP.GetPromptResult.t()

  @type notification ::
          MCP.InitializedNotification.t()
          | MCP.CancelledNotification.t()
          | MCP.RootsListChangedNotification.t()
          | MCP.ProgressNotification.t()

  @type server_reply :: {:result, result} | :stream | {:error, term}

  @doc """
  Handles an incoming MCP request and returns a result or stop the server.
  """
  @callback handle_request(request, Channel.chan_info(), state) ::
              {:reply, server_reply, state} | {:stop, reason :: term, server_reply, state}

  @doc """
  Handles an incoming MCP notification.

  Notifications are one-way messages that do not expect a response.
  """
  @callback handle_notification(notification, state) :: {:noreply, state}

  @doc """
  Handles process messages.

  Invoked when the server process receives a message that is not an MCP request
  or notification.
  """
  @callback handle_info(term, state) :: {:noreply, state}
end