Behaviour for contributing tools, resource repositories, and prompt repositories to a GenMCP.Suite based on the request.

A plain Suite serves the providers you list in its :tools, :resources, and :prompts options. An extension serves the same kinds of providers, but it computes them per request from the GenMCP.Mux.Channel.t/0 it is given. This is the place to vary what the server exposes by request context, most often the caller's authorization, since the channel carries the read-only client meta and the auth assigns merged into it.

An extension answers three questions, one per callback: which tools, which resource repositories, and which prompt repositories to add. Each callback returns provider specs (bare modules or {module, arg} tuples), not built results. The Suite invokes the extension while gathering capabilities and while listing or dispatching, then expands and runs the providers it returns.

Minimal implementation

An extension that exposes an add tool to everyone and an extra admin tool only when the request was authenticated as an administrator. The tools are ordinary GenMCP.Suite.Tool modules the application defines elsewhere; the extension only decides which ones to surface, reading the auth assign that the router merged into the channel meta (see GenMCP.Mux.Channel.from_request/3):

defmodule MyApp.AdminExtension do
  @behaviour GenMCP.Suite.Extension

  @impl true
  def tools(channel, _arg) do
    case channel.meta do
      %{current_user: %{role: :admin}} -> [MyApp.AddTool, MyApp.AdminTool]
      _ -> [MyApp.AddTool]
    end
  end

  @impl true
  def resources(_channel, _arg), do: []

  @impl true
  def prompts(_channel, _arg), do: []
end

Wiring an extension into the server

An extension is given to the Suite through its :extensions option. Because the Suite is the default :server, those options are passed straight to the transport plug in your router. Each entry is a bare module or a {module, arg} tuple, where arg is handed back to every callback as the trailing argument:

# In your router
forward "/mcp", GenMCP.Transport.StreamableHTTP,
  server_name: "My App",
  server_version: "1.0.0",
  extensions: [{MyApp.AdminExtension, []}]

Ordering

The providers configured directly on the Suite (:tools, :resources, :prompts) always come first, ahead of any extension. Extensions then contribute in the order they appear in the :extensions list. This ordering is what tools/list and the resource and prompt listings reflect, and it decides precedence when two providers define the same tool: the first one wins.

Provider arguments

The two arguments every callback receives follow the conventions shared by all Suite providers, documented in GenMCP.Suite:

• channel is the request-scoped GenMCP.Mux.Channel.t/0, carrying read-only client meta and auth assigns. Use it to tailor what the extension exposes per request.
• arg is the value configured alongside the module as {module, arg} (a bare module is treated as {module, []}). It lets one generic extension module be configured differently in different Suites.