A behaviour describing extensions to the GenMCP.Suite server.

Extensions allow modular addition of tools, resources, and prompts to the suite. They are configured via the :extensions option when plugging GenMCP.Transport.StreamableHTTP.

Lifecycle

Extensions are invoked during server initialization to gather the initial list of capabilities.

The order of invocation follows the order in the :extensions list. However, tools, resources, and prompts defined directly on the configuration (via :tools, :resources, :prompts options) are always treated as the first extension, taking precedence in the listing order.

Channel Access

Extension callbacks receive the GenMCP.Mux.Channel from the initialization HTTP request.

This allows extensions to filter or dynamically generate capabilities based on request context (e.g., user authorization).

Future implementations to support listChanged notifications will pass the channel from the current GET HTTP request streaming server notifications.

Example

defmodule MyExtension do
  @behaviour GenMCP.Suite.Extension

  @impl true
  def tools(channel, _arg) do
    if channel.assigns.admin do
      [AdminTool]
    else
      []
    end
  end

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

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

# In Router
plug GenMCP.Transport.StreamableHTTP,
  extensions: [{MyExtension, []}]