An MCP App: an HTML document the host renders in a sandboxed iframe when a tool that links it is called (the io.modelcontextprotocol/ui extension). On the wire it is a resource with a ui:// URI and the text/html;profile=mcp-app MIME type, so it is listed on a server with the other resources:

defmodule MyApp.MCP.SalesMap do
  use Sake.UI,
    uri: "ui://my-app/sales-map",
    name: "sales_map",
    title: "Sales map",
    csp: [resource_domains: ["https://tile.openstreetmap.org"]],
    prefers_border: true

  @impl true
  def render(_ctx), do: ~s(<!doctype html><html>...</html>)
end

defmodule MyApp.MCP do
  use Sake.Server, name: "my-app", version: "1.0.0"
  tools [MyApp.MCP.ShowSales]
  resources [MyApp.MCP.SalesMap]
end

A tool shows it with use Sake.Tool, ui: MyApp.MCP.SalesMap.

Options: uri (required, ui://), name (default: the module's last segment in snake case), title, description, icons, scope, ttl, meta, csp (connect_domains, resource_domains, frame_domains, base_uri_domains: the origins the document may reach), permissions (:camera, :microphone, :geolocation, :clipboard_write), domain (a dedicated sandbox origin, in the host's own format), prefers_border, and template.

The document comes from render/1, which returns a string or iodata, {:safe, iodata}, or, when the host has phoenix_html, anything that implements Phoenix.HTML.Safe (the result of a HEEx function component), or from an EEx file named by template:, relative to the module's own file and compiled into the module. A template's assigns come from assigns/1, and <%= %> escapes its output as HTML unless the value is raw/1. inline/1 reads a file into the document at compile time (a bundled script or stylesheet), and the module recompiles when the file or the template changes.

The host learns the document's CSP and permissions from the resource's _meta.ui, on its resources/list entry and on its resources/read contents alike. A client that did not declare the extension does not see UI resources listed (it can still read them).