A result asking the client for input before a request can complete: the InputRequiredResult of the multi round-trip requests pattern (MCP 2026-07-28).

A tools/call, prompts/get or resources/read handler returns one in place of an Anubis.Server.Response. The client answers each input request under its key and retries the original request; the handler then reads the answers with Anubis.Server.Frame.input_response/2 and, if it attached one, its state with Anubis.Server.Frame.request_state/1.

alias Anubis.Server.{Frame, InputRequired, Response}

@impl true
def execute(_params, frame) do
  case Frame.input_response(frame, "confirm") do
    %{"action" => "accept"} ->
      {:reply, Response.text(Response.tool(), "Sent."), frame}

    _not_yet ->
      input =
        InputRequired.new()
        |> InputRequired.elicit("confirm", "Send the message?", %{
          "type" => "object",
          "properties" => %{"ok" => %{"type" => "boolean"}}
        })

      {:reply, input, frame}
  end
end

A client may answer only some requests, or none, and may never retry; a handler asks again for what is missing rather than failing.

An input request needs the matching client capability (elicitation, sampling or roots), including its elicitation mode or sampling tool support. An unsupported request is refused with -32021 instead of being sent. Anubis.Server.Frame.client_supports?/2 checks top-level presence; handlers offering alternatives also check the required subfeatures in frame.context.client_capabilities. The result exists only in the stateless era; a handshake-era request that gets one fails with an internal error.

State is signed by Anubis.Server.RequestState, which needs config :anubis_mcp, :request_state_secret.