The interactive step of the device authorization grant (RFC 8628), grant: :device in Sake.Client.Auth: the client shows the person a user code and where to enter it, on any device, and polls the token endpoint until they decide there.

prompt/1 is the default :device_prompt: it logs the code and the URL, as Sake.Client.Auth.Browser logs the authorization URL when it cannot open a browser. A CLI shows them its own way:

auth: [
  grant: :device,
  device_prompt: fn %{user_code: code, verification_uri: uri} ->
    IO.puts("Open #{uri} and enter #{code}")
  end
]

The function receives user_code, verification_uri, verification_uri_complete (the URL with the code in it, for a link or a QR code; nil when the server gives none) and expires_in, in seconds. What it returns is ignored.