subkey-to-ssh-agent cannot handle primary authentication key
If I have a keypacket where the primary key is the sole authentication-capable key, gen-subkey will say that I already have an authentication-capable key and will discourage me from generating another one. Furthermore, monkeysphere-authentication is perfectly happy to generate authorized_keys entries from such a keypacket. However, if I try to use subkey-to-ssh-agent, it breaks with "no authentication-capable subkeys available" and asks me to run gen-subkey.
(from redmine: created on 2015-06-09)