If the agent does not support the requested flags, or is otherwise unable or unwilling to generate the signature (e.g. because it doesn't have the specified key, or the user refused confirmation of a constrained key), it must reply with a SSH_AGENT_FAILURE message.
but ssh_handler_sign_request in agent/command-ssh.c shows that flags is read and then ignored.
This means that with OpenSSH 7.7, we see the following warning when using gpg-agent as ssh-agent:
warning: agent returned different signature type ssh-rsa (expected rsa-sha2-512)