I switched from an El Capitan machine to a new MacBook Air with Sierra.
With El Capitan, when I ssh into an host from terminal, the SSH passphrase GUI dialog showed up, and then it never asked again the passphrase for the whole login session.
Now with Sierra the dialog is not showing up, I recurred to adding the following lines to
eval $(ssh-agent) ssh-add
but the drawback is that every new terminal asks for the passphrase.
How to diagnose the missing dialog?