In T4427#123774, @werner wrote:

Can you please run

gpg --debug ipc -vK

which will also start gpg-agent and print some diagnostics. You may want to redact the output. You can also run

You're very welcome. In my instance, this is "resolved" - I now get the prompt I realised I needed so to me this bug could be considered closed or wontfix, but I'll leave you to do with it as you please.

OK - so if an entry is not required in sshcontrol for a smart-card key - is the private key stub sufficiently detailed enough for the agent to realise that it can ask for that card to be inserted for an ssh connection?

Also - going back to sshcontrol - with an ssh key added to the agent with ssh-add, an entry in sshcontrol is required - but not for a key on a smartcard. Is that intentional, or just a byproduct of the smartcard diversion that happens?

Oh, wow - yes, adding to sshcontrol brings up the prompt - I do however need to stop the agent from being restarted on insertion for it to subsequently ask for the unlock.

Yes, it's running. I have a scheduled task that spawns a vbscript to ensure that gpg-agent is started on login, and restarts it on insertion of a card (specifically for two reasons: windows ssh clients don't typically start agents automatically, and windows can cause gpg-agent to get a but upset after a card is removed and re-inserted. Edit: although, I think that latter reason might be resolved now... I haven't investigated deeply. more info here and here).

When no card is inserted, usage of an ssh client simply fails to request insertion of the card for the stub keys present in ~/.gnupg/.

..... And now after looking into this a bit deeper after attempting to build gpg-agent for windows, it appears that this is a bit deeper than the logic above (which is actually sound, when I read it for the second time)