Today in "diagnostic messages I've not seen before", comes this beaut

edit: solved -- see follow-up

#debian13

thanks to a question @and1bm asked, I found the problem: it really is an older agent: I forward gpg-agent using SSH, and Debian (where the diagnostic was emitted) has a newer gpg than my workstation has.

For those wanting to know why I do that, I wrote that up a year ago: https://jpmens.net/2025/04/04/forwarding-gnupg-agent-over-ssh/

Jan-Piet Mens :: Forwarding GnuPG agent over SSH

and, fwiw, if the server side has a higher version, that appears to be acceptable: no diagnostic message is printed to the effect.
@jpmens shouldn’t you avoid gnupg 2.5.x anyway because that’s the internet / OpenPGP hostile version of gnupg ?
@letoams should I avoid it? I wasn't aware there's a hostile version ...