Defining "agent" as "thing with agency" seems legitimate to me, what with them being the same word.
I'm not saying it's not a valid definition of the term, I'm pushing back on the idea that it's THE single correct definition of the term.
Bank: financial institution, edge of a river, verb to stash something away
Spring: a season, a metal coil, verb to jump
Match: verb to match things together, noun a thing to start fires, noun a competition between two teams
Bat: flying mammal, stick for hitting things
And so on.