Caption to show `pwd` instead of shell?

I'm using this caption:

caption string "%{= 9w}%-w%{+b w}%n-%f %t%{-}%+w%{+b w}"

What I'd like to do is change each listed window to show the pwd
instead of shell. I'm using zsh.

I can't see any way of doing this except to update it every time a
command is run.

Is there a way to do this at all?

D.