I've found several solutions to avoid agents from building complexity -- the first is to not use Astra or Fable/Mythos unless absolutely required. The second is to use effort level "high", not "xhigh". The last idea would be to edit your AGENTS.md to be short, and start it with something like:

Contributors to this project must follow YAGNI, DRY, KISS, and principle of least astonishment, both from user-visible surfaces, and backend design.

Code is a liability. Do not build anything not expressly asked for.