Admittedly, the mulligan phase system prompt is the weakest part of the project. I had to add heuristics to stop the LLMs from mulliganing down to just a few cards looking for a perfect hand. The scoring for the benchmark is mostly based on if the LLM could complete legal turns, not good turns.
https://github.com/CallumFerguson/mtg-auto-deck/blob/a877c08...