Something weird is happening with LLMs and chess

I remember one of the early "breakthroughs" for LLMs in chess was that if it could actually play legal moves(!) In all of these games are the models always playing legal moves? I don't think the article says. The fact that an LLM can even reliably play legal moves, 20+ moves into a chess game is somewhat remarkable. It needs to have an accurate representation of the board state even though it was only trained on next token prediction.

The author explains what they did: restrict the move options to valid ones when possible (for open models with the ability to enforce grammar during inference) or sample the model for a valid move up to ten times, then pick a random valid move.

I think it only needs to have read sufficient pgns.

I did a very unscientific test and it did seem to just play legal moves. Not only that, if I did an illegal move it would tell me that I couldn't do it.

I think said that I wanted to play with new rules, where a queen could jump over any pawn, and it let me make that rule change -- and we played with this new rule. Unfortunately, I was trying to play in my head and I got mixed up and ended up losing my queen. Then I changed the rule one more time -- if you take the queen you lose -- so I won!