As long as it's all in memory, that's fair game. Everybody at the high level is using engines for training/preparation. This is balanced by the fact that the s

As long as it's all in memory, that's fair game. Everybody at the high level is using engines for training/preparation. This is balanced by the fact that the sheer number of possible lines is huge. Too big for any human to "remember" them all. It's also not enough to memorize "good" moves from an engine perspective. "Good" engine moves and "good" human moves are different things. An engine might see a move as "good", because it calculated 50 moves down the line and found an advantage. This means that you'd have to play the next 49 moves in a very precise manner to reach this advantage. One way to think about it, is that there are multiple positions that are analyzed as "draw" by en engine, but 9 grandmasters out of 10 will see one side winning. That's because one side would need to play perfectly, while the other side has much higher tolerance for mistakes (engine assume perfect play on both sides). TLDR: engine is fair game, during preparation. It's a big deal, it transformed high level chess, but it didn't break the game, and is unlikely to do so.