How to find the move where you left your opening preparation
After a game that went wrong early, the engine will happily tell you about a blunder on move twenty-three. The more useful question is an earlier one: what was the first move where you left what you know?
Why the first deviation matters
Everything after that move was played without preparation, so the mistakes that follow are symptoms. Fix the first deviation and the next game goes one move deeper into positions you understand. Repeat that for a season and your opening is a different thing.
Finding it by hand
- Open the finished game in an analysis board with an opening explorer. Both Chess.com and Lichess have one.
- Step through the moves next to your repertoire: a study, a file or your notes. This is the hard part, because you need a written repertoire to compare against.
- Stop at the first move where the game and the repertoire disagree, and note who left: you or your opponent.
Three different cases
- You forgot. The position was in your repertoire and you played something else. This is a memory problem: drill that position until the move comes by itself.
- Your opponent surprised you. They played a move your repertoire does not cover, and from there you were on your own. This is a gap: choose a reply and add it.
- You deviated on purpose. You tried something new. Decide now, with a calm head, whether it replaces your old move, stays as an alternative or goes.
Was the move actually bad?
Compare the engine's opinion of the move you played with its opinion of your usual move, not with zero. If the difference is small, the move is playable and the real question is which position you prefer to play. If it is large, cut it. In openings a few hundredths of a pawn do not matter; whether you understand the position does.
Doing it automatically
ChessBonsai compares every game you play on Chess.com or Lichess with your own opening tree and marks the exact move where the game left your line. You then choose: cut the move, or accept it as an alternative. The engine's opinion comes from the Lichess analysis database and from Stockfish running in your browser.