Hi, I'm a physics PhD student and the developer of [Backrank.io](https://backrank.io/), a chess website with over 1,000 users that turns the mistakes in your own games into spaced-repetition puzzles. It also has a free game review ([https://backrank.io/free-chess-game-review](https://backrank.io/free-chess-game-review)) that grades any game move by move.

I wanted the game review to have coach explanations like [Chess.com](http://Chess.com), where every move gets a description like "You missed a fork" or "This loses your bishop to a skewer." A common way to build that is using an LLM, but they hallucinate frequently, describing pieces that aren't there and praising moves that aren't even legal. A chess coach that's confidently wrong is not very helpful.

So instead I built the coach as a massive motif engine. Chess is full of recurring patterns (forks, pins, skewers, back-rank mates, hanging pieces), and the engine has a detector for each one, over a hundred in total. Each detector is a series of hard board checks, validated by thousands of tests from real user games. A "hanging" piece for example must survive a full legal capture sequence, played out in take-back order, with pins and x-rays accounted for, so a piece defended by a pinned knight correctly counts as lost if captured. When several detectors fire on the same move, the most specific one wins.

The coach analysis is free, no account needed, and works with Chess.com and Lichess.org games. Any feedback would be really appreciated, happy to answer questions!

We built a deterministic chess coach that explains every move in your games. Here's how it works