How Stockfish Game Review Works

Every game you review on ChessDeeper is analysed move by move by Stockfish, one of the strongest chess engines in the world. For each position in the game, the engine works out the best move available and how good the position is — and the gap between that and what was actually played is what every label on the review comes from.

The measurement is centipawn loss: how many hundredths of a pawn a move gave away compared with the engine's best move. Playing the top move costs nothing. A move that concedes half a pawn costs fifty centipawns. The move labels are bands of that number — Excellent and Good for small losses, then Inaccuracy, Mistake and Blunder as the loss grows past about one and a half pawns, then three.

Not every move is judged that way. Moves still inside known opening theory are marked Book and skipped entirely, because grading the first few moves of a Ruy Lopez against an engine tells you nothing useful. A move that was the only legal one is marked Forced. Neither counts towards your accuracy.

The more interesting labels need more than one number, which is why the engine looks at the two best moves in every position rather than just one. If the move you played was the engine's top choice and every alternative was far worse, it is marked Great: you found the only move that held the position. If that top move also gave up significant material and still left you better, it is marked Brilliant.

Miss is the label for an opportunity rather than an error. When your move was an inaccuracy or mistake but the engine's best move would have won material with a clear tactic — a fork, a sacrifice, a winning capture — the review calls it a Miss, because what you lost was not position but a chance you had.

Every move also gets a short explanation. Most are written instantly from what the engine found. The moments that matter most — blunders, misses, brilliant moves — get a fuller explanation written in your own language, so the review spends its effort where you are most likely to read it.

All of this runs in your browser, not on a server. Stockfish is compiled to WebAssembly and started inside the page, so there is no queue and your games never wait behind anyone else's. To make that fast, the review starts several engine workers at once and hands each of them different positions from your game, rather than analysing one position at a time. On a typical multi-core laptop that makes a full review several times quicker.

The one thing to keep in mind when reading the result is that the engine judges positions, not decisions. A move marked as an inaccuracy may have been the sensible practical choice with little time left. Use the labels to find the moments worth looking at, then look at them yourself.