I built a chess coach that explains moves like a grandmaster instead of showing engine lines — powered by LLM

The problem I wanted to solve: Stockfish tells you *what* the best move is, but never *why*. Players under 1800 don't lose because they can't read centipawns — they lose because they don't understand plans, structures, key squares. **What the tool does:** 1. Imports your games from Chess.com or Lichess 2. Stockfish 17.1 WASM runs in your browser (fully local, nothing uploaded) 3. A pattern detector finds 18 types of recurring mistakes across all your games (missed forks, exposed king, bad bishop, neglected development...) 4. An LLM generates coaching narratives in the style of a 2700+ coach **Instead of:** -89 cp · Best: Nc3 Nf6 Be3 **The AI coach says:** > "Bd3 is premature — the bishop attacks nothing and blocks d3 where the queen may want to go. Nc3 was the right move: it defends d4, prevents Black's ...e5 counterplay, and leaves the bishop free to settle on Be3 or Be2 depending on Black's plan." You can also **chat with the coach** — it knows your full game history, opening stats, specific weaknesses. Ask "why do I keep losing with Black in the French?" and it answers with data from YOUR games. Other features: spaced repetition (SM-2) on your own blunders, puzzle rush with real mistakes, 6-month progress tracking. Free tier: unlimited Stockfish. Pro ($14.99/mo, 15-day free trial): LLM coach + chat. https://chessmentorai.com Happy to discuss the prompting approach — getting the LLM to explain chess like a coach (not an engine) was the hardest part.