BetterChess

Returns an engine evaluation of a chess position: the strongest candidate moves with their scores, and the position's checkable facts — side to move, whether that side is in check, undefended pieces and the pieces attacking them, and the material balance. Accepts a FEN, a PGN, or a list of SAN moves from the starting position. Also reports what the OPPONENT would play if it were their turn, which is the difference between a position being quiet and merely looking quiet. Pass `considering` with a move you are thinking of playing and it returns what that move actually walks into — a losing move is usually not in the top candidates, so asking about it directly is the only way to find out.

Returns the exact list of squares attacked by the piece standing on a given square in a chess position, together with that piece's type and colour. Returns an empty list when the square is unoccupied. Accepts a FEN, a PGN, or a list of SAN moves from the starting position.

Replays a finished game ply by ply with the engine and returns the moves where the evaluation swung by more than a threshold (default 1.5 pawns). Each entry gives the move number, the side that moved, the move played, the engine's preferred move, the size of the swing in pawns, the evaluation before and after, and the FEN of the resulting position. Accepts a PGN or a list of SAN moves, and can be filtered to one colour. Long games are truncated at 60 plies.

Returns the JavaScript source of the in-page coaching rig for a chess.com bot game, to be run in that page by the browser's javascript tool. Once running, the page exposes window.__coachWait(ms) (resolves with events and board status), window.__coachWatch.status() (moves, fen, turn, myTurn, result) and window.__coach (beginDraw, play, dashed, label, step, say) for drawing on the board. Also returns the rig version and the URL of the chess.com bot surface.