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agents.writerslogic.com

Checks whether a move (or a sequence of moves) is legal from a FEN position under the FIDE Laws of Chess, for agents that play, annotate or verify chess games. Accepts UCI or SAN, returns the normalized move, the new FEN, check / checkmate / stalemate / insufficient-material flags and, if asked, every legal move; an illegal move names the reason (castling_through_check, ep_exposes_king, moves_into_check, blocked, bad_promotion). Does NOT suggest or evaluate moves, detect threefold repetition (no game history), detect dead positions other than insufficient material, or support Chess960 (reported as unsupported).