Every filter here is computed automatically by replaying each puzzle's real solution move-by-move with a chess engine (python-chess) and inspecting the resulting position — it isn't pulled from Lichess's crowd-sourced puzzle themes. A few tags (noted below) are deliberately approximate heuristics rather than a full tactical analysis.
Puzzle Basics
ELO Bracket — the Lichess-rated difficulty of the puzzle, grouped into 100-point brackets. This is the one value here that comes directly from the source dataset rather than being computed by us.
Sequence Length — how many moves you need to find to solve the puzzle; the opponent's replies in between don't count.
Tactics
Tactical Piece Used — select any number of piece types; matches puzzles where any of them make at least one of the moves in the solution.
Fork — the piece that just moved attacks two or more enemy pieces at once, at least one of which is undefended or worth more than the attacking piece.
Pin — a piece is pinned in front of a piece of equal or greater value on the same line (not the king) — moving it would expose that piece behind to capture.
Absolute Pin — a piece is pinned directly in front of its own king, so moving it would be illegal or expose the king to check.
Skewer — like a pin, but the more valuable piece is in front — attacking it forces it to move, exposing a lesser piece behind it.
Discovered Check — moving one piece out of the way reveals a check from a different, already-stationary piece.
Double Check — a single move checks the king from two pieces at once — it can only be answered by moving the king.
Sacrifice — the solution gives up material at some point but still ends up materially even or ahead (or delivers mate). Flagged from the capture sequence alone, so it can occasionally over- or under-flag compared to a full engine evaluation.
Checkmate — the solution ends in checkmate.
Position
Game Phase — classified automatically from material and move number: Endgame if either side has no queen or total material is very low; Opening if it's within the first 10 full moves and most material is still on the board; otherwise Middlegame. It's a standard proxy, not a judgment of piece activity or king safety.
Pawn Structure — Isolated Pawn: no friendly pawn on an adjacent file. Doubled Pawn: two or more of the same color's pawns share a file. Passed Pawn: no enemy pawn can stop it from promoting, on its own file or the two adjacent files.
King Safety — Exposed King: the king has fewer than two of its own pawns on the row directly in front of it, a rough proxy for an open, under-protected king.
Material Imbalance — one side has more pawns (Pawn Imbalance) or more of exactly one piece type (Piece Imbalance) than the other, right before the puzzle starts. Only flagged when a single piece type accounts for the entire imbalance.
Board Material Value — total material on the board, split into four bands so each contains roughly the same number of puzzles across the whole dataset — a data-driven quartile split, not fixed point thresholds.
Special Moves
All rare in the dataset — combining several with a narrow ELO bracket may return no results.
Castling, En Passant, Promotion, and Underpromotion (promoting to a knight, bishop, or rook instead of a queen) each mean the solution includes that specific move.