Sliding-tile puzzle solved with an optimal (minimum-move) plan or impossibly fast
bh.arena_slide_superhuman · convicts
What it catches
Kitsune arena SLIDING-TILE PUZZLE gate (arena/slide.go) — the KeyCAPTCHA / 15-puzzle family, reproduced as an owned 8-puzzle (3x3, no clone/asset). A scrambled board is shown; the client slides tiles (clicks a tile adjacent to the blank) until it is in order, reporting its move sequence. TWO SERVER-OBSERVED prongs: (1) OPTIMAL PLAN — the move count equals the BFS-computed MINIMUM solution length (the server solves the scramble optimally via breadth-first search) on a non-trivial scramble (optimal >= slideOptimalFloor, 8 moves). A human wanders and backtracks, effectively never hitting the exact minimum on a deep scramble; a solver (A*/IDA*/BFS) plays the optimal sequence. This is the distinctive PLAN-LENGTH tell. (2) SUPERHUMAN SPEED — the whole solve (SERVER-OBSERVED age) faster than nMoves * slidePerMoveMs (350ms), the time to physically slide the tiles: a bot computes the plan and submits all moves instantly. Either prong -> anomaly:slide_superhuman; the detector relay (/arena/slide/verify) joins it to ks_sid so a PASS is CONVICTED on coherence (the arena thesis: the solve-BEHAVIOUR discriminates, not the puzzle). FP-SAFE BY CONSTRUCTION: hitting the exact optimum on an >=8-move scramble is a hard search a human does not perform by hand, and the speed floor bounds physical slide time (hardware-independent); a human's suboptimal, time-spending solve is silent on both. EXPERIMENTAL (w0.85, calibration: raise slideOptimalFloor if a puzzle expert is seen hitting optimal by hand on the medium level; the RED evader that adds a wasted round-trip to beat the optimal-plan prong still spends the moves + time, and an instant submit still trips the speed prong). Test both directions: an A*/BFS solver playing the minimum move sequence instantly convicts; a human sliding tiles suboptimally over several seconds is silent.
Signals it reads
behavioral.arena_slide_superhuman
How it fires
present
Bypassed by 10
Frontier evaders that reach the detector uncaught (scored only suspicious, defeating every convicting tell) — this check is not one that stops them. The red-team frontier this detection still has to convict.