Spot-the-difference solved with pixel-perfect diff clicks (image-diff) or superhuman scan speed
bh.arena_spotdiff_superhuman · convicts
What it catches
Kitsune arena SPOT-THE-DIFFERENCE gate (arena/spotdiff.go) — a prevalent wild captcha style, reproduced as owned infra (no clone/asset). Two near-identical panels of coloured disks are rendered into one image; K disks are recoloured on the right panel; the client clicks each difference. The K difference centres are kept SERVER-SIDE, so passing requires actually COMPARING the panels. TWO SERVER-OBSERVED tells: (1) PIXEL-PERFECT — a bot solves by PIXEL-DIFFING the two panels, which hands it the EXACT centroid of each changed region, so every difference click lands within spotPixelFloor (3px) of the true centre; a human eyeballs the difference and clicks approximately (tens of px off). All-clicks-exact means an image-diff, not a human eye. (2) SUPERHUMAN SCAN — found all K differences faster than a human can compare the two panels (age < K * spotPerDiffMs, 1200ms/diff): a human scans back and forth for each difference over seconds, a bot diffs instantly. Either -> anomaly:spotdiff_superhuman; the detector relay (/arena/spotdiff/verify) joins it to ks_sid so a PASS is CONVICTED on coherence. FP-SAFE BY CONSTRUCTION: the acceptance radius (24px) admits human clicks while the pixel floor (3px) sits far below human aim, and the scan floor bounds human visual-comparison time (hardware-independent). This gate REUSES the click-precision + solve-speed tell families (applied to a comparison task) rather than introducing a new tell family — its value is the prevalent STYLE, not a new tell. EXPERIMENTAL (w0.85). GROUNDED LIVE BOTH DIRECTIONS: an image-diff solver (pixel-perfect clicks, instant) convicts; a human clicking approximately over several seconds is silent.
Signals it reads
behavioral.arena_spotdiff_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.