Futoshiki

Einstein Futoshiki is the hardest level: 8×8 and 9×9 grids with up to 81 cells. Every puzzle at this level is certified solvable through constraint propagation alone — no trial-and-error or backtracking needed.

That certification means every step forward can be deduced from the current state using naked singles, hidden singles, inequality forcing, and chain propagation. If you find yourself guessing, step back and look for a constraint you missed. The solution path exists, and it's purely logical.