Every puzzle in this app is a 27–28 clue grid with exactly one solution — verified in advance, so there's always a logical path through, never a guess required. "Tough" sits between "Hard" and "Diabolical": every puzzle needs at least BUG+1-tier reasoning, and the large majority (83%) call for Y-Wings, W-Wings, or colour-based logic before they crack. About 1 in 10 push further still, into full chains. On average that works out to about 8 non-Singles moves per puzzle — more than Hard's 6, short of Diabolical's 12.
Paste 81 numbers, in almost any format: comma- or space-separated, a 9×9 grid, or one long string. Use 0 or . for empty cells — box-drawing separators (| and ---+---) are fine too.
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