About this game Free Sudoku Online
Development notes
Grading by technique, not by blanks
My first grader used only the hardest technique a puzzle required. Thirty test puzzles came out 19 Easy, 4 Normal and 7 the solver could not finish, with no Hard or Expert: random carving leaves grids that fall to singles and pointing until one more removal jumps to chain techniques. The grade now also sums a weighted cost per step, and each level has a clue target (40, 32, 27, 21) so puzzles do not drift easy; 187 of 200 test puzzles then matched the requested level.
Symmetry, a fallback pass, honest labels
Clues are removed in pairs mirrored through the centre, the shape newspaper puzzles taught players to expect, but symmetric carving stalled near 27 clues and Expert overlapped Hard, so a final pass removes single cells. If six attempts miss the level, the closest puzzle is used and practice puzzles show its real grade, while your time is filed under the level you chose: about one Hard or Expert puzzle in six grades differently.
Undo is a move; mistakes stay
Undo once deleted the move from the record, so the server's replay never saw the wrong digit, and entering anything then undoing gave zero-mistake times. Undo is now a move of its own that restores the cell and notes but not the mistake count. Duels have no Undo at all, since it would empty the three-mistake rule.
Duel pressure with limits
The first sabotage counter treated every accepted move as progress, so toggling one note 30 times sent six attacks; now only a cell's first correct fill and a unit's first completion count. A test match between two bots locked 24 empty cells within 30 moves, which is why at most six cells can be locked at once. The opponent's grid is never shown, because their filled cells would reveal answers.
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