Rob
Easy — naked singles
Our solver completes the grid using only naked singles: a cell has one candidate left after excluding numbers already in its row, column and box. Full-house placements are included in this category. Easy is a local category, not a promise that every reader will find every placement obvious.
Moderate — hidden singles
Naked singles alone stall. The solver then needs a hidden single: a number has only one possible cell in a row, column or box, even if that cell still has other candidates. The entire puzzle must finish using those two single techniques.
Challenging — candidate elimination
The two single techniques alone stall. Our solver must use locked candidates or a naked pair, followed by more singles, to finish. Locked candidates use the overlap between a box and row/column to eliminate a candidate outside the overlap. A naked pair has exactly the same two candidates in two cells of one unit, so those numbers leave other cells in that unit.
How we validate the label
The fixed solver order is naked single, hidden single, locked candidates, then naked pair. Each deduction is recorded and independently replayed in tests. A puzzle is accepted only if the solver finishes without guessing, the returned category matches the requested one, and a separate solution-counting check finds exactly one answer. Tests also confirm that lower technique sets stall for the higher categories.
What the label does not mean
These are Printable Puzzle Kit categories under rubric qg-techniques-1, not a universal Sudoku rating. Different publishers and people use different orders and names. Clue count is shown only as a property of the sheet; it is not the grading rule. Puzzles that need other techniques or guessing are not offered in these categories.
Keep the level with the seed
Sudoku version qg-2 uses the seed and chosen difficulty to reproduce a grid. Changing print size or paper does not change it. Saved qg-1 PDFs remain usable, but entering an old qg-1 seed into the new version does not promise the old layout. Keep downloaded files when preserving a specific puzzle matters.
