We solve each puzzle before publishing it
The first check counts solutions. Only puzzles with exactly one solution continue.
The second check starts from the given digits and solves the whole puzzle. Every step must use a technique available in the coach.
The complete solve is recorded step by step
At each board state, the solver finds a supported move, applies it, and scans again. The puzzle passes when this process reaches the completed grid.
The saved trace includes placements and candidate removals. It also records the technique and evidence behind each step.
Techniques used for certification
The current certification set includes Last Digit, Naked Single, Hidden Single, Pointing Candidates, Claiming Candidates, Hidden Pair, Hidden Triple, Naked Pair, Naked Triple, Naked Quadruple, X-Wing, Swordfish, Skyscraper, 2-String Kite, XY-Wing, XYZ-Wing, and W-Wing.
Easy and Medium puzzles use singles. Hard adds locked candidates. Expert may require subsets, fish, or wings.
The guarantee applies to generated puzzles
Puzzles generated inside Sudoku Pilot receive both checks. Imported puzzles come from another source, so their full construction has not passed this certification.
The trace proves that at least one logical path reaches the solution. You may find the same moves in a different order.
What this means when you get stuck
You never need trial and error on a generated puzzle. There is a sequence of supported deductions from the starting grid to the solution.
Keep searching, reveal the technique name, or open the exact move. The coach can explain every technique used in the certified path.
Difficulty comes from the required techniques
The difficulty label reflects the hardest technique required by the recorded solve.
Extreme will appear when its puzzle catalog passes the same checks. Every difficulty available today already has a complete trace.