Expert Sudoku

Seventeen. In 2012 Gary McGuire, Bastian Tugemann and Gilles Civario completed an exhaustive computational search establishing that no valid sudoku with a unique solution has sixteen clues or fewer. It is a proof, not a conjecture — they checked every possibility. Expert here stops at twenty-four, which is hard without being pathological, and every puzzle is built by removing clues one symmetric pair at a time and re-checking uniqueness after each removal.

Not by scanning. With twenty-four clues the board is too empty for hidden singles to be visible by eye, so the first move is mechanical: fill in every candidate, then look for the smallest structures.

• Fill marks immediately. There is no version of this where doing it in your head is faster.
• Find the unit with the fewest candidates in total, not the fewest empty cells. That is where a pair or triple is most likely to be hiding.
• Look for a digit that appears as a candidate in only two cells of a unit. Even without resolving it, its geometry often eliminates something in a crossing unit.
• Take the eliminations, not the placements. On an expert grid you may clear thirty candidates before you legitimately write a single digit.

It is not, and if you are guessing you are playing it wrong. Every puzzle here has exactly one solution and a chain of certain deductions leading to it. Twenty-four clues is well clear of the region where puzzle-setters start needing genuinely exotic techniques — this is long, not arbitrary. The check button will tell you whether a digit is wrong, but leaning on it turns deduction into trial and error.