Notes on this puzzle
26 digits are printed in this grid and the other 55 cells are yours to fill. A solver that never guesses gets through it in 57 steps.
Most cells fall to the hidden single move: 55 of the 57 steps. Pointing pair accounts for 1 more. The toughest moment needs XY-Wing, and that single step is what sets the level.
Among the givens, 4 is the most common digit (4 times) and 1 the rarest (1). The emptiest box is the top-right one, with 1 given; the top-left box is the most crowded, with 5.
Where to beginLook at row 3, column 4 first: a hidden single is available there from the very start. Expect a stage where only a pattern across several lines moves the grid on; full notes make it visible.