No given digits — only cage sums. Fill the 6x6 grid three times against the clock.
Every row, every column and every 3x2 box must contain 1 to 6 exactly once — the ordinary sudoku rule.
The dashed outlines are cages. The small number in a cage's top-left corner is the sum of the digits inside it, and a digit cannot repeat within a cage. There are no given digits at all: the sums are the whole clue.
Tap a cell, then tap a number on the pad below to write it, or ⌫ to clear it. Anything that breaks a rule turns red straight away — a repeat in a row, column, box or cage, or a cage whose total has already gone past its target.
Three grids in a row, and the clock runs across all of them. Lower time is better.
Start with the arithmetic that has only one arrangement. In a six-digit grid a two-cell cage summing to 3 can only be 1+2, and one summing to 11 can only be 5+6; a three-cell cage summing to 6 must be 1+2+3, and one summing to 15 must be 4+5+6. Find those first and you have pinned down two or three cells before you have done any sudoku reasoning at all.
Then use the total. Every row, column and box holds 1 through 6, so each of them sums to 21. If a group of cages covers a whole row with one cell sticking out, that cell equals 21 minus the cage sums — this trick alone cracks most openings, and it works on boxes just as well as rows.
Remember the no-repeat rule inside a cage, because it does more work than it looks. A three-cell cage summing to 7 cannot be 1+3+3 or 2+2+3; it has to be 1+2+4. Whenever a sum feels ambiguous, list only the combinations with distinct digits and the list usually collapses to one or two.
Write nothing you are not sure of. There are no given digits here, so an early wrong entry has nothing to contradict it and can survive for a dozen moves before the grid falls apart. The red highlight catches a duplicate or an overshot cage immediately, but it cannot catch a digit that is merely in the wrong place — that one only shows up much later, and by then you have to unpick everything.