An ordinary sudoku, plus a number outside every row and column end: the total of the first three squares reading in from that edge. Rows, columns and boxes still hold 1 to 9 exactly once.
Frame is a free daily frame sudoku from Mind Loft. Each day gives you a 9×9 grid with a number printed outside every row and column end: the sum of the three squares nearest that edge, reading inward. Thirty-six sums, all printed, every day, and rows, columns and 3×3 boxes still hold 1 through 9 exactly once. The sums do so much of the work that the board prints far fewer digits than an ordinary sudoku needs.
The reasoning is arithmetic on a fixed group of three: a 6 is 1, 2 and 3 in some order, a 24 is 7, 8 and 9, and the middle values are where the deduction lives. Select a square and the two sums that speak about it light up. Every board has a single solution you can always reach by logic, with no guessing, and on every board the gutter is load-bearing rather than decorative: the printed digits alone would fit more than one grid. Wrong entries are never flagged, so spotting your own slips is part of the puzzle, and a clean solve scores a perfect 10 with the daily leaderboard decided on the clock.
A new board drops every day at midnight Eastern; weekdays print fewer digits as the week goes on, from 14 on Monday down to 4 on Saturday, and the Sunday Edition prints two. No app, no signup, play free in your browser. Want the rest of the family? Try Suds, the classic 9×9, Sando, the sandwich sudoku, Rim, the outside sudoku with nothing printed inside, and Diag, the one with the diagonals.