Killer Sudoku: Cellsum

Cellsum is a killer sudoku. Ordinary sudoku rules - the digits 1 to 9 once in every row, once in every column, once in every 3x3 box - except that not a single digit is given at the start.

What replaces the givens is the cages. The grid is tiled by them, each one prints a sum, the digits inside it add up to that sum, and no digit repeats inside a cage. That is the whole rule set, and it is enough to pin all eighty-one cells.

Every board is generated and then proved by our own solver: exactly one solution, reachable by logic alone. If a board would ever make you guess, it does not ship.

• DAILY - the same board for everyone, once a day. Beat the clock, keep your streak, share your result without spoiling it.
• LEVELS - 560 boards in eight chapters, from a gentle opening to cages that need real bookkeeping.
• ENDLESS - a fresh board in three difficulties. Its code sits on the board: send it to a friend and you both play the identical grid.

Made to respect your time:
• Gold, silver or bronze on every board, by how close you came to par.
• Hints come back on their own over time, or right away with a video.
• Colour-blind shapes, high contrast and a choice of palettes.
• Dark mode. Works offline. Tiny download.
• No sign-up and no accounts: progress stays on your device.

Pencil marks, an undo you can lean on, and a mistake check that only speaks when you ask.