Sudoku
Sudoku you play in the browser, on a phone, tablet or computer, with no sign-up. It is one endless ladder: every solve moves you up a level, three wrong digits end the run, and each level is graded by the reasoning it demands rather than by how many clues it leaves.
How to play
Fill the grid so every row, every column and every three-by-three box holds the digits 1 to 9 exactly once.
Tap a square, then a number. NOTES pencils in candidates. Keyboard works too: arrows, 1–9, N for notes, Ctrl+Z to undo.
ERASE clears a square and UNDO steps back. Placing a digit wipes it from the notes of every square it rules out. A wrong digit counts as a mistake, and three wrong digits ends the run.
Sudoku difficulty levels
Every solve moves you up a level. The higher you climb the fewer clues you get, and the harder the reasoning has to work.
GENTLE, from level 1: every move is staring right at you. STEADY, from level 4: scan the rows, the digit only fits one place. TRICKY, from level 7: a digit gets locked into one line of a box.
NASTY, from level 11: pairs, triples and the occasional X-wing. BRUTAL, from level 15: past every technique the grader knows. Take notes.
What's different here
Clue count is not difficulty. Thirty clues can be trivial or vicious. So every puzzle is solved by a program that works the way a person does: the easy techniques first, harder ones only when it gets stuck. The hardest technique it needed is the puzzle’s grade, and that grade is what the ladder climbs.
Puzzles are made fresh in your browser. A full grid is filled, then clues are removed in symmetric pairs, and a clue only comes out if the puzzle still has exactly one solution and still grades inside the band being aimed for.
Roughly a fifth of the hardest possible grids need something past X-wing. Those are not thrown away. They are the BRUTAL band.
Scoring and records
A solve pays 120 points per level, plus a speed bonus that shrinks the longer you take, minus 75 points per mistake. Difficulty comes first: a slow BRUTAL is worth more than a fast GENTLE, and a finished puzzle always pays something.
The menu keeps your best run score, the deepest level you have reached and your fastest solve in each band. They are stored in your browser, not on an account.
Finishing a puzzle lets the ants out. The digits shake loose and march off the board in trails.