Sudoku that explains itself
Every puzzle is proven to have exactly one solution before you see it, and its difficulty is measured from the techniques it actually needs. Hints teach the next deduction rather than filling in the answer.
How these puzzles are made, and why it takes a moment
A full grid is generated from a seed, then clues are removed one at a time in a random order. After every single removal the remaining grid is re-solved from scratch — from the givens alone, exactly as you receive them — and the removal is undone unless precisely one solution remains.
That last part is the whole thing. A solver finding an answer only proves at least one exists. Proving there is exactly one means continuing the search after the first and finding no second. A puzzle with two answers looks completely normal until you fill the grid correctly and are told you are wrong.
Difficulty is measured, not counted. The common shortcut is to label a puzzle by how many clues it has, which is very nearly meaningless: a 26-clue puzzle solvable by naked singles is easier than a 32-clue one needing an X-wing. Instead a second solver works the way a person does — naked singles, hidden singles, pointing pairs, box/line reduction, naked and hidden pairs, triples, X-wings — and the label comes from the hardest technique it genuinely needed. We have generated two puzzles with 31 clues each, one beginner and one easy, which is exactly why the count cannot be the label.
A puzzle that cannot be finished by those techniques is discarded rather than shipped as "expert". Nothing here needs trial and error.
Every hint is a reason, not an answer
Press Hint and you are told where to look. Press again and you are told the deduction — which technique applies, and why it works on those particular cells. Only the third press gives the digit. Most of the time the first two are enough, and the point of the puzzle survives.
If your grid has drifted from the solution, the hint says so instead of explaining a clever deduction about a position that can no longer be reached. That check runs first, because a hint given on a broken board is worse than no hint at all.
What the difficulty labels mean
Beginner — every step is a cell with only one digit left. Nothing else is needed.
Easy — needs hidden singles: a digit with only one remaining home in a row, column or box. Harder to see, because you are scanning for absence rather than presence.
Medium — needs pointing pairs or box/line reduction: deductions that eliminate candidates rather than place digits. The first real change in how you have to think.
Hard — needs naked or hidden pairs, which means reasoning about cells you cannot fill yet.
Expert — needs naked triples or an X-wing: patterns spanning several units at once, where the deduction has no local explanation.
Playing without a mouse
Arrow keys move, digits enter, Backspace clears. Holding Shift with a digit toggles a pencil mark instead of an entry. The grid is a proper ARIA grid, so a screen reader announces each cell's position, its digit, and whether it is a given you cannot change.
Nothing depends on colour alone. A wrong entry is underlined as well as recoloured, and the 3×3 boxes are drawn with heavier rules rather than shading — so the board works in greyscale, in high-contrast mode, and printed.
Related
The Sudoku generator uses the same engine to make printable sheets with answer keys — for a class, a puzzle book, or a long flight.
Questions people actually ask
Can a puzzle here have more than one answer?
No, and that is enforced rather than assumed. After every clue removed during generation, the remaining grid is re-solved from the givens alone and the removal is undone unless exactly one solution remains. A solver merely finding an answer proves only that at least one exists — proving uniqueness means continuing the search and finding no second solution.
Why does the same difficulty sometimes have more clues than a harder one?
Because clue count is not difficulty. We have generated two puzzles with 31 clues each, one beginner and one easy — identical counts, different labels. What decides the band is the hardest technique the puzzle actually requires, measured by a solver that works the way a person does.
Do I ever have to guess?
Never. A puzzle that cannot be finished with the modelled techniques — singles, pointing pairs, box/line reduction, naked and hidden pairs, triples and X-wings — is discarded during generation rather than shipped with an 'expert' label. If you find yourself guessing, the hint button will usually show a step you have not spotted.
Are hints just the answer?
Only if you ask three times. The first press tells you which cells to look at, the second explains the deduction and why it works, and only the third gives the digit. If your board has drifted from the solution, it says that instead, because explaining a deduction about an unreachable position helps nobody.
Is my progress saved?
In your browser's local storage, on your device. It is not sent anywhere and there is no account. Clearing your browser data clears it, and opening the page on another device starts fresh.
Why does expert take longer to generate?
Because expert puzzles are rare. Clues are removed at random and the difficulty is whatever emerges, so the generator carves, measures, and discards anything that misses the band. Expert turns up in roughly one carve in a hundred, and asymmetric carving is used for it because a symmetric grid leaves too many clues to reach that level at all.