When the answer is the point
Sometimes the right tool is the one that just tells you. Two of you finished the same grid over the weekend and disagree about one square. An early digit went in wrong, thirty entries got stacked on top of it, and seeing the wreck resolved matters more than untangling it by hand. Or the answer page for the puzzle in front of you ships in next month's issue, and you have no intention of waiting for it. This page is for those moments: put in whatever digits you have and let the grid finish itself.
It has a gentler sibling. The helper looks at the same eighty-one squares and names the move that's available, keeping the digits to itself so the solve stays yours. The two pages are the two honest ways of getting unstuck, and it pays to know which one you're reaching for — because this one closes the puzzle for good. It will not teach you anything, and it isn't trying to. It's the back of the book.
What the machine does instead of thinking
The engine behind the solve button carries no repertoire. It has never heard of an X-wing, holds no opinion about where a person should look first, and would fail a quiz on every named technique this site teaches. What it does instead is try things, at a pace nobody can match: find the square with the fewest digits still legal, commit to one of them, and push on until the grid either fills or breaks. At a break it retreats to the last commitment, crosses that digit off, and takes the next — thousands of small wrong turns, each retracted faster than the screen could have drawn it.
A person solves by refusing to guess. The machine guesses so fast that being wrong costs it nothing.
That difference is why the answer is instant on any legal grid, including positions far past what pattern logic reaches. It is also why the answer arrives mute. Exhaustive search produces digits without producing reasons — every one of them is correct, and none of them comes with a story about why. If the why is what you're missing, the ladder next door does nothing else.
The count button is a fairness meter
What a number bigger than one is telling you
A proper sudoku has exactly one completion. That is the entire contract between setter and solver — the thing that makes "the" answer grammatical — so the third button refuses to assume it and counts instead. One means the puzzle is sound and your copy of it is faithful. Zero means the position cannot finish at all, and the likeliest culprit is a digit that didn't survive the journey from the printed page to this one. A count of five, or ninety, means the grid is genuinely under-determined — a misprint, or a generator that never checked its work — and every solver faced with it, human or machine, is silently picking between endings that all fit.
The biggest possible count
Left completely blank, a nine-by-nine grid admits 6,670,903,752,021,072,936,960 completed sudokus — settled by Bertram Felgenhauer and Frazer Jarvis in 2005. Our counter retires at a thousand, which loses nothing: past the first few, every extra completion repeats the same verdict.
Testing a puzzle you made yourself
Composing a sudoku by hand is an underrated pleasure with a silent failure mode: remove one clue too many and the grid doesn't look broken, it just quietly acquires second endings. This page is the test bench for that. Build your position, press count, and hear "exactly one" before anyone spends an evening on it — if the number comes back higher, restore a clue and ask again. Once it holds, the copy button under the board turns your creation into a link, carrying the same single-solution promise that every grid on the main board is generated under.
One square, and the ink that keeps score
Between "no help" and "all of it" sits the middle button. Select the square being argued over — or the one an entire corner hinges on — and the solver fills that square alone, leaving the rest of the blanks yours. Conceding a battle doesn't have to mean conceding the war.
Whatever the machine writes, it writes in indigo; everything you entered keeps the board's dark ink, so the grid itself remembers whose digits are whose. The take-back button unwinds the machine's contributions step by step, which makes accepting one square of help reversible rather than final. And the position — yours, half-finished, or fully answered — travels as a single line of eighty-one characters through the paste box and the copy buttons, the same plain format the rest of the sudoku world already speaks.