Difficulty you can trust.

Every board is proved solvable by logic alone before you ever see it, and each level is defined by the hardest technique it actually requires. Never a guess, at any level.

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What the claim is worth

An assertion on a landing page costs nothing to write, so here is the measurement behind this one. Every board the site has ever published was solved again afterwards by a separate checker, which re-derives the hardest technique the board actually needs rather than trusting what the generator intended.

10,950: boards audited, every one published to date
0: that needed a guess at any point
0: that required a technique above their stated level
0: that turned out to have more than one solution

Those three zeroes are the product. The method is written out in full, including what the checker does, what solvable by logic alone is taken to mean, and where the argument could be attacked. It is published so it can be argued with.

Why it matters

Most apps grade puzzles by counting clues, which is a poor proxy: two boards with the same number of clues can sit at opposite ends of the range. Worse, a puzzle labelled expert can turn out to need a guess, and when you stall you cannot tell whether you missed something or the board was broken.

Here the tier is a statement about the logic. When you finish, the app tells you which technique the board genuinely required, so you can see whether you improved or just got an easier draw.

What a technique actually looks like

Difficulty here is named after the hardest technique a board needs, so it is worth seeing one. This is a Naked Pair, the first technique most players learn after singles, on a real board from the archive rather than a grid drawn to make the pattern look tidy.

Sudoku grid showing Naked Pair. r1c2, r1c3 in r1 hold only 2, 7 between them, so those two digits fill those cells and appear nowhere else in r1. — r1c2 and r1c3 hold only 2 and 7 between them. Those two digits fill those two cells in some order, so both come out of every other cell in the row.

A medium board can need that and nothing harder. A hard board can require an X-Wing. The full library covers every technique from singles up to the exotic patterns that only appear in constructed puzzles, grouped into families that share one idea.

What you get

Never a guess

Every board is checked to have exactly one solution reachable by logic alone. If you are stuck, something is always deducible.

The technique, named

Finish a board and it tells you the hardest technique that board needed, from hidden singles up to X-Wing.

Three tiers that mean three things

Easy stays within singles. Medium brings in locked candidates and subsets. Hard can require an X-Wing.

Pencil marks that clean up

Note the candidates in a cell. Placing a digit clears that note from the row, column and box for you. Undo and redo whenever.

An honest timer

The clock stops when you leave and picks up when you come back, so a time reflects time actually spent on the board.

Share the result, not the answer

Finish and you get a link showing your level and time. It cannot leak the solution, because the link never contains the board.

The same three boards, everywhere

Boards are generated from the date itself, so every player in the world gets an identical set, refreshed at midnight UTC. That is what makes one time comparable to another.

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