How the daily Sudoku challenge works

A daily puzzle is a small promise: the same grid for everyone, once a day, with something at stake. Most of the design work goes into keeping that promise honest. Here is how ours is put together and why each part is the way it is.

One puzzle, one day, one clock

Everyone gets the same grid, and the day rolls over at 00:00 UTC — a single moment worldwide, not midnight in your own timezone.

Using local midnight sounds friendlier and is worse. It would mean the puzzle you are working on is a different puzzle from the one someone two timezones away is working on, and any shared board becomes meaningless. One clock for everyone is the only version where "today's puzzle" means one thing.

The practical consequence is worth knowing: if you are far east of UTC, the new puzzle lands during your morning rather than at your midnight. Nothing is lost — you have a full 24 hours either way.

The board ranks *when*, not *how fast*

This is the part people find surprising. The completion list is ordered by the moment you finished, not by how long you took.

The reasoning is about what kind of competition is worth having. Ranking by solve time makes the leaderboard a race against a stopwatch, and a stopwatch rewards a very specific, quite narrow skill: pattern recognition at speed. It also makes every form of cheating enormously profitable, because a solved grid copied from anywhere converts directly into a top time.

Ranking by finish time changes the game into "did you show up and do it". Someone who solves in twenty unhurried minutes at 00:10 UTC places above someone who solves in four minutes at noon. That rewards the habit, which is the thing a daily puzzle is actually for.

It also makes one entire category of cheating pointless — which brings us to the solutions.

Why the solution waits a day

Every daily puzzle gets a complete, step-by-step solution published on the site. Not just the finished grid: every cell in the order a solver would fill it, with the name of the technique used and the specific reason that digit had to go there.

Those solutions are published one day late. Today's puzzle shows the grid and nothing else. Yesterday's shows everything.

The delay exists so that while a puzzle is live there is simply nothing to look up. Combined with the ranking rule above, reading a published solution buys you nothing at all: the puzzle it solves closed hours ago, and a fast time was never what the board measured.

One honest caveat: because the day boundary is UTC, someone whose local day has not ended yet can see the solution to a puzzle they might still be working on. We decided to accept that rather than build timezone machinery around it. Nobody is required to look.

The solutions are generated, not written

The walkthrough on each day's page is produced by the same hint engine that powers hints inside the app, run over the puzzle from start to finish. It is not written by hand and not written by a language model.

That has two consequences worth stating plainly. The first is that the explanation cannot disagree with the app — if a hint in the game says a cell must be a 7 because column 4 already has one, the website says exactly the same thing, because it is the same code. The second is that a puzzle only gets published if the engine can solve it by deduction alone. If it ever has to fall back on reading the answer, that puzzle does not ship.

Every technique named in a walkthrough links to the guide that teaches it, so a puzzle you struggled with turns into the specific thing to go and learn.

Where everything lives

• Today's puzzle and yesterday's solution: the daily challenge page.
• Every past day, grouped by month: the archive.
• The techniques the walkthroughs name: the guides, starting from the three rules.

No account is needed to play the puzzle or read the solutions. An account only matters if you want to appear on the completion board, since that needs a name to show.