Fill the grid so that every row, every column and every 3×3 box contains all the numbers from 1 to 9, with no repetitions.

Select an empty cell and enter a number with the keyboard or with the buttons on the right. Numbers flagged as wrong are shown in red and counted as mistakes: the two options below the buttons choose whether to flag only obvious conflicts with the visible numbers (default) or every mistake against the solution. Each flagged mistake adds a time penalty: 10 seconds if it conflicts with numbers already visible, 30 seconds if it only shows up against the solution.

Armed number: click a number button, a 1-9 key, or a cell that already holds a number to "arm" it (it stays attached to the cursor). Cells with that number turn dark green, connected rows/columns/boxes turn light green, and other filled cells turn grey: an at-a-glance view of where it can go. Click an empty cell to place it right away (it stays armed for the next ones); clicking the last placed cell again removes it if the number turned out to be wrong. If you armed the number by clicking a cell that already held it, that cell stays highlighted in yellow: it's the one Erase will clear.

✏️ Pencil: toggle notes to jot down possible candidates in a cell (automatically disarms any armed number).
❌ Erase: clear the selected cell (or the yellow one while a number is armed).
↶ Undo: undo the last move (also Ctrl+Z).
💡 Hint: highlights a cell that can be deduced from the visible numbers (or a mistake, if present) and opens a panel with three choices: Explanation shows the logical reasoning, Fill it in fills the cell, Close dismisses the hint without changing anything. Opening a hint (highlighted cell) costs 10 seconds; asking for the Explanation or choosing Fill it in adds another 20 seconds (only once, even if you use both).

The game is saved automatically: if you close the page you can resume where you left off.