Free Sudoku HTML5 Game — Single File, No Ads

SudoZip is a complete Sudoku game in one HTML file — the first build in our HTML5 Games category. The interesting part is under the hood: the puzzle generator runs entirely in your browser and mathematically guarantees that every puzzle has exactly one solution. It fills a full valid grid with randomized backtracking, then digs holes in 180° symmetric pairs (so puzzles look hand-made), re-checking uniqueness after every dig with an MRV-optimized solution counter. Generation takes under 60 ms even on Expert.

The game itself has everything you expect from the good Sudoku apps: four difficulties (Easy → Expert), pencil notes, smart note cleanup when you place a number, a hint button, mistake counter, timer that pauses when the tab is hidden, same-number and row/column/box highlighting, conflict marking, full keyboard controls (arrows, 1-9, N for notes) plus touch, and auto-save to localStorage so you can close the tab mid-puzzle and resume later.

Zero dependencies, zero network calls, zero tracking. Double-click the file and play; drop it on any static host; or embed it on your site with an iframe.

1. Unzip. You get one file: index.html.

2. Put it anywhere — a folder on your website, GitHub Pages, Netlify, a USB stick — or just double-click it and play locally.

3. To embed it on an existing page: <iframe src="/sudozip/index.html" style="width:100%;height:820px;border:0"></iframe>

There is no step 4. Player progress auto-saves in the browser's localStorage.

Paste this prompt into Claude to build this exact product:

Build a complete, polished Sudoku game called SudoZip as a SINGLE self-contained HTML file (inline CSS and JS, no dependencies, no network calls). Dark theme: near-black background, amber accent, rounded cards, system font stack. Must feel great on both desktop and phone.

The generator (this is the important part): produce puzzles with a mathematically guaranteed unique solution. Fill a complete valid grid with randomized backtracking; then dig cells in 180°-symmetric pairs, and after each dig run a solution counter that stops at 2 — if the puzzle is no longer unique, restore the pair. The counter must pick the most-constrained empty cell first (MRV) so counting stays fast. If the symmetric pass can't reach the clue target, do a second asymmetric single-cell pass. Difficulty = clue targets: Easy 40, Medium 32, Hard 27, Expert 23. Generation must stay under ~100 ms.

Gameplay: 9×9 grid with proper 3×3 box borders; tap/click to select; highlight the selected cell, its row/column/box peers, and all cells holding the same number; mark conflicts in red. Number pad 1–9 (buttons grey out when a digit is fully placed 9 times), Erase, Hint (fills one correct cell, prefers the selected one), and a pencil-Notes mode that toggles small candidate digits in a 3×3 layout inside the cell. Placing a correct number auto-removes that candidate from peer cells' notes. Count mistakes (wrong entries). Timer in m:ss that pauses while the tab is hidden. Win detection with an overlay showing difficulty, time and mistakes, and a "Play another" button.

Controls: full keyboard support — arrow keys move the selection, 1–9 enter, Backspace/Delete/0 erase, N toggles notes — plus complete touch support.

Persistence: auto-save the entire game state (puzzle, entries, notes, elapsed time, mistakes, difficulty) to localStorage every few seconds and on every move; on load, resume an unfinished game or start a fresh Medium puzzle.

Deliver the single HTML file. MIT license header in a comment.

What happened when we ran it: Claude produced the game in one file — 446 lines total. We unit-tested the engine headlessly in Node (extracted straight from the HTML): across all four difficulties every generated puzzle verified as having exactly one solution, every solution grid passed row/column/box validation, and worst-case generation time was 56 ms. One revision: the symmetric digging couldn't reach the Expert clue target (it stalled around 28 clues), so we added the asymmetric second pass described above — after which Expert averages ~24 clues.

Note: there is no demo restriction here at all — the demo IS the full game, and the download is the same single file. The prompt is for reproducing (or remixing) the build yourself with your own Claude subscription.