It started, as it so often does, on the toilet. I had five minutes, maybe six. I wanted to do a Sudoku. A simple grid of numbers. A puzzle that has existed in print form since the 1970s and requires, at its most technologically demanding, a pencil. I opened the app I'd been using for the past few months — a perfectly decent Sudoku app, or so I thought — and prepared to enjoy a brief moment of quiet mental engagement.

What I got instead was this:

  • An interstitial ad for a mobile game involving cartoon jewels.
  • A "Daily Challenge Reminder" notification prompt that I have declined no fewer than fourteen times.
  • A popup informing me that I was "on a 7-day streak!" and asking if I'd like to "keep it going" by watching a video.
  • A second ad. Same cartoon jewels. Different colour scheme.
  • The Sudoku grid. Finally. For approximately ninety seconds.
  • A third ad, triggered mid-puzzle, because I'd apparently been playing for "a while."

Let me be absolutely clear about something. I was not playing for a while. I was playing for ninety seconds. I hadn't even finished the top-left box. The app decided that ninety seconds of actual puzzle-doing was quite enough free content for one session, and that what I really needed was to watch a woman enthusiastically describe a skincare product.

The "Keep It Going" Button

I want to dwell on this for a moment, because it has become a personal nemesis. The "Keep It Going" popup appears when you complete a puzzle. You've done the thing. The grid is full. All the numbers are correct. You have, by any measurable definition, finished. You should feel a small, clean sense of satisfaction.

Instead, before you can see your completion time or move on with your life, a modal appears. It has a cheerful animated tick. It tells you that you're doing great. It says — and I am not paraphrasing here — "Keep it going! Watch a short video to unlock your next puzzle for free!"

Unlock. The next puzzle. Which is just a Sudoku. Which is just a grid. Which has been freely available in every newspaper on the planet for fifty years. Unlock.

I don't need to unlock a Sudoku puzzle. I need you to stop treating a two-kilobyte number grid like it's exclusive DLC. The only thing this popup has ever successfully done is interrupt the one single moment of the entire puzzle-doing experience where I felt calm and good. You have taken my small dopamine hit and replaced it with a sales pitch. This is not "keeping it going." This is sabotage.

A note on "free" apps: This app is free to download. The developers are, presumably, human beings who need to eat. I understand that. I do not begrudge them advertising revenue in principle. What I begrudge is the specific, deliberate placement of that advertising at the precise moments designed to cause maximum disruption to the user experience. There is a version of this that isn't awful. They chose not to build it.

The Subscription Offer I Was Not Asked For

On the fourth session — and I know it was the fourth session because the app cheerfully informed me I'd "played 4 times this week!" as if this were an achievement rather than a warning sign — it offered me a subscription. £4.99 a month. Or £29.99 a year. For an ad-free Sudoku experience.

I want you to really sit with that. Thirty pounds a year to play Sudoku without adverts. Sudoku. The puzzle. With numbers. In a grid. Thirty. Pounds.

For thirty pounds a year I could buy approximately three and a half actual physical Sudoku books — which contain hundreds of puzzles, require no battery, have no notification system, cannot display ads, and will never, ever, under any circumstances, tell me to "keep it going."

I briefly considered it. Not because I think it's a fair price, but because I was so thoroughly exhausted by the constant interruption that I was willing to pay to make it stop. And then I thought: hang on. This is exactly what they want. This is the whole mechanism. Make the free experience sufficiently miserable that the paid tier feels like relief rather than a transaction. It's a hostage situation with a nicer font.

So. Here We Are.

I closed the app. I put my phone down. I sat there for a moment, genuinely irritated by a Sudoku application — a sentence I never expected to write — and I thought about what STEN is supposed to be about.

And then I thought: how hard can it actually be to build a Sudoku app? A proper one. One that just shows you a grid, lets you fill in numbers, tells you if you've made an error, and lets you finish the puzzle in peace. No ads. No subscriptions. No streak tracking. No motivational popups. No unlocking. Just a Sudoku.

With AI tooling and a free afternoon, the answer is: not very hard at all. So that's what's happening.

What We're Building

The STEN Sudoku project is a clean, fast, browser-based Sudoku game. Here's the entire feature list, because it's deliberately short:

  • A Sudoku grid. Generates valid puzzles at several difficulty levels.
  • Note mode. Tap a cell, tap numbers to add candidate notes. Because that's a thing people use.
  • Error highlighting. Optional — you can turn it off if you're brave.
  • A timer. Just so you know how long you've been at it. No streak. No leaderboard. Just a clock.
  • Dark mode. Because it's the evening and your eyes are tired.
  • That's it. Genuinely. That's the entire app.

No account required. No sign-in. No cloud sync (your progress saves locally — in your browser, on your device, never on our server). No ads. No subscription. No "keep it going" popup. No streak counter. No daily challenge. No push notifications. No rating prompts. No jewel-matching ads. Nothing except the puzzle.

Current Status

The puzzle generator is working — a proper constraint-satisfaction solver that produces valid, uniquely-solvable Sudoku grids across four difficulty levels. The grid UI is functional. Input, notes mode, and error detection are in place.

Still to do: dark mode polish, difficulty selector, and making it look like something a human would willingly use. Also writing this whole thing up as a proper guide so anyone else who wants to do the same thing can follow along.

When it's ready — really ready, not "ready but surrounded by ads" ready — it'll live here, on this site, free to use. No app store. No install. Just a URL.

"The best consumer technology is technology that gets out of the way and lets you do the thing you came to do."

— Nobody at a mobile ad-tech company, ever.

Updates as they happen. Watch this space. And if you're currently using that Sudoku app and nodding along to any of this — you know where we are.