Idle Engine Kit — a production-tested foundation for browser idle games (with AI-ready docs)

The engine under my shipped idle RPG, extracted and documented so your AI assistant can extend it without breaking it.

I didn't set out to build an engine. I set out to build an idle RPG — and to actually finish it this time.

I'd abandoned game projects before. What changed everything was building with an AI assistant as my pair programmer. But here's what nobody tells you about AI-assisted development: the AI is only as good as the architecture you give it. Ask it to "add a feature" to a messy codebase and it will happily make the mess worse. Give it clear rules and clean boundaries, and it ships production code.

So over months of real development — real players, real save files I couldn't afford to break — I ended up with a set of patterns that just work:

• A game loop that survives background tabs. Browsers throttle timers when you switch tabs. Most idle game tutorials ignore this. This loop doesn't: a fixed-tick accumulator pumped by two sources, so time is never lost and never counted twice.
• A save system with a migration chain. Every schema change is a small migration that preserves player data. I've shipped 13 schema versions to my playtesters. Zero broken saves.
• Closed-form offline progress. No fake tick replay. Just math: capped, tunable, O(1) whether the player was gone an hour or a week.
• A strict systems/UI split over an event bus. Game logic never touches the DOM. The UI never mutates state. This one rule is why my AI assistant can work on combat without breaking the interface.
• Headless tests in plain Node. No test framework, no browser. Seed the RNG, fire synthetic events, assert on state.

The kit is all of that, extracted, cleaned up, and wrapped around a tiny demo game (one resource, three generators) that shows every pattern working together.

The part I'm most proud of

AI-INSTRUCTIONS.md — the file I wish someone had sold me.

It's the architecture rulebook written for your AI assistant. Hand it to Claude, Cursor, or Copilot at the start of a session, and "add a prestige system" produces code that respects the save migrations, the event contract, and the i18n rules — instead of code that fights them. It even includes prompt templates that name the rules they must respect, because that's the trick that makes AI output dramatically better.

Every rule in that file exists because I broke it once and paid for it. The bugs are documented next to the rules that prevent them. That's not something you can generate — it's something you earn one broken save at a time.

What you get

• Complete source: vanilla JS, ES modules, zero framework, zero build step, zero dependencies
• Working demo game you can play in 30 seconds (python3 -m http.server and you're in)
• AI-INSTRUCTIONS.md — architecture rules + working prompt templates for AI-assisted development
• Headless test suite (plain Node, no framework) showing how to test every pattern
• i18n system with English + Italian locales included
• Commented code that explains the why, not just the what

Who it's for

• You want to build an idle/incremental game without spending three weekends on save systems and loop timing
• You develop with an AI assistant and you're tired of it generating code that almost fits
• You want to learn how a real shipped browser game is structured, from someone who broke things so you don't have to

Who it's NOT for

• You want a finished game to reskin — the demo is deliberately minimal
• You need mobile-native, multiplayer, or a framework-based stack (this is proudly vanilla)

License

Build and sell your games freely — no royalties, no attribution required. Just don't resell the kit itself. One license per developer.

FAQ

Do I need to know JavaScript? Yes — intermediate level. The kit makes AI-assisted development much more reliable, but you should be able to read the code you ship.

Does it work with [Claude / Cursor / Copilot]? The AI instructions are tool-agnostic. I built the original game with Claude; the file works with any assistant that accepts context.

Updates? Bug fixes and doc improvements are free for existing buyers. Larger content additions may ship as separate modules.

Refunds?  14 days, no questions.

Built in Rome, Italy, one evening at a time, alongside a full-time job. If you finish a game with this kit, tell me — that's genuinely the best part.

14.99 — instant download, 18 files, ~35 KB of code you'll actually read.