The prompt is the design doc. Here are concrete, reusable patterns for describing a game to an AI so you get something playable, not a broken demo.
When you build a game with an AI assistant, the prompt is not a magic incantation. It is a design document. The clearer and more structured it is, the closer the first playable version lands to what you actually wanted. Most disappointing results trace back to a vague prompt, not a weak model. These patterns come from doing this loop many times and noticing what reliably works.
Pattern 1: Lead with the constraints
Start by fencing in the technology before you describe anything fun. A line like 'Build a single self-contained HTML file using canvas and vanilla JavaScript, no external libraries, that runs by opening the file in a browser' does an enormous amount of work. It stops the assistant from reaching for a framework you would then have to install, keeps everything in one file you can publish instantly, and gives the model a small, well-understood target.
Constraints are not the boring part of the prompt. They are what make the creative part achievable. A model with clear boundaries produces tighter, more finished output than one given open-ended freedom.
Pattern 2: Describe the core loop as numbered steps
Do not describe your game as a vibe. Describe it as a loop. Write the exact sequence the player repeats: '1. Blocks fall from the top. 2. The player moves a paddle left and right with arrow keys. 3. The ball bounces off the paddle and breaks blocks. 4. Clearing all blocks wins the level. 5. Missing the ball loses a life; three lives and it is game over.'
Numbered steps force you to notice gaps in your own design, and they give the AI an unambiguous spec to implement. If you cannot write the loop as steps, the game is not defined yet, and no model can rescue an undefined game.
Pattern 3: State controls, win, and lose explicitly
Three things get silently dropped when you leave them implied: the exact controls, the win condition, and the lose condition. Spell out all three. Name the keys or the touch gestures. Say precisely what ends the game in triumph and what ends it in failure. 'It should be challenging' is not a lose condition; 'the game ends when the timer reaches zero before you reach the exit' is.
This is also where you prevent the classic broken result where a game runs forever with no way to win or lose, which feels pointless within seconds. Explicit end states are what make a toy into a game.
Pattern 4: Build in layers, one prompt per layer
Do not ask for the finished game in one message. Ask for the smallest version that is still a game, get it working, then add one layer at a time: first movement, then the core mechanic, then scoring, then the fail state, then feel and polish. Each layer is its own prompt against the working file from the last one.
Layering keeps every change testable. When you add ten features at once and something breaks, you have no idea which one did it. When you add one, you always know. This single habit does more for your success rate than any clever wording. The same idea drives the full build walkthrough.
Pattern 5: Report bugs like a QA tester
Iteration prompts are as important as the first one, and the best ones read like a bug report. Give the symptom, the steps to reproduce, and what you expected instead: 'When I press the down arrow while jumping, the player freezes mid-air until I release it. I expected down to have no effect while airborne.' That gives the assistant a precise target.
Compare that to 'jumping is broken,' which forces the model to guess what you mean and often makes things worse. Precision in, precision out. Treat every fix as a small, verifiable transaction.
A reusable prompt skeleton
Here is a skeleton you can adapt for almost any small game. Fill in the brackets and you have covered the essentials: 'Build a single self-contained HTML file with canvas and vanilla JavaScript, no libraries. The game is [one sentence]. Core loop: [numbered steps]. Controls: [keys or gestures]. Win when [condition]. Lose when [condition]. Start with just the core loop working; we will add polish after.'
Keep it in a notes file and reuse it every time. When your game is ready, the publish page takes it live, and the creator guide covers what to check before you do. For more genre ideas to point these prompts at, browse thinky games and puzzle games.