# Build a playable browser game prototype in one evening with an AI agent

> A step by step pack for going from a one-paragraph idea to a playable HTML5 game in a single file, then posting it on itch.io for friends to try.

- URL: https://paperchasewebb.ai/bots/marketplace/items/build-a-browser-game-prototype-in-one-evening/
- Type: Agent use case pack
- Category: Games
- By: PaperChaseWebb
- Works with: Claude Code, Codex, Cursor (named in the listing); Plain .md
- Price: Free USD
- Difficulty: Beginner
- Setup time: 10 min
- Time to result: 2 to 3 hours to a playable prototype
- Format: Markdown (.md) file, build-a-browser-game-prototype-in-one-evening.md
- Version: 1.0, updated 2026-10-15
- License: Personal and commercial use. The games you make are yours.

## What you end up with

By the end of the evening you have a game in a browser tab that someone else can play, plus three notes on what to change next.

## What you get

- One .md playbook with an evening timeline and two worked spec examples
- A one-paragraph game spec template: core loop, controls, win and lose
- Six stage prompts from first build to polish
- Playtest note card for the five-minute test
- Checklist for licensing of generated art, sound and fonts
- Guide to uploading an HTML5 zip to itch.io

## Who it is for

Players who have always wanted to make a game, game jam entrants and developers who want to test a mechanic before committing to an engine.

## Tools needed

- Claude Code, Cursor, Codex or a chat model that writes code: writes and edits the game
- A web browser: runs the game, no build step
- itch.io account (optional): free hosting for HTML5 games
- You need: A game idea you can describe in one paragraph

Cost to run: Free with most AI coding tools' included usage.

## How it works

1. **Write the spec.** Fill in the template: what the player does every few seconds, the controls, how you win, how you lose. One paragraph, no more. Expected: A spec short enough to read in 20 seconds.
2. **First build in one file.** Ask the agent for a single index.html with a canvas, inline JavaScript and no libraries. Open it in your browser. Expected: Something moves on screen and responds to input.
3. **Five-minute playtest.** Play for five minutes. Write three notes: one thing that feels good, one that is confusing, one that is missing. Expected: Three short notes, not a feature list.
4. **Change one mechanic at a time.** Feed the agent one note per prompt. Save a copy of the file each time it still works. Expected: A folder of working versions you can go back to.
5. **Add feedback and feel.** Ask for sound effects generated in code, a small screen shake on hits and a score display. Expected: The game feels responsive even with placeholder art.
6. **Share it.** Zip index.html, upload it to itch.io as an HTML game and set it to be played in the browser. Expected: A link you can send to friends tonight.

## Sample prompt: First build (Claude Code, Cursor, Codex or a chat model)

```
Build a browser game from this spec: [paste your one-paragraph spec].
Constraints: one file named index.html, a single canvas, inline JavaScript, no external libraries or assets. Shapes and colors only for art.
Include: a start screen, a game over screen with restart, keyboard controls and touch controls for phones.
Keep the game logic in clearly named functions (update, draw, reset) so I can ask for changes later.
When done, tell me in three lines how to play it and what you left out.
```

One file with named functions makes later changes much less likely to break input handling.

## Free preview of build-a-browser-game-prototype-in-one-evening.md

````markdown
# Build a playable browser game prototype in one evening with an AI agent

PaperChase agent use case pack (playbook), version 1.0. For Claude Code, Cursor, Codex or any chat model that writes code. Time: 2 to 3 hours.

## The plan for tonight

| Time | Stage | You end with |
|---|---|---|
| 0:00 | Write the spec | One paragraph you could read aloud in 20 seconds |
| 0:15 | First build | A single index.html where something moves and responds |
| 0:45 | Five-minute playtest | Three notes: good, confusing, missing |
| 0:55 | Change one mechanic at a time | A folder of working versions |
| 1:45 | Feedback and feel | Sound, hit feedback, a score |
| 2:15 | Share it | An itch.io link you can send tonight |

## Rules that keep the evening on track

1. One file. No libraries, no build tools, no images. Shapes and colors only.
2. Save a copy every time it works: `v1.html`, `v2.html`, and so on.
3. One change per prompt. Never "and also".
4. Stop adding features at 1:45 even if you have ideas. Write them down for tomorrow.

## Contents

1. Spec template, with two filled-in examples
2. Stage prompts 1 to 6
3. Playtest note card
4. When the agent breaks it
5. Licensing checklist for art, sound and fonts
6. Posting on itch.io
````

## Worth knowing

- Agents often break controls when adding features. Save a working copy before every change.
- Do not clone a trademarked game or use its name. Make the idea yours.
- If you add generated art or music later, check the license terms of the tool that made it before you sell anything.

## Questions

**Do I need a game engine?** No. The pack uses plain HTML and JavaScript in one file, which any browser runs. Move to an engine later if the prototype is worth it.

**Why is it free?** It is a newsletter starter for the games section. Enter your email to get the full pack with all six prompts.

## How to get it

Free sample. Download it directly at https://paperchasewebb.ai/bots/marketplace/items/build-a-browser-game-prototype-in-one-evening/, no signup.

Seller: PaperChaseWebb Inc., mgmt@paperchasewebb.com