ChatGPT Codex Game Development: Give It Task Cards
Codex is the part of ChatGPT that works on your actual project: it changes your files and runs commands, while a normal chat only sends you text. For game development it works best when you hand it one small, clearly fenced job at a time, on what we call a task card.
OpenAI's pages, as seen on 2 November 2026, list Codex in the ChatGPT Free, Go ($8 a month), Plus ($20), Pro (from $100) and Business plans. The Free entry names the desktop app, subject to rollout, so check what you really get.
On my computer or in the cloud?
A game has to be played after almost every change, and that points to running Codex on your own computer, where the game is one click away. Cloud tasks suit chores you can judge from a list of changes, such as tidying level files. They cannot tell you how a jump feels.
Tell it the house rules once
Codex reads a file called AGENTS.md before doing any work. Put it in the main folder of your project, and restart Codex after you change it, because it reads the file when it starts. For Tide Market, an invented trading game in a fishing harbour, it could say:
# Tide Market
- Turn-based. One turn is one in-game day.
- Rules live in src/rules/. Screens live in src/screens/.
- Prices are whole coins, never below 1.
- game-flow.md holds the day cycle as a Mermaid diagram. Keep it current.
- Never change saved games without asking me.
Mermaid is a way to write a diagram as text, and mermaid.live draws it for you.
Why a card beats a wish
"Add weather" is a wish. Codex will decide what weather means, how often it comes and which files to touch, and its choices will sit somewhere inside a long list of changes. A card says what to do, what to leave alone and how to report back.
Codex has one more strength here. It can read the whole project, so it can draw how a day really works now, not how you remember it, and then show where storms fit. Problems found in that drawing cost you nothing. Problems found on day 40 of a playthrough do.
If the report no longer fits on one screen, the card was too big. Split it. The page on turn-based combat shows how one round breaks into small steps, and the same thinking works for cards.
Paste this to your AI
Task card for Tide Market. Do only this job.
Job: add storms. From day 5 on, each new day has a 1 in 3 chance of a storm. During a storm, fish cost double and boats stay in harbour.
Leave alone: saving and the shop screen. If you think you must touch them, stop and ask.
Step 1, before editing: read the code and update the Mermaid diagram in game-flow.md so it shows the day as it works today, with storms added. Look for trouble on paper first, such as a boat already at sea when a storm starts, or two storms in a row. Tell me what you found and wait for my OK.
Step 2: build from that diagram.
Step 3, report: each file changed and why, the rule with its file and line, what you ran and what came out. If you did not run it, write "not run". If the code does not show how something works, say what you could not find. Do not guess.
Or skip this and connect the MCP: your AI then finds all of this by itself.Connect the MCP
Keep this shape for your own jobs: job, leave alone, diagram, build, report. A card this size takes a minute to write and saves you from reading a hundred changes you never asked for.
How to check it worked
- Play 20 days. No storm before day 5, then roughly one day in three.
- Buy fish in a storm. The price should double, then return the next day.
- Have a boat at sea when a storm starts. The game should do what the diagram says.
- Read the file list. If the shop screen is on it, Codex left the fence. Ask why.