Building a game with Codex? Get one playable round working before you add content

#What changed

On Oct 14, 2026, Thomas Ricouard (@Dimillian) published a long walkthrough on X of how he is building Dark Veil, a dark-fantasy first-person roguelite with a custom Rust and wgpu renderer, mostly by prompting Codex on a personal $200 Pro plan. A playable web build is live on ChatGPT Sites, and the same Rust code also runs natively on macOS and iOS. He reports about 470 commits and roughly 154,000 lines so far.

#Who is affected

Codex users building games, or any project where look and feel need many rounds of iteration. It is most useful for solo builders who want a workflow they can copy, not a single magic prompt.

#What to do now

• Start with concept images and keep them. He had Codex generate four connected concept screens before writing code, then kept comparing real captures against them to say exactly what was still missing.
• Ask for a small vertical slice first. One arena, three weapons, a few enemies, and the full combat, loot, shop loop. Keep simulation and asset generation separate from platform code; that separation is why the iOS port reused the game instead of rebuilding it.
• Build content from shared recipes. Weapons come from reusable procedural geometry and material helpers, so a new item matches the old ones and one improvement lifts several items at once.
• Add test shortcuts early. Launch straight into a specific fight with chosen gear, record before and after a change, measure frame times, and keep test runs separate from real saves. Many of his day-to-day prompts are as short as "put me just before this boss fight."
• Use simulations as leads, not verdicts. He had Codex simulate 14,000 campaigns with the real shop prices and drop rates to find balance problems, then played to check whether the fixes were actually fun.
• Write lasting rules into AGENTS.md. Examples from his project: call equipped cards a "hand," never a "deck"; don't add UI complexity to paper over bad affix rolls; after each coherent change, run the relevant checks and make a local Git commit.
• A screenshot plus a few sentences is often the best next prompt when something looks wrong.

#What is not confirmed

This is a field report from the author, not independently verified. The commit count, line count, and the 14,000-run simulation are his own figures. We confirmed the playable web build loads, but did not test the macOS or iOS builds. It is a personal project, not a revenue case: the game is not yet released on Steam or the App Store. He also credits his own prior engine knowledge for the custom-engine choice, so results will vary for people without that background.

#Sources

• Original post and article: https://x.com/Dimillian/status/2110374345985994999
• Playable build on ChatGPT Sites: https://dark-veil.dimillian.chatgpt.site
• Codex AGENTS.md guide: https://developers.openai.com/codex/guides/agents-md