How to make an AI game mashup with Claude Code or Codex

This is the path documented by universalmodder.dev on 8 October 2026, tied to upstream snapshot 6c02e77, plus the demonstration described by MindStudio. It is not a promise that two random games will merge. Check the repository again before you copy a command. Upstream can move.

1. Start from games you own

Universal Modder needs a local install it can inspect. Buy or already own both games. Copy the saves you care about to a folder outside the game, and use a spare install if you have one. MindStudio says the Doom and Skyrim demonstration used a legitimate extra copy of Skyrim.

A Windows game is driven natively or from WSL. A Mac or Linux coding session does not, by itself, make that game’s mod loader work. For Minecraft Java, the upstream guide says to add a JDK and a matching loader workspace.

2. Install the agent and the toolkit

Install Claude Code and confirm a normal session runs. You also need Git and Python 3.10 or newer. The guide recommends uv for Python tools, ffmpeg for the media workflow, and Blender only when you are rendering 3D models into sprites.

Inside a Claude Code session, not in your system shell, the published commands are:

/plugin marketplace add rehan-remade/universal-modder
/plugin install universal-modder@universal-modder

Those commands install the toolkit. They do not install a game or a finished mod. If you would rather work from a clone:

git clone https://github.com/rehan-remade/universal-modder
cd universal-modder

The um CLI comes with the plugin or the clone. If it is missing from your PATH, the guide’s fallback is uv tool install git+https://github.com/rehan-remade/universal-modder and then um --help.

3. Codex, and optional art

The step-by-step page we checked is written for Claude Code. MindStudio describes the same skill as something you can point Claude Code, Codex, or Gemini at: open the repo with the agent and say which two games you own. If you use Codex, still read AGENTS.md in the clone before you enable integrations. We are not republishing a separate Codex install path because the public guide we verified does not include one.

You can start with placeholder art. Generated images, 3D, audio, and video on the upstream path use a Fal API key in the environment that starts the agent (FAL_KEY). MindStudio says the Doom/Skyrim demonstration did not need that step. Never put a real key in a mod, a public repo, or a screenshot. Local images can also go through an existing ComfyUI server via um comfy, which the guide describes as an image alternative, not a replacement for every Fal feature.

4. Inspect, then change one small thing

Start a new Claude Code session after installation. The guide’s first prompt asks the agent to find the mod-any-game skill, inspect your install, and report the engine, exact version, recommended route, and files it would change, without modifying anything yet. Compare that answer with the game you actually have installed.

Then pick one weapon, item, or data change. Have the agent explain the plan, keep the save backup, implement a minimal version, and test it in the running game. Only after that is it worth asking for a mashup. MindStudio’s Doom and Skyrim example took roughly an hour of agent time and still left dialogue broken because every actor was set aggressive.

5. Keep the result on your machine

Do not upload the games’ files, and do not ship a zip that contains them. MindStudio points at two cautionary cases: a browser port of GTA 5 that Rockstar took down, and a Fallout fan project that drew a cease and desist after it was distributed. Building something locally and publishing it are different risks. This page is not legal advice.

If you want a poster of the idea before you spend an hour of agent time, use the poster generator. It does not create a mod.