Universal Modder: Mod Almost Any PC Game with an AI Agent

Universal Modder is the open-source toolkit behind a lot of the AI game mashups you have seen this month. It doesn’t mod games by itself. It gives an AI coding agent, such as Claude Code, Codex or Gemini CLI, a set of skills, tools and notes for modding almost any PC game you own: finding the game, working out its engine, reading its real code, building the mod, generating art and sound, testing it in the running game and recording a showcase video. This guide explains what Universal Modder does, how to install it, and what a realistic first project looks like.

What Universal Modder actually is

Think of it as a playbook plus a toolbox that your agent reads before it starts work. It was created by Rehan Sheikh and published on GitHub under the MIT licence. It has four parts:

• Skills. Step-by-step instructions the agent follows, written in the shared Agent Skills format so several different agents can use them. The main one, mod-any-game, holds the whole process and twelve engine playbooks (Unity, Unreal, .NET and XNA games like Terraria and Stardew Valley, Godot, Source, Bethesda games, Minecraft, retro decompilations and more).
• A command-line tool called um. Helpers the agent calls for jobs like searching notes, generating assets or packaging a mod.
• An image and audio connection through fal. fal is a paid service for AI-generated images, 3D models, sound and music. The toolkit uses it to make sprites, textures and effects. Without a fal key it can use a local image generator instead.
• A shared knowledge base. Field notes written by agents after they finish a mod: which game versions worked, which route they took, and every trap they hit. The next agent reads these first.

The part most people care about is the mashup-mods skill, the one for putting one game inside another. Its worked example is the Minecraft in GTA 5 passthrough mod, which several other projects on our wall started from.

What you need before you start

• A PC game you own, installed and working. Single-player or offline only.
• An AI agent that can work on your computer. Claude Code is the most common choice in the current wave. It needs a paid Claude plan. Codex, Gemini CLI, Cursor, VS Code with Copilot and OpenCode are supported too.
• Git, Python 3.10 or newer, and ffmpeg. The agent can install these for you if you ask. Blender is needed only if you want 3D models turned into sprites.
• Optionally, a fal API key for generated art and sound. fal bills per generation, so set a spending limit in its dashboard.
• Windows, for most games. Windows games are driven natively or from WSL.

How to install Universal Modder

Installation is one or two commands, and it depends on your agent. In Claude Code you add the project’s plugin marketplace and install the plugin, both from inside Claude Code itself. Codex and Gemini CLI have equivalent one-line installs, and any agent that understands Agent Skills can add just the skills. The exact, current commands are at the top of the project’s README; copy them from there rather than from a third-party site, because plugin names and flags change.

If you just want to look around, you can also clone the repository and start your agent inside the folder. Every supported agent finds the instructions file there automatically.

The ten-step loop your agent follows

Every Universal Modder project runs the same loop. Knowing it helps you understand what the agent is doing and where you come in.

1. Search the knowledge base for anyone who has modded this game before.
2. Recon. Identify the engine and version, whether the code is managed or native, anti-cheat, existing mod loaders and the save folder. The agent writes a plan.
3. Set up a safe lab, starting with a backup of your saves.
4. Read the actual code, using decompilers and debuggers where needed.
5. Build one working slice. One weapon, one mechanic, one block, not the whole idea.
6. Generate assets such as sprites, textures, models and sound.
7. Verify in the real game. The agent launches it, takes screenshots and checks the logs.
8. Record a showcase video using only the game’s own audio.
9. Package the mod for players, with credits and no game files.
10. Write a field note so the next agent starts where this one finished.

Your job is mostly steps 5 to 7: play what it built, say exactly what feels wrong, and paste logs when something breaks. Creators keep repeating the same lesson: the agent writes the code, but only a person can tell whether it is fun.

Your first project: start small

The best first project is not “put all of Minecraft inside Elden Ring.” Pick something with a mature mod loader and one clear feature. Good starting points:

• A Terraria weapon. tModLoader is Terraria’s official modding API, and the toolkit ships a worked example with new weapons and a boss.
• A Skyrim or Fallout 4 tweak through their script extenders, which are some of the best-documented modding tools anywhere.
• A small Minecraft Fabric mod, which makes a good stepping stone toward a mashup later.

Describe the goal in plain language, the way the project’s own examples do: which game, what you want added, and what it should look and feel like. Then let the agent investigate before it builds. Our AI modding prompts guide has starter prompts you can adapt.

Going further: mashups

Once you have one working mod, the mashup skill covers four kinds of “game inside a game”:

• Content ports. Bringing a weapon, vehicle or character from one game you own into another.
• Passthrough mods. Two games running at once and linked live, like SkyCraft. Read how passthrough mods work first.
• Decompilations as libraries. Using a community decompilation, like the Super Mario 64 one behind Mario in Elden Ring, as a code library inside another game.
• Reimplementations. Rebuilding a game’s engine, as the Rust rewrites of Modern Warfare 2 and Skate 3 did.

Whether any of these is realistic depends mostly on one thing: can you run your own code inside the host game? Our mod loaders guide answers that for the games people mash up most.

Rules worth keeping

The project’s own contribution rules are a good default for anything you build: no game files in your repository, no decompiled dumps, nothing that cheats other players, and an honest status for what works. Stay offline, keep your saves backed up, and read how publishers have reacted so far before you publish. A takedown is more likely for projects that recreate or expose a publisher’s game directly than for mods that read files from copies players own.

What it doesn’t do

Universal Modder doesn’t make modding effortless. It shortens the grind of reading unfamiliar code and fixing crashes, but it stands on years of human reverse-engineering, and it can’t judge whether something feels right. Expect many rounds of playtesting, and expect your plan’s usage limits to matter more than you think. Our prompts and workflow guide covers how to keep a long project on track and what it costs.

When you finish something, publish it with credits and add your field note. That is how every mashup on our wall got here.