Universal Modder: Let an AI Coding Agent Mod Almost Any PC Game

Universal Modder is a free, open-source toolkit that turns an AI coding agent into a game modder. You point the agent at a PC game you own, describe the mod you want, and it does the rest: works out the engine, reads the game’s code, builds the mod, generates the art and sound, tests it in the running game and even cuts a showcase video.

It was released by Rehan Sheikh of fal under the slogan “every game is moddable now,” and packages everything his agent learned while modding Terraria and Age of Empires II.

What is Universal Modder?

It is a bundle of skills, tools and the fal MCP server that you add to an AI coding agent. It was built around Claude Code, but it also works with Codex, Cursor, Gemini CLI, GitHub Copilot, OpenCode and any agent that reads an AGENTS.md file.

The code is on GitHub at rehan-remade/universal-modder.

How Universal Modder works

1. Find the game: the agent locates the install of a game you own.
2. Engine recon: it identifies the engine and picks a modding route.
3. Read the code: it decompiles and maps the real game logic instead of guessing.
4. Build the mod: new weapons, enemies, mechanics or UI.
5. Make assets: sprites, 3D models and audio are generated with fal.
6. Test in game: it launches the actual game to check the mod works, and goes back to fix bugs.
7. Video and notes: it records a showcase video and writes down what it learned for the next agent.

Which games and engines does it support?

The mod-any-game skill ships with 12 engine playbooks:

• Unity
• Unreal
• .NET / XNA (Terraria, Stardew Valley, Celeste)
• Godot
• Source 1 and Source 2
• Bethesda games
• Minecraft
• Age of Empires II (Genie engine)
• RE Engine, FromSoftware, GTA, Cyberpunk 2077 and Baldur’s Gate 3
• Native C++ games
• Smaller indie engines

The toolkit came out of real sessions modding Terraria, Age of Empires II and a GTA V × Minecraft crossover. One example, the Terraria mod “Fal Arsenal,” adds a homing missile launcher, a tactical nuke, a chain-lightning rifle, a black-hole gun, an orbital strike, three new enemies and a two-phase Drone Mothership boss, all with sprites generated by fal. Another adds a “San Franciscans” civilization with Robotaxis to Age of Empires II.

How to install Universal Modder

In Claude Code, add it as a plugin:

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

To use the um command-line tool on its own, install it with uv (or pipx):

uv tool install git+https://github.com/rehan-remade/universal-modder

You will also need:

• Python 3.10+ and ffmpeg
• Blender, if you want 3D models rendered into sprites
• A fal API key for asset generation: export FAL_KEY=...

Is Universal Modder free?

The toolkit itself is free and open source. Two things cost money: your AI coding agent’s subscription or API usage, and fal, which charges for every image, 3D model or sound it generates. A mod with lots of custom art can add up, so keep an eye on your fal balance.

What Universal Modder won’t do

It is built for offline, single-player games you own. It refuses to:

• inject into online games protected by anti-cheat
• make multiplayer cheats
• bypass anti-cheat, DRM or ownership checks

If you were hoping to use it in competitive online games, it is not that tool. Using mods there can get your account banned anyway.

Should you try it?

If you have a single-player PC game you love and an idea for a mod, Universal Modder is the quickest way to get from idea to a playable build without learning the game’s engine first. Start small, like one new item or enemy, so you can see how the agent works and how much fal generation costs before you ask for a full overhaul.

Links: Universal Modder on GitHub · fal · Claude Code