Universal Modder

What it is

A free, MIT-licensed set of skills, a CLI (um) and a shared knowledge base that let an AI coding agent mod almost any PC game you own. The agent finds the game, works out the engine and the route, backs up your saves, reads the real code, builds the mod, makes art, 3D and sound with fal, and tests it in the running game. Its official example adds a homing missile launcher and a tactical nuke to Terraria (tModLoader).

Setup

1. Pick your agent. For Claude Code, run these inside it:

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

2. Codex:

   codex plugin marketplace add rehan-remade/universal-modderCopy
   codex plugin add universal-modder@universal-modderCopy

3. Gemini CLI:

   gemini extensions install https://github.com/rehan-remade/universal-modderCopy

4. Skills only, for any other agent:

   npx skills add https://github.com/rehan-remade/universal-modderCopy

5. For art, 3D and sound, get a fal API key and export it. fal bills pay-per-use. For images without a key, um comfy uses a local ComfyUI server.

   export FAL_KEY=...Copy

6. You also need Python 3.10+ and ffmpeg (uv is recommended; Blender only for 3D to sprite renders). Windows games run natively or from WSL.

7. Ask in plain words, for example: "What engine is C:\Games\Foo, and has anyone modded it before?" or "Mod Terraria: add a homing missile launcher and a tactical nuke".

Cheat sheet

• Repository and README.
• Terraria example (Fal Arsenal for tModLoader).
• Safety rules: single-player or servers you run only. It refuses to inject into online games with anti-cheat, and it never ships game files or decompiled code.
• Contribution rules: no game files, no decompiled dumps, no cheating other players, honest status.
• Knowledge base: field notes by agents, for agents. Run um kb search "<game>" before you start.
• Back up first: the agent snapshots saves with um backup, and it asks before driving your mouse and keyboard, installing loaders or publishing.
• Costs: the skills are free; fal generations are billed pay-per-use, and your coding agent's own usage applies.

Axo's take

AXO

Haven't run it myself: this page comes from the project's README and safety notes. Try it on a single-player game you own, never an online one. The agent backs up your saves first, but the bold part is still bold.