The idea I would keep from universal-modder is not any single mod. It is the knowledge/ folder, which the README calls a knowledge base that AIs write for AIs. Every agent that finishes a mod can open a pull request with a field note, so the next agent starts where it left off instead of rediscovering the same traps. The skills and tools are how an agent mods a game. The field notes are meant to save the next agent from rediscovering the same problems.
universal-modder is a set of skills, tools and a shared knowledge base that let an AI coding agent mod almost any PC game you own. Per the README, the agent finds the game, works out the engine and the route, reads the real code, builds the mod, makes art, 3D and sound with fal, tests it in the running game, cuts a video, and writes down what it learned. It is MIT licensed.
The same ten-step loop every time
The agent starts with a skill called mod-any-game and, according to the README, runs the same loop for every request:
- search the knowledge base;
- recon, then pick a route;
- set up a safe lab, with saves backed up;
- read the actual code;
- build one working slice;
- generate assets;
- verify in the real game;
- record;
- package;
- write a field note for the next agent.
Step three and step five are the ones I would point to if someone asked whether this is responsible. Backing up saves before touching anything, and building one working slice before expanding, are the habits a careful human modder has. The skill writes both habits into the workflow itself.
The README's example prompts range from a homing missile launcher in Terraria to a new Age of Empires II civilization with a unit rendered from 3D, and to a question as simple as "What engine is C:\Games\Foo, and has anyone modded it before?"
What a field note contains
Each note in knowledge/ records how a specific game was actually modded, decompiled or reverse-engineered: the exact versions that worked, the route and why, what the engine really does, how it was verified, and the gotchas written as symptom, cause and fix. The um CLI handles the workflow, with um kb search for prior art, um kb new to start a note, um kb check to validate it, and um kb pr to branch, push and open the pull request.
The comment beside that last command says to run it "after your human says OK". The contribution rules in CONTRIBUTING.md, which the README says apply to humans and AIs, are: no game files, no decompiled dumps, no cheating other players, and an honest status and verification.
The skill set
The README lists a dozen skills. game-recon covers prior notes, engine and version, anti-cheat, loaders and save folders, and writes a MODDING_PLAN.md. reverse-engineering drives ILSpy, Cpp2IL, Vineflower, Ghidra and IDA over MCP, Cheat Engine, Frida and RenderDoc, and asks the agent to prove a reverse-engineered file format with a round trip. fal-assets and asset-pipeline turn generated art into engine-exact frames. game-automation launches the game, takes screenshots and clicks safely. Others handle showcase video, game-in-a-game mashups, publishing and porting an existing mod to a newer version.
The engine playbooks inside mod-any-game name Unity, Unreal, .NET and XNA games such as Terraria and Stardew, Godot, Source, Bethesda, Minecraft and several more engine families.
Setup and costs
The install table covers Claude Code, Codex, Gemini CLI, VS Code with Copilot, Cursor and OpenCode, plus a skills-only path through npx skills add for any agent. Each gets the same skills, the fal MCP server and the um CLI. You also need Git, Python 3.10 or newer and ffmpeg, and Blender for 3D-to-sprite renders.
Using fal for assets needs a fal API key or fal's browser sign-in. The README recommends storing it with um fal key set, which puts it in the Linux desktop keyring or an owner-only file elsewhere, rather than in a shell profile or .env. Without a key, fal's OAuth relay signs you in through the browser, and um comfy can make images on a local ComfyUI server. The um sprixen command uses a separate paid service with its own API key.
If you want to try it, start with the recon question about a game you already own, read the MODDING_PLAN.md it produces, and only then let it build.
GitHub: https://github.com/rehan-remade/universal-modder
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