universal-modder

Skills, tools and a shared knowledge base that let any AI coding agent mod almost any PC game you own.

Works with Claude Code, Codex, Cursor, Gemini CLI, GitHub Copilot, OpenCode, or anything that reads AGENTS.md.

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 the video, and writes down what it learned for the next agent.

Install

Pick your agent. Each gets the same skills (Agent Skills format), the fal MCP server, and the um CLI.

Inside a clone, each agent finds the skills where it looks for them: .agents/skills (Codex and friends), .claude/skills, .gemini/skills and .github/skills all link to skills/. Instructions are in AGENTS.md, which CLAUDE.md and GEMINI.md point to. MCP config is in .mcp.json, .codex/config.toml, .cursor/mcp.json and .vscode/mcp.json.

The um CLI. Plugin installs and clones put it on PATH. Anywhere else:

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

For assets, get a fal API key. It powers both the fal MCP server and um fal:

export FAL_KEY=...

You also need Python 3.10+ and ffmpeg. uv is recommended. Blender is needed for 3D → sprite renders. Windows games are driven natively or from WSL.

Try it

Mod Terraria: add a homing missile launcher and a tactical nuke that craters the world. Make the sprites with fal.

Make a new civilization for Age of Empires II with a unique unit rendered from 3D.

Put real Minecraft inside GTA V story mode. Minecraft's camera should follow GTA's, and its TNT should blow up GTA cars.

What engine is C:\Games\Foo, and has anyone modded it before?

The agent starts with the mod-any-game skill and runs the same loop every time:

1. search the knowledge base;
2. recon, then pick a route;
3. set up a safe lab (saves backed up);
4. read the actual code;
5. build one working slice;
6. generate assets;
7. verify in the real game;
8. record;
9. package;
10. write a field note for the next agent.

A knowledge base that AIs write for AIs

knowledge/ holds field notes: how specific games were actually modded, decompiled and reverse-engineered. Each note gives:

• the exact versions that worked;
• the route, and why;
• what the engine really does;
• how it was verified;
• the gotchas (symptom → cause → fix).

Every agent that finishes a mod can open a pull request with its note, so the next agent starts where it left off instead of rediscovering the same traps.

um kb search "grand theft auto"                 # before you start: prior art (works outside the repo too)
um kb new --game "Hades II" --title "A new boon god" --from-scan hades --agent "Codex (gpt-6)"
um kb check knowledge/games/hades-ii/a-new-boon-god.md
um kb pr knowledge/games/hades-ii/a-new-boon-god.md --yes    # after your human says OK: branch, push, PR

Browse knowledge/INDEX.md. Contribution rules, for humans and AIs, are in CONTRIBUTING.md: no game files, no decompiled dumps, nothing that helps cheat online, and an honest status and verification.

What's inside

Skills (skills/, Agent Skills format)

The um CLI (Python). Every command has --help with examples.

Two no-build Windows tools ship inside the package (um/ps1/): WinDrive input and ProcLoopback game-only audio, both PowerShell with embedded C#.

Built with it

• examples/terraria-tmodloader: Fal Arsenal for tModLoader.
  • Weapons: a homing missile launcher, a tactical nuke (crater + mushroom cloud), a chain-lightning rifle, a

    black-hole gun and an orbital strike.

  • Three new enemies and a two-phase Drone Mothership boss.
  • Every sprite came from fal.
• examples/aoe2-de-civ: San Franciscans for Age of Empires II DE.
  • A new civilization with a Robotaxi unique unit and Delivery Drones, rendered from fal image-to-3D models

    at AoE2's camera angle.

  • A Transamerica Pyramid wonder.
  • A reverse-engineered .sld sprite writer.
• examples/minecraft-gta5-passthrough: real Minecraft inside

  GTA V story mode.

  • A Fabric mod and a ScriptHookV + ReShade add-on exchange camera, ground and events over a local

    WebSocket.

  • Minecraft's colour + depth are depth-composited into GTA's frame.
  • Minecraft TNT, arrows and fireworks become GTA explosions and bullets, and Minecraft mobs fight the

    police.

Each has a field note with every non-obvious lesson: knowledge/INDEX.md.

Rules it follows

• Single-player and offline, on games you own. It refuses to inject into online games with anti-cheat,

  write multiplayer cheats, or bypass anti-cheat, DRM or ownership checks.

• It never ships game files or decompiled code. Mods ship as code, your own assets, patches or

  converters.

• It backs up before touching saves, and kills processes by PID only.
• It asks before driving your mouse and keyboard, installing loaders into game folders, or publishing,

  PRs included.

Full reasoning: skills/mod-any-game/references/safety.md.

Credits

• Built from real agent sessions modding Terraria, Age of Empires II and GTA V × Minecraft.
• Assets: fal (GPT Image 2, Nano Banana 2, FLUX, Trellis 2, ElevenLabs...).
• Standing on the shoulders of tModLoader, genieutils-py, AoE2ScenarioParser, ScriptHookV, ReShade, Fabric,

  BepInEx, Harmony, UE4SS, REFramework, SKSE, ILSpy, Ghidra and every modding community that documented its game.

• The engine playbooks also draw on the September 2026 wave of AI-built mods, and on how their creators

  explained them in public.

MIT licensed. Fonts: Space Grotesk and JetBrains Mono (SIL OFL).