Universal Modder Codex: install, verify and first mod
Codex installs the same skills, fal MCP server and um CLI as Claude Code, through its own plugin marketplace commands.
Codex uses the same universal-modder package as every other supported agent: the Agent Skills set teaches the modding loop, the fal MCP server handles generated assets, and the um CLI provides the deterministic tooling. The install is two commands, and the plugin keeps the CLI on PATH.
This page covers the Codex-specific path: prerequisites, the two marketplace commands, how to verify the setup safely, and the errors that show up in practice. It mirrors the Claude Code guide so you can switch agents without relearning anything.
Before you start
- Git, Python 3.10+ and ffmpeg available on PATH.
- uv is recommended for the um CLI; pipx works too.
- A fal API key for generated art, audio or 3D (optional — um comfy can use a local ComfyUI instead).
- Blender only for um render3d, which turns 3D models into sprite frames.
- A single-player game you own, or a server you host — anti-cheat-protected online games are refused by design.
Install steps
- Add the plugin marketplace. Run codex plugin marketplace add rehan-remade/universal-modder in your terminal to register the upstream repository as a plugin source.
- Install the universal-modder plugin. Run codex plugin add universal-modder@universal-modder. This installs the skills, the fal MCP server configuration and the um CLI in one step.
- Set your fal key for asset generation. Export FAL_KEY in your shell profile (setx FAL_KEY on Windows) so um fal and the fal MCP can generate assets. Without a key, plan on a local ComfyUI server through um comfy.
- Confirm the um CLI is on PATH. Open a new terminal and run um --help. If the command is missing, run uv tool install git+https://github.com/rehan-remade/universal-modder and restart the shell.
- Start Codex near a game and describe the mod. Run Codex in or near a game you own and describe the change, for example: add a tactical nuke to Terraria with a mushroom cloud, sprites from fal. The mod-any-game skill drives the loop from there.
- Review each checkpoint. Expect the agent to back up saves, pick a route from recon, build one working slice first, then generate assets, verify in the running game, record, package and write a field note. It asks before installing loaders into game folders or publishing.
Verify the installation
Verify before modding anything: um --help prints the full CLI surface, and um kb search queries the shared knowledge base of field notes written by earlier agent sessions.
um --help
um kb search "halo"Common problems and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| codex: plugin commands are not recognized. | The plugin marketplace commands require a recent Codex build; older versions do not ship the plugin system. | Update Codex, then rerun codex plugin marketplace add rehan-remade/universal-modder. Cloning the repository and starting Codex inside it is the equivalent fallback. |
| um: command not found after the plugin install. | PATH changes only apply to new shells. | Open a new terminal. If it is still missing, run uv tool install git+https://github.com/rehan-remade/universal-modder and restart the shell. |
| Asset steps fail with FAL_KEY is not set. | The fal MCP server and um fal need a fal API key, and Codex inherits it from the environment that launched it. | Create a key at fal.ai/dashboard/keys, export FAL_KEY in your shell profile, then restart Codex from a new terminal. |
| The agent answers from general knowledge instead of following the modding loop. | The skills were not loaded because the session started outside the plugin or clone context. | Restart the session, state the game and the desired mod explicitly, and ask the agent to use the mod-any-game skill. Starting inside a clone also loads skills from the working directory. |
Plugin install versus cloning for Codex
A clone exposes the repository's AGENTS.md file, which Codex reads automatically, and the .codex/config.toml MCP configuration. For plugin installs the same pieces arrive through the marketplace. Both paths give identical skills and tools, so pick whichever fits your workflow and keep one of them.
The rules the toolkit enforces
The agent backs up saves before touching them, kills processes by PID only, and asks before driving input, installing loaders into game folders, or publishing. It refuses anti-cheat bypasses, cheats against other players in online games, game file uploads and decompiled code dumps. These rules exist to keep your accounts and machines safe, so treat any deviation as a stop signal.