Universal ModderTHE INDEPENDENT GUIDE
FIELD GUIDE / AGENT SETUP ENGLISH · SOURCE-REVIEWED OCT 11, 2026

Universal Modder for Codex.
A clear setup. A verifiable result.

Follow the upstream plugin route or work from a clone, then give Codex a small game-mod task with explicit checks and an honest field note.

Independent tutorials for the open-source project. Not affiliated with the upstream maintainers. Source-reviewed; commands and game examples have not been tested by this site.

Install Universal Modder for Codex

The upstream project documents this two-command Codex CLI route. Run it in a terminal, not as Claude Code slash commands:

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

Adding the marketplace registers a source; the second command requests the plugin itself. These are the toolkit’s published installation instructions, not a promise that every installed Codex client supports the same plugin interface.

OpenAI’s plugin packaging documentation documents marketplace registration and inspection. If your client rejects the plugin-add command, check its current plugin UI and help rather than guessing a replacement. The clone workflow below is an alternative listed upstream.

Sources: Toolkit install instructions ↗

Check the marketplace, skills and shell

Use the documented marketplace-list command to inspect configured sources:

codex plugin marketplace list

A listed marketplace confirms source discovery, not a successful Universal Modder installation. Separately confirm the plugin installation result and ask Codex to find mod-any-game and explain its rules.

Then check the CLI from the same shell the agent will use:

um --help
um scan --help

If those fail, follow the standalone CLI installation. Python tools, game paths and any Windows automation need to be reachable in that environment. A plugin does not automatically authorize protected filesystem access or install a game loader.

Use the repository workflow

Clone the repository and start Codex in it when the plugin route is unavailable or you want to inspect the project configuration:

git clone https://github.com/rehan-remade/universal-modder
cd universal-modder
codex

The upstream clone puts shared skills in .agents/skills, instructions in AGENTS.md and Codex MCP settings in .codex/config.toml. Ask the agent to identify what it actually loaded. Review project configuration before trusting or enabling it; do not overwrite your existing personal configuration just to follow this guide.

The clone is also useful when you intend to write a field note. A plugin’s installed files and a writable source checkout are different things.

Plan and verify a first Terraria mod

Give Codex a small task with a reviewable finish line. The upstream Terraria example is a source reference, not a result reproduced here.

um scan --list
um scan terraria

These scans require access to the local library. If Terraria is missing from the inventory, consult um scan --help and use the real installation directory.

Suggested Codex prompt

Read mod-any-game and search existing Terraria field notes.
Inspect my owned installation and propose one simple tModLoader weapon.
State the exact versions, test-world location and backup plan.
Keep the first change minimal. Ask before installing loaders,
driving game input or publishing. Report untested behavior honestly.
  1. Review recon. Confirm the engine, loader route and any version mismatch. Read the upstream Terraria example.
  2. Set up a test world. Back up the actual saves. The example’s terrain-changing weapons can destroy a world; start with a simpler item.
  3. Make one slice. Have Codex explain the item/projectile integration and build a small source change before adding art or more weapons.
  4. Build through tModLoader. Verify the actual ModSources folder and project. The example uses Workshop → Develop Mods → Build + Reload. Its Steam tModLoader dependency must be present.
  5. Check the game. Observe the item loading and its behavior in a disposable single-player world. The example does not claim tested multiplayer support.
  6. Prepare assets and package. Add an appropriately sized sprite, recheck behavior, then use publish checks and manually review the distributable files.

For backup, asset and testing checkpoints, follow the full ten-stage workflow.

Record what Codex actually verified

A field note should make the next session easier: record the game build, loader, route, verification steps and symptom–cause–fix details. Use a writable upstream clone or fork for knowledge-base work.

um kb new --game "Terraria" --title "A small weapon mod" --agent "Codex (gpt-6)"

The --agent "Codex (gpt-6)" label is an example used by the upstream documentation. It does not select a model or establish which model your session used. Replace it with an accurate label. The command creates a scaffold; fill in the note before validation.

um kb check --help
um kb pr --help

Validate the actual generated file path. The pr command is a dry run unless --yes is supplied; actual branching, pushing and PR creation require approval and GitHub access. Do not add --yes just to make a tutorial look complete.

Sources: Knowledge-base arguments and PR behavior ↗

Codex setup troubleshooting

Unknown plugin command

Confirm you are invoking the Codex CLI. Read your installed client’s help and current plugin documentation. The upstream route may differ from your version; use the source-clone workflow if appropriate rather than trying guessed flags.

Skills or MCP configuration are missing

For a clone, check the working directory, .agents/skills, AGENTS.md and .codex/config.toml. For an installed plugin, inspect its installation result and actual loaded capabilities. A folder existing on disk is not proof the agent loaded it.

Codex can read code but cannot reach the game

Check filesystem permissions and whether the session is on the same machine and OS environment as the game. A remote session cannot infer access to your local running game. Explain any unavailable verification instead of reporting a test passed.

Do I need fal to start?

No fal key is needed for a code-only investigation. Hosted generation requires a key and may cost money. Local ComfyUI is an image option with separate setup. See asset configuration.

Claude Code setup →CLI command reference →