Mashup Atlas
Guide · repository read at commit 5ba8150 on 12 Oct 2026

Universal Modder: set it up and plan a first game mashup

Universal Modder is a set of skills, a um command-line tool and an asset connection for AI coding agents. You ask your agent for a mod or mashup, and it works on PC games you own. It is not a website that merges games for you.

Everything below comes from the project’s GitHub repository at commit 5ba8150. We haven’t installed it or built a mod with it, and we aren’t affiliated with its authors.

Is this the right route for you?

Do you want to make a mod, or play one that exists?

Make one

Universal Modder fits if you can run a coding agent and own the games involved. You will need:

  • A supported agent: Claude Code, Codex, Gemini CLI, VS Code with Copilot, Cursor, OpenCode, or any agent that takes skills.
  • Git, Python 3.10 or newer and ffmpeg. The README recommends uv. Blender only for 3D-to-sprite renders.
  • A fal API key for generated art, set as FAL_KEY. Without one, um comfy uses a local ComfyUI server instead.
  • PC games you own. Windows games are driven natively or from WSL.
  • Single-player, offline, or a server you run yourself.

Play one

You don’t need this toolkit to play finished mashups.

Your coding agent and fal have their own accounts and pricing. The README lists no prices, so check both providers before starting.

Install it in your agent

Pick your agent. The README says each one gets the same skills, the fal MCP server for generated assets and the um CLI. Two routes are partial: OpenCode 2’s plugin and Skills only, as their cards say. Commands are copied from the README; check it for changes before you run them.

  • Claude Code

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

  • Codex

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

  • Gemini CLI

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

  • VS Code / Copilot

    Enable chat.plugins.enabled, run “Chat: Install Plugin From Source” and enter the repository URL.

  • Cursor

    Install it from the Cursor Marketplace, or clone the repository.

  • OpenCode

    opencode plugin add 'github:rehan-remade/universal-modder#main::path:plugins/opencode'

    This OpenCode 2 plugin adds a um tool and, when FAL_KEY is set, the fal MCP server, but not the skills: add those with the Skills only command. It also doesn’t put um on your PATH. Or clone the repository and run opencode inside it.

  • Skills only (any agent)

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

    Adds the skills. The README doesn’t say this route sets up the fal MCP server or the um CLI; add um with the command below.

  • Anything else

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

    Then start your agent inside the cloned folder.

The um command comes with the plugin installs and clones, except OpenCode’s plugin. Elsewhere, the README installs it with uv tool install git+https://github.com/rehan-remade/universal-modder.

Choose how the two games meet

The toolkit’s mashup skill describes five patterns and says to “pick the lightest one that delivers the idea”. The README’s skill list names only the first four kinds; the fifth is in the skill itself. Each finished example has its own page with what it needs to run.

The five mashup patterns, when to pick them and a finished example
Pattern What runs Pick it when Finished example
1. Port the content Only the host game. The guest’s enemy, weapon or block is rebuilt as new host content. One thing from the guest is enough. The skill calls this the lightest pattern: “cheap and robust, with no IPC”. None in this catalog yet.
2. Passthrough Both games at once, exchanging state, rendering and collision over 127.0.0.1. You want the guest’s real behaviour, so the real guest has to run. SkyCraft Filed by the toolkit as “passthrough + geometry transfer” over named shared memory. Also ArkWeb. Our reading: two games run together over a shared-memory link. Not in the toolkit’s case list.
3. Embed a decomp as a library The host game plus a library built from a decompiled game. The host feeds it collision and input. A decomp or recomp of the guest already exists. You supply your own ROM for its assets. ER Mario Filed by the toolkit as a “rebuilt mechanic, in-process”, built on libsm64 from the Super Mario 64 decomp.
4. Reimplement, then fuse Rewritten game runtimes joined in one process. The skill’s example reads MW2’s files from your own install. You want the most control and accept the most work. The skill calls it the heaviest pattern. GameMash Our reading: its creator says every mechanic is an original implementation, and it uses no game files. Not in the toolkit’s case list.
5. Guest rules headless, host as the view The real host game. Only the guest’s rules are rewritten, as a simulation that the host displays. The guest’s gameplay is numbers (tower defense, card, puzzle, top-down 2D, turn-based), or the guest can’t or shouldn’t run, such as an anti-cheat-protected client. None in this catalog yet. The skill’s own example is Bloons TD 6 inside Minecraft, a note in its repository.

Sources: mashup-mods skill and its case list.

Plan your first mashup before building

These are the toolkit’s own rules, in the order its skills apply them. Your agent should follow them; it helps to know what to expect and what to approve.

  1. Say what you want from the guest game. The agent confirms the game, platform and idea, agrees what “done” means and starts a MODLOG.md journal.
  2. Recon. It searches the shared notes with um kb search "<game>" and scans the install with um scan "<game>" for engine, anti-cheat, loaders and save paths.
  3. Pick the lightest route and write the contract. It names the strategy, records why, and writes down what crosses each boundary between the games. Traffic between them stays on 127.0.0.1.
  4. Set up a lab. It backs up saves with um backup create "<saves folder>" --name <game>-saves and uses a separate profile or save folder where the loader allows.
  5. Build one small slice first. The skill asks for a first slice per route. For passthrough it is “a cube from process A drawn in B at the right spot”, then per-frame positions, collision and real content.
  6. Prove it in the running game. “The running game is the oracle”: a log plus a screenshot someone actually looked at. A README or a passing test is not a real run.
  7. Change course after repeated failure. If the same failure happens three times, the agent stops and changes approach or asks you.

Example first request and plan

The request is one of the README’s own “Try it” prompts. The plan fills in the steps above with facts from the toolkit’s Minecraft × GTA V example. Example only; not run by us.

Put real Minecraft inside GTA V story mode. Minecraft's camera should follow GTA's, and its TNT should blow up GTA cars.
Host / guest
GTA V Legacy in story mode / Minecraft Java 26.3
Pattern
2, passthrough: both games run. The case list files this example as “frame compositing + state”
First slice
One cube from Minecraft drawn in GTA at the right spot
Proof
The cube appears in the running game: a log line plus a screenshot you look at

A worked example: Minecraft inside GTA V

The repository ships this as a buildable example. It uses a WebSocket for commands and shared memory for frames, not one or the other.

What runs

  • GTA V side: a ScriptHookV script with an ASI loader, plus a ReShade 6.8.0 add-on that draws Minecraft into the frame.
  • Minecraft side: a Fabric mod for Minecraft Java 26.3 (Fabric Loader 0.19.5 or newer).

How they talk

  • Commands and game events: a WebSocket on 127.0.0.1, port 25599.
  • Frames: named shared memory with three slots, carrying Minecraft’s colour, depth and HUD.
  • ReShade draws Minecraft wherever it is nearer than GTA’s depth, so both worlds overlap correctly.

What it needs

  • Windows 10 or 11 with WSL, your own Minecraft Java and GTA V Legacy on Steam. The Enhanced edition isn’t supported.
  • Story mode only, never GTA Online.
  • Build tools: Visual Studio 2022 or newer with C++, JDK 25 and Python 3.12.

Its stated limits

  • At most 400 placed-block props; GTA crashes at around 1,500 script objects.
  • A GTA update may need a new ScriptHookV before the plugin works again.
  • The local link has no authentication, so any program on the PC can send it commands.
  • Its README says it was built and tested in GTA V Legacy (build 3889) in September 2026, and written with Claude Code. We haven’t run it.

Hard stops

From the toolkit’s safety rules. It says plainly that none of this is legal advice; read the publisher’s EULA or mod policy too.

It will

  • Back up saves, profiles and config before any modded launch.
  • Ask before driving your mouse and keyboard, installing loaders or publishing anything.
  • Ship only its own code and assets, plus patches or converters that work on your own files.
  • End processes by exact ID, so it doesn’t close your other programs.

It won’t

  • Inject into games with anti-cheat, or disable or bypass anti-cheat.
  • Write cheats for multiplayer games, or connect a hooked game to official online services.
  • Get past DRM or ownership checks.
  • Download or publish ROMs, ISOs, game files or decompiled source.

Next steps