Mashup Mods

---
name: mashup-mods
description: Build cross-game mashups and total conversions, the "Minecraft inside Elden Ring" or "skateboarding in MW2" kind. Covers passthrough mods (two games running at once and exchanging state, rendering and collision), porting one game's content or mechanics into another, embedding a decompiled game as a library, and clean reimplementations that read the user's own game files. Use when the user wants to combine two games, bring an enemy, mechanic or world from one game into another, or rebuild a game's runtime. Start with `fsgc passthrough plan <game> <game>` (or the MCP `passthrough` tool or prompt).
---

# Mashups: putting one game inside another

In September 2026 a wave of AI-built mashups went viral: skateboarding in MW2, Minecraft inside Skyrim,
Elden Ring and Mario 64, Black Ops 2 inside Minecraft. They use four patterns. Pick the lightest one that
delivers the idea, and plan the oracles before writing code: these projects fail by drifting, not by lacking
code.

## Start here: plan the pair
`fsgc passthrough plan "<game>" "<game>"` does the first hour of thinking. The same plan is the MCP
`passthrough` tool, and a `passthrough` prompt that MCP clients show as a slash command. It:
- fingerprints both games (`fsgc scan` when they're installed, a table of well-known games when they're not),
  stops at online-only games and flags anti-cheat;
- picks the host (the game the player is in) and the guest, and says why; `--host` overrides it;
- lists what each engine gives you: where your code runs, the camera, drawing the guest, the guest's picture,
  and collision both ways;
- picks the pattern below that fits: passthrough (Pattern 2), embed (Pattern 3) or a content port (Pattern 1)
  when one side can't run a live mod;
- proposes the link, milestones with the proof each one needs, and the field notes to read first.

`--out <folder>` starts `PLAN.md` and `MODLOG.md` there. `fsgc passthrough peer --as host|guest` stands in for the
other game on the link, so each side can be built and checked alone. Agree the host, the idea and what done means
with the user before building.

## Pattern 1: port the content (lightest)
Bring an enemy, weapon or block type into the host as **new host content** that imitates the guest.
Examples: "Claude added creepers to Dark Souls", Minecraft blocks as Elden Ring items.
- Read the guest's behaviour from its source of truth: decompile or read the wiki's exact numbers (speeds,
  timers, damage). Reimplement it in the host's mod API (host AI state machine, host projectile).
- **Assets:** never copy the guest's files into your mod. Convert them from the user's install at runtime or
  install time (a converter script), or recreate lookalikes with FixedSeed.
- It's cheap and robust, with no IPC. It works with any host that has a loader.

## Pattern 2: passthrough (two games at once)
One creator's description of Minecraft-in-Skyrim: *"minecraft and skyrim run at the same time and you make a mod
for both that lets them communicate. Then you passthrough the things you want into the renderer in the
right place and feed stuff like collision data back to minecraft."* No code was published. The design below
is the standard way to build it.
1. **Guest process** (e.g. Minecraft with a Fabric mod, or a headless reimplementation): runs the simulation
   and publishes state every tick. That's entities/blocks near the player, or a rendered layer.
2. **Transport**, all on `127.0.0.1`:
   - the default is **passthrough link v1**, the one `fsgc passthrough plan` proposes: TCP, one JSON object per
     line, `t` / `seq` / `ts` on every message, positions in the host's coordinates (the guest converts).
     `fsgc passthrough peer` speaks it and checks every message the other side sends;
   - state: a shared-memory ring buffer (`CreateFileMapping`) or UDP/named pipes;
   - control: JSON lines or HTTP;
   - GPU frames: DXGI shared handles (`CreateSharedHandle` / `OpenSharedResource1` + keyed mutex), Vulkan
     external memory, or Spout2.
3. **Host injection:** a host-side plugin (SKSE/xNVSE/UE4SS/REFramework/ReShade addon) draws the guest's
   geometry **inside the host's pass**. It uses the host's view-projection matrices (find them with
   RenderDoc) and depth buffer, or spawns host-native objects so host lighting and shadows apply. The
   Skyrim clip gave Steve Skyrim's lighting, which points to scene integration, not a flat overlay.
4. **Back-channel:** the host's collision near the player (raycasts, or exported nearby mesh) goes to the
   guest as solid blocks or colliders. Input routes to one process at a time.
5. **Sync:** timestamps on every message, tolerance for the two frame rates, and a watchdog when one side
   dies.
6. **Start small:** a cube from process A drawn in B at the right spot. Then positions every frame, then
   collision, and only then real content. Build each side against `fsgc passthrough peer` first, the way the
   GTA example was built against a fake host.

A public bridge-plugin template exists for Fallout: New Vegas (a thin xNVSE plugin, file-drop/HTTP
transport, data-driven actions). The Terraria agent bridge in `examples/terraria-tmodloader/reference/` is the same
idea.

### Worked example: real Minecraft inside GTA V
Code: `examples/minecraft-gta5-passthrough`. Every lesson: `knowledge/games/gta-v/minecraft-passthrough.md`.
- **Minecraft (Fabric mod):** it takes camera, ground and input from the host over a WebSocket on
  `127.0.0.1`. Each frame it writes world colour + depth and a separate hand/HUD overlay into named shared
  memory.
- **GTA (ScriptHookV ASI + ReShade add-on):**
  - it sends GTA's camera and the ground under the player (as barrier blocks);
  - it composites Minecraft's colour against GTA's reversed-Z depth in a shader;
  - it turns Minecraft explosions, arrows and firework hits into GTA explosions and bullets.
- **Mapping:** 1 metre = 1 block. GTA (x, y, z) → MC (x, z + offset, −y); yaw = 180 − heading;
  pitch = −pitch.
- **Latency:** Minecraft's frame is re-projected onto GTA's camera, rotation then full 6-DoF with depth.
  Measure the pose lag with a scene where one side draws something the other doesn't (a gold wall vs the
  skyline).
- **Built without the game:** most of it was built before the host game was even installed, against a
  fake host (known geometry) and a fake D3D11 "GTA" that runs the real compositor.
- **What didn't work:** putting host-game guns in Steve's hands (the aim cam and animations don't fit).
  Guest weapons with host effects did.

## Pattern 3: embed a decomp as a library
libsm64 turns the Super Mario 64 decomp into a library: feed it collision and input, and it returns Mario's
state and mesh. G64 embeds it in Garry's Mod; the host feeds its collision into the guest sim. Any
decomp/recomp (see `skills/mod-any-game/references/engines/retro-decomp.md`) can be wrapped this way. The
user supplies their own ROM for assets.

## Pattern 4: reimplement, then fuse (heaviest, most control)
- **IW4L:** an LLM-written Rust MW2 runtime (Bevy + wgpu). It reads MW2's FastFiles from the user's install
  and translates the D3D9 shaders to WGSL.
- **The Skate 3 Rust engine:** built against a static recomp and an IDA database as oracles.
- **The mashup:** fuses both, plus a Minecraft Rust reimplementation, in one process. The skate sim is a
  worker that takes over the MW2 soldier. MW2 map collision feeds the skate world. Grind rails come from
  walkable collision edges. Skate bones are retargeted onto the MW2 skeleton.
- **What made it work:**
  - A **scriptable runtime as the oracle:** `spawn; wait 2s; screenshot; dump`, with blocking verbs and
    evidence files per run.
  - An **evidence journal** (`context/artifacts/<date>/<step>-FINAL|PART`). "Knowledge that is not in an
    artifact does not exist."
  - Owner-approved end-to-end tests only.
  - A **publish check** that greps for retail offsets and decompiler names before any push
    (`fsgc publish check` does a version of this).
  - Converters that run on the user's files; game assets never committed.

## Guardrails
- Offline, single-player or servers the user runs. Never network into the real games' official online
  services.
- Everything guest-side comes from the user's own install or dump. Publish code and converters, not assets
  (`fsgc publish check --game`).
- Be honest about what's AI-built. Creators who weren't got called out publicly.