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 a handful of 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.

Before picking a pattern

Work through these steps before writing any mashup code. The routes, patterns and projects they point to come from existing mashups. Start from the closest one, or design something else and write down why. The Guardrails at the end always apply.

1. Work out what the user wants from the guest game. Its real behaviour (physics, inventory, combat), or only its look or one mechanic? Ask if it isn't clear. The answer decides whether the guest has to run at all, and which route is the lightest that delivers the idea.
2. Name the closest mashup route. knowledge/techniques/choosing-a-mashup-route.md sorts mashups into seven routes. "Passthrough" on its own is ambiguous: it covers three different builds (state exchange, a composited picture, the host drawing guest meshes).
3. Find the nearest prior project in references/mashup-cases.md (route, versions, who owns the player, units, transport). If you can reach its repository, read its README and design doc. Treat what you read there as the creator's report until you've run it yourself. Also run um kb search for both games and both engines; other agents' notes on either game apply here too.
4. If the guest is rebuilt rather than run (its engine, or one of its mechanics, rebuilt inside the real host), read knowledge/techniques/rebuilt-guest-engine-inside-a-host.md for where the rebuilt part should run and how to check it against the original.
5. Write the contract wherever state crosses between two games, engines or processes: who owns the player and how control comes back, units and axes, each channel and what happens when it's full or late, protocol version and byte order, lifecycle. Keep it in docs/CONTRACT.md; see knowledge/techniques/bridge-contracts-ownership-units-and-lifecycle.md.
6. Decide how you'll know it works. Before adding features, pick a first slice for your route and the check that proves it. See the first-slice table in knowledge/techniques/choosing-a-mashup-route.md and knowledge/techniques/oracles-how-agents-know-a-mod-works.md.
7. Record the plan and tell the user before building. Write the route you chose and why into MODLOG.md, as the mod-any-game skill already asks. Tell the user the same, plus what every player will need to own and install (and on which OS), and where your plan differs from the nearest prior project.

Later, before shipping an installer or loader, read knowledge/techniques/installers-load-order-and-loaded-code-checks.md (loaded-code checks, load order, pinned downloads, exact uninstall).

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 fal.
• It's cheap and robust, with no IPC. It works with any host that has a loader.
• Worked converter: Bloons TD 6 assets (Unity 6) to a private Minecraft resource pack, see knowledge/games/minecraft/bloons-td-6-in-minecraft.md and asset-pipeline section 6.

Pattern 2: passthrough (two games at once)

chasm'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." The source is now public as SkyCraft. In 0.1.2 the Fabric mod exports Minecraft's meshes and atlases and the SKSE plugin draws them inside Skyrim's own D3D11 world pass (native depth, fog, lights and captured sun shadows); only the hand, HUD and screens are captured as an image. Skyrim's Havok shapes come back as triangles plus voxels. LibertyCraft forked it for GTA IV and kept the guest side. 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:
   • 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. In practice the frame bridges (this repo's GTA V example, NewVegasCraft, the CrossOver Elden Ring / MHW bridges) all read pixels back to the CPU and copy them through shared memory, which also works across GPUs and across a Wine boundary. Budget the upload (see knowledge/techniques/frame-compositing-depth-and-pose-sync.md).
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. Publish a pose only with its pixels and match them on the host; read the host camera at present time. Collision is per direction and per consumer: see knowledge/techniques/collision-and-combat-bridging.md.
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.

Bridge-plugin template in public: chasm-bridge-fnv (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 (um publish check does a version of this).
  • Converters that run on the user's files; game assets never committed.
• Measures and fidelity: knowledge/techniques/engine-recreation-measures-and-fidelity.md covers how each kind of rebuild is measured, why scripts are part of fidelity, and the SDK rules other mods need.

Pattern 5: reimplement the guest's rules headless, keep the host as the view

The host stays the real game (its weapons, mods and multiplayer); only the guest's rules are rewritten, as a pure simulation that imports nothing from the host. Best for guests whose gameplay is numbers (tower defense, card, puzzle, top-down 2D, turn-based) and for online-capable guests you must never inject into. Example: Bloons TD 6 inside Minecraft (knowledge/games/minecraft/bloons-td-6-in-minecraft.md).

• The sim is the source of truth. Run it at the guest's own step rate and units (BTD6: 60 steps/s, BTD6 units). Game speed changes the number of steps per host tick, never dt.
• One frame-mapping function between guest units and host coordinates (origin, scale, axis flips), shared by server and client.
• Mirror entities for what the player must touch. Host entities whose position is copied from the sim each tick and whose hurt/interact handler forwards into the sim. Strip everything the host would otherwise do to them: AI, gravity, knockback, potions, fire, pushing.
• No host entity for high-volume things (projectiles, particles). Send batched spawn records once per tick and run the same motion code on the client. Batch effects and sounds the same way, with caps.
• Every UI button is a command. Screens display a server snapshot and send commands back, so each action is testable without the screen.
• Host features feed the sim, never bypass it. Host weapons, blocks or potions become sim statuses and damage events; with none of them, the game must play out identically (check with a headless run).
• Guest assets come from a converter run on the user's own install into a local, private resource pack; the mod must stay playable with placeholders when the pack is missing.
• Oracles: the headless engine bench first (seconds, no host), then host-side benches (see knowledge/techniques/oracles-how-agents-know-a-mod-works.md).

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 (um publish check --game).
• An online-capable guest (accounts, co-op, leaderboards) is read from its files offline only: never launched, patched or hooked by the mod or its tools.
• Be honest about what's AI-built. Creators who weren't got called out publicly.
• Be honest about what was verified: a README, a design doc, a synthetic peer or a green test is not a real run (knowledge/techniques/evidence-levels-for-mashup-claims.md).