What Is a Passthrough Mod? How Two Games Run as One

A passthrough mod links two real games that run on your PC at the same time. One game draws the world you see. The other runs hidden in the background and supplies the gameplay: movement, blocks, weapons, physics. The two swap information every frame, so it looks like one game. It is the technique behind SkyCraft, which puts Minecraft inside Skyrim, and behind most of the “Minecraft inside X” projects on our wall. This page explains how a passthrough mod works and what it takes to build one.

Host and guest

Every passthrough mod has two roles.

• The host is the world you look at. It draws everything on screen and supplies the terrain, the characters and the story. In SkyCraft, the host is Skyrim.
• The guest supplies the gameplay. It runs with its window hidden and decides how the player moves and what they can do. In SkyCraft, the guest is Minecraft.

Because both games really run, every player needs a copy of both. That is also why passthrough mods feel authentic: the Minecraft you play inside Skyrim is real Minecraft, with its own physics, inventory and crafting.

How the two games talk

The core of a passthrough mod is a small, fast channel between two programs. The design SkyCraft documents, and that most projects since have copied, looks like this:

• Shared memory. Both games open the same named block of memory. When one writes, the other sees it immediately, without sending anything over a network.
• A header with a version check. Both sides confirm they speak the same protocol version and are both still alive. If anything looks wrong, they stop rather than act on garbage.
• “Latest value” slots for things that change every frame, such as the player’s position, the camera and frame timing. A simple locking trick lets the reader detect a half-written value and read again.
• Event queues in each direction for one-off things: a block placed, a hit landed, a save requested. A queue means events aren’t lost when the game is busy.
• One message definition, generated for both languages. The host plugin is usually C++ and the guest mod Java or C#, so defining messages once and generating both sides avoids subtle mismatches.

Then come the four jobs every passthrough mod has to solve:

1. Collision. The guest needs to know where the host’s ground and walls are. SkyCraft streams Skyrim’s collision into Minecraft as invisible blocks, which is why Steve can walk up Skyrim’s mountains.
2. Camera. The guest renders from the host’s camera, or the other way round, so both pictures line up.
3. Compositing. The guest’s picture is drawn into the host’s frame, using depth so a Minecraft block correctly hides behind a Skyrim wall.
4. Interactions. Hits, damage, explosions and doors have to be translated between two different rule systems, such as a Minecraft sword hit becoming Skyrim damage scaled to the NPC’s level.

Good projects also handle failure: if the guest crashes, the host hands control back to the player instead of leaving a puppet with no brain.

Passthrough mods you can study

The fastest way to understand the technique is to read a working one. These are all open source and listed on our wall:

Project	Host	Guest	Worth reading for
SkyCraft	Skyrim	Minecraft	The reference design and the clearest design doc
FalloutCraft	Fallout 4	Minecraft	Reusing SkyCraft’s Minecraft side with a new host plugin
OWCraft	Outer Wilds	Minecraft	A Unity host, plus a development log
ValCraft	Valheim	Minecraft	A BepInEx host and experimental multiplayer
Killcraft	ULTRAKILL	Minecraft	A fast action host and Nether portals
Ultracraft	Minecraft	ULTRAKILL	The same pair with the roles reversed
ArkWeb	Arkham Knight	Spider-Man	Two non-Minecraft games, and pose copying in combat
Minecraft in GTA 5	GTA V	Minecraft	A WebSocket link and depth compositing in GTA
Garry’s Redemption	Red Dead Redemption 2	Garry’s Mod	Physgun and toolgun on another game’s world

Notice how many start from SkyCraft. Once the hard design existed, other modders, working with AI agents, replaced only the host side. That is a big part of why so many passthrough mods appeared within a week.

Passthrough vs other kinds of game merging

Passthrough is one of three approaches we label on every cartridge. A passthrough mod (our “live bridge”) runs both games for real. An engine transplant loads a rebuilt engine from one game into another, which is lighter but only works where someone has already rebuilt that engine, like the Skate 3 mods. A mechanics port re-creates one game’s rules inside the other, like GrandTheftMinecraft. The trade-offs are explained in how AI game merging works.

Passthrough mods give you the genuine guest game, but they are heavy: two games in memory, usually about 3 GB of extra RAM for Minecraft alone, and they are tied to exact game versions.

What you need to build a passthrough mod

The single question that decides whether your idea is realistic: can you run your own code inside the host game? That means a script extender (SKSE for Skyrim, F4SE for Fallout 4), a mod loader (BepInEx for many Unity games, the Outer Wilds Mod Loader) or a plugin SDK. Our mod loaders guide lists them for the games people mash up most.

You don’t usually need to decompile anything. The loader gives your plugin access to the game, and the guest side, typically Minecraft with Fabric, has a mature mod API.

How people build one with AI

The pattern from the community guides and from creators’ own notes:

1. Pick a host with a loader and a single-player guest. Skip anything online or with anti-cheat.
2. Install both games and test the loader with an existing mod before writing anything.
3. Open an AI coding agent in an empty project folder and point it at SkyCraft’s source as the example.
4. Ask for the smallest proof first: get your plugin loading in the host and writing a line to a log, then get both sides to open the shared memory and agree on a version.
5. Add one system at a time: collision, then camera, then compositing, then combat. Playtest after each step.
6. Keep a design doc and a log so a fresh chat can pick up where the last one stopped.

Our prompts and workflow guide has a starter prompt for exactly this, and Universal Modder packages the process for several agents.

Common problems, and what causes them

• Falling through the floor. The guest hasn’t received the host’s collision for that spot yet. Projects fix this by sampling ahead of the player and double-checking the ground under their feet every frame.
• The two pictures don’t line up. The cameras disagree, often on field of view or on which way is up. Getting the coordinate conversion right is usually the first real bug.
• Stutter. Two games compete for your CPU and GPU. Capping the guest’s frame rate and giving it its own memory budget helps.
• Losing control after Alt+Tab. Both games want keyboard focus. Mature mods add a key that hands control back and forth.
• Broken saves. Scripted story moments assume the normal player. SkyCraft recommends skipping Skyrim’s opening, and FalloutCraft says to start after Vault 111.

Before you share it

Publish only your own code. Players should supply both games, and your setup should read files from their copies. Credit the projects you built on, SkyCraft above all if you reused its design. And keep it offline: nothing in a passthrough mod should touch a game’s online mode.