MCCTP - Minecraft Control Transfer Protocol

A Fabric mod that exposes a WebSocket server on your Minecraft client. It streams game state out every tick and accepts action commands in — letting any program, in any language, control the player.

Features

• WebSocket API — JSON over WebSocket, language-agnostic
• 13 action types — movement, combat, inventory, camera, and more
• Game state every tick — health, position, held item, combat context
• Item categorization — automatic classification (SWORD, BOW, BLOCK, FOOD, etc.)
• Hotbar wheel overlay — circular HUD showing all 9 slots, controlled via commands or V key
• Thread-safe — WebSocket runs on its own Netty thread group, actions dispatch to the game thread

Requirements

• Minecraft 1.21.11
• Fabric Loader >= 0.18.0
• Fabric API
• Java 21+

Install

./gradlew build

Copy build/libs/mcctp-1.0.0.jar to your .minecraft/mods/ folder (alongside Fabric Loader + Fabric API).

How It Works

When you join a world, the mod starts a WebSocket server on port 8765. Any client can connect to ws://<host>:8765/mcctp and:

1. Receive game state JSON every tick (50ms by default)
2. Send action commands as JSON to control the player

The server runs on its own Netty NioEventLoopGroup, separate from Minecraft's networking. Incoming actions are dispatched to the game thread via MinecraftClient.execute(), so they're safe and behave identically to real input. The server stops when you leave the world.

Protocol

Connecting

ws://<host>:8765/mcctp

Standard WebSocket connection. No authentication. Multiple clients can connect simultaneously.

Game State (Server → Client)

Broadcast to all connected clients every tick. All fields are always present.

{
  "type": "game_state",
  "timestamp": 1700000000000,
  "selectedSlot": 0,
  "heldItem": {
    "name": "minecraft:diamond_sword",
    "category": "SWORD",
    "stackCount": 1,
    "maxDurability": 1561,
    "currentDurability": 1500
  },
  "offhandItem": {
    "name": "minecraft:shield",
    "category": "SHIELD",
    "stackCount": 1,
    "maxDurability": 336,
    "currentDurability": 336
  },
  "playerState": {
    "health": 20.0,
    "maxHealth": 20.0,
    "hunger": 20,
    "saturation": 5.0,
    "x": 100.5,
    "y": 64.0,
    "z": -200.3,
    "yaw": 45.0,
    "pitch": -10.0,
    "onGround": true,
    "sprinting": false,
    "sneaking": false,
    "swimming": false,
    "flying": false,
    "inWater": false,
    "onFire": false
  },
  "combatContext": {
    "isUsingItem": false,
    "isBlocking": false,
    "activeHand": "MAIN_HAND",
    "crosshairTarget": "ENTITY",
    "crosshairEntityType": "minecraft:zombie",
    "crosshairBlockPos": null
  }
}

Game State Fields

heldItem / offhandItem:

playerState:

combatContext:

Actions (Client → Server)

Send a JSON message with action and params:

{"action": "<action_name>", "params": {<params>}}

Action Reference

Action Examples

// Walk forward
{"action": "move", "params": {"direction": "forward", "state": "start"}}

// Stop walking
{"action": "move", "params": {"direction": "forward", "state": "stop"}}

// Turn 90 degrees right
{"action": "look", "params": {"yaw": 90.0, "pitch": 0.0, "relative": true}}

// Face north, looking straight ahead
{"action": "look", "params": {"yaw": 180.0, "pitch": 0.0, "relative": false}}

// Draw and fire a bow
{"action": "use_item", "params": {"state": "start"}}
// ... wait ~1 second ...
{"action": "use_item", "params": {"state": "stop"}}

// Throw a snowball
{"action": "throw_item", "params": {}}

// Switch to slot 3 and attack
{"action": "select_slot", "params": {"slot": 3}}
{"action": "attack", "params": {}}

Item Categories

Every held item is classified into one of these categories:

Configuration

Config file: .minecraft/config/mcctp.json (created on first launch)

{
  "port": 8765,
  "tickInterval": 1
}

Architecture

Any Client ←── WebSocket (JSON) ──→ Fabric Mod
                                       │
                                       ├── WebSocketServer (Netty, own thread group)
                                       │     ├── WebSocketServerInitializer (HTTP → WS upgrade)
                                       │     ├── WebSocketFrameHandler (parse inbound JSON)
                                       │     └── ConnectionManager (track channels, broadcast)
                                       │
                                       ├── GameStateCollector (END_CLIENT_TICK → JSON)
                                       │     ├── GameStatePayload
                                       │     ├── HeldItemInfo + ItemCategorizer
                                       │     ├── PlayerStateInfo
                                       │     └── CombatContextInfo
                                       │
                                       ├── ActionDispatcher (JSON → ActionHandler)
                                       │     ├── MoveHandler, LookHandler, JumpHandler
                                       │     ├── AttackHandler, UseItemHandler, ThrowItemHandler
                                       │     ├── SelectSlotHandler, SneakHandler, SprintHandler
                                       │     ├── DropItemHandler, SwapHandsHandler
                                       │     ├── OpenInventoryHandler, ToggleWheelHandler
                                       │     └── KeyReleaseScheduler (timed key pulses)
                                       │
                                       └── HotbarWheelRenderer (HUD overlay)

Key implementation details:

• The WebSocket server uses Netty's NioEventLoopGroup, completely separate from Minecraft's network thread
• All action handlers run on the game thread via MinecraftClient.execute() — same as real player input
• Pulse actions (jump, attack, throw, swap, inventory) use KeyReleaseScheduler which holds the key for 3 ticks and increments timesPressed, matching how Minecraft processes single key presses
• Held actions (move, sneak, sprint, use_item) set KeyBinding.pressed directly and stay held until a "stop" command
• State collection hooks into END_CLIENT_TICK and serializes with Gson
• The mod uses a Mixin accessor (KeyBindingAccessor) to access KeyBinding.pressed and KeyBinding.timesPressed

Building from Source

# Build
./gradlew build

# Output jar
build/libs/mcctp-1.0.0.jar

Build targets Java 21. Tested with Gradle 9.2.0 and Fabric Loom 1.15.3.

Python Client

A reference Python client is included in python/. Install with:

cd python
pip install -e .

Provides async and sync WebSocket clients with typed dataclasses for game state:

import time
from mcctp import SyncMCCTPClient, Actions

with SyncMCCTPClient("localhost", 8765) as client:
    client.on_state(lambda s: print(s.player_state.health))
    client.send(Actions.move("forward", "start"))
    time.sleep(2)
    client.send(Actions.move("forward", "stop"))

Also includes remote_play.py — a keyboard/mouse passthrough that lets someone play on your world from another computer.

License

MIT