diff --git a/static/extensions/ThatBobo/MultiplayerCore.js b/static/extensions/ThatBobo/MultiplayerCore.js
new file mode 100644
index 00000000..b77aeb2f
--- /dev/null
+++ b/static/extensions/ThatBobo/MultiplayerCore.js
@@ -0,0 +1,434 @@
+(async function (Scratch) {
+  let socket = null;
+
+  let serverURL = "";
+  let myId = "";
+
+  let players = {};        // id -> player data
+  let playerList = [];     // array of IDs
+  let sharedVars = {};     // global shared variables
+  let lobbies = {};        // lobby storage (optional)
+
+  let lastMsg = "";
+  let lastSender = "";
+
+  let currentLobbyName = "";
+  
+  class MultiplayerCore {
+    getInfo() {
+      return {
+        id: "multiplayercore",
+        name: "Multiplayer Core",
+        color1: "#0000FF",
+        blocks: [
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Server"
+          },
+          {
+            opcode: "setServer",
+            text: "set multiplayer server to [URL]",
+            blockType: Scratch.BlockType.COMMAND,
+            arguments: {
+              URL: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "wss://example.com"
+              }
+            }
+          },
+          {
+            opcode: "connect",
+            text: "connect to server",
+            blockType: Scratch.BlockType.COMMAND
+          },
+          {
+            opcode: "disconnect",
+            text: "disconnect from server",
+            blockType: Scratch.BlockType.COMMAND
+          },
+          {
+            opcode: "connectionStatus",
+            text: "server connection status",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Lobby"
+          },
+          {
+            opcode: "joinLobby",
+            text: "join lobby [name]",
+            blockType: Scratch.BlockType.COMMAND,
+            arguments: {
+              name: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "main"
+              }
+            }
+          },
+          {
+            opcode: "leaveLobby",
+            text: "leave lobby",
+            blockType: Scratch.BlockType.COMMAND
+          },
+          {
+            opcode: "currentLobby",
+            text: "current lobby",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Players"
+          },
+          {
+            opcode: "playerId",
+            text: "my player ID",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            opcode: "allPlayers",
+            text: "all players",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            opcode: "playerCount",
+            text: "player count",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            opcode: "isPlayerConnected",
+            text: "player [id] is connected?",
+            blockType: Scratch.BlockType.BOOLEAN,
+            arguments: {
+              id: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "player1"
+              }
+            }
+          },
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Messaging"
+          },
+          {
+            opcode: "sendMessage",
+            text: "send message [msg]",
+            blockType: Scratch.BlockType.COMMAND,
+            arguments: {
+              msg: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "hello"
+              }
+            }
+          },
+          {
+            opcode: "sendToPlayer",
+            text: "send message [msg] to player [id]",
+            blockType: Scratch.BlockType.COMMAND,
+            arguments: {
+              msg: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "hi"
+              },
+              id: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "player1"
+              }
+            }
+          },
+          {
+            opcode: "lastMessage",
+            text: "last received message",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            opcode: "lastSender",
+            text: "last message sender",
+            blockType: Scratch.BlockType.REPORTER
+          },
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Sync"
+          },
+          {
+            opcode: "setSharedVar",
+            text: "set shared variable [name] to [value]",
+            blockType: Scratch.BlockType.COMMAND,
+            arguments: {
+              name: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "score"
+              },
+              value: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "0"
+              }
+            }
+          },
+          {
+            opcode: "getSharedVar",
+            text: "shared variable [name]",
+            blockType: Scratch.BlockType.REPORTER,
+            arguments: {
+              name: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "score"
+              }
+            }
+          },
+          {
+            opcode: "onSharedVarChange",
+            text: "when shared variable [name] changes",
+            blockType: Scratch.BlockType.HAT,
+            arguments: {
+              name: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "score"
+              }
+            }
+          },
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Movement"
+          },
+          {
+            opcode: "setPlayerPos",
+            text: "set my position to x: [x] y: [y]",
+            blockType: Scratch.BlockType.COMMAND,
+            arguments: {
+              x: {
+                type: Scratch.ArgumentType.NUMBER,
+                defaultValue: 0
+              },
+              y: {
+                type: Scratch.ArgumentType.NUMBER,
+                defaultValue: 0
+              }
+            }
+          },
+          {
+            opcode: "getPlayerX",
+            text: "x position of player [id]",
+            blockType: Scratch.BlockType.REPORTER,
+            arguments: {
+              id: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "player1"
+              }
+            }
+          },
+          {
+            opcode: "getPlayerY",
+            text: "y position of player [id]",
+            blockType: Scratch.BlockType.REPORTER,
+            arguments: {
+              id: {
+                type: Scratch.ArgumentType.STRING,
+                defaultValue: "player1"
+              }
+            }
+          },
+          {
+            blockType: Scratch.BlockType.LABEL,
+            text: "Events"
+          },
+          {
+            opcode: "onPlayerJoin",
+            text: "when player joins",
+            blockType: Scratch.BlockType.HAT
+          },
+          {
+            opcode: "onPlayerLeave",
+            text: "when player leaves",
+            blockType: Scratch.BlockType.HAT
+          },
+          {
+            opcode: "onMessage",
+            text: "when message received",
+            blockType: Scratch.BlockType.HAT
+          },
+          {
+            opcode: "onConnect",
+            text: "when connected to server",
+            blockType: Scratch.BlockType.HAT
+          },
+          {
+            opcode: "onDisconnect",
+            text: "when disconnected from server",
+            blockType: Scratch.BlockType.HAT
+          }
+        ]
+      }
+    }
+
+    // --- Server ---
+    setServer(args) {
+      this.serverURL = args.URL;
+    }
+
+    connect() {
+      if (!this.serverURL) return;
+
+      this.socket = new WebSocket(this.serverURL);
+
+      this.socket.onopen = () => {};
+
+      this.socket.onclose = () => {};
+
+      this.socket.onmessage = (event) => {
+        let data;
+        try {
+          data = JSON.parse(event.data);
+        } catch {
+          return;
+        }
+
+        if (data.type === "id") {
+          this.myId = data.id;
+        }
+
+        if (data.type === "players") {
+          this.players = data.players;
+        }
+
+        if (data.type === "message") {
+          this.lastMsg = data.message;
+          this.lastSender = data.sender;
+        }
+
+        if (data.type === "sharedVar") {
+          this.sharedVars[data.name] = data.value;
+        }
+
+        if (data.type === "position") {
+          const id = data.id;
+          this.sharedVars[`pos_${id}_x`] = data.x;
+          this.sharedVars[`pos_${id}_y`] = data.y;
+        }
+      };
+    }
+
+    disconnect() {
+      if (this.socket) {
+        this.socket.close();
+        this.socket = null;
+      }
+    }
+
+    connectionStatus() {
+      if (!this.socket) return "disconnected";
+      return this.socket.readyState === 1 ? "connected" : "connecting";
+    }
+
+    // --- Lobby ---
+    joinLobby(args) {
+      const name = args.name;
+      this.currentLobbyName = name;
+      this.socket?.send(JSON.stringify({ type: "joinLobby", name }));
+    }
+
+    leaveLobby() {
+      this.socket?.send(JSON.stringify({ type: "leaveLobby" }));
+      this.currentLobbyName = "";
+    }
+
+    currentLobby() {
+      return this.currentLobbyName;
+    }
+
+    // --- Players ---
+    playerId() {
+      return this.myId;
+    }
+
+    allPlayers() {
+      return JSON.stringify(this.players);
+    }
+
+    playerCount() {
+      return this.players.length;
+    }
+
+    isPlayerConnected(args) {
+      const id = args.id;
+      return this.players.includes(id);
+    }
+
+    // --- Messaging ---
+    sendMessage(args) {
+      const msg = args.msg;
+      this.socket?.send(JSON.stringify({
+        type: "message",
+        message: msg
+      }));
+    }
+
+    sendToPlayer(args) {
+      const msg = args.msg;
+      const id = args.id;
+      this.socket?.send(JSON.stringify({
+        type: "direct",
+        message: msg,
+        id
+      }));
+    }
+
+    lastMessage() {
+      return this.lastMsg;
+    }
+
+    lastSender() {
+      return this.lastSender;
+    }
+
+    // --- Sync ---
+    setSharedVar(args) {
+      const name = args.name;
+      const value = args.value;
+
+      this.sharedVars[name] = value;
+
+      this.socket?.send(JSON.stringify({
+        type: "sharedVar",
+        name,
+        value
+      }));
+    }
+
+    getSharedVar(args) {
+      const name = args.name;
+      return this.sharedVars[name] ?? "";
+    }
+
+    // --- Movement ---
+    setPlayerPos(args) {
+      const x = args.x;
+      const y = args.y;
+
+      this.socket?.send(JSON.stringify({
+        type: "position",
+        x,
+        y
+      }));
+    }
+
+    getPlayerX(args) {
+      const id = args.id;
+      return this.sharedVars[`pos_${id}_x`] ?? 0;
+    }
+
+    getPlayerY(args) {
+      const id = args.id;
+      return this.sharedVars[`pos_${id}_y`] ?? 0;
+    }
+
+    // --- Events (hat blocks) ---
+    onPlayerJoin() {}
+    onPlayerLeave() {}
+    onMessage() {}
+    onConnect() {}
+    onDisconnect() {}
+  }
+  
+  Scratch.extensions.register(new MultiplayerCore());
+})(Scratch);