diff --git a/server/index.js b/server/index.js
new file mode 100644
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--- /dev/null
+++ b/server/index.js
@@ -0,0 +1,190 @@
+/* eslint-env node */
+/**
+ * Sovereign self-hosted real-time WebSocket relay for the lego-builder rooms.
+ *
+ * Replaces the Liveblocks SaaS dependency. Pure room broadcast relay:
+ *   - one WebSocket connection == one client in exactly one room
+ *   - the room keeps the current shared `storage` snapshot
+ *     ({ bricks, cursorColors }) plus a Map of connected clients
+ *     ({ connId, presence }).
+ *
+ * Wire protocol (all JSON):
+ *   server -> client on connect:
+ *     { t: 'init', connectionId, storage, others: [{ connectionId, presence }] }
+ *   server -> client on peer join / leave:
+ *     { t: 'join', connectionId, presence }
+ *     { t: 'leave', connectionId }
+ *   server -> client on peer presence / storage update:
+ *     { t: 'presence', connectionId, presence }
+ *     { t: 'storage', storage }
+ *   server -> client on relayed ephemeral event:
+ *     { t: 'event', from: connectionId, event }
+ *
+ *   client -> server:
+ *     { t: 'presence', presence }   // updates this client's presence, fan-out to others
+ *     { t: 'storage', storage }     // replaces the shared snapshot, fan-out to others
+ *     { t: 'event', event }         // ephemeral (cursors), relayed to OTHERS only, NOT persisted
+ *
+ * Runs standalone: `node server/index.js` (no build step). Listens on
+ * process.env.PORT || 8090, accepts ws(s)://HOST/?room=<id>.
+ */
+
+import { WebSocketServer } from "ws";
+import { parse as parseUrl } from "node:url";
+
+const PORT = process.env.PORT || 8090;
+const HEARTBEAT_INTERVAL_MS = 30000;
+
+// roomId -> { storage: { bricks, cursorColors }, clients: Map<connId, ws> }
+const rooms = new Map();
+
+let connCounter = 0;
+function nextConnectionId() {
+  connCounter += 1;
+  return connCounter;
+}
+
+function getRoom(roomId) {
+  let room = rooms.get(roomId);
+  if (!room) {
+    room = { storage: { bricks: [], cursorColors: {} }, clients: new Map() };
+    rooms.set(roomId, room);
+  }
+  return room;
+}
+
+function send(ws, payload) {
+  if (ws.readyState === ws.OPEN) {
+    ws.send(JSON.stringify(payload));
+  }
+}
+
+function broadcastToOthers(room, exceptWs, payload) {
+  const data = JSON.stringify(payload);
+  for (const peer of room.clients.values()) {
+    if (peer !== exceptWs && peer.readyState === peer.OPEN) {
+      peer.send(data);
+    }
+  }
+}
+
+function othersList(room, exceptWs) {
+  const others = [];
+  for (const peer of room.clients.values()) {
+    if (peer !== exceptWs) {
+      others.push({ connectionId: peer.connectionId, presence: peer.presence });
+    }
+  }
+  return others;
+}
+
+const wss = new WebSocketServer({ port: PORT });
+
+wss.on("connection", (ws, req) => {
+  const { query } = parseUrl(req.url, true);
+  const roomId = (query && query.room) || "default";
+
+  const room = getRoom(roomId);
+  const connectionId = nextConnectionId();
+
+  ws.isAlive = true;
+  ws.connectionId = connectionId;
+  ws.roomId = roomId;
+  ws.presence = null;
+
+  room.clients.set(connectionId, ws);
+
+  // Send the current snapshot + roster to the newcomer.
+  send(ws, {
+    t: "init",
+    connectionId,
+    storage: room.storage,
+    others: othersList(room, ws),
+  });
+
+  ws.on("pong", () => {
+    ws.isAlive = true;
+  });
+
+  ws.on("message", (raw) => {
+    let msg;
+    try {
+      msg = JSON.parse(raw.toString());
+    } catch {
+      return; // ignore malformed frames, never crash
+    }
+    if (!msg || typeof msg !== "object") return;
+
+    switch (msg.t) {
+      case "presence": {
+        const first = ws.presence === null;
+        ws.presence = msg.presence;
+        // First presence == this client announcing itself => `join`.
+        // Subsequent updates => `presence`.
+        broadcastToOthers(room, ws, {
+          t: first ? "join" : "presence",
+          connectionId,
+          presence: msg.presence,
+        });
+        break;
+      }
+      case "storage": {
+        // Last-writer-wins replace of the shared snapshot.
+        if (msg.storage && typeof msg.storage === "object") {
+          room.storage = msg.storage;
+          broadcastToOthers(room, ws, { t: "storage", storage: room.storage });
+        }
+        break;
+      }
+      case "event": {
+        // Ephemeral, relayed to others only, never persisted.
+        broadcastToOthers(room, ws, {
+          t: "event",
+          from: connectionId,
+          event: msg.event,
+        });
+        break;
+      }
+      default:
+        break;
+    }
+  });
+
+  const cleanup = () => {
+    if (!room.clients.has(connectionId)) return;
+    room.clients.delete(connectionId);
+    broadcastToOthers(room, ws, { t: "leave", connectionId });
+    if (room.clients.size === 0) {
+      rooms.delete(roomId);
+    }
+  };
+
+  ws.on("close", cleanup);
+  ws.on("error", cleanup);
+});
+
+// Heartbeat: drop dead sockets so rooms get cleaned up reliably.
+const heartbeat = setInterval(() => {
+  for (const ws of wss.clients) {
+    if (ws.isAlive === false) {
+      ws.terminate();
+      continue;
+    }
+    ws.isAlive = false;
+    try {
+      ws.ping();
+    } catch {
+      // ignore
+    }
+  }
+}, HEARTBEAT_INTERVAL_MS);
+
+wss.on("close", () => clearInterval(heartbeat));
+
+// Join/leave semantics: a newcomer learns existing peers via `init.others`,
+// then announces itself by sending its first `presence` frame right after
+// `init` — the relay forwards that first frame as `join` (with presence) and
+// any later presence changes as `presence`. Departures are broadcast as
+// `leave` on socket close.
+
+console.log(`[realtime] lego-builder relay listening on :${PORT}`);