feat: add protocol version checking

- Welcome message now includes protocolVersion field
- Optional hello message: clients can send { type: "hello", protocolVersion }
  to verify compatibility — server rejects mismatched versions
- Backwards compatible: clients that skip hello still work
- Add BAD_PROTOCOL error code

Author: mavisakalyan

src/index.ts

@@ -1,4 +1,4 @@
-import { decodeMessage, encodeMessage, ErrorCodes } from './protocol';
+import { decodeMessage, encodeMessage, ErrorCodes, PROTOCOL_VERSION } from './protocol';
 import { Room } from './room';
 import { RateLimiter } from './rate-limit';
 import { getHealthResponse, getMetricsResponse } from './health';
@@ -123,6 +123,20 @@ const server = Bun.serve<WSData>({
         return;
       }
 
+      // Handle hello (optional protocol version check)
+      if (decoded.type === 'hello') {
+        const clientVersion = decoded.protocolVersion;
+        if (typeof clientVersion === 'number' && clientVersion !== PROTOCOL_VERSION) {
+          ws.sendBinary(encodeMessage({
+            type: 'error',
+            code: ErrorCodes.BAD_PROTOCOL,
+            message: `Protocol mismatch. Server=${PROTOCOL_VERSION}, Client=${clientVersion}`,
+          }));
+          ws.close(1008, 'BAD_PROTOCOL');
+        }
+        return; // hello is consumed, not relayed
+      }
+
       // Handle ping
       if (decoded.type === 'ping' && typeof decoded.nonce === 'string') {
         ws.sendBinary(encodeMessage({

src/protocol.ts

@@ -1,9 +1,14 @@
 import { encode, decode } from '@msgpack/msgpack';
 
+// ─── Protocol Version ─────────────────────────────────────────────
+
+export const PROTOCOL_VERSION = 1;
+
 // ─── Server → Client Messages ─────────────────────────────────────
 
 export type WelcomeMessage = {
   type: 'welcome';
+  protocolVersion: number;
   playerId: string;
   peers: string[];
 };
@@ -83,4 +88,5 @@ export const ErrorCodes = {
   RATE_LIMITED: 'RATE_LIMITED',
   ROOM_FULL: 'ROOM_FULL',
   INVALID_MESSAGE: 'INVALID_MESSAGE',
+  BAD_PROTOCOL: 'BAD_PROTOCOL',
 } as const;

src/room.ts

@@ -1,5 +1,5 @@
 import type { ServerWebSocket } from 'bun';
-import { encodeMessage, type ServerMessage, ErrorCodes } from './protocol';
+import { encodeMessage, PROTOCOL_VERSION, type ServerMessage, ErrorCodes } from './protocol';
 import type { WSData } from './index';
 
 export interface Peer {
@@ -67,6 +67,7 @@ export class Room {
     // Send welcome to the new peer with list of existing peers
     ws.sendBinary(encodeMessage({
       type: 'welcome',
+      protocolVersion: PROTOCOL_VERSION,
       playerId: id,
       peers: existingPeerIds,
     }));