Why Your Unity WebGL Multiplayer Breaks on Some Networks

You test your Unity WebGL multiplayer game. It works perfectly. You share it with a friend — it works. You share it with someone on a corporate network, or a phone hotspot, or after pushing a new build — and it silently fails to connect. No useful error. No crash. Just... nothing. This happens because Unity WebGL multiplayer has three distinct failure modes that don't exist in standalone builds: CORS blocks the connection before it opens, WebSocket gets intercepted by a network proxy, and browser cache serves your old build to players days after you deployed a fix. Here's how to diagnose and fix all three. 1. CORS — the most common first failure What it is CORS (Cross-Origin Resource Sharing) is a browser security rule. When your WebGL game at https://yoursite.com tries to connect to https://api.example.com , the browser first sends a preflight OPTIONS request asking "are you allowed to talk to me?" Your Socket.IO server must answer yes. If it does not, the browser blocks the WebSocket