How Unity WebGL Talks to a WebSocket Server (The Hard Way)

When I built socketio-unity, the hardest part wasn't the Socket.IO protocol. It wasn't binary packet assembly or reconnect logic. It was making WebGL work. Not "works on my machine" works. Production-verified, binary-safe, multi-socket, reconnect-safe works. This is the full technical writeup of how the WebGL bridge is built — every design decision, every edge case, and every gotcha that will silently destroy you in production if you miss it. Why WebGL Needs a Bridge at All Desktop Unity runs on the .NET runtime — you call System.Net.WebSockets directly and it just works. WebGL is different. Unity compiles your C# to WebAssembly via IL2CPP, which means your code runs inside a browser sandbox with no access to: No raw sockets from C# No background threads No System.Net anything Every WebSocket operation has to go through the browser's native WebSocket API. And the only way to reach browser APIs from C# in a WebGL build is through a .jslib — a JavaScript file Unity compiles into the fina