If you're into this I've been pulling apart the agar.io client in this repo: https://github.com/ibash/agar

Within 'sandbox' in the repo you can start up a proxy server to the real agar.io backend. When you visit the web server it will connect to the real agar.io backend and let you play the game. The proxy also can parse some of the agar.io messages being sent to/from the backend.