MCP Reborn Multiplayer

[Icefox]

I am generally new to Forge and MCP but I am trying to make a client for my server. The client works fine in single player but when I try to join any server I get a “Failed to log in: Invalid Session (Try restarting your game and the launcher).” Why could this be? This is before I built the jar for installation, so it’s still using the runclient gradle task in IntelliJ. Also if anyone has a good resource for making a launcher for the custom client please let me know.