Both the server and the HC try to bind to Steam's default ports: 8766 (Steam3) and 27016 (Steam queries).
A Headless Client is not a real player. It has no graphical user interface (GUI), no rendered viewport, and ideally, it should not launch Steam's overlay. By default, when you run arma3server.exe with the -client parameter, it attempts to initialize a full Steam client context. If Steam isn't running, or if the HC executable isn't properly recognized by Steam, authentication fails. arma 3 headless client steam authentication failed
If running multiple HCs, ensure each has a unique communication port assigned via the -port= launch parameter. 🔍 Common Root Causes Both the server and the HC try to
The "Steam Authentication Failed" error in typically occurs when a Headless Client (HC) —a separate instance of the game used to offload AI processing—fails to verify its connection with Steam's servers or the game host . This most often happens during setup on a local machine or a dedicated server due to missing configuration parameters or mismatched mod lists. Core Fixes for Steam Authentication Errors By default, when you run arma3server
By default, a Headless Client may be blocked if it hasn't "agreed" to the BattlEye terms. Locate your Headless Client's file.
Add the local IP of the HC to the headlessClients[] and localClient[] arrays:
You launch your HC shortcut. The console boots. It connects to the server. It loads the mission. Then, silence. The HC sits in the lobby as a "Player" or disconnects entirely with the dreaded red text: