Need to @SIDE method that adds tooltip to item class. Cannot load this method type on server and causes crash.

[10:01:25] [Server thread/ERROR] [FML]: An error occurred trying to load an EventBusSubscriber com.aetheriumashenarmor.items.ItemAetheriumAshenCloak for modid aetheriumashenarmor
java.lang.NoClassDefFoundError: net/minecraft/client/util/ITooltipFlag