Probably Ehud’s first message Judges 3:19 had been delivered to the attendants, and by them carried to the king. Now Ehud is admitted to the king’s presence, into the cool upper chamber.
I have a message from God unto thee - Ehud believed himself to be accomplishing the divine mandate, and so his words were true in a certain sense. But it was also a stratagem to cause the king to rise, that the thrust might be sure. (The king rose at once, in true Oriental respect for a divine message, or from fear, compare Joshua 9:24.)