Prompted refers to something that has been prompted or initiated, while the antonyms of un-prompted would be prompted, encouraged, persuaded or impelled. The word un-prompted is often used to describe actions or statements that are spontaneous, unplanned, or not coordinated in any way. Its antonyms would imply that something was initiated or influenced by external factors. For example, if a person's speech was prompted, it would mean that the person had either received some kind of cue or encouragement to speak. Similarly, if an action was encouraged, it would mean that someone else had played a role in inspiring the person to act in a certain way.