alt Hacker News... then all elements of the prompt that come from your human creativity and that survive into the final result remain under your copyright. Which in most cases should be almost as good as owning the full copyright, although it might influence fair use defenses if even a full copy infringes on only a minor portion of the work.
If your prompt is a novel, it's probably copyright but you don't have copyright over someone else's thing they made using a different novel-length prompt, even if the thing is similar to yours, since they didn't copy the prompt. If your prompt is a sentence, it's probably too minimal to qualify for copyright.