Why one sentence is enough
The best short story prompts are constraints, not blueprints. One good sentence contains a situation, an implied character, and an unresolved tension — everything a story needs. 'A lighthouse keeper finds a message in a bottle addressed to herself' already has mystery, isolation, self-discovery, and temporal dislocation baked into it. The AI's job is to find the story that was already there. Your job is to write a sentence interesting enough to contain one.
What makes the ending instruction so important
The 'satisfying but not predictable' instruction is the difference between an AI short story that feels complete and one that feels like it just stopped. AI defaults to predictable resolutions — the lovers reunite, the mystery is solved neatly, the character learns their lesson and everything is fine. 'Not predictable' pushes against that default toward something more honest: an ending that fits the story's logic without being obvious from the first paragraph. You'll still want to read the ending and decide if it earns it, but the instruction dramatically raises the baseline.