Prompting·intermediate
System prompt
The invisible instruction that shapes how the AI behaves all conversation long.
Most AI conversations have two participants. There's a third, silent one: the system prompt. A block of instructions the model reads before anything else — the rules for everything that follows.
On claude.ai you don't see it. Anthropic sets it. When you build with the API (like Mosaic's familiar does), you write your own.
Good system prompts define:
- The role the AI should play ("you are a patient tutor").
- Constraints ("never write more than three sentences").
- Voice and tone ("use plain language, no jargon").
- What to refuse ("don't give the answer — only hints").
Mosaic's familiar has a system prompt that keeps it in character. Peek into app/api/familiar/route.ts to read it.