system prompt design

A system prompt is a standing set of instructions placed at the very top of a conversation, before the user ever types. It sets the assistant's role, its rules, its tone, and what it should or should not do. Think of it as the briefing you give an employee on their first day: who they are working as, what the house style is, which lines must not be crossed. The user then chats normally, and every reply is shaped by that briefing.

Designing one well is about being explicit and durable. Good system prompts state the persona, the goals, the format expectations, the boundaries, and how to handle requests that fall outside them, all in plain, ordered instructions. Because the system prompt usually carries more weight than a passing user line, it is also the natural place to put safety and policy rules. Its limits are the same as any prompt: it is words, not a hard lock, and a clever or persistent user may still try to talk the model past it, so it should be combined with other guardrails rather than trusted alone.