What is System prompt?

A system prompt is the top-level instruction given to an AI model before your message — it sets the model's role, rules and tone for the whole conversation.

Where your message asks for a specific thing, the system prompt establishes the ground rules that apply to everything the model does in that session: who it should act as, what it must and must not do, how to format answers, and how to handle requests it can't fulfil. It's set once and quietly shapes every reply that follows.

Getting the system prompt right is one of the highest-leverage moves in prompting, because it fixes recurring problems at the source rather than re-explaining them in every message. A clear role and a few firm constraints there will often do more for output quality than a long, detailed user prompt.