System prompt
The standing instruction sent with every request that sets the model's role, rules, and output conventions.
A system prompt is the first message in a request, tagged with the role `system`. Unlike the user's message, it is not part of the visible conversation — it is the persistent policy the model reads before every reply: who it is, what it may and may not do, and how it should format answers.
In an application the system prompt is written once and runs on every turn, which makes it the highest-leverage prompt you own. The key discipline is keeping stable policy in the system prompt and the specific request in the user turn, so neither leaks into the other.