System Prompt
Also known as · system message
High-level instructions that set a model's role, rules, and behavior for a session.
A system prompt is a special, high-priority instruction that sets the stage for a model before the user's messages begin. It typically defines the assistant's role, tone, constraints, and any standing rules — 'You are a careful financial analyst; never give legal advice; always cite sources.'
Because it frames the entire conversation, the system prompt has outsized influence on behavior. Applications use it to enforce consistent persona and guardrails across every user interaction without repeating those instructions each turn.
It's also a security consideration: a well-designed system prompt separates trusted instructions (from the developer) from untrusted content (from users or documents), which is part of defending against prompt injection.