System Prompt

Also known as: system message, system instruction

A system prompt is a privileged message injected at the start of a conversation to establish the model's role, behavioral constraints, output format, and any persistent context. Unlike user turns, the system prompt is controlled by the developer rather than the end user and is often invisible in the interface. It is the primary mechanism for customizing a general-purpose model into a specialized application agent with a specific persona and guardrail policy.