Concept
System Prompt
Highest-priority instruction block that sets a model's persona, rules, and output format.
Definition
The system prompt is the first message in a chat conversation, hidden from the user, that sets global behaviour: persona, tone, allowed tools, safety policies, output format. Providers weight it more heavily than user turns, so it is where you encode immutable rules.
Common use cases
- Persona definition
- Safety constraints
- Output formatting