In plain language
It is the job description the AI follows before it sees the user’s specific message.
Example
“You classify support tickets into Billing, Technical, or Sales and reply with only the category.”
Related terms
Prompt · Large Language Model (LLM) · Structured Output · Guardrails
Common misconception
A system prompt is not a security boundary; crafted inputs can still persuade a model to ignore it.
Implementation guidance
State the role, rules, and output format clearly, include examples, keep it version-controlled, and test changes against saved sample inputs.