System prompt
A system prompt is the initial, hidden instruction that defines the behaviour, persona and constraints of an LLM before the user asks anything.
System prompts sit 'above' the conversation and define identity, tone, purpose, constraints and formats. For a customer-service bot: 'You are Webrock Media's assistant. Always respond in Dutch, refer technical questions to [email protected], never use jargon without explanation.' System prompts are powerful but not unbreakable — prompt injections can bypass them.
Example
An e-commerce chatbot has system prompt: 'You only advise on products from our catalogue [list]. For off-topic questions: politely redirect to FAQ. Never promise discounts beyond official offers.' This ensures consistent, safe behaviour.
Frequently asked questions
System or user prompt?
System: one-time structural instruction (persona, limits). User: the actual per-turn question. For API calls always separate them, don't dump everything into user.
How long can a system prompt be?
From one sentence to thousands of tokens. Keep it as short as possible but specific; long system prompts raise cost and can dilute focus.
Related terms
Further reading
- → Our service: GEO