System prompt: definition

A system prompt is the standing instruction given to a model before the conversation starts, setting its role, rules, tone, and the boundaries it must keep across every reply.

If the user message is the question, the system prompt is the job description the model reads first. It says who the model is acting as, what it may and may not do, how to format answers, and when to hand off to a human.

In production this is where a lot of reliability lives. A tight, well-tested system prompt is what keeps an assistant on-brand and inside its lane across thousands of conversations you will never read.

Related terms

Prompt engineering: Prompt engineering is the craft of writing the instructions, examples, and context you give a model so it produces reliable, correctly formatted output for a specific task.
Guardrails: Guardrails are the rules and checks that keep an AI system inside safe, correct behaviour, blocking off-topic, unsafe, or out-of-scope responses before they reach the user.
Large language model (LLM): A large language model is a neural network trained on huge amounts of text to predict the next token, which lets it write, summarise, classify, and answer questions across almost any topic.