What is a system prompt?

A system prompt is a set of instructions given to an AI model before every conversation, usually hidden from the person using the app. Whoever built the product writes it once. It tells the model what its job is, what tone to use, what it must not do and how to format its answers.

Why it matters for a business

The system prompt is one of the biggest reasons 2 products on the same model behave differently. It is why one support tool says “the policy does not cover this case” while another guesses. It is a request rather than a lock: a model usually follows it and can sometimes be talked out of it, so anything that must never happen needs a permission the AI lacks or a software check as well. It is also sent and billed on every request, which is where caching helps.

Example

Let’s say an internal assistant’s system prompt says to answer only from company policy documents, quote the clause used, and say so when the policy does not cover the case. Every question anyone types arrives at the model with those instructions ahead of it.

Go deeper

AI wrappers: what gets built around the model

Related terms

• Prompt engineering
• Context engineering
• AI guardrails
• Prompt injection

All terms, A to Z