A system prompt is a block of instructions given to an AI model at the start of every conversation, separate from what the user types. It defines who the assistant is, what it may and may not do, how it should sound and what information it can draw on.
The model reads the system prompt first and treats it as the highest-priority guidance for the conversation. Developers typically include the business name, tone of voice, opening hours, escalation rules, formatting preferences and instructions such as "only answer from the supplied information".
For a retailer or venue, the system prompt is where brand and policy live. It can say that the assistant must never quote a price it has not been given, should offer a callback for complaints, and must not give medical or allergy advice beyond the approved menu. It is also where you set the language and spelling style.
System prompts are not perfectly secure. A determined user can sometimes coax a model into revealing or ignoring them, so never place passwords, API keys or confidential pricing rules inside one. Back important rules with real controls, such as limiting which systems the assistant can access. Test with awkward questions before launch.
A well-written system prompt is the main lever for making a general-purpose model behave like a dependable, on-brand member of the team.