System prompt

A system prompt is a hidden set of instructions a tool gives an AI model before your message, defining its role, tone and rules.

In practice: When a company builds a support bot or writing assistant on a general model, the system prompt tells the model who it is, what it may and may not say, and how to format answers. You usually do not see it, but it shapes every reply.

Why it matters when picking a tool

Many simple AI tools are mostly a system prompt on top of a big model. That is not necessarily bad, but it means you could often get similar results in a general assistant with your own instructions. Tools that let you set your own system prompt or custom instructions give you more control.