System Prompts: Setting Behavior

System prompt

Part of: Chat Roles & Messages

Hand the same model two different first lines and you get two different assistants. "You are a terse senior engineer" produces clipped, technical answers. "You are a warm kindergarten teacher" produces gentle, simple ones. Same model, same question, the only difference is one message at the top. That message is the system prompt , and it's the cheapest, most powerful lever you have. What it is The system prompt is the first message in the list, with role system. It's not part of the back-and-forth chatter, it's the setup. It tells the model who it is and how it should behave for the entire conversation that follows. The user never sees it; the app supplies it. Think of it as the director's note to an actor before the scene starts: persona, tone, rules, and the format of the lines they should deliver. How it works A system prompt is just content on a message with role system, placed first: Because the model reads top to bottom, the system message colors how it interprets and answers everything below it. A good system prompt usually covers a few things: - Persona : who the assistant is ("a careful legal assistant"). - Rules : what it must or must not do ("never give medical advice").

Challenge: The System-Prompt Router