What is a system prompt?
A system prompt is the standing instruction the app applies before your message. It sets role, rules, and boundaries that a single chat message should not have to repeat.
Also called: system message, developer message
In plain English
Your message is today's job. The system prompt is the employee handbook for the assistant: 'You draft, you do not send. You never invent a price. You write like our office, short, no fluff.' Some products call this custom instructions, a project brief, or org instructions. Keep the standing rules shorter than the job. Five lines beat a novel nobody updates. Put voice, the draft-only rule, and the ban on invented fees there. Leave the customer's name and today's date in the message. When the rules and the message fight, the model may follow either one. Read both before you blame the tool. Date the rules. An undated handbook is how last season's offer keeps leaking into new drafts.
How an owner would use it
Write five rules and stop. Put the voice, the ban on invented numbers, and the 'draft only' rule in the standing instructions. Put today's customer facts in the normal prompt.
Common mistakes
Owners paste today's customer into the standing instructions, so the next chat still talks about that customer. Standing instructions are for rules that survive the week: voice, draft-only, never invent a price. A product's custom instructions on your own account are a related slot, not always the same screen as a builder's system prompt. Do not hide a refund policy only in a system prompt nobody else can see. Put the policy in the knowledge base too.
A worked example
A small marketing agency
House rules on the account
This is an illustration, not a customer result. The agency sets a system prompt: drafts only, no discounts unless the human typed one, client names stay inside that project. The producer then prompts, 'Write the status email from these three bullets.'
Related terms
Prompts and guides
Questions owners ask
What is a system prompt?
It is the standing instruction an AI app uses on every turn, before the user's message. It holds the rules you do not want to retype.
How do I use a system prompt?
Put stable rules there: tone, draft-only, do not invent prices, which files to trust. Put the facts of today's job in the regular prompt.
How is a system prompt different from custom instructions?
A system prompt is the standing note the app applies before the user talks, sometimes hidden from the staff. Custom instructions, when a product offers them, are the note on that person's account. Either way, they hold rules, not today's invoice.
Product guides are on AI apps for business.