Guide
What is a system prompt?
A system prompt is a standing instruction sent with the conversation, separate from the user’s question. It sets role, rules, and output habits that should apply on every turn. The user prompt is the specific request for this turn.
Updated 2026-09-23
Before and after
Before
You are a helpful assistant. Also, here is today's ticket, please reply and remember to be brief every time.
After
System: You answer billing questions for Northwind. Use only the policy in the user message. Under 120 words. If the policy does not cover the case, say so. Do not invent amounts. User: Policy: refunds within 14 days if unused. Ticket: customer asks for a refund on day 20. Draft the reply.
How to do it
1. Write rules, not the task
“You are a billing assistant. Answer in under 120 words. Never invent refund amounts.” That belongs in the system prompt.
2. Put the case in the user message
The ticket text, the policy excerpt, and “draft the reply” belong in the user turn.
3. Keep it short enough to reread
A system prompt that nobody can scan will contradict itself. One screen is a practical limit for a product assistant.
4. Test by changing only the user message
If behavior changes when you rephrase the task but the rules hold, the split is right. If the rules vanish, they were only in the user message.
When to use this
Use a system prompt for rules you do not want to repeat: tone, tools the model may use, topics it must refuse, and the shape of every answer. Put the task, the documents, and the question in the user message. If you only have a chat box with no system field, the first message can act as a stand-in, but it is easier for a later message to override it.
Where system prompts actually exist
APIs from OpenAI, Anthropic, and Google accept a system (or developer) message that is separate from user content. ChatGPT custom instructions, Claude projects, and Gemini Gems are product versions of the same idea: text that rides along with every chat.
A system prompt is not a jailbreak and it is not hidden from the developer who set it. Treat it as product copy. If it conflicts with the user message, different models resolve the conflict differently, so put safety and format rules in the system prompt and do not contradict them later.
Sources
Questions
What is a system prompt?
A system prompt is a standing instruction applied to the whole conversation. It is separate from the user message, which carries the task for this turn.
What should go in a system prompt?
Role, tone, standing constraints, and output rules you want on every turn. Put the specific question and source text in the user message.
Can I use a system prompt in ChatGPT?
In the API, yes, as the system or developer message. In the chat product, custom instructions are the equivalent standing text. A normal chat has no separate system field.