What is System prompt?
Also called: system message · custom instructions
A system prompt is the standing instruction set for an AI assistant — who it is, how it behaves, what it must never do — applied before any user message.
Most chat models accept two kinds of input: the system prompt, which sets the assistant's identity, rules and style for the whole conversation, and user messages, which carry each request. ChatGPT's "custom instructions" and the "system" field in the API are system prompts.
A good system prompt reads like an employee handbook for one role: identity and purpose, what it always does, what it never does, how it handles edge cases (out of scope, uncertainty, hostile users), and the default format. Because it applies to every turn, it's the highest-leverage prompt you'll write for any product or assistant.
You are Nova, the support assistant for Chairside (dental scheduling software). Always answer in under 120 words, cite the relevant help-centre article, and never promise refund dates. If asked anything outside Chairside, say: "I can only help with Chairside — here's how to reach a human."
- Write rules as observable behaviour, not vibes ("under 120 words", not "be concise").
- Include what to do when unsure — otherwise the model guesses confidently.
- Add a line that fixes the instructions against override attempts (see prompt injection).
Use it right now
Ask our brain anything on the homepage — it remembers the whole conversation — or write a brief in the Studio and see the prompt it compiles to.