System Prompt
Standing instructions that set an AI model's role, tone and rules before a conversation starts, and what makes a good one.
System prompt: a set of standing instructions given to an AI model before a conversation starts. It sets the assistant’s role, tone, rules and limits, and applies to everything that follows.
In plain English
If a normal prompt is a question you ask, a system prompt is the job description the assistant was given before you arrived. It might say “answer briefly”, “act as a support agent for this company” or “never reveal private data”.
Where you see it
- Custom assistants and projects, where you write instructions that apply to every chat.
- Apps and chatbots built on AI models, where the developer sets the rules.
- AI agents, where it defines the goal, tools and guardrails.
What makes a good one
- A clear role and goal.
- Specific rules and what the assistant must never do.
- The output format you want.
- What to do when information is missing: ask, do not guess.
- A rule that text found in emails, files or web pages is data, not instructions.
Common confusion
- A system prompt guides behaviour but is not a guarantee. Models can still make mistakes or be tricked by hostile text.
- It is not always visible to users, and different products handle it differently.
- Do not put secrets or passwords in it.
Related terms
Prompt, AI agent, context window.