How to write a system prompt
A system prompt is the standing instruction an AI reads before every message. Write it in five parts: role, rules, tone, output format and examples. Keep it short, and put one-off details in your message instead. Prompt Builder Skill House builds one from a plain-language brief, free in your browser.
The five parts
| Part | What to write | Example Example |
|---|---|---|
| Role | Who the AI is and who it helps. | You are the support assistant for a small bakery. |
| Rules | What it must always or never do, in plain sentences. | Never promise a delivery date. Send refund requests to a person. |
| Tone | How it should sound. | Warm, brief, no jargon. |
| Output format | The shape of every answer. | Two short paragraphs, then one next step. |
| Examples | One or two good answers to copy the style from. | A sample question and the answer you want. |
A complete example Example
You are the support assistant for Rosa's Bakery, a small shop that sells cakes and bread. Rules: - Answer questions about orders, ingredients and opening hours. - Never promise a delivery date. Say the team will confirm it. - Send refund requests and allergy emergencies to a person at [email protected]. Tone: warm and brief. No jargon. Format: two short paragraphs, then one clear next step.
Where to put it
| Tool | Where the standing instruction goes |
|---|---|
| Claude app | Custom instructions in settings, or the instructions of a Project. |
| ChatGPT | Custom instructions in settings, or the instructions of a Project. |
| Gemini | The instructions of a Gem. |
| Any AI through an API | The system field of the request. |
Five common mistakes
- Too long. Long prompts hide the rules that matter. Start short.
- One-off details inside it. Put today's task in your message, not in the standing instruction.
- A vague role. "Be helpful" tells the AI nothing. Name the job and the person it helps.
- No output format. Say what a good answer looks like.
- Rules that disagree. Read it once as a stranger and look for pairs that conflict.
Build yours in a minute
Open the System prompt tool, write what you want in a sentence or two, and press Generate. It is free and runs in your browser. Polishing the result with an AI is optional: use your own API key from any AI company, or a Lova plan.
Questions
What is a system prompt?
A system prompt is a standing instruction that an AI reads before your messages. It sets the role, the rules, the tone and the format of every answer. When it conflicts with a single message, the system prompt usually wins.
How long should a system prompt be?
As short as possible while it still produces the behavior you need. Start with a few lines, test it on real questions, and add a line only when you see a problem that line fixes.
Is a system prompt the same as custom instructions?
They do the same job. Chat apps call it custom instructions or project instructions, and the API calls it the system parameter. Each is a standing instruction applied to every chat.
Can I write a system prompt for free?
Yes. The System prompt tool in Prompt Builder Skill House runs in your browser and builds one from a short brief at no cost. Polishing it with an AI is a paid step, using your own API key or a Lova plan.
Source
Anthropic's guidance on writing clear, direct instructions: Be clear and direct.