Set a role, tone, output format, and guardrails — and get a clean, copy-ready system prompt for a Custom GPT, Claude Project, or API. The standing brief that turns a generic chatbot into a reliable specialist. Free, no sign-up.
Tone
Output format
Rules & guardrails
You are a helpful expert assistant. ## How you respond - Be clear, accurate, and helpful.
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A system prompt (also called instructions or a system message) is the standing brief that defines how an AI assistant behaves across a whole conversation — its role, tone, output format, and rules. It's what powers Custom GPTs, Claude Projects, and API assistants. A good one is the difference between a generic chatbot and a reliable specialist.
Fill in the role, expertise, and audience, click the tone, output format, and guardrails you want, and add any custom rules. It assembles a clean, well-structured system prompt you can paste straight into a Custom GPT's Instructions field, a Claude Project, or an API system message.
Yes, completely free with no sign-up. It runs entirely in your browser — nothing is uploaded. For ready-made persona configs and Custom Instructions packs, there's a Pro plan.
Yes. Custom GPT mode adds an assistant name and opening message and is formatted for the Instructions field. Claude / API mode outputs a clean system message. Generic mode gives you a portable, well-structured prompt for any model.
Be specific about the role and expertise, define the audience, set the tone and output format explicitly, and add guardrails (what to do when unsure, what to refuse, whether to cite sources). Clear structure and explicit rules produce consistent, reliable behavior.
Browse Custom Instructions packs and 30,000+ prompts built by experts — or explore the full free toolkit.