A system prompt is the standing instruction that defines how an AI behaves for a whole conversation — its role, tone, rules and output format. To write one that works, cover three layers: who the AI is (character), how it operates (rules), and what it is working with right now (context). Keep it specific, keep it short, and test it on a real task. Here is the practical version, with examples.
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A persona skill is a professionally written system prompt you can study and reuse. Ryan sets the role, method and guardrails of a chief of staff — open it to see exactly how a strong system prompt is structured.
See the skill →What a system prompt actually does
Every AI reply is shaped by two things: the message you just typed, and a set of background instructions that persist across the whole chat. That background is the system prompt. In ChatGPT it lives in custom instructions; in Claude it is the skill or project instructions; in an API call it is the system field. Get it right and you stop repeating yourself — the AI holds the role for you.
The three layers of a good system prompt
1. Character — who the AI is
Define the role, expertise, tone and boundaries. Not “be helpful” — that is too vague to change anything. Instead: “You are a senior financial analyst. You are precise, you show your working, and you flag assumptions rather than hiding them.” This layer rarely changes.
2. Rules — how it operates
Spell out the workflow, constraints and output format. State what to do and what to avoid, and be explicit about format: “Always answer in three parts: summary, detail, next step. Never invent figures; if data is missing, say so.” This layer evolves as you learn what works.
3. Context — what it is working with
Give the situation: who the user is, the goal, any relevant facts. This is the layer that changes per conversation, and it is often better supplied in the user message or an attached file than baked into the system prompt.
A worked example
Here is a compact system prompt that uses all three layers:
“You are an experienced email copywriter for a B2B software company (character). Write in plain, confident language; no hype words like ‘revolutionary’ (rules). Every email must have one clear call to action and stay under 120 words (rules). Output the subject line, then the body (format). Ask me for the audience and the offer before writing if I have not given them (rules).”
Notice what it does not do: it does not say “be creative” or “do your best.” Every line changes the output.
Five mistakes that quietly ruin system prompts
Vague adjectives. “Professional,” “engaging” and “helpful” mean nothing measurable — replace them with specific behaviour. No output format. If you do not specify structure, you get a different shape every time. Too long. Overloaded prompts bury the important rules; cut anything that is not pulling its weight. Never tested. You cannot judge a system prompt by reading it — give the AI a real job and look at where the output is wrong. Contradictions. “Be concise” and “explain everything in detail” cancel out; check for rules that fight each other.
The shortcut: start from one that works
Writing a strong system prompt from a blank page is a real skill. The faster path is to start from a proven one and adjust. Our persona skills are exactly that — complete, professionally written system prompts for specific roles, which you can open, study and adapt. For more on prompt craft, see how to write better AI prompts and our library of AI prompt examples and templates.
$29 one-time
If writing a full system prompt sounds like a project, start from one that already works. Ryan is a complete role — character, playbook and output rules — ready to paste into Claude, ChatGPT or Gemini.
Get Ryan →Frequently asked questions
What is a system prompt?
A system prompt is the standing instruction that tells an AI who it is and how to behave for an entire conversation — its role, tone, rules and output format. Unlike a normal message, which asks for one thing, the system prompt shapes every reply until you change it.
What is the difference between a system prompt and a user prompt?
The system prompt sets the persona and the rules once; the user prompt is the specific task you type each time. Think of the system prompt as the job description and the user prompt as the day’s assignment.
How long should a system prompt be?
As short as it can be while still covering role, rules and output format. A useful rule of thumb: if it fits on one page, keep it all in the system prompt. If it grows past a page, split it — core instructions in the prompt, reference material in attached files.
Can I reuse the same system prompt across ChatGPT, Claude and Gemini?
Mostly, yes. The structure — role, rules, output format, examples — transfers across models with minor edits. That is why a well-built persona skill works whether you paste it into Claude’s Skills, ChatGPT’s custom instructions or Gemini. See what Claude skills are.
In short
A system prompt sets the AI’s role, rules and output format for the whole conversation. Cover character, rules and context; be specific; keep it short; and test it on a real task. To skip the blank page, start from a ready-made persona skill in the skills marketplace, or refine a draft with the free prompt optimizer.