Writing a great System Prompt

The four-part structure (Identity · Expertise · Tone · Boundaries), three worked examples, and the do's and don'ts of a system prompt that gives your AI Avatar

Home › Help Center › AI Avatars › Writing a great System Prompt

The system prompt is the single most important field when building a persona. It's a plain-English description of who your avatar is — its name, role, expertise, tone, and limits. Most avatars that feel "off" have a weak or vague system prompt; most that feel sharp have a focused one. This guide shows you what to put in it.

What the system prompt does

The system prompt is read by the intelligence model behind your avatar at the start of every conversation. It shapes every reply: how the avatar introduces itself, what it talks about, what it refuses to talk about, and the tone it uses. The avatar will speak the answers aloud, so write the prompt as if you were briefing a new employee — natural sentences, no bullet points, no markdown, no headers.

A simple structure that always works

You can write your own from scratch, but a good prompt almost always has four parts in this order:

Identity — who is the avatar? Give it a name, a role, and the company / product it represents. Example: "You are Maya, a friendly support specialist for ACME Bikes." Expertise — what does it know about? Be specific. Example: "You know our product catalogue inside out — bike models, sizing, accessories, shipping policy, warranty." Tone — how does it speak? Pick 2–3 traits. Example: "Speak warmly, with short sentences. Use simple words. Never sound corporate. Sprinkle in a bit of dry humour when it fits." Boundaries — what does it refuse or redirect? Example: "If asked about competitor brands, gently redirect. If asked anything medical or legal, say it's outside your scope and suggest contacting a professional."

Three examples that work

Customer support avatar

You are Lina, a warm and patient support agent for Glow Skincare. You know every product we sell, our return policy (30 days, full refund), and how shipping works (3–5 business days within the US, 7–14 internationally). Speak conversationally and use the customer's first name when they share it. Keep replies short — under three sentences when possible. If you don't know something specific, say so and offer to escalate. Never recommend medical treatments. Never discuss competitor brands.

Sales / qualifier avatar

You are Ben, an upbeat sales rep for Quill Software. Your job is to understand whether the visitor is a good fit for our team plan ($299/month, 10 seats minimum). Always open by asking what they're trying to solve. Listen first, pitch second. Don't push — if someone isn't a fit, recommend our self-serve plan instead. Be honest about pricing the moment it comes up. Avoid jargon. Never promise discounts or custom features.

Educational tutor avatar

You are Mr. Carter, a patient high-school physics teacher. Explain concepts with everyday analogies before reaching for formulas. Always check whether the student followed along before moving on. Ask a quick comprehension question at the end of each topic. Encourage them when they get something right. Never give the answer to a homework problem directly — guide them to it instead.

Do

Use a name. A persona with a name feels like a person, not a chatbot.
Be concrete. "Use short sentences" beats "be conversational". "Don't quote prices" beats "be careful with sales claims".
Write the boundaries explicitly. What the avatar should NOT do is just as important as what it should do.
Use second person ("you are…"). The model reads it as instructions to itself.

Don't

Don't paste a job description. "Senior support engineer with 5 years of experience…" reads like a résumé and the avatar will quote it.
Don't use bullet points or markdown. The avatar may read formatting symbols out loud.
Don't put product facts in the prompt. Pricing, hours, policies — those belong in the Knowledge Base so they stay updated and accurate.
Don't write a novel. Sweet spot is 150–500 words. Longer prompts dilute focus.

Length sweet spot

Aim for 150–500 words. The field accepts more, but prompts past 500 words start to lose focus — the model has to weigh too many instructions and conflicts creep in.

When the prompt isn't enough

The system prompt is great for personality and behaviour. It's a bad place for facts that change. If you need the avatar to know specific pricing, an FAQ, a product spec, or hundreds of pages of training material, use the Knowledge Base instead. If you need strict rules the avatar must never break (legal language, compliance language, banned topics), use Guardrails on top of the prompt.

Test your persona by opening a Share Link to yourself and just talking to it for 5 minutes. Note every reply that feels off — that's your editing list for the next pass at the prompt.