How to Write a System Prompt That Actually Sticks

A system prompt sits above the conversation and shapes every reply. When you put your rules there instead of repeating them in each message, the model stays on track far longer. This guide turns a loose instruction into a tight system prompt you can reuse.

What you need

• Access to a chat tool that exposes a system or custom-instructions field
• A clear idea of the role you want the model to play
• About 8 minutes

Step 1: Name the role first

Start with one sentence that fixes who the model is and who it serves. A specific role narrows the whole conversation before you ask for anything.

You are a senior editor for a finance newsletter aimed at busy founders. You value plain language and short sentences.

Step 2: List rules as imperatives

Below the role, add short commands rather than prose. Each line should be testable so you can tell at a glance whether a reply followed it.

Rules:
- Keep paragraphs under three sentences.
- Define any acronym the first time it appears.
- Never invent statistics; say what you do not know.
- End with one concrete next step.

Step 3: Show one example

A single worked example pins down tone better than another paragraph of rules. Add a short before and after so the model can pattern-match.

Step 4: Save and reuse it

Paste the finished prompt into the custom-instructions or system field so it loads for every new chat. Keep a copy in a note so you can refine it over time.

Result: your rules live in one place and apply to every message, so you stop repeating yourself and the model stays in character.