System prompt
Also called: system message
A system prompt is the standing instruction a model receives before your message ever arrives — who it should be, what rules it follows, what it's for.
A system prompt is the standing instruction a model receives before your message ever arrives — who it should be, what rules it follows, what it’s for. Every serious AI product has one; it’s a large part of the difference between a raw model and a finished product, and it’s why the same underlying model behaves differently inside different tools.
Why it matters: Two things. For your own work, it’s the highest-leverage configuration surface most people never touch — a Claude Project’s instructions, a custom GPT’s setup, an assistant’s persona are all system prompts, and a good one means your standards are ambient instead of re-explained every session. And for evaluating products: a system prompt steers, it doesn’t guarantee. It’s influence, not law — worth remembering when a vendor offers “we’ve instructed it never to do X” as the entire safety story.