System prompt / custom instructions

What is a system prompt?

A system prompt is the standing instruction given to a model before a conversation begins — the rules it should follow throughout, as distinct from the request you type. Custom instructions are the consumer-facing version of the same idea. Their weakness is that they are hand-written and static: you have to know your own working rules well enough to articulate them, and you have to remember to update them when the rules change. In practice most people write theirs once and never revisit it, so it drifts out of date silently.