System Prompt

When you chat with an AI product, your message usually isn’t the first thing the model sees. Before it comes a system prompt written by whoever built the product: who the assistant is, what it should and shouldn’t do, which tools it can use, and how to format answers. Custom assistants and agents are largely defined by their system prompts.

A good system prompt reads like a brief for a capable new colleague: the goal, the audience, the constraints and what a good result looks like. It counts toward the context window and is billed as input on every call, so long ones add cost; that is one reason providers offer prompt caching. Prompt vs. context vs. memory engineering shows where the system prompt sits among everything else a model sees.