system prompt
The block of instructions the harness puts in front of your conversation before the model sees any of it: who it is, what it may do, what tools exist and how to ask for them. You do not type it, and it is charged against the context window like everything else.
It is a plain text file in most local harnesses, which means specialising a model is editing a file rather than training anything. The same weights become a tax assistant in one folder and a code reviewer in another because the paragraph above the conversation is different.
It is also the first thing re-sent on every turn, so a long one is a tax paid once per request for the whole life of the session.