How Models Learn

System message

Instructions an app places at the start of the conversation, usually hidden from the user, which also use up tokens.

Everything counts against it: the hidden system message the app adds, every message so far, and the reply being written.
From Tokens and context windows, step “The context window is a fixed budget”

See it in action

Try system message hands-on in the lesson Tokens and context windows, part of the How AI assistants work course.

Open the lesson