System prompt

The instructions the harness prepends to every model provider request — the agent's standing brief: who it is, how to behave, which tools it can call, what conventions to follow. Usually stable across a session.

Usage:

"Two harnesses, same model, totally different behavior on the same prompt."

"Different system prompts. One's tuned for terse code edits, the other for explaining — that's where the divergence lives, before your message even arrives."