principles.fyi · the brain · concept

system prompt

Hidden instructions set before the chat that tell the model how to behave.

[system instructions] + [user message] -> response

A system prompt is a special block of instructions placed at the very start of the conversation, before the user's messages, that sets the model's persona, rules, and boundaries — like 'You are a helpful assistant; be concise; refuse harmful requests.' Mechanically it is just more conditioning text the decoder attends to, but it is given a privileged position and the model is trained (via instruction-tuning and RLHF) to weight it heavily. It is how an app developer steers the model's overall behavior separately from whatever the end user types.