System prompt
A system prompt is the standing set of instructions given to a language model before a conversation starts. It defines who the model is, what it may and may not do, and how it should behave.
Also called: instructions, agent instructions, persona prompt
The system prompt is the closest thing a language model has to a job description. It typically sets the agent’s name and role, the business it represents, the facts it may state, the questions it should ask, the tone it should take, and the things it must never do, quote prices it has not been given, promise a callback time, discuss competitors.
Because the model treats the prompt as authoritative, the prompt is both where behavior is shaped and where mistakes are made. Vague instructions produce inconsistent calls; contradictory ones produce unpredictable calls. Persona settings such as name, voice and personality are, underneath, contributions to this prompt.
The prompt is also a boundary that callers will occasionally probe, deliberately or not, which is why it is paired with guardrails rather than relied on alone.