A system prompt is a developer- or application-provided instruction that defines a model’s overall behavior, role, constraints, tone, and output expectations. It typically applies across user requests and is designed to guide model behavior consistently.
The system prompt is provided by the application developers and defines the model's overall behavior. It can set the model's role (e.g. "You're a writing assistant"), expected tone, output format (such as a strict JSON schema), and any global constraints. This prompt remains stable across requests.