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Hidden, high-priority instructions that set a model's role, tone and rules before the conversation begins, framing every reply that follows.
A system prompt is a privileged instruction layer that conditions a model for the whole session — defining its persona, constraints and refusal rules. It is one practical mechanism for encoding the kind of behavioural principles explored in Constitutional AI (Bai et al., 2022), where a written set of rules guides model behaviour.
Because system prompts sit above user input in priority, they are also a target for prompt-injection attacks that try to override them.
Primary, peer-reviewed and archival sources for this definition.
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