The system prompt is the top-level instruction set that shapes how a model behaves across an entire conversation: its role, tone, allowed topics, output format, and refusal rules. Most providers give it slightly higher priority than user messages.
Good system prompts are short, specific, and structured. Bad ones are long lists of "don't do X" that the model dutifully ignores under pressure. Use positive instructions ("Reply in JSON with these fields...") rather than negative ones ("Don't reply in prose..."). Put the most important rules first — models pay more attention to the start. Build yours with our System Prompt Builder.
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