A special instruction given to a model at the start of a conversation that sets its behavior, personality, and rules. Unlike user messages, the system prompt is meant to be persistent and authoritative — it defines who the model is for this session. "You are a helpful coding assistant. Always use TypeScript."
Why it matters
System prompts are the primary tool for customizing AI behavior without fine-tuning. They're how companies make Claude act as a customer support agent, a code reviewer, or a medical information assistant — same model, different system prompt.