A system prompt is a set of instructions provided to an LLM at the start of a session — before any user interaction — that shapes how the model behaves throughout the conversation. System prompts can define the model’s role and persona, establish guardrails, specify output formats, provide background context, and set the tone and style of responses. They are the primary mechanism through which developers customize foundation models for specific applications without fine-tuning. Well-crafted system prompts are essential for reliable, consistent AI behavior.