System Prompt

A system prompt is the set of instructions given to a language model before the user's input—defining the model's persona, constraints, output format, and behavioral rules. In the OpenAI and Anthropic APIs, it occupies the "system" role in the message array. The user never sees it, but it shapes everything the model does. System prompts are where enterprises encode their business logic: "You are a customer support agent for Acme Corp. You may only reference information from the provided knowledge base. Never discuss competitor pricing. Always respond in the customer's language." A well-written system prompt is the cheapest, fastest way to control model behavior. A poorly written one is why your AI chatbot told a customer they could get a full refund on a non-refundable ticket. System prompt engineering is a core skill that most teams underinvest in.