3.2.1. Chat Completions

💡 First Principle: Chat completions simulate conversation by maintaining context through message history—like an email thread where each reply knows what came before. The system role is special: it's instructions the model reads first but the user never sent. Think of it as CC'ing a supervisor who sets ground rules. This is why system messages are powerful for controlling behavior—the model treats them as authoritative context, not user requests to potentially ignore.