System prompts

A system prompt is a special instruction block that you send to Claude at the start of an API call to establish persistent behavioral guidelines, role, tone, and constraints for the entire conversation. Unlike user messages, which represent the back-and-forth dialogue, the system prompt acts as a foundational ruleset that shapes how Claude responds throughout a session — defining things like persona, output format, topic restrictions, and task-specific context. System prompts are passed via the `system` parameter in the Anthropic Messages API (or as a top-level string in the request payload). They are processed before any user turn and take precedence over conversational context. Because the API is stateless, the system prompt must be included on every API call to maintain consistent behavior across multiple requests. Importantly, system prompts used in Claude's web interface (claude.ai) are separate and distinct from API system prompts. Anthropic periodically updates claude.ai's built-in system prompt, but those changes do not affect API calls — API developers retain full control over what system prompt (if any) their application sends.