---
title: System Prompt — AI Glossary
description: A system prompt is a top-level instruction that sets a model's role, persona, rules, and constraints for a conversation before any user message. It establishes standing behavior — tone, boundaries, output format, and priorities — that the model should honor across all subsequent turns.
canonical: https://prompeteer.ai/glossary/system-prompt
---

# System Prompt

A system prompt is a top-level instruction that sets a model's role, persona, rules, and constraints for a conversation before any user message. It establishes standing behavior — tone, boundaries, output format, and priorities — that the model should honor across all subsequent turns.

## Definition

Chat models distinguish message roles, and the system role carries the highest-priority framing instructions. A well-written system prompt defines who the model is acting as, what it must and must not do, and how it should format responses, giving stable behavior without repeating instructions in every user turn.

Because it governs the whole session, the system prompt is also central to safety and guardrails. Providers weight it heavily, but it is not an absolute security boundary: adversarial user input can still attempt prompt injection, so system instructions should be combined with other controls for sensitive applications.

## Sources

- [OpenAI — Prompt engineering (roles and system messages)](https://platform.openai.com/docs/guides/prompt-engineering)
- [Anthropic — Giving Claude a role with a system prompt](https://docs.anthropic.com/en/docs/build-with-claude/prompt-engineering/system-prompts)

## Related terms

- [role-prompting-glossary](https://prompeteer.ai/glossary/role-prompting-glossary)
- [prompt-injection](https://prompeteer.ai/glossary/prompt-injection)
- [guardrails](https://prompeteer.ai/glossary/guardrails)
- [context-engineering](https://prompeteer.ai/glossary/context-engineering)