# System Prompt

> A system prompt is a privileged instruction block, placed before the conversation, that sets a model's role, rules, tone, and constraints for every subsequent turn.

- Identifier: PTL-0003
- Category: Foundations
- Canonical URL: https://protologue.com/t/system-prompt/
- Also known as: system message, developer message, system instruction

## Description

Chat APIs typically expose the system prompt as a separate message role. Models trained with an instruction hierarchy are taught to give system instructions precedence over conflicting user or tool content, which makes the system prompt the main lever for operator control and a frequent target of prompt-leaking attacks.

## Broader terms

- [Prompt](https://protologue.com/t/prompt/)

## Related terms

- [Instruction Hierarchy](https://protologue.com/t/instruction-hierarchy/)
- [Role Prompting](https://protologue.com/t/role-prompting/)
- [Prompt Leaking](https://protologue.com/t/prompt-leaking/)

## Sources

- Ouyang et al. (2022). Training language models to follow instructions with human feedback. https://arxiv.org/abs/2203.02155
- Wallace et al. (2024). The Instruction Hierarchy: Training LLMs to Prioritize Privileged Instructions. https://arxiv.org/abs/2404.13208

## Cite this entry

Protologue. (2026). System Prompt. In Protologue: A Taxonomy of Prompting and LLM Techniques (v1.0.0, PTL-0003). https://protologue.com/t/system-prompt/

License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)