# System prompt

**Source:** https://promtable.com/glossary/system-prompt

> A system prompt is the high-priority instruction block that defines a model's role, constraints, and default behaviors for an entire conversation.

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A system prompt is the high-priority instruction block that defines a model's role, constraints, and default behaviors for an entire conversation.

Most chat APIs (OpenAI, Anthropic, Google) accept a separate "system" message that the model treats as higher-trust than the user turn. The system prompt is where you set persona ("You are a senior contract lawyer."), output rules ("Always answer in markdown."), refusal policies, and tool-use guidance. Strong system prompts are concise, declarative, and example-light — examples belong in user messages. Anthropic and OpenAI both publish that the system prompt is the most cost-effective lever for behavior steering before reaching for fine-tuning.

## When to use

- Any production deployment of a chat or agent model.
- Enforcing safety, refusal, or compliance rules.
- Locking output format across many user inputs.

## Common mistakes

- Cramming all examples into the system prompt (use user-turn few-shot instead).
- Writing a 2000-word essay — long system prompts increase token cost on every turn.
- Forgetting that some open-source models have weaker system-prompt adherence.

## Related terms

- [prompt-engineering](https://promtable.com/glossary/prompt-engineering)
- [few-shot-prompting](https://promtable.com/glossary/few-shot-prompting)
- [instruction-following](https://promtable.com/glossary/instruction-following)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/system-prompt
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