System prompt
Instructions given to the model outside the conversation, which apply to every turn rather than one message.
A system prompt sets standing rules: who the model is acting as, what it must always or never do, and what shape the output takes. It is passed separately from user messages, and models are trained to weight it more heavily and to keep following it as the conversation moves on.
The practical test for whether something belongs in a system prompt: would you otherwise paste it into every message? Role, tone, output format, domain constraints and refusal rules all pass. The actual task does not.
Different products expose this differently - custom instructions in ChatGPT, a system parameter in the API, a CLAUDE.md or AGENTS.md file in a repository - but the mechanism is the same.
Tagged
Where this comes up
Prompts, configs and tutorials in the library that touch this term.
- GuideUniversal
Moving your rules into a config file
How to stop repeating the same instructions in every message, and what belongs in a system prompt against what belongs in the task.
No-padding everyday default
The one to set if you set only one. Answer first, nothing before it, nothing after it, and no offer to help further.
Tested on Claude Sonnet · May 2026
- PromptUniversal
Extract structured data as JSON
A strict extraction prompt that returns valid JSON only, with nulls for missing fields instead of invented values.
- PromptMidjourneyStable Diffusion
Image prompt skeleton
A fill-in structure for image models that keeps subject, environment, light, lens and style in the order the models weight them.
- ConfigClaude
CLAUDE.md for a PHP project
A project file that stops Claude Code rewriting your conventions, reformatting untouched files or inventing a framework you do not use.
- GuideUniversal
Working with long documents and code
Why long sessions drift, what a large context window actually buys you, and how to keep a model on task across a big input.