System Prompt
A high-priority instruction block before user messages that sets persona, rules, and format.
- Tools layer their own system prompt before yours.
- Most prompt engineering wins live here.
- Short and specific outperforms long and fluffy.
When to use: For any repeatable task where output should follow specific rules or formats.
Example: "You are a code reviewer. Output a JSON array of {file, line, severity, suggestion}. Never modify files."