Chapter 1 · Practice 2 of 4
Explain why a rule exists instead of only stating it
Why it matters
A bare rule gives the model one instruction. A rule with its reason lets it handle cases you did not list.
Anthropic's guide shows the difference with a formatting rule:
| Less effective | More effective |
|---|---|
| NEVER use ellipses | Your response will be read aloud by a text-to-speech engine, so never use ellipses, since the engine cannot pronounce them |
With the reason attached, the model can also avoid other things a speech engine would stumble over, such as tables or emoji, without being told about each one.
Apply this to every rule that matters:
- "Keep answers under 80 words because they are shown in a mobile notification."
- "Do not mention prices because they change daily and this text is cached for a week."
Avoid
- Stacking capital-letter warnings in place of a one-line reason