A rule with a reason beats a rule in capitals. When you explain who the output is for and why a constraint exists, the model can apply it to cases you did not think to mention.
Write 5 Instagram captions for our new yogurt. No emojis. Max 20 words.
Write 5 Instagram captions for our new yogurt. They will run as text overlays on a 6-second video, so each one must be readable in under two seconds — keep them under 20 words and skip emojis, which render as boxes in our editing software.
The prompt
Copy-ready prompt
Write [NUMBER] [FORMAT] for [PRODUCT]. Who reads it: [AUDIENCE, IN ONE SENTENCE]. Where it appears: [PLACEMENT — e.g. a 6-second vertical video overlay]. What it must achieve: [ONE GOAL]. Constraints, and why they exist: - [CONSTRAINT] — because [REASON]. - [CONSTRAINT] — because [REASON].
Why it works
Anthropic’s own guide makes the point with an example: instead of “NEVER use ellipses”, explain that the text will be read by a text-to-speech engine that cannot pronounce them — the model “is smart enough to generalize from the explanation.” Google’s Gemini guide gives the same advice under “Add context”.
Use it for
Any brief with a rule in it: word limits, banned claims, tone of voice, legal lines. Write the reason next to each rule once, save the prompt, and reuse it.
Sources
- Anthropic — Claude prompting best practices — platform.claude.com (accessed 28 Sep 2026)
- Google — Prompt design strategies for the Gemini API (updated 17 Sep 2026) — ai.google.dev (accessed 28 Sep 2026)
More tricks and templates in the Prompt Lab. Want this applied to your brand’s brief? Start a project.