Adding 'when not to use' to a skill description, sparingly
A short line saying when a skill does not apply can stop a stubborn near-miss. Used everywhere it bloats the description and blurs the job. When one line helps.
Sometimes a well-written description still catches one kind of request it should leave alone. A single "not for" line fixes that. A paragraph of them makes things worse.
Try the positive fix first
Most over-triggering comes from broad verbs, fixed by naming the artefact and outcome instead. Do that first, as described in a skill that fires too often. Reach for an exclusion only when a specific near-miss survives.
One line, at the end
description: Restyles web UI in the macOS native dark style: system colours, SF type,
sidebar app windows. Use for Mac app windows and Apple-style landing pages.
Not for Windows or Linux-first apps.It names the near-miss once and stops. The body of the skill can then say what to do if it is invoked anyway, such as recommending a different approach, but selection is decided by the description, so that is where the line lives.
Keep a note of which near-miss the exclusion was added for, and rerun your trigger test set to confirm it helped rather than just feeling tidier.
Questions
Should every skill description say when not to use it?
No. Most do not need it. Add an exclusion only for a specific near-miss that keeps triggering the skill after you have tightened the positive wording.
Where should the exclusion go?
At the end of the description, as one short sentence. The start should say what the skill does and when to use it.
Can the body of SKILL.md handle exclusions instead?
The body can tell Claude to hand back work that is out of scope once the skill is active. It does not prevent the skill being picked, which is decided from the description.