Skills fail silently
A broken skill does not error. It simply never triggers, and you conclude the mechanism does not work. That makes mechanical validation unusually valuable here — there is no other signal.
The most common cause is a description written as a title rather than a condition. "A tool for reviewing prompts" matches almost nothing, because nobody phrases a request that way.
The two halves have different economics
The description is always in context, on every request, whether or not the skill fires. It must be short and specific.
The body loads only when the skill triggers, so it can afford to be thorough. Getting this split wrong in either direction is what makes skills either useless or expensive.