A good Skill is not just a prompt with a name. The real challenge is routing. If the model cannot reliably recognize when to activate it, even a well-written Skill will underperform in practice. A few things have worked well for us: keeping every SKILL.md short, splitting Write for exactly that reason, adding “not for” negative examples to reduce false activation, listing all skills in CLAUDE.md, and keeping descriptions under 1024 characters. These are small constraints, but they seem to improve activation quite a bit. One issue we did find is that descriptions should be written in third person. The official docs call this out as important for routing, and it matches the examples they give. So instead of starting with imperative wording like “Use when…”, it is better to start with a capability statement first, then the trigger condition. For example: “Extracts text and tables from PDF files, fills forms, and merges documents. Use when working with PDF files or when the user mentions PDFs.” Small detail, real difference.(Tw93)