Write a SKILL.md description as a routing rule, not a summary — it's the only line loaded at startup

Claude Platform Docs·medium signal

The description is the sole part of a skill Claude sees before deciding to fire it, so write it in third person, name the concrete trigger phrases users actually say, and state the inputs and outputs ("Extract text and tables from PDF files... Use when the user mentions PDFs, forms, or document extraction"). Vague descriptions ("Helps with documents") mean the skill silently never activates. With 100+ skills competing, this one field is what makes progressive disclosure actually work.