Writing Effective Claude Skills

Claude skills are reusable, prompt-based automation shortcuts stored as `SKILL.md` files with YAML frontmatter and Markdown instructions. Best practices: narrow scope (one job per skill), specific instructions with examples, separate process (SKILL.md) from knowledge (REFERENCE.md), use XML tags for structure, control invocation via `disable-model-invocation` and `user-invocable` flags, and iterate based on real outputs.