creating-skill

# Creating Skills Create portable, reusable expertise that extends Claude's capabilities across contexts. ## What a skill is for — read before writing one A skill is a **procedural anchor**. Measured across 528 paired executions, 65.7% of skill effects come from supplying a usable procedure — setup steps, tool sequence, intermediate checks, pitfalls — and 4.5% from supplying facts the agent lacked (Jiang et al. 2026, arXiv:2608.14036). Three consequences shape every rule below: 1. **The procedure is the deliverable.** A high-level plan of three to five steps measured at 47.7% success against a 50.0% no-skill baseline. The same experience written as a full SKILL.md measured 79.2%. Vagueness scores worse than silence. 2. **Abstraction costs something.** `skill_guidance_misapplied_or_ignored` runs at 0.8% without a skill and **10.0%** with one. A skill that never says when it does not apply has not paid for itself. 3. **Facts are a reference, not a skill.** Writing one is fine. Call it a reference and do not expect it to change behaviour. Load [references/skill-utility-evidence.md](references/skill-utility-evidence.md) before authoring or reviewing a SKILL.md. It carries the failure-mode tables behind every requirement here, and reading it is what stops these rules from being followed as ritual. ## When to Create Skills Skills are appropriate when: - Capability needed across multiple projects/conversations - Procedural knowledge that applies broadly (no...