Skill Builder

Skill Builder — the principles behind a skill that triggers and stays focused

For interactive scaffolding, evaluation and benchmarking of a skill, use Anthropic's official Skill Creator (claude.com/plugins/skill-creator). This skill is the complementary judgment layer: the opinionated principles that make a skill trigger reliably and stay focused, plus a review checklist and the packaging path. Use it to design, review, or debug a skill, not to generate boilerplate.

The root virtue: predictability

A skill exists to pull determinism out of a stochastic system. What you make repeatable is the process, not the output — the agent should take the same approach every run, even when the result differs. Design the procedure; don't script the answer. Two tests fall out of this:

• Completion criteria the agent can check. The skill must give it a way to tell done from not-done — otherwise it stops early and calls it finished.
• One leading word. Anchor the skill on a single concept the model already knows well (e.g. tracer bullet, receipt, lesson, seam) and repeat it in the name, the description and the body. A word the model thinks with anchors both triggering and execution — far more than a phrase it has to decode.

First: is a skill even the right tool?

Claude Code has five extension primitives. Pick the one that fits before writing anything.

• Skill — a reusable workflow or body of knowledge, pulled in when its description matches the request. For "how to do X well" that recurs across projects. (This is what you're building here.)
• CLAUDE.md — always-on project context (conventions, commands, layout). For facts every session in THIS repo needs. Not triggered; always loaded.
• Subagent — a delegated task run in its own context window. For fan-out, isolation, or a specialized role.
• Hook — deterministic automation on an event (PostToolUse, Stop…). For "always run X when Y", enforced by the harness, not the model.
• MCP server — external tools/data over a protocol. For reaching a system the model otherwise can't.

If the answer is "knowledge or a procedure the model should apply when the situation arises", it's a skill. Otherwise stop and use the right primitive.

The description is everything (the #1 lever)

A skill only helps if it triggers at the right moment. The description in the frontmatter decides that almost entirely. Spend most of your effort here — and know the real failure mode: Claude UNDER-triggers skills far more than it over-triggers (it only consults a skill when it can't trivially handle the task itself). So lean inclusive and a little "pushy".

A strong description:

1. "Use when…" + real trigger phrases someone would actually type, and be pushy: "use this whenever the user mentions X, Y or Z, even if they don't say '' explicitly". Anthropic's own docx/xlsx skills phrase it exactly that way.
2. What it does, in one breath — enough for the model to know it's the right tool.
3. "NOT for…" — only if a sibling skill competes. Anthropic's official skills mostly OMIT this, because their domains are distinct (pdf ≠ xlsx, no confusion possible). Add it when you have adjacent skills that could both match (a "review changes" skill vs a "ship a feature" skill) — there it disambiguates. Don't add it just to look careful: an over-restrictive description makes the skill under-trigger, which is the worse problem.

---
name: my-skill
description: Use when <situations>, e.g. "<real phrase>", "<real phrase>" — be a little pushy
  ("whenever the user mentions …, even if they don't say '<skill>'"). <what it does in one line>.
  [NOT for <adjacent case> — add only if a sibling skill competes.]
---

Name: lowercase, hyphens, a verb-or-noun that reads as a command (book-distill, senior-review). Add argument-hint if the skill takes arguments.

Model-invoked vs user-invoked. By default a skill is model-invoked: its description sits in context and other skills can reach it — right for reusable knowledge that should fire on its own. Set disable-model-invocation: true to make it user-invoked (/skill only): the description leaves the model's reach, saving context. Reserve it for skills nobody else needs to auto-trigger — heavy, domain-specific, run-on-demand (a tax-dossier writer, a one-off migration).

Keep the body skimmable (progressive disclosure)

Three loading levels: metadata (name + description, always in context) → SKILL.md body (loaded when the skill fires) → bundled resources (loaded or executed on demand). Keep the body lean so it doesn't crowd the context.

• < 500 lines is the target, not a law. Anthropic's own docx skill is 590. Go longer when the domain warrants it, but past ~500 add a layer of hierarchy and clear pointers ("for X, read references/x.md").
• Externalize the heavy stuff — when it pays. Two thirds of Anthropic's official skills are a single SKILL.md; split only when it earns it. When you do, scripts/ (executable, deterministic ops) earns its place far more often than references/ (docs) — the document skills (pdf, xlsx, docx) are mostly scripts. Use references/<variant>.md for multi-framework domains; a reference over ~300 lines gets a table of contents.
• Imperative and concrete. "Do X. Never Y." Minimal example over paragraphs of theory.
• A checklist at the end beats a wall of prose.

Folder shape

my-skill/
└── SKILL.md            # required: frontmatter (name, description) + body
    (optional alongside:)
    scripts/*           # executable code for deterministic ops (earns its place most often)
    references/*.md     # detail loaded on demand (multi-framework, heavy domains)
    assets/*            # templates, fonts, icons used in the output

Personal skills live in ~/.claude/skills/<name>/ (or a project's .claude/skills/). To SHARE one, wrap it in a plugin.

Package for distribution (plugin + marketplace)

To make a skill installable by others:

my-plugin/
├── .claude-plugin/
│   └── plugin.json          # { "name", "description", "version", "author" }
└── skills/
    └── my-skill/
        └── SKILL.md

A marketplace is a repo with .claude-plugin/marketplace.json listing plugins (each source a relative path like ./plugins/my-plugin). Users add and install with:

/plugin marketplace add owner/repo
/plugin install my-plugin@marketplace-name

Validate before publishing: claude plugin validate <path>. Pin a version (omit it and every commit becomes a new auto-updating version). Keep all files inside the plugin root (no ../shared).

The loop to author one

1. Decide it's a skill (see the five primitives).
2. Write the description first — Use-when + real trigger phrases, leaning pushy; add NOT-for only if a sibling competes. This is the product.
3. Write the lean body — core principle, the steps/loop, a minimal example, a checklist.
4. Externalize the heavy stuff to reference/.
5. Test triggering: does it fire on the real phrases? Does it stay silent on the adjacent cases? Tune the description, not the body.
6. Package + validate if sharing (plugin.json, marketplace.json, claude plugin validate).

Anti-patterns

• Vague description ("helps with code") — it never triggers. The most common failure, and worse than over-triggering.
• Over-restrictive description — too many caveats and the skill under-triggers (the bigger risk). Lean pushy; reserve "NOT for" for genuine sibling competition.
• Monster SKILL.md — only a problem if the bulk is detail that belongs in scripts/ or references/; size alone isn't the sin (docx is 590 lines).
• Hardcoding one project's specifics in a skill meant to be reused — keep it stack-agnostic, discover specifics live.
• Renaming the concept mid-body — one term, used consistently (the model anchors on it).
• Wrong primitive — an always-on rule should be CLAUDE.md or a hook, not a skill.
• Sediment — stale instructions left from past versions that no longer match the skill; prune them on every edit instead of layering on top.
• No-op line — a sentence the agent would follow anyway; delete the whole sentence, don't trim its words.

Checklist

• A skill is the right primitive (not CLAUDE.md / subagent / hook / MCP)
• description has Use-when + real trigger phrases, leans pushy (under-triggering is the bigger risk); NOT-for only if a sibling competes
• Name is lowercase-hyphen, reads like a command
• Body is skimmable; heavy detail lives in reference/
• One leading word the model already knows, repeated in name + description + body; imperative, minimal examples
• The skill gives the agent a checkable "done" signal (completion criteria)
• Triggers on the intended phrases, silent on adjacent cases
• If shared: plugin.json + marketplace.json, version pinned, claude plugin validate passes