writing-skills

name	writing-skills
description	Use when creating, editing, or reviewing a skill before it ships. Covers when to write a skill, file layout, the SKILL.md shape, and how to verify it before relying on it.
user-invocable	false

Writing Skills

A skill is process documentation that another agent (or future you) loads when the trigger fits. Treat it like any other piece of code: write it for the reader who'll use it, verify it does what you think, and don't ship until you've watched it work.

Foundations live in writing-claude-directives (token efficiency, discovery, compliance). This skill is about skill-shaped directives specifically: when to make one, how to structure it, how to test it.

When a skill is the right tool

A skill earns its place when:

• The technique isn't obvious from the codebase or the task description.
• It's a procedure — steps, decision points, recipes — rather than a fact.
• Someone hitting the same situation later would benefit from the same playbook.

Don't write a skill for:

• One-off solutions to a specific bug.
• Standard practices already documented in the language/framework docs.
• Facts and conventions ("we use bun", "domains don't import each other") — those go in CLAUDE.md / AGENTS.md.

The carve is facts vs. procedures, not global vs. project: a project-specific procedure still wants to be a skill, just in the project's .claude/skills/ rather than the shared collection. When a section of CLAUDE.md has grown into a procedure, move it into a skill.

If you're unsure, the question to ask is "would I link a colleague to this six months from now?"

Skill types

Most skills land in one of three shapes:

Type	What it is	Examples
Technique	Concrete method with steps	review-subagent, debug
Pattern	Mental model for a class of problem	prompt-security-hardening (safe/unsafe pairs)
Reference	API / syntax / tool docs in skill form	jj

The shape changes how you write it (technique skills have steps; reference skills have tables). Pick early so the structure follows.

Directory layout

skills/
  skill-name/
    SKILL.md              # main entry (required)
    reference.md          # optional: heavy reference content
    helper.sh             # optional: scripts the skill calls

Default to a single SKILL.md. The official cap is a 500-line SKILL.md body; house preference is tighter — split out reference files when the main file crosses ~150 lines and the extra content is genuinely "load on demand" (rare flags, advanced syntax, edge-case recipes). Don't preemptively split.

When to add a script

Default to inline instructions. Pull logic into a script when:

• The operation is deterministic — validation, formatting, file generation, anything where the output is fully determined by inputs.
• The same code would be regenerated by every agent that loads the skill.
• Errors need explicit handling rather than the agent improvising recovery.

Scripts save tokens (the agent invokes helper.sh, doesn't re-emit the body) and improve reliability (deterministic output, no per-invocation drift). Trade-off: a script is another thing to maintain — if the logic might change with the language version, the harness, or the surrounding context, an inline pattern is cheaper to update.

Document script invocations by absolute path ("$HOME/dotfiles/agents/skills/<name>/helper.sh"): the agent's cwd is the project, not the skill directory. (Claude Code also exposes ${CLAUDE_SKILL_DIR}, but the $HOME form works in every harness.)

SKILL.md template

---
name: skill-name
description: "Use when [trigger/symptom][, optional: — what it does, third person]"
user-invocable: false   # omit if the user can invoke via slash command
---

# Skill Name

One or two sentences on what this skill is for and when it earns its keep.

## When to use

Symptoms or situations that should make the agent reach for this. If there
are common cases where it does *not* apply, name them.

## Core idea

The one thing that, if internalized, makes the rest follow. Often a
before/after, a key technique, or a pithy framing.

## Steps / patterns / reference

The body of the skill. Format depends on the type — steps for techniques,
tables for reference, before/after for patterns.

## Common mistakes

The things that bite people. Each one with a short fix.

Frontmatter notes:

• name: lowercase, hyphenated, gerund-ish for techniques (writing-skills, debugging-flakes).
• description: starts with "Use when…", reads in third person. This is the string the agent searches against — load it with the symptoms a future agent would actually type. Include both halves: the triggers and a short "what it does" (official guidance wants both, and capability words double as search keys). What to avoid is a description so complete it substitutes for the body — summarize the purpose, never the protocol.
• user-invocable: false for skills that the model loads on its own, not via /skill-name. Omit (or set true) for slash-command skills. The inverse control also exists: disable-model-invocation: true makes a skill slash-only (the model can't auto-load it) — right for heavyweight commands like polish or tour.

Verifying a skill before relying on it

Before shipping a discipline-enforcing skill, run it through testing-skills-with-subagents:

1. Run the realistic scenario without the skill loaded. See what the agent actually does. Capture the wording verbatim.
2. Write the skill to address those specific failures.
3. Re-run with the skill loaded. Watch it work.
4. Find the loopholes the agent finds, close them, re-run.

For pure reference skills (API docs, syntax) the test is simpler: ask the skill the kind of question you'd ask a colleague, and see if it answers.

The cheap version of all of this: read the skill back as if you'd never seen it before and weren't the one who wrote it. If the trigger conditions are fuzzy, or you'd skim past the key constraint, fix that before shipping.

Anti-patterns to avoid

• Narrative dated examples. "In session 2025-10-03 we found…" — too specific, doesn't reuse.
• Multi-language dilution. Writing the same example in JS, Python, Rust to "be inclusive" usually means three mediocre examples instead of one good one. Pick one language and label the conceptual point.
• Code embedded in flowcharts. Can't copy-paste, hard to read. Code goes in code blocks.
• Generic placeholder names. step1, helper2, thing — give them semantic names so the reader can follow what they do.
• Copy of project-specific lore. That belongs in CLAUDE.md / AGENTS.md, not here.

Cross-references

When a skill leans on another, link to the sibling SKILL.md by relative path:

See [testing-skills-with-subagents](../testing-skills-with-subagents/SKILL.md)
for the pressure-testing methodology.

Plain markdown links keep the skills portable across plugins and dotfiles.