Skill Builder

What This Skill Does

Guides the creation and optimization of Claude Code skills using official best practices. Use this whenever:

• Building a new skill from scratch
• Optimizing or auditing an existing skill
• Deciding on advanced features (subagent execution, hooks, dynamic context, etc.)
• Troubleshooting a skill that isn't working correctly

For the complete technical reference on all frontmatter fields, advanced patterns, and troubleshooting, see reference.md.

Quick Start: What Is a Skill?

A skill is a reusable set of instructions that tells Claude Code how to handle a specific task. Skills live in .claude/skills/[skill-name]/SKILL.md inside your project. When you type /skill-name or describe what you need in natural language, Claude loads the skill's instructions and follows them.

Think of skills as SOPs for Claude. Instead of re-explaining a workflow every conversation, you write it once and invoke it forever.

How they work under the hood:

• Your project's CLAUDE.md instructions are always loaded, every conversation
• Skill descriptions (from frontmatter) are always loaded so Claude knows what's available
• The full skill content only loads when the skill is actually invoked
• Once loaded, Claude follows the skill's instructions while still respecting your CLAUDE.md rules

Mode 1: Build a New Skill

When building a new skill, run the Discovery Interview first. Do NOT start writing files until discovery is complete.

Discovery Interview

Ask questions using AskUserQuestion, one round at a time. Each round covers one topic. Move to the next round only after the user answers. Keep going until you're 95% confident you understand the skill well enough to build it without further clarification.

Round 1: Goal & Name Why this matters: A clear goal prevents scope creep. The name becomes the /slash-command, so it needs to be memorable and specific.

• What does this skill do? What problem does it solve or what workflow does it automate?
• What should we call it? (Suggest a name based on their answer -- lowercase, hyphens, max 64 chars)

Round 2: Trigger Why this matters: The description field is how Claude decides whether to load your skill. Bad trigger words mean Claude never uses it. Too broad means Claude fires it when you don't want it.

• What would someone say to trigger this? (Get 2-3 natural language phrases)
• Should it be user-only (/slash-command), Claude-auto-invocable, or both?
• Does it accept arguments? If so, what? (e.g., a topic, a URL, a file path)

Round 3: Step-by-Step Process Why this matters: Claude follows instructions literally. Vague steps produce vague results. Specific steps produce consistent output every time.

• Walk me through exactly what should happen from trigger to output. What's step 1? Step 2? Keep going.
• For each step: Does Claude do it directly, or delegate to a subagent/script?
• Does this need to be conversational (back-and-forth with the user) or is it a fire-and-forget task?

Round 4: Inputs, Outputs & Dependencies Why this matters: Skills that don't specify where to find inputs or where to put outputs produce inconsistent results. Nailing this down makes the skill reliable.

• What inputs does the skill need? (Files, API responses, user arguments, live data)
• What does it produce? (Files, text output, structured data) Where do outputs go?
• Does it need external APIs, scripts, or tools? Which ones?
• Does it need reference files, style guides, templates, or examples?

Round 5: Guardrails & Edge Cases Why this matters: Skills without guardrails can produce unexpected behavior -- wrong outputs, unnecessary API costs, or actions you didn't intend.

• What could go wrong? What are the common failure modes?
• What should this skill NOT do? Any hard boundaries?
• Are there cost concerns? (API calls, AI image generation, etc.)
• Any ordering or dependency constraints? (e.g., "must check X before doing Y")

Round 6: Confirmation Why this matters: Misunderstandings caught here save you from rebuilding the skill later.

After all rounds, summarize your understanding back to the user in this format:

## Skill Summary: [name]

**Goal:** [one sentence]
**Trigger:** `/name` + [natural language phrases]
**Arguments:** [what it accepts, or "none"]

**Process:**
1. [step]
2. [step]
...

**Inputs:** [what it reads/needs]
**Outputs:** [what it produces + where]
**Dependencies:** [APIs, scripts, agents, reference files]
**Guardrails:** [what can go wrong, what to avoid]

Ask: "Does this capture it? Anything to add or change?" Only proceed to building once the user confirms.

Skipping rounds: If the user provides enough context upfront (e.g., they describe the full workflow in their first message), skip rounds that are already answered. Don't re-ask what you already know.

Build Phase

Once discovery is complete, build the skill following these steps:

Step 1: Choose the skill type

• Task skills (most common) give step-by-step instructions for a specific action. Invoked with /name or natural language. Examples: generate a report, summarize a PR, deploy code.
• Reference skills add knowledge Claude applies to current work without performing an action. Examples: coding conventions, API patterns, style guides.

Step 2: Configure frontmatter

Set these fields based on what you learned in discovery:

• name -- Matches the directory name. Lowercase, hyphens, max 64 chars.
• description -- Written as: "Use when someone asks to [action], [action], or [action]." Include natural keywords from the trigger phrases.
• disable-model-invocation: true -- Set if the skill has side effects (file generation, API calls, costs money). Prevents Claude from auto-invoking.
• argument-hint -- Set if the skill accepts arguments. Shows in the / menu autocomplete.
• context: fork + agent -- Set if the skill is self-contained and doesn't need conversation history.
• model -- Set if a specific model capability is needed.
• allowed-tools -- Pre-approves the listed tools to run without permission prompts while the skill is active. This is a GRANT, not a sandbox -- all other tools remain available under normal permissions.
• disallowed-tools -- Set if the skill should actually remove tools while active. This is the real restriction mechanism; allowed-tools never takes anything away.

Only set fields you actually need. Don't add frontmatter just because you can.

For the full field reference and invocation control matrix, see reference.md.

Step 3: Baseline without the skill (eval-first)

Before writing any content, run 2-3 representative tasks WITHOUT the skill and note where Claude falls short. Those observed failures are your eval scenarios -- write only the minimal instructions needed to fix them, not instructions for failures you never saw. Keep it lightweight: a few bullet notes are enough.

Step 4: Write the skill content

Structure task skills as:

1. Context -- Files to read, APIs to call, reference material to load
2. Step-by-step workflow -- Numbered steps. Each step tells Claude exactly what to do.
3. Output format -- What the result looks like. Include templates, file paths, structured formats.
4. Notes -- Edge cases, constraints, what to delegate, what NOT to do.

Content rules:

• Keep SKILL.md under 500 lines. Move detailed reference material to supporting files.
• Use $ARGUMENTS / $N for dynamic input from arguments.
• Use !command`` for dynamic context injection (preprocessing).
• Be specific about agent delegation -- include exact prompt text.
• Specify all file paths (inputs, outputs, scripts, references).
• Tell Claude to RUN bundled scripts, not READ them -- executing keeps the code out of context (only the output costs tokens), and tested code beats regenerated code.
• For fragile, repeatable, or consistency-critical operations (validation, transforms, scaffolding), bundle a tested script and instruct Claude to run it instead of re-deriving the steps as prose.

Step 5: Add supporting files (if needed)

If your skill needs detailed reference docs, examples, or scripts, add them alongside SKILL.md in the same directory. Reference them from SKILL.md so Claude knows they exist. Supporting files are NOT loaded automatically -- they load only when Claude needs them. See reference.md for the full pattern.

Step 6: Document in CLAUDE.md (one line, not four)

Your project's CLAUDE.md is re-read every conversation, so its cost compounds -- keep its footprint tiny. The skill's description is already auto-loaded into context, so don't restate the trigger phrases or what it does here; that would duplicate what Claude already sees (and the Quality Audit flags duplication). Add at most a single-line pointer -- the skill name and a one-line purpose -- so you and your team can see what's available.

This isn't required for the skill to work. Follow your project's CLAUDE.md budget protocol: one line plus a pointer to the canonical file, detail lives in the skill, not in CLAUDE.md.

Step 7: Test

Test both invocation methods:

1. Natural language -- Say something matching the description. Does Claude load the skill?
   • If not, revise the description field to include the keywords you used
   • Try 2-3 different phrasings to verify it triggers reliably
2. Direct invocation -- Run /skill-name with test arguments
   • Verify $ARGUMENTS / $N are substituting correctly
   • Check that outputs go where expected
3. Edge cases -- Try invoking with missing arguments, unusual input, or empty input
4. Fresh session -- Re-run the 2-3 baseline scenarios from Step 3 in a NEW session, separate from the one that authored the skill. Leftover authoring context masks gaps the written instructions must cover on their own.
5. Character budget -- If you have many skills, run /context to confirm your skill's description is being loaded. If it's not, your total descriptions may exceed the budget (see reference.md for details).

If issues arise, see Troubleshooting in reference.md.

Complete Example

Here's a minimal but complete skill you can use as a starting template:

File: .claude/skills/meeting-notes/SKILL.md

---
name: meeting-notes
description: Use when someone asks to summarize meeting notes, recap a meeting, or format meeting minutes.
argument-hint: [topic or date]
---

## What This Skill Does

Takes raw meeting notes and produces a structured summary with action items.

## Steps

1. Ask the user to paste their raw meeting notes (or provide a file path).
2. Extract the following from the notes:
   - **Attendees** -- Who was in the meeting
   - **Key decisions** -- What was decided
   - **Action items** -- Who owes what, with deadlines if mentioned
   - **Open questions** -- Anything unresolved
3. Format the output using the template below.
4. If $ARGUMENTS is provided, use it as the meeting title. Otherwise, infer a title from the content.

## Output Template

# Meeting: [title]
**Date:** [date if mentioned, otherwise "Not specified"]
**Attendees:** [comma-separated list]

## Key Decisions
- [decision]

## Action Items
- [ ] [person]: [task] (due: [date or "TBD"])

## Open Questions
- [question]

## Notes

- Keep summaries concise. Don't add commentary or embellish.
- If notes are too vague to extract action items, flag that to the user instead of making them up.

Mode 2: Audit an Existing Skill

Use this checklist to audit any existing skill. Read the skill file first before running through the checklist. Fix issues before marking the audit complete.

Frontmatter Audit

• name matches the directory name
• description uses natural keywords someone would actually say when they need this skill
• description is specific enough to avoid false triggers but broad enough to catch real requests
• disable-model-invocation: true is set if the skill has side effects (generates files, calls APIs, sends messages, costs money)
• argument-hint is set if the skill accepts arguments via /name
• allowed-tools is used only as a grant (pre-approves listed tools without prompts) -- not mistaken for a sandbox; all other tools remain available under normal permissions
• disallowed-tools is set if the skill should actually remove tools while active
• context: fork is used if the skill is self-contained and produces verbose output
• model is set only if a specific model capability is needed
• No unnecessary fields are set (don't add frontmatter just because you can)

Content Audit

• Total SKILL.md is under 500 lines (detailed reference moved to supporting files)
• Clear step-by-step workflow with numbered steps (for task skills)
• Output format is specified with templates or examples
• All file paths and locations are documented
• Agent delegation instructions include the actual prompt text to send
• Notes section covers edge cases, constraints, and what NOT to do
• No vague instructions -- every step tells Claude exactly what to do
• String substitutions ($ARGUMENTS, $N) are used where the skill takes input
• Bundled scripts are RUN, not read into context -- and fragile/repeatable operations use a tested script instead of regenerated prose steps

Integration Audit

• Skill is documented in CLAUDE.md (recommended, not required)
• Supporting files (if any) are referenced from SKILL.md, not orphaned
• Scripts (if any) have correct file paths and are executable
• API keys (if any) are stored in environment variables, never hardcoded

Quality Audit

• A beginner could follow the instructions without prior context
• Instructions are actionable, not abstract
• Content is anchored to observed failures (2-3 baseline tasks run WITHOUT the skill), not guessed instructions
• The finished skill was re-tested in a fresh session, separate from the one that authored it
• Delegates to subagents when appropriate to keep main context clean
• Doesn't duplicate information that lives elsewhere (CLAUDE.md, other skills)
• Output paths follow a predictable convention

Optimization Opportunities

After running the audit, check reference.md for advanced features that could improve the skill: context: fork, allowed-tools, dynamic context injection, hooks, and supporting files.

Recommended Conventions

Adapt these to fit your project:

• Skills live in .claude/skills/[skill-name]/SKILL.md
• Output files go in a predictable location (e.g., output/[skill-name]/)
• API keys go in environment variables, never hardcoded in skill files
• Document all active skills in your project's CLAUDE.md
• Frontmatter description is written as: "Use when someone asks to [action], [action], or [action]."

Important Notes

• Always read an existing skill before optimizing it. Never propose changes to a skill you haven't read.
• When building a new skill, check if a similar skill already exists that could be extended instead.
• For advanced patterns (subagent execution, hooks, permissions), see reference.md.