drn/write-skill

Skill Author

Create or improve Claude Code skills (slash commands) following established patterns and avoiding known pitfalls.

Arguments

• $ARGUMENTS - What the skill should do, or the name of an existing skill to improve

Context

• Global skills: !find ~/.claude/commands -maxdepth 1 -name "*.md" -exec basename {} .md \; 2>/dev/null | sort | head -20
• Project skills (Agent Skills standard): !find . -maxdepth 3 -path "*/skills/*/SKILL.md" 2>/dev/null | head -20
• Project skills (Claude Code standard): !find . -path "./claude/commands/*.md" 2>/dev/null | head -20
• Project type: !find . -maxdepth 1 \( -name go.mod -o -name Gemfile -o -name package.json -o -name Cargo.toml -o -name pyproject.toml \) 2>/dev/null | head -3

Instructions

Step 1: Understand the Request

Determine whether the user wants to:

• Create a new skill from a description
• Improve an existing skill (read it first, then apply changes)

If creating, ask the user what the skill should do if $ARGUMENTS is vague.

Step 2: Design the Skill

Plan the skill structure:

1. Frontmatter (YAML between --- fences)
2. Context section (dynamic context via shell commands)
3. Instructions (what Claude should do when the skill is invoked)

Step 3: Write the Skill File

Detect the project skill convention from the context above:

• If the project has skills matching */skills/*/SKILL.md (Agent Skills standard), create <skills-dir>/<name>/SKILL.md
• Otherwise, create claude/commands/<name>.md (Claude Code standard)

Follow all rules below carefully.

---

Skill File Reference

Frontmatter

All fields are optional. Only description is strongly recommended.

---
description: One-line summary of what this skill does and when to use it
allowed-tools: Bash(git *), Bash(npm test)
---

| Field | Purpose | |---|---| | description | Drives auto-activation and / menu. Write in third person. Max 1024 chars. Include trigger keywords. | | allowed-tools | Tools Claude can use without asking permission. Glob patterns supported. | | context | Set to fork to run in an isolated subagent (no conversation history). | | agent | Subagent type when context: fork (e.g., Explore, Plan). | | model | Override model (e.g., sonnet for cheaper tasks). | | disable-model-invocation | true to prevent auto-loading (manual /name only). Use for destructive skills. | | user-invocable | false to hide from / menu (background knowledge only). |

Dynamic Context

Inject live data by writing an exclamation mark followed by a command wrapped in backticks. The command runs locally and its output replaces the placeholder before Claude sees the prompt.

In the examples below, !{command} represents the actual syntax (exclamation mark + backtick + command + backtick). Curly braces are used here to prevent this skill file from executing its own examples.

## Context

- Current branch: !{git branch --show-current}
- Git status: !{git status --short}
- Project type: !{ls -1 go.mod package.json Cargo.toml 2>/dev/null | head -3}

CRITICAL: No $() Command Substitution

Claude Code blocks $() inside dynamic context expressions for security reasons. This is the single most common authoring mistake.

# BROKEN — $() is blocked
- Branch commits: !{DEFAULT=$(git symbolic-ref ...) && git log origin/$DEFAULT..HEAD}
- Changes: !{git diff --stat HEAD...$(git symbolic-ref ...)}

# FIXED — detect base branch portably, provide both branches
- Base ref: !{git branch -r 2>/dev/null | grep -oE 'origin/(main|master)' | head -1}
- Commits vs main: !{git log origin/main..HEAD --oneline 2>/dev/null | head -20}
- Commits vs master: !{git log origin/master..HEAD --oneline 2>/dev/null | head -20}
- Changes vs main: !{git diff --stat HEAD...origin/main 2>/dev/null | head -50}
- Changes vs master: !{git diff --stat HEAD...origin/master 2>/dev/null | head -50}

Alternatives to $():

• Pipe chains: command1 | command2 | command3
• Error suppression: command 2>/dev/null (empty output on failure is fine)
• Detect base branch: git branch -r | grep -oE 'origin/(main|master)' | head -1 — portable, no custom tools needed
• Provide both branches: include origin/main and origin/master variants — one will be empty, the agent uses whichever has output
• Separate context lines: split a complex command into multiple simpler dynamic context lines

Output Size Management

Always bound output to avoid blowing up the context window:

# GOOD — bounded output
- Files: !{find . -maxdepth 3 -name "*.go" 2>/dev/null | head -20}
- Commits: !{git log --oneline -10}

# BAD — unbounded, could be massive
- Files: !{find . -name "*.go"}
- Log: !{git log}

Error Handling and Exit Codes

Never use || or && — Claude Code's permission system treats these as multiple operations and blocks them.

Use 2>/dev/null to suppress stderr, but it does not fix exit codes. When a command fails, the skill loader sees the non-zero exit code and treats it as an error, even though stderr is suppressed. This breaks skill loading entirely.

Fix: always pipe through | head -N after 2>/dev/null. In a pipeline, the exit code is that of the last command (head), which exits 0 even when the upstream command fails and produces no output.

# BAD — 2>/dev/null suppresses stderr but exit code is still non-zero
- Tag: !{git describe --tags --abbrev=0 2>/dev/null}
- Commits: !{git log origin/master..HEAD --oneline 2>/dev/null}

# BAD — || treated as multiple operations by permission system
- Tag: !{git describe --tags --abbrev=0 2>/dev/null || echo "No tags"}

# GOOD — pipe neutralizes exit code, empty output on failure is fine
- Tag: !{git describe --tags --abbrev=0 2>/dev/null | head -1}
- Commits: !{git log origin/master..HEAD --oneline 2>/dev/null | head -50}

String Substitutions

| Variable | Description | |---|---| | $ARGUMENTS | All arguments the user passed after /skill-name | | $ARGUMENTS[0], $1 | First argument (0-indexed) |

Skill Body Best Practices

1. Be concise. Only add context Claude does not already have. The context window is shared.
2. Use imperative instructions. Tell Claude what to do, not what the skill "can" do.
3. Structure with numbered steps. Makes execution predictable and debuggable.
4. Include abort conditions. Define when to stop (e.g., "if no changes, tell the user and stop").
5. Set loop limits. If the skill loops (CI check/fix), cap retries (e.g., "if stuck 3+ times, stop and report").
6. Avoid backticks in prose. The skill content passes through shell evaluation; inline code fences can break parsing. Use plain text or code blocks (triple backtick fences) instead of inline backticks where possible.
7. Avoid contractions. Words like "don't" or "can't" can break due to single-quote shell interpretation.
8. Match specificity to fragility. Give high freedom for flexible tasks, exact commands for fragile operations.
9. Use disable-model-invocation: true for skills with side effects (deploy, merge, send).
10. Keep skills generic. This is a public repo. Never embed company names, internal tool names, proprietary patterns, or org-specific conventions. Skills should work for any user. Put org-specific knowledge in private CLAUDE.md files or project-local config instead.

Existing Skill Patterns

Skills follow this general structure (path varies by convention -- see Step 3):

---
description: ...
allowed-tools: ...  (optional)
---

## Context
- Key info: !{command}

## Your task / Instructions
Step-by-step instructions for Claude.

### Step 1: ...
### Step 2: ...

Multi-agent skills (like /dev, /debug, /explore) use the Task tool to spawn subagents with specific roles.

---

Step 4: Validate

After writing, run two validation passes. Fix issues silently — only flag if the fix would change the skill's intent.

Syntax and Safety

• [ ] No $() in any dynamic context line
• [ ] No || or && operators in any dynamic context line
• [ ] Error-prone commands use 2>/dev/null | head -N (suppresses stderr AND neutralizes exit code)
• [ ] No bare 2>/dev/null without a trailing pipe (exit code breaks skill loading)
• [ ] Description is present and includes trigger keywords
• [ ] No inline backticks in prose that could break shell evaluation
• [ ] No contractions (don't, can't, won't) in the skill body
• [ ] Destructive skills have disable-model-invocation: true
• [ ] No company-specific names, internal tools, or proprietary patterns (this is a public repo)

Content Quality

Scan the skill for these failure patterns. Fix any you find — these are the patterns that cause skills to waste tokens, loop endlessly, or produce wrong results.

Task clarity

• [ ] No vague verbs ("help with", "handle", "deal with") — replace with precise operations ("refactor", "extract", "migrate")
• [ ] Single responsibility — skill does one thing, not two unrelated tasks bundled together
• [ ] Success criteria defined — the skill states what "done" looks like, not just what to do

Scope control

• [ ] Skills that edit code specify file/directory boundaries, not global instructions
• [ ] Skills that spawn agents scope each agent to specific files or responsibilities
• [ ] No unbounded work — complex tasks are broken into numbered steps with clear boundaries

Stop conditions

• [ ] Agentic skills (using Task tool, running commands, editing files) have explicit stop/abort conditions
• [ ] Looping skills cap retries (already covered in best practice #5, but verify it is present)
• [ ] Skills that call external services have failure modes defined ("if X fails, do Y" not infinite retry)

Context efficiency

• [ ] Dynamic context pulls only what the skill needs — no exploratory commands that dump large output
• [ ] Instructions tell Claude what to do, not background theory on how it works
• [ ] No redundant context — if CLAUDE.md already provides information, the skill does not re-derive it

Report the validation results and the file path of the new/updated skill.