Writing Skills

When This Skill Activates

• Creating a new skill (SKILL.md file)
• Editing or improving an existing skill
• Verifying a skill works before deployment
• Reviewing skill quality or effectiveness

Overview

Writing skills IS Test-Driven Development applied to process documentation.

Personal skills live in agent-specific directories (~/.claude/skills for Claude Code, ~/.codex/skills for Codex)

You write test cases (pressure scenarios with subagents), watch them fail (baseline behavior), write the skill (documentation), watch tests pass (agents comply), and refactor (close loopholes).

Core principle: If you didn't watch an agent fail without the skill, you don't know if the skill teaches the right thing.

REQUIRED BACKGROUND: You MUST understand shipyard:shipyard-tdd before using this skill. That skill defines the fundamental RED-GREEN-REFACTOR cycle. This skill adapts TDD to documentation.

Official guidance: For Anthropic's official skill authoring best practices, see anthropic-best-practices.md. This document provides additional patterns and guidelines that complement the TDD-focused approach in this skill.

What is a Skill?

A skill is a reference guide for proven techniques, patterns, or tools. Skills help future Claude instances find and apply effective approaches.

Skills are: Reusable techniques, patterns, tools, reference guides

Skills are NOT: Narratives about how you solved a problem once

TDD Mapping for Skills

The entire skill creation process follows RED-GREEN-REFACTOR.

When to Create a Skill

Create when:

• Technique wasn't intuitively obvious to you
• You'd reference this again across projects
• Pattern applies broadly (not project-specific)
• Others would benefit

Don't create for:

• One-off solutions
• Standard practices well-documented elsewhere
• Project-specific conventions (put in CLAUDE.md)
• Mechanical constraints (if it's enforceable with regex/validation, automate it -- save documentation for judgment calls)

Skill Types

Technique

Concrete method with steps to follow (condition-based-waiting, root-cause-tracing)

Pattern

Way of thinking about problems (flatten-with-flags, test-invariants)

Reference

API docs, syntax guides, tool documentation (office docs)

Directory Structure

skills/
  skill-name/
    SKILL.md              # Main reference (required)
    supporting-file.*     # Only if needed

Flat namespace - all skills in one searchable namespace

Separate files for:

1. Heavy reference (100+ lines) - API docs, comprehensive syntax
2. Reusable tools - Scripts, utilities, templates

Keep inline:

• Principles and concepts
• Code patterns (< 50 lines)
• Everything else

SKILL.md Structure

Frontmatter (YAML):

• Only two fields supported: name and description
• Max 1024 characters total
• name: Use letters, numbers, and hyphens only (no parentheses, special chars)
• description: Third-person, describes ONLY when to use (NOT what it does)
  • Start with "Use when..." to focus on triggering conditions
  • Include specific symptoms, situations, and contexts
  • NEVER summarize the skill's process or workflow (see CSO section for why)
  • Keep under 500 characters if possible

---
name: Skill-Name-With-Hyphens
description: Use when [specific triggering conditions and symptoms]
---

# Skill Name

## Overview
What is this? Core principle in 1-2 sentences.

## When to Use
[Small inline flowchart IF decision non-obvious]

Bullet list with SYMPTOMS and use cases
When NOT to use

## Core Pattern (for techniques/patterns)
Before/after code comparison

## Quick Reference
Table or bullets for scanning common operations

## Implementation
Inline code for simple patterns
Link to file for heavy reference or reusable tools

## Common Mistakes
What goes wrong + fixes

## Real-World Impact (optional)
Concrete results

Claude Search Optimization (CSO)

Critical for discovery: Future Claude needs to FIND your skill

1. Rich Description Field

Purpose: Claude reads description to decide which skills to load for a given task. Make it answer: "Should I read this skill right now?"

Format: Start with "Use when..." to focus on triggering conditions

CRITICAL: Description = When to Use, NOT What the Skill Does

The description should ONLY describe triggering conditions. Do NOT summarize the skill's process or workflow in the description.

Why this matters: Testing revealed that when a description summarizes the skill's workflow, Claude may follow the description instead of reading the full skill content. A description saying "code review between tasks" caused Claude to do ONE review, even though the skill's flowchart clearly showed TWO reviews (spec compliance then code quality).

When the description was changed to just "Use when executing implementation plans with independent tasks" (no workflow summary), Claude correctly read the flowchart and followed the two-stage review process.

The trap: Descriptions that summarize workflow create a shortcut Claude will take. The skill body becomes documentation Claude skips.

CSO Description Examples

Skill Content Examples

Last week I was working on a project where tests were flaky. I found that using sleep(5) was unreliable. After some experimentation, I discovered that polling for conditions works better...

Narrative style, no activation triggers, no XML structure, not reusable.
</example>

</examples>

<instructions>

### CSO Continued

**Content:**
- Use concrete triggers, symptoms, situations -- describe the *problem* not language-specific symptoms
- Technology-agnostic unless the skill itself is technology-specific
- Third person (injected into system prompt)
- **NEVER summarize the skill's process or workflow**

### 2. Keyword Coverage

Use words Claude would search for:
- Error messages: "Hook timed out", "ENOTEMPTY", "race condition"
- Symptoms: "flaky", "hanging", "zombie", "pollution"
- Synonyms: "timeout/hang/freeze", "cleanup/teardown/afterEach"
- Tools: Actual commands, library names, file types

### 3. Descriptive Naming

**Use active voice, verb-first:**
- `creating-skills` not `skill-creation`
- `condition-based-waiting` not `async-test-helpers`

### 4. Token Efficiency (Critical)

**Targets:** getting-started <150 words, frequently-loaded <200 words, others <500 words.

**Key techniques:** Move details to `--help`, use cross-references instead of repeating content, compress examples to minimal form, eliminate redundancy. See EXAMPLES.md for detailed token efficiency patterns.

**Naming:** Use active verb-first names (`condition-based-waiting` not `async-test-helpers`). Gerunds work well for processes (`creating-skills`, `testing-skills`).

### 5. Cross-Referencing Other Skills

**When writing documentation that references other skills:**

Use skill name only, with explicit requirement markers:
- Good: `**REQUIRED SUB-SKILL:** Use shipyard:shipyard-tdd`
- Good: `**REQUIRED BACKGROUND:** You MUST understand shipyard:shipyard-debugging`
- Bad: `See skills/testing/test-driven-development` (unclear if required)
- Bad: `@skills/testing/test-driven-development/SKILL.md` (force-loads, burns context)

**Why no @ links:** `@` syntax force-loads files immediately, consuming 200k+ context before you need them.

## Flowchart Usage

```dot
digraph when_flowchart {
    "Need to show information?" [shape=diamond];
    "Decision where I might go wrong?" [shape=diamond];
    "Use markdown" [shape=box];
    "Small inline flowchart" [shape=box];

    "Need to show information?" -> "Decision where I might go wrong?" [label="yes"];
    "Decision where I might go wrong?" -> "Small inline flowchart" [label="yes"];
    "Decision where I might go wrong?" -> "Use markdown" [label="no"];
}

Use flowcharts ONLY for:

• Non-obvious decision points
• Process loops where you might stop too early
• "When to use A vs B" decisions

Never use flowcharts for:

• Reference material -- Tables, lists
• Code examples -- Markdown blocks
• Linear instructions -- Numbered lists
• Labels without semantic meaning (step1, helper2)

See @graphviz-conventions.dot for graphviz style rules.

Code Examples

One excellent example beats many mediocre ones

Choose most relevant language:

• Testing techniques -- TypeScript/JavaScript
• System debugging -- Shell/Python
• Data processing -- Python

Good example:

• Complete and runnable
• Well-commented explaining WHY
• From real scenario
• Shows pattern clearly
• Ready to adapt (not generic template)

Don't:

• Implement in 5+ languages
• Create fill-in-the-blank templates
• Write contrived examples

You're good at porting - one great example is enough.

File Organization

Self-Contained Skill

defense-in-depth/
  SKILL.md    # Everything inline

When: All content fits, no heavy reference needed

Skill with Reusable Tool

condition-based-waiting/
  SKILL.md    # Overview + patterns
  example.ts  # Working helpers to adapt

When: Tool is reusable code, not just narrative

Skill with Heavy Reference

pptx/
  SKILL.md       # Overview + workflows
  pptxgenjs.md   # 600 lines API reference
  ooxml.md       # 500 lines XML structure
  scripts/       # Executable tools

When: Reference material too large for inline

The Iron Law (Same as TDD)

NO SKILL WITHOUT A FAILING TEST FIRST

This applies to NEW skills AND EDITS to existing skills.

Write skill before testing? Delete it. Start over. Edit skill without testing? Same violation.

No exceptions:

• Not for "simple additions"
• Not for "just adding a section"
• Not for "documentation updates"
• Don't keep untested changes as "reference"
• Don't "adapt" while running tests
• Delete means delete

REQUIRED BACKGROUND: The shipyard:shipyard-tdd skill explains why this matters. Same principles apply to documentation.

Testing All Skill Types

Different skill types need different test approaches:

Discipline-Enforcing Skills

Test with academic questions, pressure scenarios (time + sunk cost + exhaustion combined), and rationalization tracking. Pass: Agent follows rule under maximum pressure.

Technique Skills

Test with application scenarios, edge-case variations, and gap detection. Pass: Agent applies technique correctly to new scenarios.

Pattern Skills

Test with recognition scenarios, application tasks, and counter-examples. Pass: Agent identifies when/how to apply pattern and when NOT to.

Reference Skills

Test with retrieval tasks, application scenarios, and coverage gaps. Pass: Agent finds and correctly applies reference information.

Common Rationalizations for Skipping Testing

All of these mean: Test before deploying. No exceptions.

Bulletproofing Skills Against Rationalization

Discipline skills must resist rationalization. Key strategies:

1. Close every loophole explicitly -- don't just state the rule, forbid specific workarounds with "No exceptions" lists
2. Address "Spirit vs Letter" -- add early: "Violating the letter of the rules is violating the spirit of the rules"
3. Build rationalization tables from baseline testing -- every excuse agents make gets a counter
4. Create red flags lists for agent self-check (e.g., "Code before test", "This is different because...")
5. Update CSO descriptions with violation symptoms as triggers

See EXAMPLES.md for detailed good/bad examples of each strategy.

RED-GREEN-REFACTOR for Skills

Follow the TDD cycle:

RED: Write Failing Test (Baseline)

Run pressure scenario with subagent WITHOUT the skill. Document exact behavior:

• What choices did they make?
• What rationalizations did they use (verbatim)?
• Which pressures triggered violations?

This is "watch the test fail" - you must see what agents naturally do before writing the skill.

GREEN: Write Minimal Skill

Write skill that addresses those specific rationalizations. Don't add extra content for hypothetical cases.

Run same scenarios WITH skill. Agent should now comply.

REFACTOR: Close Loopholes

Agent found new rationalization? Add explicit counter. Re-test until bulletproof.

Anti-Patterns

• Narrative Example -- session-specific stories are not reusable
• Multi-Language Dilution -- one excellent example beats five mediocre translations
• Code in Flowcharts -- can't copy-paste, hard to read (use markdown blocks)
• Generic Labels -- helper1/step3 have no semantic meaning; use descriptive names

See EXAMPLES.md for detailed anti-pattern examples.

STOP: Before Moving to Next Skill

After writing ANY skill, you MUST STOP and complete the deployment process.

Do NOT:

• Create multiple skills in batch without testing each
• Move to next skill before current one is verified
• Skip testing because "batching is more efficient"

The deployment checklist below is MANDATORY for EACH skill.

Deploying untested skills = deploying untested code. It's a violation of quality standards.

Skill Creation Checklist (TDD Adapted)

Use TodoWrite to create todos for EACH item below.

RED: Create pressure scenarios (3+ for discipline skills) -- run WITHOUT skill -- document baseline rationalizations

GREEN:

• Name: letters/numbers/hyphens only; YAML frontmatter (name + description, max 1024 chars)
• Description: "Use when..." + triggers/symptoms, third person, no workflow summary
• Keywords for search, clear overview, address baseline failures from RED
• Code inline or linked; one excellent example (not multi-language)
• Run WITH skill -- verify compliance

REFACTOR: Find new rationalizations -- add counters -- build rationalization table + red flags -- re-test until bulletproof

Quality: Flowcharts only for non-obvious decisions; quick reference table; common mistakes; no narrative; supporting files only for tools/heavy reference

Deploy: Commit and push; consider contributing via PR if broadly useful

Discovery Workflow

How future Claude finds your skill:

1. Encounters problem ("tests are flaky")
2. Finds SKILL (description matches)
3. Scans overview (is this relevant?)
4. Reads patterns (quick reference table)
5. Loads example (only when implementing)

Optimize for this flow - put searchable terms early and often.