Creating Skills

Creating Skills

Overview

Write a skill file in .claude/skills/. Test it with TDD.

Core Principle: If you did not test a skill, it is useless.

Process: The RED-GREEN-REFACTOR Cycle for Skills

RED: Write Failing Tests First

1. Create failing test cases that demonstrate edge cases or common mistakes
2. Document assertions for each test case (expected behavior in expected.md)

GREEN: Make Tests Pass

3. Write the skill that addresses all test cases
4. Test skill with subagent using the failing test cases
5. Analyze results: Did the subagent catch what was expected?
6. Iterate on skill until all tests pass

REFACTOR: Improve While Tests Stay Green

7. Run code review on the skill (token optimization, clarity)
8. Regression test with all examples to ensure nothing broke
9. Document changes and test outcomes

Examples

Example 1: Python Code Review Skill

This section documents the complete TDD process used to create the Python Code Review skill (.claude/skills/collaboration/python-code-review.md).

Initial Requirements

User requested: "A skill for reviewing python code. Make sure that we always use python 3.12 types. No untyped code is allowed. Make sure that we always use list, type, dict and not List, Type, Dict - this is old syntax."

Iteration 1: RED-GREEN (v1.0.0)

1. RED - Created Failing Test Cases

Write examples.

.claude/skills/collaboration/python-code-review/
└── examples/
    ├── example-1-mixed-syntax/         # Medium: Mixed old/new syntax
    │   ├── scenario.md                 # Metadata for humans
    │   ├── input.py                    # Code mixing List/list inconsistently
    │   └── expected.md                 # Assertions: Should catch List, Dict, Optional issues
    ├── example-2-untyped-code/         # Easy: Most common real-world case
    │   ├── input.py                    # Completely untyped Python class
    │   └── expected.md                 # Assertions: Should catch 0% type coverage
    └── example-3-complex-generics/     # Hard: Edge cases with TypeVar
        ├── input.py                    # Code with TypeVar, Generic, Protocol
        └── expected.md                 # Assertions: Should NOT flag legitimate typing imports

Copy the input.py into ai/scratch/example-N.

Run general subagents with this prompt pattern:

You are a specialized code reviewer. Apply the following skill to review the specified files.

**Files to Review**: ai/scratch/example-N/input.md

**Output Format**:

For each issue found, output:

---
**File**: [file path]
**Lines**: [line range, e.g., "45-52" or "78"]
**Issue**: [brief description]
**Suggestion**: [exact multiline replacement; omit if no specific change]
**Reason**: [why this improves the code]
---

Return ONLY the issues found. No commentary or summary.

2. Created the Skill

• Focused on Python 3.12+ modern typing conventions
• Enforced strict typing: no List/Dict/Type, use built-in generics
• Ensured X | None instead of Optional[X]
• Included 6-step process with examples and anti-patterns

3. GREEN - Ran the Tests (3 parallel subagents)

Launched 3 subagents with this prompt pattern:

You are a specialized code reviewer. Apply the following skill to review the specified files.

**Skill to Apply**: Read and follow .claude/skills/collaboration/python-code-review.md

**Files to Review**: ai/scratch/example-N/input.md

**Output Format**:

For each issue found, output:

---
**File**: [file path]
**Lines**: [line range, e.g., "45-52" or "78"]
**Issue**: [brief description]
**Suggestion**: [exact multiline replacement; omit if no specific change]
**Reason**: [why this improves the code]
---

Return ONLY the issues found. No commentary or summary.

Test Results:

• ✅ Example 1: PASS (with minor over-strictness - caught MORE than expected)
• ✅ Example 2: PASS (100% coverage gap identified correctly)
• ✅ Example 3: PASS (critical test: correctly distinguished deprecated vs legitimate typing imports)

Failing Behavior Discovered: When testing without explicit constraints, subagents provided extra non-typing feedback (design suggestions, code quality improvements) - scope creep!

Iteration 2: REFACTOR (v1.1.0)

Problem: Skill lacked explicit scope boundaries, causing subagents to provide general code review feedback.

Fix:

1. Added explicit scope statement in Overview:

   **Scope**: This skill focuses exclusively on type annotations,
   not code logic, design patterns, performance, or general quality.
   

2. Added directive at start of Process section:

   Focus only on type annotation issues.
   

Regression Testing:

• Launched same 3 tests without "typing only" constraint in prompt
• ✅ All tests pass: Subagents maintained focus on typing issues
• ✅ Zero design/quality feedback
• Refactoring successful!

Iteration 3: REFACTOR with Code Review Orchestrator (v1.2.0)

Self-Review Process:

Applied /pr-review command then incorporated All Feedback:

Regression Testing After Refactor:

Reran all 3 test cases with v1.2.0:

• ✅ Example 1: PASS (all issues caught, proper scope)
• ✅ Example 2: PASS (0% coverage identified, pure typing review)
• ✅ Example 3: PASS (nuanced TypeVar/Generic distinction correct)

Zero regression - all tests still pass after refactoring!

Anti-patterns

• ❌ Don't: Skip creating test cases (RED phase)

  • ✅ Do: Write failing tests before finalizing the skill

• ❌ Don't: Only test happy-path scenarios

  • ✅ Do: Create test cases that cover edge cases

• ❌ Don't: Test once and move on

  • ✅ Do: Regression test after each refactor

• ❌ Don't: Accept vague skill behavior

  • ✅ Do: Iterate until all tests pass with precise behavior

• ❌ Don't: Skip the refactor step

  • ✅ Do: Improve the skill while tests keep you safe

• ❌ Don't: Create test cases without assertions

  • ✅ Do: Document expected behavior in expected.md

Example Skill Template

---
name: Skill Name
description: Brief one-line description
when_to_use: When [specific trigger or scenario]
version: 1.0.0
category: [meta|testing|debugging|collaboration|workflow]
---

# Skill Name

## Overview

2-3 sentence summary of what this skill does and why it matters.

## When to Use

- Trigger condition 1
- Trigger condition 2
- Trigger condition 3

## Process

1. First step with specific action
2. Second step with clear instruction
3. Third step with validation

## Examples

### Example 1: [Scenario Name]

**Context**: Describe the situation
**Application**: Show how to use the skill
**Outcome**: What success looks like

## Anti-patterns

- ❌ **Don't**: Common mistake 1
  - ✅ **Do**: Correct approach instead

- ❌ **Don't**: Common mistake 2
  - ✅ **Do**: Correct approach instead

## Testing This Skill (TDD Validation)

How to validate this skill works:
1. Create test case with failing example
2. Apply skill to test case
3. Verify assertions pass

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