test-design

name	test-design
description	Design test cases following Kent Beck and t-wada principles. Use this skill when the user asks what to test, wants to design test cases, needs a test strategy, or wants to identify edge cases and boundary conditions for a feature. Focuses on behavior-driven testing with the 0-1-N rule.
disallowed-tools	Edit, Write, NotebookEdit

Test Design

Design tests that improve design resilience, not just find bugs.

Inspired by Kent Beck (TDD creator) and t-wada (Takuto Wada).

Workflow

1. Investigate Existing Conventions

Before designing test cases, understand the context:

• Read the target code or specification to understand expected behavior
• Check the codebase for existing naming conventions, patterns, and common values (e.g., directory structures that map to identifiers, commit history patterns)
• Use actual values from the codebase in test cases rather than generic placeholders — this catches real-world edge cases and ensures tests match actual usage

2. Design Test Cases

Apply the core principles below to generate test cases in the output format.

Core Principles

Principle	Description
Fear into Tests	Prioritize boundaries and edge cases where "this might break"
Test Behavior	Focus on external behavior, not internal implementation details
Refactoring Resistance	Extract essential specs that survive code restructuring
0-1-N Rule	Always include empty(0), single(1), multiple(N), and boundary vals

Output Format

When analyzing code or specifications, provide:

1. Core Behavior Tests

• Happy path scenarios that deliver direct user value

2. Boundary and Error Tests (0-1-N)

• 0: Empty, null, unset
• 1: Single item, minimum value
• N: Multiple items, maximum, max+1, timeout

3. Design Concerns

• Complex dependencies
• Side effects (DB, API, external services)

4. Design Insights

• How these tests help "tidy" the code
• Suggestions for loose coupling