| 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
