# Test Design

> 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. Use when this capability is needed.

- Skill: `tomevault-io/test-design` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/test-design`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/test-design/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/test-design

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# 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

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<!-- tomevault:4.0:skill_md:2026-04-13 -->