# Test Design Patterns

> |

- **Type:** Skill
- **Install:** `agentstack add skill-aeyeops-aeo-skill-marketplace-test-design-patterns`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [AeyeOps](https://agentstack.voostack.com/s/aeyeops)
- **Installs:** 0
- **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [AeyeOps](https://github.com/AeyeOps)
- **Source:** https://github.com/AeyeOps/aeo-skill-marketplace/tree/main/aeo-tdd-workflow/skills/test-design-patterns

## Install

```sh
agentstack add skill-aeyeops-aeo-skill-marketplace-test-design-patterns
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# Test Design Patterns

## The Test Pyramid

```
        /  E2E  \          Few, slow, expensive
       /----------\        Test critical user journeys
      / Integration \      Moderate count, moderate speed
     /----------------\    Test component interactions
    /    Unit Tests     \  Many, fast, cheap
   /--------------------\  Test individual behaviors
```

### Layer Guidelines

```markdown
Unit Tests (70% of test suite):
  Speed:  with >=
   - Replace + with -
   - Remove a function call
   - Change a constant
   - Negate a condition
3. Run the test suite against the mutant
4. If tests fail → mutant "killed" (tests caught the change) ✓
5. If tests pass → mutant "survived" (tests missed the change) ✗

Mutation score = killed mutants / total mutants × 100%
Target: >80% mutation score
```

### Common Mutation Operators

```markdown
Arithmetic:     + → -, * → /, % → *
Relational:     > → >=, == → !=, = 18): return "adult"
  Mutant:    if (age > 18): return "adult"
  Survived because: No test checks age == 18 (boundary)
  Fix: Add test_classify_age_18_returns_adult()
```

## Coverage Interpretation

### What Coverage Measures

```markdown
Line coverage:    Which lines were executed
Branch coverage:  Which conditional branches were taken
Path coverage:    Which execution paths were followed
Function coverage: Which functions were called

Line coverage is necessary but not sufficient.
100% line coverage does NOT mean the code is well-tested.
```

### Coverage Traps

```markdown
False confidence from high coverage:
  - Lines executed but results not asserted
  - Happy path covered but edge cases missing
  - Implementation tested but behavior not verified

Example of misleading 100% coverage:
  def divide(a, b):
      return a / b

  def test_divide():
      divide(10, 2)  # 100% line coverage, but:
                      # - No assertion on result
                      # - Division by zero not tested
                      # - Float precision not tested
```

### Healthy Coverage Practices

```markdown
Guidelines:
- Aim for 80%+ line coverage as a baseline
- Focus on branch coverage for conditional logic
- Use coverage to find UNTESTED code, not to prove quality
- Never game coverage metrics (writing tests just to hit lines)
- High-risk code (payments, auth, data) deserves near-100% coverage
- Generated code, configuration, and glue code can have lower coverage
- Review uncovered lines: are they dead code or missing tests?

Coverage as a ratchet:
- Set a minimum threshold (e.g., 80%)
- Never allow coverage to decrease
- Increase the threshold as the suite matures
- Fail CI if coverage drops below threshold
```

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [AeyeOps](https://github.com/AeyeOps)
- **Source:** [AeyeOps/aeo-skill-marketplace](https://github.com/AeyeOps/aeo-skill-marketplace)
- **License:** MIT

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-aeyeops-aeo-skill-marketplace-test-design-patterns
- Seller: https://agentstack.voostack.com/s/aeyeops
- Browse the marketplace: https://agentstack.voostack.com/browse

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Listed on AgentStack — the marketplace for AI agent skills and MCP servers. Every listing is security-reviewed. Creators keep 70%.