Review testability

Testability Review Skill

Review code (a diff or the entire codebase) with a focus on testability, simplicity, technical-debt management, and sustainable software development.

Review Target

1. If there are uncommitted changes or staged diffs, review those.
2. If the user specifies files or a PR, review those.
3. Otherwise, ask the user what to review.

Review Perspectives

Evaluate the code against the following principles:

1. Testability

• Is the code easy to test? Can each unit be tested in isolation?
• Are dependencies injectable rather than hard-coded?
• Are side effects separated from pure logic?
• Would a test for this code be testing behavior (good) or implementation details (fragile)?

2. Test Quality

• Do existing tests cover the meaningful behaviors?
• Are tests written at the right level of abstraction?
• Do test failure messages provide enough information to diagnose the problem? (Would power-assert style assertions help?)
• Are there tests that would break on harmless refactoring? (over-mocking, asserting on internal state, etc.)

3. Simplicity & YAGNI

• Is there unnecessary abstraction, indirection, or generalization?
• Is there code written for hypothetical future requirements rather than actual current needs?
• Could the same goal be achieved with less code or fewer concepts?
• Are there premature optimizations that sacrifice readability?

4. Quality and Speed

• Does the change support the principle that quality and speed reinforce each other?
• Will this code be easy to change next week? Next month?
• Are there shortcuts that will slow the team down later?

5. Technical Debt

• Does the change introduce new technical debt? If so, is it intentional and documented?
• Are there opportunities to pay down existing debt nearby?
• Is the debt/shortcut ratio appropriate for the context?

6. Refactoring Opportunities

• Are there structural improvements that can be made without changing behavior?
• Is there duplication that should be consolidated?
• Are names (variables, functions, classes) clear and intention-revealing?
• Is the code organized so that related things are together and unrelated things are apart?

Output Format

## Testability Review

### Summary
<Overall assessment in 2-3 sentences — would this code pass a rigorous testability-focused review?>

### Findings

#### [Severity] Finding title
- **Where**: file:line
- **What**: Description of the issue
- **Why it matters**: Impact on testability, maintainability, or development speed
- **Suggestion**: Concrete improvement

...

### Positive Points
- <What the code does well from these perspectives>

### Key Takeaway
<The single most important thing to address, framed as actionable advice>

Severity levels:

• Critical: Actively harmful — will cause bugs, block testing, or create significant debt
• Warning: Not ideal — reduces testability, readability, or maintainability
• Note: Minor improvement opportunity or style suggestion

Guidelines

• Be direct and specific. Reference actual code, not abstract principles.
• Praise good design decisions — don't only point out problems.
• Prioritize findings by impact. A few important findings are better than an exhaustive list of nitpicks.
• Frame suggestions in terms of concrete benefit (easier to test, safer to refactor, clearer intent), not dogma.
• Do NOT make changes. This skill only produces the review.