# Tdd

> Implements features using strict test-driven development — red-green-refactor cycle. Writes a failing test first, then minimal code to pass, then refactors. Use when building features that need high reliability.

- Skill: `viknesh20-20/tdd` (Agent Skill)
- Install (CLI): `npx skillmds@latest add viknesh20-20/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/viknesh20-20/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: viknesh20-20 (https://skillmd.com/u/viknesh20-20)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/viknesh20-20/tdd

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# Test-Driven Development

## Detect Test Environment
!`ls package.json pytest.ini pyproject.toml go.mod Cargo.toml *.csproj Gemfile mix.exs 2>/dev/null`
!`find . -type f \( -name "*.test.*" -o -name "*.spec.*" -o -name "test_*" -o -name "*_test.*" \) -not -path "*/node_modules/*" -not -path "*/.git/*" 2>/dev/null | head -5`

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## The TDD Cycle

For the requested feature, repeat this cycle until complete:

### RED — Write a Failing Test
1. Write the smallest possible test that describes the next piece of behavior
2. The test must import/reference the function or module (even if it doesn't exist yet)
3. Run the test — it MUST fail
4. Verify the failure message is meaningful (not just a compilation error)

### GREEN — Make It Pass
1. Write the absolute minimum code to make the test pass
2. Hardcoding is acceptable at this stage — it will be cleaned up in refactor
3. Don't add functionality the test doesn't require
4. Run the test — it MUST pass
5. Run all related tests — nothing else should break

### REFACTOR — Clean Up
1. Remove duplication in both production and test code
2. Improve naming, extract methods, simplify logic
3. Run tests after every refactoring step — they must stay green
4. Stop when the code is clean

## Test Progression
Start simple, add complexity gradually:
1. Null/empty/zero case
2. Single item / simplest valid input
3. Multiple items / typical case
4. Edge cases / boundary values
5. Error cases / invalid input
6. Async / concurrent behavior (if applicable)

## Output
- Show each cycle clearly: RED (test code + failure), GREEN (impl + pass), REFACTOR
- Display test runner output at each step
- Final summary: all tests, coverage of the feature

## Rules
- NEVER write implementation before a failing test
- NEVER skip the refactor step
- One behavior per cycle — don't batch multiple features
- If the test framework isn't set up, set it up first