# Developerinlondon Agentkit Test Driven Development > Test-Driven Development (TDD) - Skill: `tomevault-io/developerinlondon-agentkit-test-driven-development` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/developerinlondon-agentkit-test-driven-development` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/developerinlondon-agentkit-test-driven-development/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/developerinlondon-agentkit-test-driven-development --- # Test-Driven Development (TDD) ## The Cycle (NON-NEGOTIABLE) Every code change follows this exact sequence: 1. **RED** — Write a test that describes the desired behavior. Run it. It MUST fail. 2. **GREEN** — Write the MINIMAL code to make the test pass. Nothing more. 3. **REFACTOR** — Clean up while keeping tests green. Remove duplication, improve naming. **No code without a test. No test without seeing it fail first.** ## When to Apply TDD applies to ALL code changes: - **New features**: Test the behavior before writing it - **Bug fixes**: Write a test that reproduces the bug, THEN fix it - **Refactoring**: Ensure tests cover the behavior before changing implementation - **API endpoints**: Test request/response contract before implementing the handler - **CLI commands**: Test arg parsing and output before wiring the command - **Data models**: Test serialization/deserialization before defining structs ## Workflow Steps (Detailed) ### Step 1: Write the Test - Write a test that describes WHAT the code should do, not HOW - Use descriptive test names: `user_login_returns_token`, not `test1` - Test one behavior per test function - Include edge cases and error paths ### Step 2: Verify RED - Run the test suite - Confirm the new test FAILS - Confirm it fails for the RIGHT REASON (missing function, wrong return value — not a syntax error) - If the test passes immediately, it's not testing new behavior — rewrite it ### Step 3: Implement (Minimal) - Write ONLY the code needed to make the failing test pass - Do not add extra features, optimizations, or "while I'm here" changes - Do not write code that isn't covered by a test ### Step 4: Verify GREEN - Run the FULL test suite (not just the new test) - ALL tests must pass — new AND existing - If existing tests break, fix the implementation, not the tests ### Step 5: Refactor - Clean up duplication, improve names, extract helpers - Run tests after EVERY refactor step to ensure nothing breaks - This is where you improve code quality — not during GREEN ## Anti-Patterns (BLOCKING violations) - Writing implementation before tests - Writing tests that pass immediately (never saw RED) - Skipping the RED verification step ("I know it'll fail") - Writing tests after the fact to "backfill coverage" - Deleting or modifying tests to make them pass instead of fixing code - Writing too much implementation at once (should be incremental) - Testing implementation details instead of behavior - Skipping the refactor step ## Test Quality Standards - Each test should be independent — no shared mutable state between tests - Tests should be fast — mock external dependencies (network, filesystem, databases) - Test names should read like specifications - Prefer `assert_eq!` over `assert!` for better failure messages - Test error paths, not just happy paths - One logical assertion per test (multiple `assert_eq!` on the same result is fine) ## Bug Fix Protocol When fixing a bug: 1. Write a test that REPRODUCES the bug (fails with current code) 2. Verify the test fails — this proves you understand the bug 3. Fix the bug with minimal changes 4. Verify the test passes — this proves the fix works 5. The test now permanently guards against regression --- > Source: [developerinlondon/agentkit](https://github.com/developerinlondon/agentkit) — distributed by [TomeVault](https://tomevault.io).