Common Tdd Agent Skill

Test-Driven Development (TDD) Standard

Priority: P0 (CRITICAL)

> Iron Law: NO PRODUCTION CODE WITHOUT FAILING TEST FIRST.

> Code written before test MUST deleted. Start over.

Step 1: RGR Loop (Red-Green-Refactor)

> [!TIP]

> Orchestration: If sub-agents are available, delegate each AC implementation to specialist-tdd-implementer.

• RED: Write minimal failing test. Verify failure (Expected error, not typo).
• GREEN: Write simplest code to pass. Verify pass.
• REFACTOR: Clean up code while staying green.

Red Flags

• Stop if code exists before test: Delete it. Restart from RED.
• Stop if test passes first run: You tested old behavior.
• Stop if "tests after" appears: That is not TDD.

AAA Structure (Mandatory)

Every test must follow Arrange-Act-Assert:

• Arrange: Set up inputs, stubs, mocks, and expected values.
• Act: Call single unit under test.
• Assert: Verify output and side effects. One logical assertion per test.

(See AAA Example for code structure).

Step 3: Verification & Thresholds

• Minimum Coverage: 80% (Stat/Func/Line), 75% (Branch).
• Mocks:
• Always mock: HTTP, Time/Date, Filesystem.
• Never mock: Fast internal services (<200ms), pure domain logic.
• See Test Runner Reference for environment-specific commands.

Step 4: Principles & Mocks

• Watch it Fail: Prove test works before writing code.
• Minimalism: Don't add features/options beyond current test (YAGNI).
• Isolation: Mock external APIs (HTTP) and Time.
• Realism: Prefer real DBs (test containers) and fast internal services (<200ms).

Rationalization Prevention

• "Too small to test": Small code still regresses. Write the test.
• "Manual testing is enough": Manual checks do not prove the RED step.
• "Keep code as reference": Pre-test code biases the implementation. Delete it.
• "Tests after are equivalent": Passing immediately proves little.

Verification Checklist

• [ ] Every new function/method failing test first?
• [ ] Failure message expected?
• [ ] Minimal code implemented passed?
• [ ] AAA structure followed?
• [ ] Coverage thresholds met?

Expert References

• AAA Example
• AAA Methodology
• Test Runners
• TDD Patterns
• Testing Anti-Patterns

Anti-Patterns

• No test-after: Writing tests post-implementation defeats TDD. Delete and restart.
• No assertion-free tests: test without assert not test.
• No testing implementation: Test behavior and contracts, not internal calls.

• A passing test alone is not enough: the failing test must demonstrate missing behavior before implementation.