Implements a strict Red-Green-Refactor loop to ensure zero production code is written without a prior failing test. Use when: creating new features, fixing bugs, or expanding test coverage.
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---
name: common-tdd
description: 'Implements a strict Red-Green-Refactor loop to ensure zero production code is written without a prior failing test. Use when: creating new features, fixing bugs, or expanding test coverage.'
metadata:
triggers:
files:
- '**/*.test.ts'
- '**/*.spec.ts'
- '**/*_test.go'
- '**/*Test.java'
- '**/*_test.dart'
- '**/*_spec.rb'
keywords:
- tdd
- unit test
- write test
- red green refactor
- failing test
- test coverage
---
# Test-Driven Development (TDD) Standard
## **Priority: P0 — Iron Law**
> **NO PRODUCTION CODE WITHOUT FAILING TEST FIRST.**
> Code written before test MUST deleted. Start over.
## **Step 1: RGR Loop (Red-Green-Refactor)**
1. **RED**: Write minimal failing test. **Verify failure** (Expected error, not typo).
2. **GREEN**: Write simplest code to pass. **Verify pass**.
3. **REFACTOR**: Clean up code while staying green.
## **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](references/aaa_example.md) 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](references/test_runners.md) 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).
## **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](references/aaa_example.md)
- [AAA Methodology](references/aaa_methodology.md)
- [Test Runners](references/test_runners.md)
- [TDD Patterns](references/tdd_patterns.md)
- [Testing Anti-Patterns](references/testing_anti_patterns.md)
## 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.
