Skill: tdd-first

What I do

I enforce the non-negotiable discipline of writing tests before implementation. Where bdd-workflow teaches what to test and how to structure specs, I enforce when — test FIRST, always. No exceptions. No shortcuts. No "I'll add tests later".

When to use me

• Before writing ANY implementation code — load me and follow the mandatory sequence
• When an agent is tempted to "just write the function first"
• During code review to verify test-first discipline was followed
• When pairing with bdd-workflow, ginkgo-gomega, jest, or any test framework
• When fixing a bug — write the failing test that reproduces it BEFORE the fix

Core principles

1. Test first is non-negotiable — No implementation code exists without a failing test that demanded it
2. RED means the right failure — The test fails because the behaviour doesn't exist yet, not because of a compilation error or wrong assertion
3. Minimum to GREEN — Write only enough implementation to make the failing test pass, nothing more
4. Refactor under GREEN — Improve design only when all tests pass; never refactor on RED
5. Small cycles — Each RED-GREEN-REFACTOR cycle should take minutes, not hours

MANDATORY SEQUENCE (NON-NEGOTIABLE)

Every agent MUST follow this sequence. Violation is a blocking failure.

STEP 1: WRITE THE TEST
  → Describe the behaviour you want
  → Use your framework's syntax (It/Expect, test/assert, etc.)
  → The test MUST reference code that doesn't exist yet

STEP 2: MAKE IT COMPILE (stub only)
  → Create the function/method/type signature
  → Return zero values — Do NOT add any logic

STEP 3: RUN THE TEST — VERIFY RED
  → Confirm the test FAILS for the RIGHT REASON:
    ✅ "expected 42 but got 0" — correct RED (behaviour missing)
    ❌ "undefined: CreateUser" — compilation error, NOT red
    ❌ "nil pointer dereference" — crash, NOT red
  → If the test passes: your test is wrong

STEP 4: IMPLEMENT — MINIMUM TO GREEN
  → Write the simplest code that makes the test pass
  → Do NOT add extra features or optimisations
  → Run ALL tests — they must pass

STEP 5: REFACTOR UNDER GREEN
  → Improve naming, extract functions, remove duplication
  → Run tests after every change — stay GREEN

STEP 6: REPEAT — next behaviour, back to Step 1

Patterns & examples

WRONG (implementation first) — VIOLATION:

func CalculateDiscount(price float64, tier string) float64 {
    switch tier {
    case "gold": return price * 0.8
    case "silver": return price * 0.9
    default: return price
    }
}
// Then struggles to test rigid code — wasted tokens, poor design

RIGHT (test first):

// STEP 1: Write the test
It("applies 20% discount for gold tier", func() {
    result := CalculateDiscount(100.0, "gold")
    Expect(result).To(Equal(80.0))
})

// STEP 2: Stub
func CalculateDiscount(price float64, tier string) float64 { return 0 }

// STEP 3: RED — "expected 80 but got 0" ✅

// STEP 4: Minimum to GREEN
func CalculateDiscount(price float64, tier string) float64 {
    if tier == "gold" { return price * 0.8 }
    return 0
}

// STEP 5: Refactor after more cases pass

The cost of implementation first

Writing implementation before tests produces: coupled code, hard-to-mock dependencies, tests that conform to implementation rather than driving design, and wasted tokens fixing untestable code. Test-first produces injectable dependencies, clean interfaces, and safe refactoring from the first line. See KB doc for the full comparison table and interface stub pattern.

Anti-patterns to avoid

• "I'll add tests later" — You won't. And if you do, they'll test the implementation, not the behaviour
• GREEN without RED — If you never saw the test fail, you don't know it tests anything
• Compilation errors as RED — undefined: Foo is not a failing test; stub the function first, then see the behaviour fail
• Over-implementing on GREEN — Write ONLY enough to pass. Extra code is untested code
• Skipping REFACTOR — The refactor phase is where clean design emerges; skipping it accumulates debt
• Testing after debugging — When you find a bug, write a failing test FIRST, then fix it

KB Reference

~/vaults/baphled/3. Resources/Knowledge Base/AI Development System/Skills/Testing-BDD/TDD First.md

Related skills

• bdd-workflow - Teaches BDD syntax and outside-in approach; tdd-first enforces the discipline
• clean-code - Apply during the REFACTOR phase of each cycle
• ginkgo-gomega - Go BDD framework; tdd-first ensures you use it test-first
• jest - JavaScript test framework; tdd-first ensures you use it test-first
• rspec-testing - Ruby BDD framework; tdd-first ensures you use it test-first
• discipline - General step discipline; tdd-first is the testing-specific enforcement