Go TDD Workflow

Apply TDD end-to-end: write failing tests first, implement minimal code, refactor safely, and prove quality with coverage plus risk-path checks.

Hard Rules

• Start with tests, not implementation.
• Preserve visible Red -> Green -> Refactor evidence in commands/output.
• Keep test files co-located and named <target>_test.go.
• Assertion strategy (adapt to project):
  • If project uses testify: require for fatal preconditions, assert for value checks.
  • If project uses standard library only: use t.Fatalf for fatal, t.Errorf for value checks; include got/want: t.Errorf("Name = %q, want %q", got, want).
  • If project uses go-cmp: use cmp.Diff for deep struct comparison.
  • Detection: Check existing _test.go files for "github.com/stretchr/testify" imports. Follow project convention.
• Prefer table-driven tests with t.Run.
• Prefer real deps or lightweight fakes; avoid heavy mock chains by default.
• Do not add speculative production code not required by failing tests.

New Mandatory Gates

1) Defect Hypothesis Gate

Before writing tests, list concrete defect hypotheses from target code:

• boundary/index (n-1, i+1, last-item behavior)
• error propagation/wrapping
• mapping loss/data mismatch
• concurrency/order/timing
• idempotency/retry behavior

Each hypothesis must map to at least one test case name. For detailed defect-hypothesis patterns and BAD/GOOD examples, cross-reference the unit-test skill: references/bug-finding-techniques.md, references/killer-case-patterns.md, and Fixed Boundary Checklist.

2) Killer Case Gate

Per changed method/use-case, add at least one killer case that:

• targets a high-risk defect hypothesis
• includes assertion(s) that must fail on known bad mutation/path
• is explicitly marked in report

3) Coverage Gate (Line + Risk Path)

• Line coverage gate: changed package(s) >=80% by default.
• Risk-path gate: all high-risk branches/hypotheses must be covered even if line coverage already passes.
• If gate waived, require explicit user approval and documented risk.

4) Execution Integrity Gate

Never claim tests/coverage were run unless actually executed.

• If executed: report exact commands and key result lines.
• If not executable in current environment: report Not run in this environment and provide exact run commands.

5) Concurrency Determinism Gate

For concurrency-sensitive code:

• avoid time.Sleep for synchronization
• use channels/barriers/waitgroups/atomics to control ordering
• run with -race

6) Change-Size Test Budget Gate

Choose test depth by change size to avoid test bloat. Use concrete criteria first, then case budget:

If exceeding range, justify by distinct logic paths. For security-sensitive code (auth, input validation, SSRF guards, crypto), the budget may be doubled — document the security rationale in the output contract.

Workflow

1. Classify change size (S/M/L) and target scope.
2. Write behavior contract (Given/When/Then bullets).
3. Build defect hypothesis list and map each to tests.
4. Red: write failing tests first (include killer case).
   • Characterization testing (for pre-existing code): when adding tests AFTER implementation already exists, satisfy the Red evidence gate via one of:
     • Mutation: temporarily break the production code, verify new tests fail, then revert.
     • Hypothesis: document the specific defect hypothesis each test targets — the hypothesis itself serves as Red evidence that the test guards a real risk.
5. Green: implement minimal code to pass failing tests.
6. Refactor: improve structure without behavior change.
7. Validate quality gates:
   • focused tests
   • coverage (line + risk-path)
   • -race when concurrency is relevant
8. Report with evidence and residual risks.

Command Playbook (Go)

# Red phase (expect at least one fail)
go test ./path/to/pkg -run TestXxx -v

# Green phase
go test ./path/to/pkg -v

# Coverage (line)
go test ./path/to/pkg -coverprofile=coverage.out
go tool cover -func=coverage.out

# Concurrency safety (when applicable)
go test ./path/to/pkg -race -v

# Broader regression (as needed)
go test ./...

Anti-Examples (Core TDD Mistakes)

For the full set of 7 anti-examples, read references/anti-examples.md. The most critical TDD mistakes are inlined here so the contract is self-contained:

Mistake 1: Writing all tests before any implementation (Big-Bang Red)

// BAD: 15 test cases written at once before any production code
// You lose the tight feedback loop — hard to tell which test drives which behavior
func TestUserService(t *testing.T) {
	t.Run("create", func(t *testing.T) { /* ... 5 subtests */ })
	t.Run("update", func(t *testing.T) { /* ... 5 subtests */ })
	t.Run("delete", func(t *testing.T) { /* ... 5 subtests */ })
}
// Then implement everything at once — this is test-first, NOT TDD

// GOOD: one failing test → minimal implementation → next failing test
// Iteration 1: TestUserService/create/success → implement Create()
// Iteration 2: TestUserService/create/duplicate_email → add uniqueness check
// Each cycle is Red → Green → Refactor before moving on

Mistake 2: Testing implementation details instead of behavior

// BAD: locks test to implementation details and blocks safe refactor
repo.AssertCalled(t, "Save", mock.Anything, user)
// GOOD: assert observable behavior such as returned state, persisted fields, and domain errors
require.Equal(t, "active", got.Status)

Mistake 3: Refactor phase changes observable behavior

// BAD: "refactor" silently changes API behavior
func NormalizeEmail(s string) string { return strings.TrimSpace(strings.ToLower(s)) }
// GOOD: keep behavior unchanged during refactor; add a new Red cycle for any behavior change
func normalizeEmail(s string) string { return strings.TrimSpace(s) }

Mistake 4: Skipping Red evidence — "it compiles so it works"

// BAD: write test and implementation simultaneously, never see a failure
func TestAdd(t *testing.T) {
	// Written AFTER Add() was already implemented
	assert.Equal(t, 3, Add(1, 2))
}

// GOOD: write test FIRST, run it, see it FAIL, then implement
// Step 1: write test → go test → FAIL (Add undefined)
// Step 2: stub: func Add(a, b int) int { return 0 }
//         go test → FAIL (got 0, want 3)
// Step 3: implement: return a + b → go test → PASS
// Red evidence proves the test actually validates something

Mistake 5: Change-size mismatch

// BAD: S change but 40 test cases and 6 helper files added without new logic paths
func TestTinyBugfix(t *testing.T) { /* massive matrix */ }
// GOOD: size the regression to the change budget, then justify any extra cases with distinct risk paths
func TestTinyBugfix_LastElementBoundary(t *testing.T) { /* focused killer case */ }

Mistake 6: Speculative helper extraction before tests demand it

// BAD: extract helpers and abstractions before any failing test proves the need
type orchestrator struct{ repo Repo; audit Audit; cache Cache }
// GOOD: add the smallest code needed for the current Red case, then refactor after Green
func createUser(repo Repo, in CreateUserInput) (User, error) { /* minimal path */ }

Quality Scorecard

Mark each as PASS / FAIL / N/A (reason).

Critical (all must pass for overall PASS)

Standard (≥4/5 must pass)

Hygiene (≥3/4 must pass)

Scoring:

• PASS: All Critical pass AND ≥4/5 Standard AND ≥3/4 Hygiene
• FAIL: Any Critical fails → overall FAIL regardless of other scores

Output Contract

• Changed files
• Change size: S/M/L with reason
• Defect hypotheses -> test mapping
• Killer cases
• Red -> Green evidence (or Not run in this environment)
• Coverage (line + risk-path)
• Scorecard (3-tier: Critical / Standard / Hygiene)
• Residual risks / follow-ups — must include:
  • untested edge cases beyond budget
  • design limitations of the target function
  • security implications for sensitive code (e.g., bypass vectors, upstream stdlib assumptions)
  • dependencies on upstream behavior (stdlib, libraries, environment)

References (Load Selectively)

Always read (every TDD task):

• references/boundary-checklist.md — defect hypothesis patterns and killer case design

Read for API/service layer TDD (skip for pure-function or utility TDD):

• references/api-3layer-template.md — Handler/Service/Repo TDD template
• references/fake-stub-template.md — fake design and error injection

Read for first-time TDD or complex refactors:

• references/tdd-workflow.md — end-to-end walkthrough, Outside-In vs Inside-Out, characterization testing

Read when reviewing or generating TDD code:

• references/anti-examples.md — 5 additional TDD mistakes with BAD/GOOD examples

Read for characterization testing (adding tests to pre-existing code):

• references/golden-characterization-example.md — full output contract with mutation-based Red evidence for a security-sensitive function

Skill Maintenance

Run regression before publishing changes:

scripts/run_regression.sh

Validates frontmatter, gates, anti-examples, scorecard, references, and golden fixtures.