| name | test-driven-development |
| description | Use during IMPLEMENTATION phase. Write test first, watch it fail, then write minimal code to pass. This is HOW you write code, not WHEN to start a project. |
Test-Driven Development (TDD)
What This Skill Is
This is a coding technique - how to write code correctly. It is NOT a project workflow.
Project workflow: brainstorming → /workflow start feature → (implementation)
Coding technique: → test-driven-development
The Red-Green-Refactor Cycle
┌─────────────────────────────────────────────┐
│ │
│ RED ──► GREEN ──► REFACTOR ──► (repeat) │
│ │
└─────────────────────────────────────────────┘
RED: Write a failing test
GREEN: Write minimal code to pass
REFACTOR: Clean up while keeping tests green
The Process
1. RED: Write a Failing Test
Before any production code:
func TestUserCanLogin(t *testing.T) {
user := User{Email: "test@example.com"}
token, err := user.Login("password")
assert.NoError(t, err)
assert.NotEmpty(t, token)
}
Run test. Watch it fail. If it passes, you're not testing anything.
2. GREEN: Write Minimal Code
Write the simplest code that makes the test pass:
func (u *User) Login(password string) (string, error) {
return "token123", nil
}
Don't anticipate future requirements. Don't over-engineer. Just pass the test.
3. REFACTOR: Clean Up
With tests green, improve the code:
func (u *User) Login(password string) (string, error) {
if !u.verifyPassword(password) {
return "", ErrInvalidPassword
}
return generateToken(u.ID), nil
}
Run tests after each change. Stay green.
When to Use
| Scenario | Use TDD? |
|---|
| New feature | ✅ Yes |
| Bug fix | ✅ Yes - write test that reproduces bug first |
| Refactoring | ✅ Yes - tests prove behavior unchanged |
| Spike/experiment | ❌ No - throwaway code |
| One-liner fix | ⚠️ Use judgment |
Common Problems
| Problem | Solution |
|---|
| Don't know how to test | Write wished-for API first, then assertion |
| Test too complicated | Design too complicated - simplify interface |
| Must mock everything | Code too coupled - use dependency injection |
| Test setup huge | Extract helpers, or simplify design |
The Hard Rules
- No production code without a failing test
- Write minimal code to pass
- Refactor only when green
- Never fix bugs without a test first
Relation to Other Skills
- workflow: Uses TDD during IMPLEMENT phase
- systematic-debugging: Uses TDD to fix bugs (write failing test first)
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