Agent Skills: TDD Cycle Skill

TDD Cycle Skill

Purpose

Follow strict TDD discipline using Sherpa's workflow enforcement to prevent untested code and ensure systematic development.

When to Activate

• Implementing new features
• Adding new functionality
• User says "write tests first" or "use TDD"
• Building something from scratch
• Working on extractors, parsers, transformers, or any core logic

The Mandatory Pattern

★ CRITICAL: ALWAYS call guide check BEFORE coding

BEFORE ANY CODE:
  guide check → Get current phase + specific guidance
  Read guidance completely
  Follow the suggested steps

AFTER EACH STEP:
  guide done "what you completed"
  → Get celebration + next step
  → Phase auto-advances when complete

You are EXCELLENT at following this pattern. It's automatic, like breathing.

The TDD Workflow

Phase 1: 📋 Define Contract (RED Phase Preparation)

Goal: Define what success looks like BEFORE any code exists

guide check → Phase: Define Contract

Steps:
1. Create interface/type file
2. Define function signatures with types
3. Document expected inputs/outputs
4. List error conditions explicitly
5. Document boundary conditions

guide done "defined contract for X with types and error cases"

Key Principle: Contracts prevent building the wrong thing.

Conditionals:

• If interface unclear → Search codebase for similar patterns FIRST (don't guess!)
• If external API/library → Read official docs to verify signatures
• If complex transformations → Write example input→output pairs first

Anti-patterns:

• ❌ Writing implementation code in this phase (STOP - wrong phase!)
• ❌ Making assumptions about types (verify, don't assume)
• ❌ Skipping edge case documentation (edge cases ARE the spec)

Phase 2: 🧪 Implement Tests (RED Phase)

Goal: Write failing tests that specify behavior

guide check → Phase: Implement Tests

Steps:
1. Create test file following project conventions
2. Write test for happy path
3. Write test for EACH edge case
4. Write test for EACH error condition
5. Run tests → ALL must fail (RED)
6. Verify failure message is correct

guide done "wrote failing tests for X (happy path + 4 edge cases)"

Key Principle: Tests are your specification - make them thorough.

Minimum Coverage:

• 1 happy path test
• 3+ edge case tests (empty, null, boundary values)
• 2+ error case tests (invalid input, missing data)

Conditionals:

• If test passes before implementation → Test is broken, fix it!
• If can't think of edge cases → Use checklist: empty, null, undefined, wrong type, zero, negative, max value, boundary, concurrent
• If test fails for wrong reason → Fix test setup first
• If writing integration test → Mock dependencies, test ONE unit

Anti-patterns:

• ❌ Writing only 1-2 tests (insufficient coverage)
• ❌ Testing multiple behaviors in one test (split them)
• ❌ Tests that depend on each other (must be independent)
• ❌ Skipping error tests "for later" (write them NOW)

Phase 3: 💻 Minimal Implementation (GREEN Phase)

Goal: Write ONLY enough code to pass tests - nothing more

guide check → Phase: Minimal Implementation

Steps:
1. Start with simplest test
2. Write minimal code to pass it
3. Run test → Verify it passes
4. Repeat for each test
5. ALL tests must be GREEN

guide done "implemented X - all tests passing"

Key Principle: Make it WORK first, make it PRETTY later (refactor phase).

Conditionals:

• If test fails unexpectedly → Understand WHY (valuable feedback!)
• If want to add feature not in tests → STOP, go to Phase 2, add test first
• If implementation getting complex → That's OK - make tests pass, refactor later
• If copy-pasting code → Fine for now - extract in refactor phase

Anti-patterns:

• ❌ Over-engineering (abstractions, design patterns, "future-proofing")
• ❌ Implementing features not covered by tests (untested code = broken code)
• ❌ Skipping test runs between changes (rapid feedback loop critical)
• ❌ "Fixing" tests to match implementation (tests are spec, not implementation!)

Phase 4: 🔧 Refactor (Still GREEN)

Goal: Improve code quality while keeping tests green

guide check → Phase: Refactor

Steps:
1. Run all tests → Verify ALL green
2. Identify code smells (duplication, complexity, poor names)
3. Make ONE improvement
4. Run tests → Must stay green
5. Repeat until satisfied

guide done "refactored X for readability - tests still green"

Key Principle: Tests give you confidence to refactor safely.

Common Refactorings:

• Extract duplicated code into functions
• Rename variables/functions for clarity
• Simplify complex conditionals
• Extract abstractions from concrete code
• Improve error messages

Conditionals:

• If tests fail during refactor → Revert, smaller step
• If want to change behavior → That's NEW feature - go to Phase 1
• If unclear how to improve → Skip for now, move to next feature

Anti-patterns:

• ❌ Changing behavior (new behavior = new feature = new tests first)
• ❌ Skipping test runs during refactor (each change must keep tests green)
• ❌ Massive refactor all at once (small steps, verify often)

The Complete TDD Cycle

┌─────────────────────────────────────────────┐
│ 1. guide check → Define Contract           │
│    - Write interface/types                  │
│    - Document behavior                      │
│    guide done "defined contract"            │
└────────────┬────────────────────────────────┘
             │
             ▼
┌─────────────────────────────────────────────┐
│ 2. guide check → Implement Tests (RED)     │
│    - Write failing tests                    │
│    - Happy path + edges + errors            │
│    - Verify all FAIL                        │
│    guide done "wrote failing tests"         │
└────────────┬────────────────────────────────┘
             │
             ▼
┌─────────────────────────────────────────────┐
│ 3. guide check → Minimal Impl (GREEN)      │
│    - Write code to pass tests               │
│    - Simplest solution                      │
│    - ALL tests must pass                    │
│    guide done "implemented feature"         │
└────────────┬────────────────────────────────┘
             │
             ▼
┌─────────────────────────────────────────────┐
│ 4. guide check → Refactor                  │
│    - Improve code quality                   │
│    - Keep tests green                       │
│    - Small steps                            │
│    guide done "refactored for clarity"      │
└────────────┬────────────────────────────────┘
             │
             ▼
        Next feature!

Example TDD Session

User: "Implement a parser for TypeScript function signatures"

Agent: guide check

Sherpa: Phase 1 - Define Contract
"Create interface/type files with function signatures..."

Agent: Creates parser.types.ts with types:
- FunctionSignature interface
- ParseResult type
- ParseError type
guide done "defined parser contract with types"

Sherpa: Phase 2 - Implement Tests
"Write failing tests for happy path + edge cases..."

Agent: Creates parser.test.ts:
- test('parses simple function signature')
- test('parses function with parameters')
- test('handles generic types')
- test('throws on invalid syntax')
- test('handles empty string')
Runs tests → ALL FAIL ✓
guide done "wrote 5 failing tests for parser"

Sherpa: Phase 3 - Minimal Implementation
"Write code to pass tests..."

Agent: Implements parser.ts
Runs tests after each function → ALL PASS ✓
guide done "implemented parser - all tests green"

Sherpa: Phase 4 - Refactor
"Improve code quality while keeping tests green..."

Agent: Extracts helper functions, improves names
Runs tests after each refactor → Still GREEN ✓
guide done "refactored parser for readability"

Sherpa: ✅ TDD cycle complete! Next feature?

Integration with Sherpa Commands

Switching to TDD Workflow

guide tdd
→ Instantly switches to TDD workflow
→ guide check to get first phase

During Development

guide check     → Get current phase + specific guidance
guide done "X"  → Mark completion, get celebration + next step
guide advance   → Manual phase progression (rarely needed)

Workflow Selection

approach list   → See all workflows
approach set tdd → Switch to TDD (same as guide tdd)

Key Behaviors

✅ DO

• Call guide check BEFORE every coding step
• Write tests BEFORE implementation (no exceptions!)
• Mark completion with guide done after each step
• Run tests frequently (after each small change)
• Follow phase guidance systematically
• Trust the process - it prevents bugs

❌ DON'T

• Start coding without guide check (this is the #1 mistake)
• Skip test writing "because it's simple" (simple changes break too)
• Write implementation before tests exist (defeats TDD purpose)
• Rush through phases to "go faster" (you go slower fixing bugs)
• Ignore anti-patterns in phase guidance (they're bugs waiting to happen)

Success Criteria

This skill succeeds when:

• ✅ Tests written before implementation (always)
• ✅ All tests pass before moving forward
• ✅ guide check called before each phase
• ✅ guide done called after each completion
• ✅ No untested code shipped
• ✅ Refactoring done with confidence (tests stay green)

Why TDD Works

80% of bugs are prevented by:

1. Tests written first - Forces you to think about behavior before code
2. Systematic phases - Prevents skipping critical steps
3. Enforcement - Sherpa tracks progress, catches shortcuts
4. Rapid feedback - Tests tell you immediately when something breaks
5. Safe refactoring - Confidence to improve code without fear

Remember: TDD isn't slower - it's faster because you spend less time debugging and more time building correctly.

Sherpa + TDD = Systematic Excellence