Incremental Implementation

Philosophy

"The simplest thing that could work."

Build in thin vertical slices. Each increment leaves the system working and testable.

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When to Apply

Use this skill when:

• Multi-file changes
• New features
• Refactoring work
• Any change > 100 lines

Skip for:

• Single-file, small changes
• Simple bug fixes
• Configuration updates

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The Increment Cycle

┌─────────────────────────────────────────┐
│  1. Pick smallest complete piece        │
│                 ↓                       │
│  2. Write failing test (RED)            │
│                 ↓                       │
│  3. Implement minimal code (GREEN)      │
│                 ↓                       │
│  4. Refactor if needed                  │
│                 ↓                       │
│  5. Run all tests                       │
│                 ↓                       │
│  6. Commit with clear message           │
│                 ↓                       │
│  7. Repeat for next piece               │
└─────────────────────────────────────────┘

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Vertical vs Horizontal Slicing

Vertical Slices (Correct)

Each slice delivers end-to-end functionality:

Task 1: User can create a task
        └── DB model + API route + UI component

Task 2: User can view task list
        └── DB query + API endpoint + List component

Task 3: User can complete a task
        └── DB update + API handler + Toggle UI

Horizontal Slices (Anti-pattern)

Layers completed separately:

Task 1: Create all DB models
Task 2: Create all API routes
Task 3: Create all UI components

❌ Problem: Nothing works until everything is done

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Slicing Strategies

1. Happy Path First

Slice 1: Basic flow works
Slice 2: Add validation
Slice 3: Add error handling
Slice 4: Add edge cases

2. Risk-First

Slice 1: Uncertain/complex piece (reduce risk early)
Slice 2: Dependent pieces (build on verified foundation)
Slice 3: Polish (now safe to invest time)

3. Contract-First

Slice 1: Define API contract (types, endpoints)
Slice 2: Backend implements contract
Slice 3: Frontend implements against contract

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Rules

The 100-Line Rule

Test before writing more than ~100 lines.

If you've written 100+ lines without running tests, stop and verify.

Touch Only What's Needed

Don't refactor adjacent code. Don't add unrequested features.

Stay focused on the current task.

Keep It Building

Project must compile and tests must pass after each increment.

Never leave the codebase broken between commits.

Feature Flags for Incomplete Work

// Use flags when merging incomplete features
if (featureFlags.newCheckout) {
  return <NewCheckoutFlow />;
}
return <LegacyCheckout />;

Safe Defaults

New code defaults to conservative, disabled behavior:

• New features off by default
• New permissions denied by default
• New validations strict by default

Rollback-Friendly

Each increment should be independently revertable:

# If this commit breaks something, revert just this
git revert HEAD

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Red Flags

Stop and reassess if you're:

• Writing > 100 lines without testing
• Mixing unrelated changes in one commit
• Expanding scope mid-task
• Breaking the build between increments
• Creating abstractions "for later"
• Touching files outside the task scope

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Commit Strategy

Each increment = one commit:

# Good: Atomic, focused commits
git commit -m "feat(tasks): add Task model with title and status"
git commit -m "feat(tasks): add POST /api/tasks endpoint"
git commit -m "feat(tasks): add CreateTaskForm component"

# Bad: Large, unfocused commits
git commit -m "Add task feature"  # 500 lines across 10 files

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When Stuck

If an increment fails:

1. Stop — Don't push through
2. Diagnose — What specifically failed?
3. Reduce scope — Can you make a smaller increment?
4. Ask for help — If truly blocked

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Verification Checklist

After each increment:

• Tests pass
• Build succeeds
• Code compiles
• Feature works (manual check if UI)
• Commit is atomic and focused
• Message follows conventions