incremental-implementation

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# Incremental Implementation

## Overview

Build in thin vertical slices — implement one piece, test it, verify it, then expand. Avoid implementing an entire feature in one pass. Each increment should leave the system in a working, testable state. This is the execution discipline that makes large features manageable.

## When to Use

- Implementing any multi-file change
- Building a new feature from a task breakdown
- Refactoring existing code
- Any time you're tempted to write more than ~100 lines before testing

**When NOT to use:** Single-file, single-function changes where the scope is already minimal.

## The Increment Cycle

```
┌──────────────────────────────────────┐
│                                      │
│   Implement ──→ Test ──→ Verify ──┐  │
│       ▲                           │  │
│       └───── Commit ◄─────────────┘  │
│              │                       │
│              ▼                       │
│          Next slice                  │
│                                      │
└──────────────────────────────────────┘
```

For each slice:

1. **Implement** the smallest complete piece of functionality
2. **Test** — run the test suite (or write a test if none exists)
3. **Verify** — confirm the slice works as expected (tests pass, build succeeds, manual check)
4. **Commit** -- save your progress with a descriptive message (see `git-workflow-and-versioning` for atomic commit guidance)
5. **Move to the next slice** — carry forward, don't restart

## Slicing Strategies

### Vertical Slices (Preferred)

Build one complete path through the stack:

```
Slice 1: Create a task (DB + API + basic UI)
    → Tests pass, user can create a task via the UI

Slice 2: List tasks (query + API + UI)
    → Tests pass, user can see their tasks

Slice 3: Edit a task (update + API + UI)
    → Tests pass, user can modify tasks

Slice 4: Delete a task (delete + API + UI + confirmation)
    → Tests pass, full CRUD complete
```

Each slice delivers working end-to-end functionality.

### Contract-First Slicing

When backend and frontend need to develop in parallel:

```
Slice 0: Define the API contract (types, interfaces, OpenAPI spec)
Slice 1a: Implement backend against the contract + API tests
Slice 1b: Implement frontend against mock data matching the contract
Slice 2: Integrate and test end-to-end
```

### Risk-First Slicing

Tackle the riskiest or most uncertain piece first:

```
Slice 1: Prove the WebSocket connection works (highest risk)
Slice 2: Build real-time task updates on the proven connection
Slice 3: Add offline support and reconnection
```

If Slice 1 fails, you discover it before investing in Slices 2 and 3.

## Implementation Rules

### Rule 0: Simplicity First

Before writing any code, ask: "What is the simplest thing that could work?"

After writing code, review it against these checks:
- Can this be done in fewer lines?
- Are these abstractions earning their complexity?
- Would a staff engineer look at this and say "why didn't you just..."?
- Am I building for hypothetical future requirements, or the current task?

```
SIMPLICITY CHECK:
✗ Generic EventBus with middleware pipeline for one notification
✓ Simple function call

✗ Abstract factory pattern for two similar components
✓ Two straightforward components with shared utilities

✗ Config-driven form builder for three forms
✓ Three form components
```

Three similar lines of code is better than a premature abstraction. Implement the naive, obviously-correct version first. Optimize only after correctness is proven with tests.

### Rule 0.5: Scope Discipline

Touch only what the task requires.

Do NOT:
- "Clean up" code adjacent to your change
- Refactor imports in files you're not modifying
- Remove comments you don't fully understand
- Add features not in the spec because they "seem useful"
- Modernize syntax in files you're only reading

From kevinnft/ai-agent-skills, MIT.

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