vertical-slice-planning

# Vertical Slice Planning Skill

This skill guides the decomposition of features into vertical slices - thin, end-to-end pieces of functionality that can be shipped independently.

## When to Use

Apply this skill when:
- Breaking down a feature into sub-issues
- Deciding implementation order for a feature
- Planning PR structure for a feature
- Users ask "how should we break this down?"
- Discussing what to build first
- Reviewing feature decomposition plans

## What is a Vertical Slice?

A vertical slice cuts through ALL layers of the application to deliver a thin piece of complete functionality.

```
┌─────────────────────────────────────────┐
│              HORIZONTAL LAYERS           │
├─────────────────────────────────────────┤
│  UI Layer      │ █ │     │     │        │
├────────────────┼───┼─────┼─────┼────────┤
│  API Layer     │ █ │     │     │        │
├────────────────┼───┼─────┼─────┼────────┤
│  Service Layer │ █ │     │     │        │
├────────────────┼───┼─────┼─────┼────────┤
│  Data Layer    │ █ │     │     │        │
└────────────────┴───┴─────┴─────┴────────┘
                  ↑
            Vertical Slice
         (Complete feature)
```

## Vertical vs Horizontal

### Horizontal Approach (Avoid)
Building all of one layer before moving to the next:
1. Build all database models
2. Build all API endpoints
3. Build all UI components
4. Wire everything together

**Problems:** Nothing works until everything is done, late integration issues, hard to show progress.

### Vertical Approach (Prefer)
Building thin, complete features:
1. User can view empty product list (UI → API → DB)
2. User can add a product (UI → API → DB)
3. User can edit a product (UI → API → DB)

**Benefits:** Each slice is shippable, continuous integration, visible progress.

## How to Identify Vertical Slices

### 1. Start with User Actions
What can the user DO? Each action is often a slice.

### 2. Find the Thinnest Version
For each action, what's the minimal implementation?
- Skip