planning-and-task-breakdown

# Planning and Task Breakdown

## Overview

Decompose work into small, verifiable tasks with explicit acceptance criteria. Good task breakdown is the difference between an agent that completes work reliably and one that produces a tangled mess. Every task should be small enough to implement, test, and verify in a single focused session.

## When to Use

- You have a spec and need to break it into implementable units
- A task feels too large or vague to start
- Work needs to be parallelized across multiple agents or sessions
- You need to communicate scope to a human
- The implementation order isn't obvious

**When NOT to use:** Single-file changes with obvious scope, or when the spec already contains well-defined tasks.

## The Planning Process

### Step 1: Enter Plan Mode

Before writing any code, operate in read-only mode:

- Read the spec and relevant codebase sections
- Identify existing patterns and conventions
- Map dependencies between components
- Note risks and unknowns

**Do NOT write code during planning.** The output is a plan document saved to `tasks/plan.md` and a task list recorded in the task list target (see Output Files; default `tasks/todo.md`), not implementation.

### Step 2: Identify the Dependency Graph

Map what depends on what:

```
Database schema
    │
    ├── API models/types
    │       │
    │       ├── API endpoints
    │       │       │
    │       │       └── Frontend API client
    │       │               │
    │       │               └── UI components
    │       │
    │       └── Validation logic
    │
    └── Seed data / migrations
```

Implementation order follows the dependency graph bottom-up: build foundations first.

### Step 3: Slice Vertically

Instead of building all the database, then all the API, then all the UI — build one complete feature path at a time:

**Bad (horizontal slicing):**
```
Task 1: Build entire database schema
Task 2: Build all API endpoints
Task 3: Build all UI components
Task 4: Connect everything
```

**Good (vertical slicing):**
```
Task 1: User can create an account (schema + API + UI for registration)
Task 2: User can log in (auth schema + API + UI for login)
Task 3: User can create a task (task schema + API + UI for creation)
Task 4: User can view task list (query + API + UI for list view)
```

Each vertical slice delivers working, testable functionality.

### Step 4: Write Tasks

Each task follows this structure, whether it lands in the markdown task list or as an item in an external tracker (see Output Files):

```markdown
## Task [N]: [Short descriptive title]

**Description:** One paragraph explaining what this task accomplishes.

**Acceptance criteria:**
- [ ] [Specific, testable condition]
- [ ] [Specific, testable condition]

**Verification:**
- [ ] Tests pass: [the repository's focused-test command]
- [ ] Build succeeds: [the repository's build command]
- [ ] Manual check: [description of what to verify]

**Dependencies:** [Task numbers this depends on, or "None"]

**Files likely touched:**
- `src/path/to/file.ts`
- `tests/path/to/test.ts`

**Estimated scope:** [Small: 1-2 files | Medium: 3-5 files | Large: 5+ files]
```

### Step 5: Order and Checkpoint

Arrange tasks so that:

1. Dependencies are satisfied (build foundation first)
2. Each task leaves the system in a working state
3. Verification checkpoints occur after every 2-3 tasks
4. High-risk tasks are early (fail fast)

Add explicit checkpoints to the task list target:

```markdown
## Checkpoint: After Tasks 1-3
- [ ] All tests pass
- [ ] Application builds without errors
- [ ] Core user flow works end-to-end
- [ ] Review with human before proceeding
```

## Task Sizing Guidelines

| Size | Files | Scope | Example |
|------|-------|-------|---------|
| **XS** | 1 | Single function or config change | Add a validation rule |
| **S** | 1-2 | One component or endpoint | Add a new API endpoint |
| **M** | 3-5 | One feature slice | User registration flow |
| **L** | 5-8 | Multi-component feature | Search with filtering and pagination |
| **XL** | 8+ | **Too large — break it down further** | — |

If a task is L or larger, it should be broken into smaller tasks. An agent performs best on S and M tasks.

**When to break a task down further:**
- It would take more than one focused session (roughly 2+ hours of agent work)
- You cannot describe the acceptance criteria in 3 or fewer bullet points
- It touches two or more independent subsystems (e.g., auth and billing)
- You find yourself writing "and" in the task title (a sign it is two tasks)

## Output Files

- **Plan document:** Save the implementation plan to `tasks/plan.md`. This is always a markdown file — design decisions, risks, and open questions don't map cleanly onto individual tracker issues.
- **Task list:** Record each task in the **task list target** (defined below).

Create the `tasks/` directory if it does not exist.

**Never overwrite an incomplete plan.** Before writing `tasks/plan.md` or `tasks/todo.md`, check whether they already exist and still contain unchecked tasks:

- Same work being replanned (the user asked to revise or extend this plan) → update the existing files in place.
- Different work → **stop and ask.** The unchecked tasks may be mid-build in another session. Do not delete, overwrite, or rename the existing files on your own; present the conflict and let the user decide (finish the old plan first, explicitly discard it, or tell you where the new plan should go).

The same rule applies to an external task list target: never bulk-close or delete another plan's open tracker items to make room for new ones.

### Task List Target

The task list target is where tasks and checkpoints are recorded. It is defined once, here; every other reference in this skill defers to it.

- **Default: a checklist-style markdown file at `tasks/todo.md`.** This is the convention the `/build` command and other downstream tooling expect. Use it unless the project says otherwise.
- **External tracker:** if