planning-and-task-breakdown

name	planning-and-task-breakdown
description	Produces a read-only implementation plan: vertical slices, acceptance criteria, verification commands, and checkpoints — no code. Use when a spec is too large to start, scope must be estimated, or work can be paralleled. Not for /goal GOAL.md autonomous loops (goal), single-file obvious edits, or writing the implementation during planning.
version	1.0.1

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.

The output of this skill is a plan document, not implementation. Do NOT write code during planning.

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.

Prerequisites

• A spec, requirements document, or clear description of the desired outcome
• Read access to the relevant codebase sections (you will need to read existing patterns and conventions before planning)
• No code changes are permitted during planning — operate in read-only mode

Procedure

Step 1: Enter Plan Mode (Read-Only)

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

1. Read the spec and relevant codebase sections
2. Identify existing patterns and conventions
3. Map dependencies between components
4. Note risks and unknowns

Do NOT write code during planning. The output is a plan document, not implementation.

Step 2: Identify the Dependency Graph

Map what depends on what. Implementation order follows the dependency graph bottom-up: build foundations first.

Example dependency graph:

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

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:

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

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

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

**Verification:**
- [ ] Tests pass: `npm test -- --grep "feature-name"`
- [ ] Build succeeds: `npm run build`
- [ ] 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:

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

Step 6: Produce the Plan Document

Assemble everything into a plan document using the template below. This is the final deliverable of the planning process.

# Implementation Plan: [Feature/Project Name]

## Overview
[One paragraph summary of what we're building]

## Architecture Decisions
- [Key decision 1 and rationale]
- [Key decision 2 and rationale]

## Task List

### Phase 1: Foundation
- [ ] Task 1: ...
- [ ] Task 2: ...

### Checkpoint: Foundation
- [ ] Tests pass, builds clean

### Phase 2: Core Features
- [ ] Task 3: ...
- [ ] Task 4: ...

### Checkpoint: Core Features
- [ ] End-to-end flow works

### Phase 3: Polish
- [ ] Task 5: ...
- [ ] Task 6: ...

### Checkpoint: Complete
- [ ] All acceptance criteria met
- [ ] Ready for review

## Risks and Mitigations
| Risk | Impact | Mitigation |
|------|--------|------------|
| [Risk] | [High/Med/Low] | [Strategy] |

## Open Questions
- [Question needing human input]

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)

Parallelization Opportunities

When multiple agents or sessions are available:

• Safe to parallelize: Independent feature slices, tests for already-implemented features, documentation
• Must be sequential: Database migrations, shared state changes, dependency chains
• Needs coordination: Features that share an API contract (define the contract first, then parallelize)

Pitfalls

Common Rationalizations That Cause Failure

Rationalization	Reality
"I'll figure it out as I go"	That's how you end up with a tangled mess and rework. 10 minutes of planning saves hours.
"The tasks are obvious"	Write them down anyway. Explicit tasks surface hidden dependencies and forgotten edge cases.
"Planning is overhead"	Planning is the task. Implementation without a plan is just typing.
"I can hold it all in my head"	Context windows are finite. Written plans survive session boundaries and compaction.

Red Flags

• Starting implementation without a written task list
• Tasks that say "implement the feature" without acceptance criteria
• No verification steps in the plan
• All tasks are XL-sized
• No checkpoints between tasks
• Dependency order isn't considered
• Writing code during the planning phase instead of producing a plan document
• Horizontal slicing instead of vertical slicing (building all of one layer, then all of the next)

Verification

Before starting implementation, confirm every item below:

• Every task has acceptance criteria
• Every task has a verification step
• Task dependencies are identified and ordered correctly
• No task touches more than ~5 files
• Checkpoints exist between major phases
• The human has reviewed and approved the plan

To self-check the plan document is complete, run through this checklist mentally or in writing:

1. Can I read the task list top-to-bottom and know exactly what to build first?
2. Does every task have a testable acceptance criterion?
3. Would a second agent be able to pick up any single task and implement it without asking questions?
4. Are checkpoints placed so that the system is in a working state at each one?
5. Are high-risk tasks scheduled early enough to fail fast?

If any answer is "no," revise the plan before proceeding to implementation.