Phase Planning Skill

This skill provides methodology for creating implementation plans that are optimized for sub-agent execution, with properly sized phases that respect context window constraints.

Overview

A good phase plan:

1. Breaks work into independently executable chunks
2. Sizes phases to fit within sub-agent context budgets
3. Minimizes dependencies between phases where possible
4. Provides clear acceptance criteria for verification
5. Uses whole number phases only (no 1.1, 1.2 sub-phases)

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Step 1: Gather Requirements (Always Ask Questions)

Never skip this step. Even if the description seems complete, clarifying questions:

• Reveal implicit assumptions
• Uncover edge cases
• Establish scope boundaries
• Identify existing constraints

Question Categories

Scope & Boundaries

• What's explicitly OUT of scope?
• Is this greenfield or integrating with existing code?
• Are there existing patterns/conventions to follow?
• What's the target completion state? MVP or production-ready?

Technical Context

• What's the tech stack? (language, framework, database)
• Are there existing models/services this builds on?
• What authentication/authorization exists?
• Are there performance requirements?

Integration Points

• What external services/APIs are involved?
• Are there existing interfaces to conform to?
• What other systems will consume this?
• Are there upstream dependencies not yet built?

User Experience

• Who are the users? (end users, admins, developers, APIs)
• What's the primary workflow/happy path?
• What error states need handling?
• Are there accessibility requirements?

Constraints

• Security requirements? (OWASP, compliance, data sensitivity)
• Testing requirements? (coverage, E2E, specific frameworks)
• Documentation requirements?
• Deployment constraints?

Question Presentation Format

Present 3-5 targeted questions based on the description:

Before I create the implementation plan, I have a few questions:

1. **Existing Code**: Is this a new feature in an existing codebase, or greenfield?
   If existing, what patterns should I follow?

2. **Auth Context**: You mentioned user roles - is there an existing auth system
   to integrate with, or is that part of this work?

3. **Data Layer**: What database are you using? Are there existing models
   this relates to?

4. **Scope Boundary**: Should this include the admin UI for managing X,
   or just the core functionality?

5. **Testing**: What level of test coverage do you need? Unit only,
   or integration/E2E as well?

Wait for answers before proceeding.

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Step 2: Phase Sizing Guidelines

Context Budget Per Phase

Target each phase to consume 30-50k tokens of sub-agent context:

• ~10k tokens: Phase spec + project context (CLAUDE.md, conventions)
• ~15-25k tokens: File reads and code analysis
• ~10-15k tokens: Implementation work and verification

Sizing Heuristics

RIGHT-SIZED Phase (~30-50k tokens):

• Creates/modifies 2-5 files
• Implements 1-2 closely related features
• Can be verified with a clear test or check
• Completes in one sub-agent session without compacting

TOO LARGE Phase (>60k tokens - split it):

• Creates/modifies 6+ files
• Implements multiple unrelated features
• Requires reading large portions of codebase
• Description exceeds ~500 words
• Contains words like "and also", "as well as", "plus"

TOO SMALL Phase (<15k tokens - combine it):

• Single file change
• Config-only changes
• Simple additions with no logic
• Could be done in 5 minutes manually

Splitting Strategy

When a phase is too large, split by:

1. Layer: Separate data model, business logic, API, UI
2. Entity: One phase per core entity/resource
3. Operation: Separate CRUD operations if complex
4. Concern: Separate core logic from error handling, logging, etc.

Example - Too Large:

Phase 1: Build user management with registration, login, profile
editing, password reset, email verification, and admin user listing

Split Into:

Phase 1: User model and registration endpoint
Phase 2: Login and session management
Phase 3: Password reset flow
Phase 4: Email verification
Phase 5: Profile editing
Phase 6: Admin user listing

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Step 3: Dependency Planning

Dependency Types to Track

Hard Dependencies (must complete first):

• Schema/model that other phases import
• Auth middleware other phases use
• Shared utilities or helpers
• Base classes being extended

Soft Dependencies (preferred order, but parallelizable):

• Related features that share patterns
• Test setup that other tests use
• Documentation that references implementation

No Dependencies (fully parallel):

• Isolated features
• Different layers of same feature (if interfaces defined upfront)
• Independent utilities

Minimizing Dependencies

Strategies to reduce coupling between phases:

1. Define interfaces early: First phase exports types/interfaces, later phases implement against them

2. Stub dependencies: Phase can stub what it needs, later phase replaces stub with real implementation

3. Feature flags: Phases can merge independently, enable via flag

4. Vertical slices: Each phase is a thin vertical slice through all layers rather than horizontal layers

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Step 4: Plan Document Structure

File Location

docs/plans/{feature-name}.md

Slugify the feature name:

• "User Authentication System" -> user-authentication-system.md
• "API Rate Limiting" -> api-rate-limiting.md

Document Template

# {Feature Name} Implementation Plan

## Overview
{2-3 sentence summary of what this plan delivers}

## Goals
- {Primary goal}
- {Secondary goal}
- {Tertiary goal}

## Non-Goals (Out of Scope)
- {Explicit exclusion 1}
- {Explicit exclusion 2}

## Technical Context
- **Stack**: {language, framework, database}
- **Existing Code**: {relevant existing modules/patterns}
- **Integration Points**: {external services, APIs}

---

## Phase 1: {Phase Name}

### Objective
{One sentence describing what this phase accomplishes}

### Specification
{Detailed description of the work. Include:}
- What to create/modify
- Specific requirements
- Edge cases to handle
- Error handling expectations

### Files to Create/Modify
- `path/to/file.ts` - {purpose}
- `path/to/other.ts` - {purpose}

### Acceptance Criteria
- [ ] {Verifiable criterion 1}
- [ ] {Verifiable criterion 2}
- [ ] {Verifiable criterion 3}

### Dependencies
- **Requires**: {None | Phase X}
- **Blocks**: {Phase Y, Phase Z}

### Estimated Scope
- Files: {2-5}
- Complexity: {Low | Medium | High}

---

## Phase 2: {Phase Name}

{Same structure as Phase 1}

---

## Execution Strategy

### Dependency Graph

Phase 1 --+-- Phase 3 | Phase 2 --+ | Phase 4 <-+-- Phase 5

### Recommended Execution
- **Parallel Group 1**: Phase 1, Phase 2
- **Sequential**: Phase 3 (after Phase 1)
- **Parallel Group 2**: Phase 4, Phase 5 (after Phase 3)

---

## Verification Checklist

After all phases complete:
- [ ] {Integration verification 1}
- [ ] {Integration verification 2}
- [ ] {End-to-end test}

## Open Questions

- {Any unresolved decisions to revisit}

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Step 5: Phase Writing Guidelines

Phase Names

Use action-oriented names:

Good:
- "Create User Model and Repository"
- "Implement JWT Authentication"
- "Add Rate Limiting Middleware"
- "Build Password Reset Flow"

Bad:
- "User Stuff"
- "Part 1"
- "Backend Work"
- "Misc Improvements"

Specification Writing

Be specific enough that a sub-agent can implement without guessing:

Vague:
"Add user authentication"

Specific:
"Create POST /api/auth/login endpoint that:
- Accepts { email, password } body
- Validates against User model
- Returns JWT token with 24h expiry on success
- Returns 401 with { error: 'Invalid credentials' } on failure
- Rate limits to 5 attempts per 15 minutes per IP
- Logs failed attempts with IP and email (not password)"

Acceptance Criteria

Write testable criteria:

Not Testable:
- "Works correctly"
- "Handles errors"
- "Is secure"

Testable:
- "POST /api/users returns 201 with user object (excluding password)"
- "Invalid email format returns 400 with validation error"
- "Passwords are hashed with bcrypt cost factor 12"
- "JWT tokens expire after 24 hours"

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Anti-Patterns to Avoid

No Sub-Phases

Wrong:
Phase 1: Setup
  Phase 1.1: Database schema
  Phase 1.2: Model classes
  Phase 1.3: Repository layer

Right:
Phase 1: Database Schema and Migrations
Phase 2: User Model and Repository
Phase 3: Authentication Service

No Kitchen Sink Phases

Wrong:
Phase 3: Implement all remaining features including search,
filtering, pagination, sorting, export, and batch operations

Right:
Phase 3: List Endpoint with Pagination
Phase 4: Search and Filtering
Phase 5: Sorting Options
Phase 6: Export Functionality
Phase 7: Batch Operations

No Vague Phases

Wrong:
Phase 2: Handle edge cases and fix bugs

Right:
Phase 2: Input Validation and Error Handling
- Validate email format, password strength
- Handle duplicate email registration
- Return structured error responses

No Dependency Spaghetti

Wrong:
Phase 1 depends on Phase 3
Phase 3 depends on Phase 2
Phase 2 depends on Phase 4
Phase 4 depends on Phase 1  (circular!)

Right:
Phase 1: Foundation (no dependencies)
Phase 2: Core Logic (depends on 1)
Phase 3: Extended Features (depends on 2)
Phase 4: Polish and Edge Cases (depends on 3)

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Final Checklist

Before delivering plan:

• Asked clarifying questions and incorporated answers
• Each phase is 30-50k tokens of work (2-5 files)
• No sub-phases (whole numbers only)
• Every phase has clear acceptance criteria
• Dependencies are explicit and acyclic
• Specifications are detailed enough to implement without guessing
• File saved to docs/plans/{feature-name}.md

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IMPORTANT: Planning Only — Do Not Implement

This skill is for planning only. After creating the plan, STOP.

Do NOT:

• Start implementing any phases
• Write any code
• Create any files other than the plan document
• Begin execution automatically

After the plan is complete:

1. Present the plan document to the user
2. Ask if they want to make any revisions
3. Inform them to use /workflow-implement-phases or the implement-phases skill when ready to execute

The user decides when to proceed with implementation.