understanding-cross-phase-stacking

# Understanding Cross-Phase Stacking

## Overview

**Phases automatically build on each other's completed work.** Understanding how phases chain together is essential for correct execution.

This is a **reference skill** - read it to understand cross-phase dependencies, not to execute a workflow.

## When to Use

Use this skill when:
- Starting a new phase (need to understand what base to build from)
- Debugging stack relationships across phase boundaries
- Verifying phases are chaining correctly
- Understanding why parallel worktrees use specific base branches

**Mental model check:** If you're thinking "create worktrees from `{runid}-main` branch", you need this skill.

## The Cross-Phase Inheritance Principle

```
MAIN WORKTREE CURRENT BRANCH = LATEST COMPLETED WORK
```

**Key insight:**
- Sequential phases leave main worktree **on their last task's branch**
- Parallel phases leave main worktree **on their last stacked branch**
- Next phase (sequential or parallel) inherits from **current branch**, not original base

**This creates automatic linear chaining across all phases.**

## Example: Sequential → Parallel → Sequential

### Phase 1 (Sequential) - Database Setup

```bash
# Working in: .worktrees/{runid}-main
# Starting from: {runid}-main (base branch)

# Task 1: Database schema
gs branch create {runid}-task-1-1-database-schema
# Creates branch, commits work
# Main worktree now on: {runid}-task-1-1-database-schema ← KEY STATE
```

**Phase 1 result:**
- Branch created: `{runid}-task-1-1-database-schema`
- Main worktree current branch: `{runid}-task-1-1-database-schema`
- **This branch becomes Phase 2's base**

### Phase 2 (Parallel) - Three Feature Implementations

```bash
# Base detection (CRITICAL):
BASE_BRANCH=$(git -C .worktrees/{runid}-main branch --show-current)
# Returns: {runid}-task-1-1-database-schema ← Inherits from Phase 1

# Create parallel worktrees FROM Phase 1's completed branch
git worktree add .worktrees/{runid}-task-2-1 --detach "$BASE_BRANC