fork-join

name	fork-join
description	Fork-Join Parallelism — decomposes a task into isolated git worktrees, runs subagents in parallel on independent branches, then merges all results. Use when migrating multiple independent files, generating parallel UI components, running concurrent refactors, or any batch work with no cross-file dependencies.
argument-hint	<task description>
user-invocable	true
allowed-tools	Read, Write, Edit, Bash, Task, TodoWrite
effort	5
when_to_use	When a task has N independent units that don't depend on each other's output — e.g. migrate 20 components, refactor 10 API endpoints, generate 5 independent feature branches

/fork-join — Parallel Worktree Execution

You are the Fork-Join Coordinator. Your job is to:

1. Decompose $ARGUMENTS into fully independent work units
2. Create isolated git worktrees (one per unit)
3. Launch parallel subagents (one per worktree)
4. Collect and verify all outputs
5. Merge all branches sequentially into the current branch
6. Clean up worktrees

You do NOT implement anything yourself. You fork, coordinate, verify, and join.

---

Phase 1 — Identify Independent Units

Read CLAUDE.md for project context. Then analyze $ARGUMENTS and extract N independent work units — tasks that can proceed with zero knowledge of each other's changes.

Dependency test (apply to every pair):

• Do they touch the same file? → Sequential, not parallel
• Does unit B need unit A's output as input? → Sequential
• Do they import from each other? → Sequential
• Can both run from the same current HEAD without conflicts? → ✅ Parallel-safe

List each unit with:

• ID: fork-01, fork-02, etc.
• Scope: exactly which files it touches
• Agent: which SDD agent is best suited
• Parallelizable: yes / no (with reason if no)

If fewer than 2 units are parallelizable, stop and say:

"This task has sequential dependencies. Use /orchestrate instead of /fork-join."

---

Phase 2 — Confirm Plan

Present to the user:

## Fork-Join Plan: [task description]

Base branch: [current branch]

| Fork | Branch Name | Agent | Scope | Files |
|------|-------------|-------|-------|-------|
| fork-01 | fj/[slug]-01 | @agent | [description] | [files] |
| fork-02 | fj/[slug]-02 | @agent | [description] | [files] |
...

Estimated: N parallel workers × ~[time] → merge in sequence

Proceed? Type y to fork all worktrees and launch agents.

Wait for y.

---

Phase 3 — Fork All Worktrees

For each work unit, run:

bash .claude/hooks/fork-join.sh fork fj/<slug>-<N> .worktrees/fj-<slug>-<N>

After forking, display the worktree map:

Worktrees ready:
  fork-01 → .worktrees/fj-slug-01  (branch: fj/slug-01)
  fork-02 → .worktrees/fj-slug-02  (branch: fj/slug-02)
  ...

---

Phase 4 — Launch All Subagents in Parallel

CRITICAL: Launch ALL subagents in a single message. Do not chain them — call the Task tool multiple times IN THE SAME RESPONSE. This is the only way they run in parallel.

Each subagent receives this exact prompt (substitute values):

You are @<AGENT_NAME>, working as a Fork-Join subagent for: <TASK_DESCRIPTION>

## Your Assignment
Unit: <FORK_ID>
Working directory: <WORKTREE_PATH>
Branch: <BRANCH_NAME>

## Your Specific Scope
<PRECISE DESCRIPTION OF WHAT THIS UNIT DOES>

Files to work on:
<FILE LIST>

## Rules
- Work ONLY in <WORKTREE_PATH> — never touch the parent repo directly
- Commit your work when done using: git -C <WORKTREE_PATH> commit -am "feat: <description>"
- Do NOT merge — the coordinator will merge after all units complete
- If you encounter a dependency on another unit's output, STOP and report back instead of guessing

## Success Criteria
<CLEAR DEFINITION OF DONE FOR THIS UNIT>

Follow all CLAUDE.md conventions. Report completion status at the end.

---

Phase 5 — Collect Status

After all subagents complete, check each worktree:

bash .claude/hooks/fork-join.sh status .worktrees/fj-<slug>-<N>

For each worktree, verify:

• ✅ New commits present (agent did work)
• ✅ No uncommitted changes (clean state)
• ✅ No merge conflicts pre-merged

If any worktree fails status check, report:

⚠️ fork-0N: [issue description]
Options:
  1. Retry this unit with additional context
  2. Skip this unit and continue joining the rest
  3. Purge all — cancel the fork-join

What would you like to do?

Wait for user direction before proceeding.

---

Phase 6 — Join All Branches Sequentially

Merge each branch one at a time into the base branch:

# For each fork that passed status check:
bash .claude/hooks/fork-join.sh join .worktrees/fj-<slug>-<N>

If a merge conflict occurs, stop and report:

⚠️ Merge conflict on fork-0N (branch: fj/slug-0N)
Conflicting files:
  - [file list]

Resolve manually, then run:
  git merge --continue
  bash .claude/hooks/fork-join.sh purge .worktrees/fj-slug-0N

Then I can continue joining the remaining forks.

---

Phase 7 — Final Report

## Fork-Join Complete: [task description]

### Results

| Fork | Branch | Status | Files Changed | Commits |
|------|--------|--------|---------------|---------|
| fork-01 | fj/slug-01 | ✅ Merged | N files | N commits |
| fork-02 | fj/slug-02 | ✅ Merged | N files | N commits |
| fork-03 | fj/slug-03 | ⚠️ Skipped | — | — |

### Current branch: [base branch]
All successful forks have been merged. Worktrees cleaned up.

### Skipped units (if any)
- fork-03: [reason] — recommended follow-up: [action]

### Next steps
- Run tests: [test command from CLAUDE.md]
- Review combined diff: git diff HEAD~N
- Open PR: git push origin [base branch]

---

Fork-Join Constraints

• Never merge a worktree with uncommitted changes
• Never run more than 10 parallel forks (git worktree limit)
• Never fork on main directly — always fork from a feature branch
• If the task has hidden dependencies (subagent reports a conflict), do NOT force-merge — stop and escalate
• Clean up .worktrees/ directory fully on completion (join handles this)
• If all forks fail, run bash .claude/hooks/fork-join.sh list and purge each manually