using-git-worktrees

name	using-git-worktrees
description	Use when starting feature work that needs isolation from current workspace or before executing implementation plans - ensures an isolated workspace exists via native tools or git worktree fallback

The isolation step for every tier — Tier 1 included. The flow adds three things to a plain worktree: a plan-gate reminder, a base-commit check (Step 0.5), and an in-place branch fallback (Step 1c), backed by the helper scripts/new-branch.sh.

Using Git Worktrees

Overview

Ensure work happens in an isolated workspace. Prefer your platform's native worktree tools. Fall back to manual git worktrees only when no native tool is available.

Core principle: Detect existing isolation first. Then use native tools. Then fall back to git. Never fight the harness.

dev-stack — three rules ride on top of the port below:

1. Plan gate. Don't reach this skill to start coding until your tier's plan is presented and approved (Tier 1: a few lines in chat + a "go"; Tier 2: the spec + tickets chain). The worktree is where approved work goes — not a way around the gate.
2. Base commit. Before creating the worktree, confirm the commit you're branching off — the tip of the current branch (normally the default branch, which already carries the domain docs and any merged blockers), and local not silently behind origin — Step 0.5 below.
3. No worktree ≠ no isolation. When a worktree can't be made (sandbox denial) or the user declines one, fall back to a dedicated in-place branch — Step 1c. Never work on the default branch.

Announce at start: "I'm using the using-git-worktrees skill to set up an isolated workspace."

Step 0: Detect Existing Isolation

Before creating anything, check if you are already in an isolated workspace.

GIT_DIR=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
GIT_COMMON=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
BRANCH=$(git branch --show-current)

Submodule guard: GIT_DIR != GIT_COMMON is also true inside git submodules. Before concluding "already in a worktree," verify you are not in a submodule:

# If this returns a path, you're in a submodule, not a worktree — treat as normal repo
git rev-parse --show-superproject-working-tree 2>/dev/null

If GIT_DIR != GIT_COMMON (and not a submodule): You are already in a linked worktree. Skip to Step 2 (Project Setup). Do NOT create another worktree.

Report with branch state:

• On a branch: "Already in isolated workspace at <path> on branch <name>."
• Detached HEAD: "Already in isolated workspace at <path> (detached HEAD, externally managed). Branch creation needed at finish time."

If GIT_DIR == GIT_COMMON (or in a submodule): You are in a normal repo checkout.

Has the user already indicated their worktree preference in your instructions? If not, ask for consent before creating a worktree:

"Would you like me to set up an isolated worktree? It protects your current branch from changes."

Honor any existing declared preference without asking. If the user declines consent, don't work on the current branch as-is — cut a dedicated branch in place (Step 1c, dev-stack), then skip to Step 2.

Step 0.5: Confirm the base commit (dev-stack)

A worktree's base is a commit you choose and then verify — not a default you accept. Two silent traps put it on the wrong commit; check both before creating, and verify after.

Resolve the default branch and where you are:

DEF=$(git symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>/dev/null | sed 's#^origin/##')
[ -z "$DEF" ] && { git show-ref -q refs/heads/main && DEF=main || DEF=master; }
CUR=$(git branch --show-current)

Trap 1 — wrong branch. If CUR != DEF, STOP and ask before creating anything:

"You're on <CUR>, not the default <DEF>. Branch this worktree from here, or from <DEF>?"

Set BASE to their choice ($CUR or $DEF).

Trap 2 — local ahead of origin. Native worktree tools (e.g. EnterWorktree) often default to branching from origin/<DEF> (a worktree.baseRef=fresh-style default), NOT your local branch. If your local base carries unpushed commits, that base silently drops them — including the foundation you're building on.

git rev-list --count "origin/$DEF..$DEF" 2>/dev/null   # >0 → local $DEF is ahead of origin
git rev-list --count "@{u}..HEAD"        2>/dev/null   # >0 → current HEAD has unpushed commits

If either is non-zero, choose the base deliberately:

• keep the unpushed foundation → base off local (EnterWorktree: set worktree.baseRef=head; git fallback: pass the local ref as the start point in Step 1b).
• want a clean base off the pushed tip → base off origin — only if nothing local is load-bearing.

Then verify — do NOT trust the config. A baseRef setting can silently fail to apply, so the tool may still branch from origin after you asked for local. After the worktree exists (Step 3), confirm it sits on the base you intended:

# FOUND = a commit that MUST be present (your foundation). Fails if the base was wrong.
git merge-base --is-ancestor "$FOUND" HEAD && echo "base ok" || echo "WRONG BASE — re-point"

If the base is wrong and the worktree has no commits of its own, re-point its branch onto the intended base (git reset --hard <local-base>), then re-verify.

Branching off the wrong commit is silent — the worktree looks fine and quietly carries the wrong history. This check is the cheap insurance.

Step 1: Create Isolated Workspace

You have two mechanisms. Try them in this order.

1a. Native Worktree Tools (preferred)

The user has asked for an isolated workspace (Step 0 consent). Do you already have a way to create a worktree? It might be a tool with a name like EnterWorktree, WorktreeCreate, a /worktree command, or a --worktree flag. If you do, use it and skip to Step 2.

Native tools handle directory placement, branch creation, and cleanup automatically. Using git worktree add when you have a native tool creates phantom state your harness can't see or manage.

Only proceed to Step 1b if you have no native worktree tool available.

1b. Git Worktree Fallback

Only use this if Step 1a does not apply — you have no native worktree tool available. Create a worktree manually using git.

Directory Selection

Follow this priority order. Explicit user preference always beats observed filesystem state.

1. Check your instructions for a declared worktree directory preference. If the user has already specified one, use it without asking.

2. Check for an existing project-local worktree directory:

   ls -d .worktrees 2>/dev/null     # Preferred (hidden)
   ls -d worktrees 2>/dev/null      # Alternative
   

   If found, use it. If both exist, .worktrees wins.

3. If there is no other guidance available, default to .worktrees/ at the project root.

Safety Verification (project-local directories only)

MUST verify directory is ignored before creating worktree:

git check-ignore -q .worktrees 2>/dev/null || git check-ignore -q worktrees 2>/dev/null

If NOT ignored: Add to .gitignore, commit the change, then proceed.

Why critical: Prevents accidentally committing worktree contents to repository.

Create the Worktree

# Determine path based on chosen location
path="$LOCATION/$BRANCH_NAME"

git worktree add "$path" -b "$BRANCH_NAME" "${BASE:-$DEF}"   # BASE chosen in Step 0.5
cd "$path"

Sandbox fallback (dev-stack): If git worktree add fails with a permission error (sandbox denial), tell the user the sandbox blocked worktree creation, then fall back to a dedicated in-place branch — Step 1c. Never settle for working in the current directory as-is: the current branch is not isolation.

1c. In-Place Branch Fallback (dev-stack)

No worktree, still isolation. Reach this section on exactly two triggers:

• Sandbox denial — worktree creation failed with a permission error (Step 1a or 1b).
• User declined — the user said no to a worktree (Step 0).

Cut a dedicated branch in place and do the work there.

Never work on the default branch — not for a one-line fix, not "just to test". When a worktree can't be made, the floor is a dedicated in-place branch, cut before the first edit. The plan gate at the top of this file still applies on this path.

Confirm the branch origin first. Branching off the wrong base is silent and expensive — Step 0.5's traps apply unchanged:

• On the default branch — cut the branch off it, as normal.
• Not on the default — STOP and ask: "You're on <cur>, not <def>. Branch from here, or switch to <def> first?" The script refuses in this case unless you pass --from-here, so ask the user before you do.

Run:

${DEV_STACK_ROOT}/scripts/new-branch.sh <type> <slug> [--from-here]

If $DEV_STACK_ROOT is unset in this context, the inline equivalent is:

git switch -c <type>/<slug>   # off the confirmed base — origin checked above

• type — fix for a bug, feat/chore/refactor as the task fits. Default chore.
• slug — a short kebab description of the work; the script sanitises it.
• --from-here — branch off the current branch instead of refusing when it isn't the default. Only after the user has said so.

Name from the task: a crash → fix/<slug>, a small addition → feat/<slug>.

A branch-guard PreToolUse hook denies edits and commits on the default branch in any repo carrying a .branch-guard marker — this fallback is the clean path the guard points to. The obligation lives in the hook, not in this text.

Once on the branch, continue to Step 2 — setup and baseline run in place.

Step 2: Project Setup

Auto-detect and run appropriate setup:

# Node.js
if [ -f package.json ]; then npm install; fi

# Rust
if [ -f Cargo.toml ]; then cargo build; fi

# Python
if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
if [ -f pyproject.toml ]; then poetry install; fi

# Go
if [ -f go.mod ]; then go mod download; fi

Step 3: Verify Clean Baseline

Base check first (dev-stack). Before trusting any baseline, confirm the worktree is founded on the intended commit — run Step 0.5's git merge-base --is-ancestor "$FOUND" HEAD check. A wrong base is the most common silent worktree failure: a green test suite on the wrong foundation still ships the wrong history.

Run tests to ensure workspace starts clean:

# Use project-appropriate command
npm test / cargo test / pytest / go test ./...

If tests fail: Report failures, ask whether to proceed or investigate.

If tests pass: Report ready.

Report

Worktree ready at <full-path>
Tests passing (<N> tests, 0 failures)
Ready to implement <feature-name>

Quick Reference

Situation	Action
Already in linked worktree	Skip creation (Step 0)
In a submodule	Treat as normal repo (Step 0 guard)
Native worktree tool available	Use it (Step 1a)
No native tool	Git worktree fallback (Step 1b)
.worktrees/ exists	Use it (verify ignored)
worktrees/ exists	Use it (verify ignored)
Both exist	Use .worktrees/
Neither exists	Check instruction file, then default .worktrees/
Directory not ignored	Add to .gitignore + commit
Permission error on create	In-place branch fallback (Step 1c, dev-stack)
User declines a worktree	In-place branch fallback (Step 1c, dev-stack)
Tests fail during baseline	Report failures + ask
No package.json/Cargo.toml	Skip dependency install

Common Mistakes

Fighting the harness

• Problem: Using git worktree add when the platform already provides isolation
• Fix: Step 0 detects existing isolation. Step 1a defers to native tools.

Skipping detection

• Problem: Creating a nested worktree inside an existing one
• Fix: Always run Step 0 before creating anything

Skipping ignore verification

• Problem: Worktree contents get tracked, pollute git status
• Fix: Always use git check-ignore before creating project-local worktree

Assuming directory location

• Problem: Creates inconsistency, violates project conventions
• Fix: Follow priority: explicit instructions > existing project-local directory > default

Proceeding with failing tests

• Problem: Can't distinguish new bugs from pre-existing issues
• Fix: Report failures, get explicit permission to proceed

Red Flags

Never:

• Create a worktree when Step 0 detects existing isolation
• Create a worktree off a non-default branch without asking first (Step 0.5)
• Work on the default branch when a worktree falls through — the fallback is a dedicated in-place branch (Step 1c, dev-stack), never the current checkout as-is
• Trust a native worktree tool's default base — it may branch from origin, behind your local commits. Verify the base (Step 0.5), don't assume it.
• Use git worktree add when you have a native worktree tool (e.g., EnterWorktree). This is the #1 mistake — if you have it, use it.
• Skip Step 1a by jumping straight to Step 1b's git commands
• Create worktree without verifying it's ignored (project-local)
• Skip baseline test verification
• Proceed with failing tests without asking

Always:

• Run Step 0 detection first
• Prefer native tools over git fallback
• Follow directory priority: explicit instructions > existing project-local directory > default
• Verify directory is ignored for project-local
• Auto-detect and run project setup
• Verify clean test baseline