Using Git Worktrees

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.

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 ` on branch `."
• Detached HEAD: "Already in isolated workspace at `` (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, work in place and skip to Step 2.

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.

> Brain mode: In a Brain vault (a .brain/ directory exists in this > directory or an ancestor), prefer a worktree inside the repo — > /.worktrees/. Brain detection walks up from the > working directory looking for .brain/, and .brain/ is untracked, so a > worktree placed outside the repo has no .brain/ ancestor: the session > drops out of Brain mode mid-workflow, after the design and plan are already > in the vault. If your native tool can be pointed at .worktrees/, point it > there. If it cannot and it places the worktree elsewhere, say so, keep the > vault plan path in hand across the hop, and call brain_session again from > the new working directory before continuing.

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.

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

``bash ls -d .worktrees 2>/dev/null # Preferred (hidden) ls -d worktrees 2>/dev/null # Alternative ` If found, use it. If both exist, .worktrees` wins.

1. 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"
cd "$path"

Sandbox fallback: If git worktree add fails with a permission error (sandbox denial), tell the user the sandbox blocked worktree creation and you're working in the current directory instead. Then run setup and baseline tests 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

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 
Tests passing ( tests, 0 failures)
Ready to implement 

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 | Sandbox fallback, work in place | | Tests fail during baseline | Report failures + ask | | No package.json/Cargo.toml | Skip dependency install |

Common Rationalizations

| Excuse | Reality | |--------|---------| | "I'm obviously not in a worktree — no need to check" | Run Step 0. Harness-created isolation and submodules both fool eyeballing; the detection commands settle it. | | "git worktree add is quicker than hunting for a native tool" | A native tool (e.g. EnterWorktree) owns placement, branching, and cleanup. Bypassing it is the #1 mistake — it creates phantom state your harness can't see or manage. | | "The worktree directory is surely ignored already" | Run git check-ignore. An unignored worktree directory commits the whole tree into the repo. | | "Any directory name works" | Explicit instructions beat an existing project-local directory, which beats the .worktrees/ default. | | "The workspace is fresh — baseline tests can wait" | A dirty baseline makes every later failure ambiguous. Run the tests now; proceeding past failures is your human partner's call. |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Rob-Morris
• Source: Rob-Morris/agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.