Git Expert

Git Expert Skill

Installation

The skill invokes the git CLI. Install Git if not present:

• Windows: Download from git-scm.com or winget install --id Git.Git -e --source winget
• macOS: brew install git or Xcode CLI tools: xcode-select --install
• Linux: apt-get install git (Debian/Ubuntu), dnf install git (Fedora), pacman -S git (Arch)

Verify: git --version

Cheat Sheet & Best Practices

Essential commands (token-efficient):

• git status -s — short status; git add -p — stage hunks; git diff --cached — review staged
• git switch -c <branch> or git checkout -b <branch> — new branch; git branch — list
• git log --oneline -5 — compact history; git log --follow <file> — track renames
• git restore <file> — discard unstaged; git reset --soft HEAD~1 — undo last commit (keep changes)
• git fetch then git merge or git pull — prefer fetch+merge over blind pull

Hacks: Set git config --global color.ui auto and user.name/user.email. Use .gitignore aggressively. Prefer git merge --squash for clean history on feature merge. Use git cherry-pick <commit> to bring single commits. Never rebase pushed commits without team agreement.

Certifications & Training

Free / official: Atlassian Git Tutorials (beginner–advanced). Microsoft Learn – GitHub Training (GitHub Foundations path). GitHub Learn (Git + GitHub). No single “Git cert”; GitHub Foundations aligns with fundamentals.

Skill data: Focus on branching, undo (reset/restore/revert), merge vs rebase, remote workflow, and safety (no force-push, no secrets).

Hooks & Workflows

Suggested hooks: Pre-commit: run commit-validator (conventional commits). Pre-push: run tests (reference tdd / verification-before-completion). Post-merge: optional memory/learnings update.

Workflows: Use with developer (primary), devops (always). Flow: branch → edit → add → validate commit message → commit → push; use github-ops or github-mcp for PR/create. See .claude/workflows for feature-development and code-review workflows that use git-expert.

⚡ Token-Efficient Workflow

Do not use git status repeatedly. Use this workflow:

1. Check State: git status -s (Short format saves tokens)
2. Diff: git diff --cached (Only check what you are about to commit)
3. Log: git log --oneline -5 (Context without the noise)

🔄 Common Patterns

Safe Commit

git add <file>
git diff --cached # REVIEW THIS!
git commit -m "feat: description"

Undo Last Commit (Soft)

git reset --soft HEAD~1

Fix Merge Conflict

1. git status to see conflict files.
2. Edit file to resolve markers (<<<<, ====, >>>>).
3. git add <file>
4. git commit --no-edit

Iron Laws

1. NEVER use git push --force on shared branches — force-pushing rewrites history for all collaborators, causes lost commits, and breaks in-progress rebases silently; use --force-with-lease when truly necessary.
2. NEVER commit secrets, credentials, or tokens to the repository — once pushed, secrets are visible in all forks and clones forever even after deletion from HEAD.
3. ALWAYS run the test suite and verify it passes before pushing to shared branches — broken code in shared branches blocks everyone and triggers emergency rollbacks.
4. ALWAYS rebase feature branches onto the base branch before merging — merging with a stale base creates avoidable conflicts and produces noisy merge commits in shared history.
5. NEVER rebase or rewrite history on commits that have already been pushed to a shared remote — teammates' local branches will diverge and produce duplicate commits on next pull.

Anti-Patterns

Git 2.45–2.50 Features (2024–2025)

Reftable Backend (Git 2.45+)

The reftable backend completely replaces the legacy files reference format with a binary format that is faster, atomic, and storage-efficient:

Benefits:

• Atomic multi-ref updates (all-or-nothing transactions)
• Faster single-ref lookup and iteration over ref ranges
• Consistent reads — never reads a partial update state
• More efficient reflog storage

Enable for a new repository:

git init --ref-format=reftable my-repo

Check current backend:

git rev-parse --show-ref-format
# outputs: files  OR  reftable

Migration note: Converting an existing repo from files to reftable requires re-cloning or using git clone --ref-format=reftable. Clients using files repos are unaffected — the backend is server/local only.

Incremental Multi-Pack Indexes (Git 2.47+)

Multi-pack indexes (MIDXs) allow Git to maintain a single index across multiple packfiles without repacking. Git 2.47 adds incremental MIDX updates — only newly added packs are indexed rather than rebuilding the full MIDX:

# Generate or update multi-pack index
git multi-pack-index write --stdin-packs

# Verify the MIDX
git multi-pack-index verify

# Enable via maintenance
git maintenance start

Multi-pack reachability bitmaps extend MIDX with precomputed reachability data, dramatically speeding up git clone, git fetch, and garbage collection on large repositories.

Sparse Checkout for Monorepos (Git 2.25+ cone mode)

Sparse checkout lets you check out only the directories you need from a large monorepo:

# Clone without checking out any files
git clone --no-checkout --filter=blob:none https://github.com/org/monorepo.git
cd monorepo

# Initialize in cone mode (recommended — uses fast prefix matching)
git sparse-checkout init --cone

# Check out only specific directories
git sparse-checkout set frontend docs shared/utils

# See what is currently checked out
git sparse-checkout list

# Add more directories without losing current ones
git sparse-checkout add backend/api

# Disable sparse checkout (restore full working tree)
git sparse-checkout disable

One-command clone (Git 2.25+):

git clone --filter=blob:none --sparse https://github.com/org/monorepo.git
cd monorepo
git sparse-checkout set services/payments

Cone mode vs. non-cone mode:

Git Scalar — Large Repository Optimization

Scalar is a repository management tool bundled with Git (since Git 2.38) that configures and maintains all recommended performance settings automatically:

# Clone a large repo with all performance features enabled
scalar clone https://github.com/org/large-monorepo.git

# Register an existing repo with Scalar
scalar register

# Run all maintenance tasks manually
scalar run all

# View configured enlistments
scalar list

What Scalar configures automatically:

• Partial clone (--filter=blob:none)
• Sparse checkout (cone mode)
• File system monitor (core.fsmonitor)
• Commit graph generation
• Background maintenance (hourly fetch, daily gc)
• Multi-pack index

Sign Commits with SSH Keys (Git 2.34+)

SSH key signing is simpler than GPG and works with keys you already use for authentication:

# Configure SSH signing globally
git config --global gpg.format ssh
git config --global user.signingKey ~/.ssh/id_ed25519.pub

# Sign all commits automatically
git config --global commit.gpgSign true

# Or sign a single commit manually
git commit -S -m "feat: add payment service"

# Verify a commit
git verify-commit HEAD

Set up an allowed-signers file for team verification:

# ~/.config/git/allowed_signers
alice@example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAA...
bob@example.com   ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAA...

git config --global gpg.ssh.allowedSignersFile ~/.config/git/allowed_signers
git log --show-signature

GitHub verification: Add your SSH signing key to GitHub under Settings > SSH and GPG keys > New SSH key > Signing Key. Commits signed with that key will show "Verified" on GitHub.

FIDO2 / hardware security key signing:

# Generate a resident key on a FIDO2 security key (e.g., YubiKey)
ssh-keygen -t ed25519-sk -O resident -f ~/.ssh/id_ed25519_sk_fido2
git config --global user.signingKey ~/.ssh/id_ed25519_sk_fido2.pub

Improved Merge Strategies (Git 2.38+ ort)

The ort merge strategy has been the default since Git 2.34. It is significantly faster than recursive for large trees and more deterministic:

# ort is the default; explicitly select if needed
git merge -s ort feature/my-feature

# Rename detection threshold (default 50%)
git merge -X rename-threshold=80 feature/rename-heavy-branch

# Conflict style: diff3 (shows common ancestor) — strongly recommended
git config --global merge.conflictStyle diff3
# Or zdiff3 (even cleaner context in conflicts, Git 2.35+)
git config --global merge.conflictStyle zdiff3

Performance Commands for Large Repos

# Enable file system monitor (avoids scanning entire tree for status)
git config core.fsmonitor true
git config core.untrackedCache true

# Precompute commit graph for faster log/blame
git commit-graph write --reachable --changed-paths
git config fetch.writeCommitGraph true

# Partial clone — skip large blobs on clone, fetch on demand
git clone --filter=blob:none <url>

# Shallow clone (CI use case — last N commits only)
git clone --depth=1 <url>

# Check repository health and performance
GIT_TRACE_PERFORMANCE=1 git status

Related Skills

• gitflow - Branch workflow patterns (feature, release, hotfix branches)

Memory Protocol (MANDATORY)

Before starting: Read .claude/context/memory/learnings.md

After completing:

• New pattern -> .claude/context/memory/learnings.md
• Issue found -> .claude/context/memory/issues.md
• Decision made -> .claude/context/memory/decisions.md

ASSUME INTERRUPTION: If it's not in memory, it didn't happen.