git-cow

Git worktrees that share their build output. git-cow keeps one extra checkout of each repository and makes every new worktree a copy-on-write clone of it. So target/, node_modules and other ignored folders take no extra disk until a build changes them. A worktree that would cost gigabytes starts at a few megabytes, and its first build is incremental.

git cow add -b feat/login                 # ~/.worktrees/<repo>/login
git cow add -b feat/login ../login        # any path, like git worktree add
git cow list                              # your worktrees, without the extra checkout
git cow remove --warm ~/.worktrees/<repo>/login   # after a merge: give its build output to the extra checkout, then remove it

Install

cargo install git-cow   # into ~/.cargo/bin

Git runs any program named git-<name> on your PATH as git <name>, so git cow works right after the install. If git says cow is not a git command, add ~/.cargo/bin to your PATH:

export PATH="$HOME/.cargo/bin:$PATH"   # in your shell profile

For a different name, add an alias: git config --global alias.wt '!git-cow' makes git wt add -b feat/x work.

Commands

These are normal git worktrees, so git worktree list and git worktree remove work too. git worktree list also shows the extra checkout. If you remove it by hand, the next add makes a new one.

How it works

1. git-cow keeps one extra checkout per repository, at origin/HEAD. It is a git worktree of your repository, so it uses your .git and the history is not stored twice. git-cow makes it by cloning your checkout and removing your edits and untracked files, so it takes disk only for the files that differ from origin/HEAD.
2. The extra checkout gets copy-on-write clones of your checkout's ignored folders, such as target/ and node_modules. This runs at most once an hour by default (cow.warmInterval).
3. A new worktree is a copy-on-write clone of the extra checkout. macOS clones each folder in one clonefile(2) call. Linux uses cp --reflink=always.
4. Git then rewrites only the files where your branch differs from the extra checkout.

If cloning is not possible, git-cow cleans up and runs plain git worktree add with your arguments.

Loose ignored files such as .env are never copied. Hidden folders are copied only when they are on the share list, because most of them hold tool or agent settings (.claude, .codex, .aws). The list starts with these build caches: .uv-cache, .next, .turbo, .nuxt, .svelte-kit, .parcel-cache, .gradle, .mypy_cache, .ruff_cache and .pytest_cache. Add more with git cow share <dir>. A .venv only works after a clone if it was created relocatable, because it stores absolute paths, so it is not on the list.

Configuration

All settings are git config, per repository or --global. In a path, {repo} becomes the repository's name.

The extra checkout sits next to the worktree, so tools with different layouts each get their own: ~/.worktrees/<repo>/.git-cow/<repo> for git cow add, <repo>/.claude/worktrees/.git-cow/<repo> for Claude Code. Another extra checkout takes almost no disk, but each is refreshed on its own. To share one extra checkout per repository across tools:

git config --global cow.base '~/.orch/state/templates/{repo}'

Your checkout, the extra checkout and the worktrees must sit on one volume that supports cloning: APFS on macOS, btrfs or XFS formatted with reflink=1 on Linux. On ext4 and HFS+, git-cow falls back to git worktree add.

Library

git_cow::add(repo, checkout, "origin/main", worktree, Some("feat/x"), None)?;
git_cow::warm(merged_worktree, checkout)?;

let opts = git_cow::Options { share: vec![".venv".into()], ..Default::default() };
git_cow::add_with(repo, checkout, "origin/main", worktree, Some("feat/x"), None, &opts)?;

Options::from_repo(repo) returns the options the CLI uses: the defaults plus cow.share and cow.warmInterval from git config. A warm_interval of Duration::MAX never warms the extra checkout from your checkout, the same as cow.warmInterval never.

git_cow::remove_template(repo, checkout) removes the extra checkout, its entry in git worktree list, and the lock and stamp files next to it. Call it only when no add or warm on that extra checkout can run.

git_cow::add_or_plain takes the arguments of add_with and runs git worktree add with the same path, branch and commit when cloning fails. It returns Added::Cloned, or Added::Plain with the reason the clone failed.

The library is synchronous and has no dependencies. A lock file next to the extra checkout stops two clones, from threads or processes, from seeing it half updated.

Benchmarks

One run each on an Apple Silicon Mac (APFS). Disk counts the bytes written after the worktree was made, because du counts every clone as a full copy.

orch (Rust), then cargo test --no-run:

The dependencies come from the extra checkout. Cargo still rebuilds the repository's own crates.

bean (Python), then uv sync:

Costs

• A cloned file's blocks are copied the first time either side writes to it.
• Making the extra checkout the first time is slower than a plain checkout, about 30 s on a repository with large data folders, because the clone is cleaned before the build folders come back.
• Clones have new inodes, so git re-hashes every tracked file once. That took about 3 s on a repository with large tracked data.
• A big shared cache still takes time to clone: about 4 s for a 6.7 GB uv cache.
• The extra checkout can be up to an hour behind your checkout, so a new worktree may rebuild a little more.
• Tools that store absolute paths, such as virtualenvs, point at the extra checkout after a clone.
• A folder where every file is ignored is shared whole, hidden subfolders included, because git lists it as one entry.
• Repositories with submodules always fall back.

Origin

Extracted from orch, where every agent task gets a worktree.

MIT licensed.