git worktreesとは？複数ブランチを同時にチェックアウトして並行開発を効率化するWhat are git worktrees, and why should I use them?

Git worktrees are one of the older features in Git's toolset, yet they have only recently moved into the spotlight. Introduced in 2015 with Git 2.5, worktrees let you check out multiple branches at the same time, each in its own directory, while still sharing a single underlying repository. For developers who routinely juggle several lines of work, this can remove a great deal of friction from everyday version control.

The core problem worktrees solve is the single-checkout limitation of a traditional Git working directory. Normally, a repository has one working tree, so switching between branches means running git checkout or git switch and overwriting the files in place. If you have uncommitted changes, you either commit prematurely or stash them, and any running build, test process, or development server tied to those files is disrupted. Worktrees sidestep this by allowing each branch to live in a separate folder on disk, so you can move between them simply by changing directories.

Creating a worktree is straightforward. From within an existing repository, running git worktree add ../feature-branch feature-branch creates a new directory containing a checkout of that branch. You can then open it in a separate editor window or terminal and work as if it were an independent clone. Behind the scenes, however, the worktrees share the same .git object database, so they remain lightweight compared with making full copies of the repository. Supporting commands include git worktree list to see all active worktrees, git worktree remove to delete one cleanly, and git worktree prune to tidy up references to directories that have been deleted manually.

The practical workflows this enables are easy to picture. A common example is the urgent hotfix. If you are deep into building a feature and a production bug needs immediate attention, you can spin up a new worktree based on the main branch, fix the issue there, and return to your feature work without ever stashing changes or losing your place. Another scenario is comparing two branches side by side, for instance to verify how a refactor behaves against the previous implementation, or to run two long test suites concurrently. Reviewers can also use a dedicated worktree to check out a colleague's pull request while keeping their own branch untouched.

Part of the renewed interest in worktrees appears to stem from the rise of AI coding agents and assistants such as GitHub Copilot. When an autonomous or semi-autonomous agent is asked to work on a task, it benefits from an isolated environment where it can edit files, run commands, and iterate without colliding with the developer's active session. Worktrees provide exactly that kind of isolation cheaply, which is likely why they are increasingly mentioned in the context of parallel and agent-driven development. Running several agents or experiments at once, each in its own worktree, lets a developer keep multiple streams of work progressing simultaneously.

It is worth understanding how worktrees relate to adjacent approaches. Before they became popular, developers often maintained multiple full clones of a repository to achieve a similar effect, but that wastes disk space and means fetches and remotes must be managed separately. Stashing remains useful for quick, temporary context switches, though it does not let you see two states at once. Worktrees sit between these options, offering true parallel checkouts without duplicating the entire object store.

There are some constraints to keep in mind. The same branch cannot be checked out in two worktrees at once, which prevents conflicting edits but occasionally requires planning. Worktrees stored outside the main project folder can be easy to forget, so periodic cleanup with prune and remove is advisable. Tooling, editor extensions, and continuous integration setups may also need configuration to recognize the separate directories.

For teams adopting Copilot or other AI-assisted workflows, worktrees pair naturally with feature branching, pull requests, and short-lived branches. They are not a replacement for those practices but a complement that reduces the cost of context switching. Anyone comfortable with basic Git commands can begin experimenting with a single git worktree add, and the feature's long presence in Git means it is stable, well documented, and available in any reasonably current installation.