Why run multiple Claude Code agents in parallel instead of one at a time?

A large rollout - dozens of new pages, a multi-file wiring change, several independent content pieces - splits into non-overlapping chunks of work that finish much faster run concurrently than in sequence, as long as no two agents touch the same file.

What is a git worktree and why does it matter here?

A git worktree checks out a second working copy of the same repository on its own branch and directory, so an agent can build and even run a local server in isolation without its uncommitted changes colliding with a different agent - or a human - working in the main checkout at the same time.

What happens if two agents edit the same file?

Whichever agent commits last silently overwrites or merges over the other’s changes, often without either agent - or the human supervising them - noticing until CI fails or a route breaks in production, because both git and each agent report success individually.

What actually went wrong when this was skipped?

A shared file picked up an unrelated, uncommitted change from a second concurrent session during a routine `git add`, which broke CI for over a day before anyone traced it back to the file collision rather than the intended change.

What is the minimum discipline needed to do this safely?

An explicit, written file-ownership table handed to every agent before it starts, isolation via git worktrees so uncommitted work never collides, and a serial verification step after all parallel agents finish, before anything gets merged or pushed.

Running Parallel Claude Code Agents Safely

Running several Claude Code agents on one codebase at the same time speeds up a large rollout, but two agents touching the same file is a silent, hard-to-diagnose failure mode. This guide covers git worktree isolation, non-overlapping file ownership tables, and a real incident where skipping both broke CI for over a day.