Run 5 Claude Code instances in parallel using git worktree

When five Claude Code instances share a single working directory, `git stash` becomes a silent data-loss trap. The author illustrates a concrete failure mode: a PS instance runs `git stash → git pull --rebase → git stash pop` while a Win instance has uncommitted edits to `lib/pages/horse_racing.dart`. Because `git stash` operates on the entire working directory with no concept of process ownership, the Win instance's changes are captured and effectively lost. The root cause is architectural — shared state between independent agents.

The fix is to give each instance a dedicated worktree using `git worktree add .claude/worktrees/instance-ps -b claude/ps-wip origin/main` (repeated per instance).

The fix is to give each instance a dedicated worktree using `git worktree add .claude/worktrees/instance-ps -b claude/ps-wip origin/main` (repeated per instance). Each worktree is a fully independent working directory on its own branch, so a stash or rebase in the PS worktree has zero effect on the Win worktree. The author also replaces `git stash` with WIP commits (`git add -A && git commit -m "WIP: $(date +%H:%M)"`), reverting with `git reset HEAD~1` when needed, making the workflow entirely stash-free. A critical operational guard is verifying the working root with `git rev-parse --show-toplevel` — if it returns the main repo path instead of the instance-specific worktree path, the instance must `cd` to its worktree immediately. File writes to the wrong path (e.g., `/repo/docs/...` instead of `/repo/.claude/worktrees/instance-ps/docs/...`) can corrupt another instance's in-progress work. At five or more parallel agents, the author concludes, worktree isolation is "not optional — it's the foundation."

Key facts

1. 01Running 5 Claude Code instances in a shared working directory causes `git stash` in one instance to capture and destroy uncommitted changes belonging to other instances.
2. 02`git worktree add .claude/worktrees/instance-<name> -b claude/<name>-wip origin/main` gives each instance a fully isolated working directory on its own branch.
3. 03A stash or rebase inside one worktree has zero effect on files in any other worktree.
4. 04WIP commits (`git add -A && git commit -m "WIP: $(date +%H:%M)"`) replace `git stash` entirely; changes are unstaged later with `git reset HEAD~1`.
5. 05`git rev-parse --show-toplevel` must return the instance-specific worktree path, not the main repo path, before any file writes.
6. 06Writing files to the main repo path instead of the correct worktree path can corrupt another instance's in-progress work.