Parallel agents overwrite the same checkout: give each one its own worktree with `claude --worktree`

Symptom

Two agents editing the same checkout clobber each other; in the control test the second write to a.txt silently replaced the first.

Environment and tested versions

Debian GNU/Linux 13 (trixie) x86_64, node 20.19.2, npm 11.21.1, git 2.47.3; Claude Code not logged in

• @anthropic-ai/claude-code 2.1.316
• git 2.47.3

Client used in the test: Partial test: only the worktree setup was tested for real (Claude Code 2.1.316 creates the worktree before checking login). Two live agent sessions were NOT run; their edits were simulated by writing into each worktree.

Root cause

A single working tree is shared state.

Fix steps

1. Make sure the repo has at least one commit.
2. Start each agent with claude --worktree <name> (or -w <name>); each gets .claude/worktrees/<name> on its own worktree-<name> branch.
3. Merge the branches when the agents are done.

Verification

Tested 31 Oct 2026 in a throwaway directory on a Linux box (fake credentials only). Before (A) and after the fix:

no commits: Error creating worktree: Failed to resolve base branch "HEAD": git rev-parse returned no commit for HEAD
one commit: git worktree list -> .../.claude/worktrees/feat-a [worktree-feat-a] (and feat-b)
feat-a: agent A edit [branch worktree-feat-a]
feat-b: agent B edit [branch worktree-feat-b]
control, same checkout: a.txt now: agent B edit (agent A's edit lost)

Sources

• https://code.claude.com/docs/en/common-workflows