[BUG] Race condition in claude task list management

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

If two different Claude sessions attempt to create a task in the same todo list, they will override reach other.

What Should Happen?

Creating tasks should be done atomically. Most modern file system support atomic file creation, so I am hoping this would be straight-forward (for example, using open('file.txt', 'x') in Python).

Steps to Reproduce

In one session:

CLAUDE_CODE_TASK_LIST_ID=bug-demo claude

Then ask it to create three tasks.

In another session:

CLAUDE_CODE_TASK_LIST_ID=bug-demo claude

Ask it to create a new task called "foo bar". Then go back to the original session, ask it to create a new task called "bar baz", and you will see it will override the previous one.

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.19

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_