Anthropic: Claude Code v2.1.133 brings worktree.baseRef and race condition fix
Anthropic has released Claude Code v2.1.133 with new parameters worktree.baseRef, sandbox.bwrapPath/socatPath and the environment variable CLAUDE_EFFORT in hooks. The version fixes a race condition in parallel sessions and issues with Windows drive root paths. The third release this week after v2.1.131 and v2.1.132.
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Anthropic released Claude Code v2.1.133 on 7 May 2026, the third release this week after v2.1.131 and v2.1.132. The version brings several configuration parameters oriented towards team development and fixes a number of issues in parallel sessions and on the Windows platform.
New configuration parameters
The most significant is worktree.baseRef with values fresh or head. A worktree is a Git mechanism for working on multiple branches from the same repository in parallel directories. The default value fresh restores the EnterWorktree behaviour of branching from origin/<default>, while head retains unpublished commits from the local HEAD in the new worktree.
Next are sandbox.bwrapPath and sandbox.socatPath as managed parameters for Linux and WSL that allow custom locations for bubblewrap and socat binaries. parentSettingsBehavior is an administrative key (first-wins or merge) that controls how the SDK merges managedSettings between levels.
Hooks now receive the active effort level through the JSON field effort.level and the environment variable $CLAUDE_EFFORT, which Bash commands can also read.
Which bugs are fixed?
The most important fix is the race condition in parallel sessions that caused 401 errors after a token-refresh race deleted shared credentials. The Windows drive root path issue is also fixed (C:\) and POSIX /, which previously matched incorrectly for Edit/Write rules. The MCP OAuth flow now respects HTTP(S)_PROXY, NO_PROXY and mTLS configuration variables.
Additional fixes include mapped network drive reading via --add-dir, Remote Control session interruption from claude.ai, synchronisation of the /effort command between parallel sessions (no longer modifies another session), and subagent skill detection at project, user and plugin levels. The VSCode extension no longer throws an “Unsupported platform” error.
Frequently Asked Questions
- What does the worktree.baseRef parameter do?
- It controls whether new worktrees branch from origin/<default> (value 'fresh', now the default) or from the local HEAD ('head'); 'head' retains unpublished commits.
- Which race condition issues does the version fix?
- It fixes 401 errors caused by a token-refresh race that deleted shared credentials between parallel sessions, and unwanted synchronisation of the /effort command between concurrent sessions.
- How is $CLAUDE_EFFORT used in hooks?
- Hooks now receive the effort level through the effort.level field in the JSON input and the environment variable $CLAUDE_EFFORT, which Bash commands can also read.