[BUG] mkdir fails in claude headless

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?

● Claude Code 2.1.2 Sandbox Bug: Blocks File Operations in Subdirectories of Allowed Working Directory

Version: Claude Code 2.1.2

Platform: Linux (Fedora/RHEL based, kernel 6.12.0)

Description:
The sandbox incorrectly blocks all file operations (touch, mkdir, write) to subdirectories of the allowed working directory. The error message correctly identifies the allowed directory, but the path comparison fails to recognize that subdirectory paths are inside the allowed directory.

Reproduction Steps:

# 1. Create empty test directory
mkdir -p /tmp/claude-sandbox-test

# 2. Run Claude Code with --add-dir pointing to test directory
cd /tmp/claude-sandbox-test
claude -p "Run: touch test.txt" --add-dir /tmp/claude-sandbox-test --verbose --output-format stream-json

Expected Result: File /tmp/claude-sandbox-test/test.txt is created.

Actual Result:
Error: touch in '/tmp/claude-sandbox-test/test.txt' was blocked.
For security, Claude Code may only create or modify files in the
allowed working directories for this session: '/tmp/claude-sandbox-test'.

The path /tmp/claude-sandbox-test/test.txt IS inside /tmp/claude-sandbox-test, but the sandbox blocks it anyway.

Attempted Workarounds (None Work):

• --add-dir /path/to/project - still blocked
• settings.json with "sandbox": {"allowedDirectories": ["/path"]} - still blocked
• settings.json with "sandbox": {"enabled": false} - still blocked
• Using relative paths instead of absolute - still blocked

Only Working Workaround:
claude -p "prompt" --dangerously-skip-permissions

Impact: This completely breaks any automated/orchestrated use of Claude Code that needs to write files in project subdirectories. The sandbox path comparison logic appears to check if paths are equal rather than if one path is a prefix of another.

What Should Happen?

mkdir should work as it did previous.

Error Messages/Logs

Steps to Reproduce

See above.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.2

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_