EINVAL error writing task output files on Windows

Bug Description

Background tasks/agents fail with EINVAL: invalid argument when trying to write task output files on Windows.

Error Message

Error: EINVAL: invalid argument, open 'C:\Users\omkco\AppData\Local\Temp\claude\C--Users-omkco-PycharmProjects-scube-quantpy\tasks\bouhwyo9n.output'

Environment

• OS: Windows 11 Pro 10.0.26200
• Claude Code version: 2.1.58
• Shell: bash (Git Bash)

Steps to Reproduce

1. Open Claude Code on Windows
2. Run a task that triggers a background agent (e.g., a Bash command via the Task tool)
3. The task fails with the EINVAL error when Claude Code tries to write the .output file

Troubleshooting Already Attempted

• The tasks directory exists and is writable (verified manually with Python open()/write())
• Disk space is not an issue (1.1TB free)
• Adding a Windows Defender exclusion for the Claude temp directory did not resolve the issue
• Updated to latest version (2.1.58) — issue persists
• Error is intermittent and appears across multiple Claude Code sessions

Appears to be an internal Node.js file handle / race condition issue specific to Windows.