[BUG] Cowork returns 403 on macOS 26.3.1 but works on macOS 26.3

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?

Cowork/Code mode returns 403 "Request not allowed" on one Mac, but works perfectly on another Mac with the same account, same Claude Desktop version, and same proxy setup.

Error message:
Failed to authenticate. API Error: 403 {"error":{"type":"forbidden","message":"Request not allowed"}}

Regular Chat mode works fine on both machines. Only Cowork/Code mode fails on the affected machine.

What Should Happen?

Cowork should authenticate and work the same as it does on my other Mac, since both use the same Pro account, same Claude Desktop version, and same proxy configuration.

Error Messages/Logs

Steps to Reproduce

1. Open Claude Desktop on the affected MacBook (macOS 26.3.1)
2. Chat mode works fine
3. Switch to Cowork mode
4. Submit any task
5. 403 error appears immediately

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.81 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

The only difference between the two machines is macOS 26.3.1 (25D771280a) vs macOS 26.3 (25D125). This may be related to sandbox or network permission changes in the macOS 26.3.1 update affecting Cowork's VM environment.