[BUG] Local plugin upload fails in Cowork — "Upload failed" error on every attempt

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?

Local plugin uploads consistently fail in Cowork (research preview). Clicking Upload after selecting a .plugin file always returns "Upload failed. You can try again." — retrying does not help.

What Should Happen?

Plugin installs and appears under Local uploads

Error Messages/Logs

Upload failed. You can try again.

Steps to Reproduce

1. Open Cowork → Customize → Plugins → Personal tab
2. Click + next to "Local uploads"
3. Select a .plugin file and click Upload
4. "Upload failed" error appears immediately

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Desktop 1.2.234

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

<img width="1353" height="808" alt="Image" src="https://github.com/user-attachments/assets/5db47dd5-af36-42f5-ad6c-c36a14be0a82" />