[BUG] Skills upload returns "Failed to fetch" on Claude Desktop

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?

When attempting to upload a valid ZIP file for a skill, the upload fails and throws a Failed to fetch error. This issue occurs consistently even though the ZIP file is properly formatted and contains no errors.

What Should Happen?

The valid ZIP file should be uploaded successfully without any network or fetch errors, and the skill should be deployed/registered properly.

Error Messages/Logs

Steps to Reproduce

1. Navigate to the skill upload page.
2. Click the upload button and select a valid ZIP file.
3. Observe the Failed to fetch error message.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Version 1.9659.2 (390d6c)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

<img width="497" height="389" alt="Image" src="https://github.com/user-attachments/assets/d73ea0bf-9348-48e7-a6f4-d157316976b3" />

<img width="926" height="71" alt="Image" src="https://github.com/user-attachments/assets/a0cfebfb-0c70-4b61-9eb1-38f3934d0e22" />