[BUG] .skill file upload always fails: "スキルのアップロードに失敗しました" for new skills

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?

Installing a new skill via .skill file (double-clicking from Finder) always fails with the error toast: "スキルのアップロードに失敗しました。再度お試しください。"

Varied file size (11354 bytes → 1394 bytes), skill name (mercari-listing → mercari), and ZIP format — all failed.

Log from ~/Library/Logs/Claude/main.log:

The error occurs inside the web app after this point. No further error detail is logged.

Existing skills (npb-roster-update, moneyforward-to-sheets) installed on April 12 work fine. The skill IS available in Claude Code (Code tab) via ~/.claude/skills/ — only the chat UI registration via Anthropic's server is failing.

What Should Happen?

New skill should be registered and appear under Customize > Skills in the chat UI.

Error Messages/Logs

[info] Handling skill file: /Users/macbookairkenji/Claude/claude-skills/mercari-listing/mercari-listing.skill
[info] Skill file "mercari-listing.skill" sent to web app for preview
(no further log — error occurs silently in web app)

Steps to Reproduce

1. Create a .skill ZIP file containing a folder with SKILL.md inside
2. Double-click the .skill file from Finder
3. Claude Desktop App opens the skill preview dialog
4. Attempt to install the skill
5. Error toast appears: "スキルのアップロードに失敗しました。再度お試しください。"

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.101

Platform

Other

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

• Tried varying file size: 11354 bytes → 1394 bytes (all failed)
• Tried varying skill name: mercari-listing → mercari (all failed)
• Tried varying ZIP format and metadata (all failed)
• Other skills (npb-roster-update, moneyforward-to-sheets) were successfully installed on April 12 and still work fine
• The skill IS available in Claude Code (Code tab) via ~/.claude/skills/ directory — only registration via Anthropic's server fails
• GitHub repo KenjiTodoroki/claude-skills is connected, but mercari-listing does not appear in Customize > Skills