[BUG] Custom skills show "skill not found" / cannot delete phantom skills on Mac - Internal server error on upload

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?

Custom skills uploaded via claude.ai (browser) and Claude desktop app (Mac)
show "skill not found" with no files inside when clicked. Skills cannot be
deleted ("Failed to delete skill" error) and cannot be replaced (replacement
silently fails). Upload of new skills returns "Internal server error".

Confirmed via the Skills API that these skills do NOT exist server-side, yet
they appear as phantom entries in the UI on both browser and desktop app.

Clearing all local app cache (IndexedDB, Local Storage, WebStorage, Session
Storage) did not resolve the issue. The phantom skills persist across browser
and desktop app.

What Should Happen?

Skills should upload successfully, be viewable, and be deletable.

Error Messages/Logs

Steps to Reproduce

Upload a custom skill ZIP via Settings > Capabilities

1. Upload fails with "internal server error" but skill appears in the list
2. Click on the skill → "skill not found", no files visible
3. Try to delete → "Failed to delete skill"
4. Try to replace → Asked to confirm replacement, confirm, nothing happens
5. Check via API (GET /v1/skills) → skills do not exist server-side
6. Clear all local cache → phantom skills still visible in UI

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

N/A - Using Claude desktop app (Cowork). Version : Claude 1.1.3189 (1b7b58) 2026-02-14T00:09:46.000Z

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

#26172 - [BUG] Skills are not loading to Claude Cowork on Mac (filed Feb 16 2026)