Plugin validation failed on save — cannot overwrite existing personal plugin in account-scoped marketplace (HTTP 400 plugin_validation)

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?

Description
When saving changes to an existing personal plugin in Cowork (research preview), the save operation fails with "Plugin validation failed". The plugin cannot be updated.
Environment

Claude Desktop, macOS
Cowork mode (research preview)

Steps to reproduce

Install a personal plugin via file upload in Cowork
Make any change to the plugin files
Click the Save button in the project tree

Expected result: Plugin is saved and updated.
Actual result: "Plugin validation failed" error. Plugin is not saved.
What we've ruled out

File content: fails even with a minimal plugin.json containing only name, version, description, author
Plugin name format: valid kebab-case
File size: 85 KB, well under the 50 MB limit
Marketplace plugin limit: only 1 personal plugin installed
Client-side state: restarting Claude does not resolve the issue

Additional context
The error originates from a server-side HTTP 400 response with error code plugin_validation during the upload to the account-scoped marketplace. The actual server error message is not surfaced in the UI or in any log file. There is no option in the UI to delete a plugin from the personal marketplace — only local uninstall is available, which does not reset the server-side state.

What Should Happen?

Plugin is saved and updated.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce

1. Install a personal plugin via file upload in Cowork
2. Make any change to the plugin files
3. Click the Save button in the project tree

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Claude 1.5354.0 (9a9e3d) 2026-04-29T01:14:34.000Z

Claude Code Version

Claude 1.5354.0 (9a9e3d) 2026-04-29T01:14:34.000Z

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_