[BUG] Unable to install Cowork plugins in MacOS local

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:
All 11 official Anthropic plugins fail to install in Cowork mode. When clicking "Add plugin" on any plugin (Product Management, Marketing, Customer Support, etc.), a red toast notification appears in the top right saying "Failed to install plugin." No additional error detail is shown.
Environment:

Claude Desktop App version: 1.1.3647 (8f7c53), build date 2026-02-19
Mode: Cowork
Plan: Max
OS: (add your macOS version — you can find it in Apple menu → About This Mac)
Network: (mention if you're on a VPN, corporate network, or regular home Wi-Fi)

<img width="986" height="745" alt="Image" src="https://github.com/user-attachments/assets/3ff3716d-8274-4d6a-9bff-ff8904d29e3f" />

What Should Happen?

Expected behavior: Plugin installs successfully and becomes available in the session.
Actual behavior: Red toast error "Failed to install plugin" appears. No plugin is installed. This happens for all 11 Anthropic plugins.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

Open Claude Desktop App
Start a Cowork session
Click "+" in the sidebar → Plugins
Browse "By Anthropic" plugins
Click "Install" on any plugin

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Desktop App version: 1.1.3647 (8f7c53), build date 2026-02-19

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_