[BUG] Claude for mac desktop "Failed to attach prompt" with attach_prompt returning 400

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 I try to use a prompt from a local MCP server, I get a popup message "Failed to attach prompt. You can try again." and the prompt is not added.

What Should Happen?

I should be able to use my prompt.

Error Messages/Logs

[REACT_QUERY_CLIENT] QueryClient error: {"type":"invalid_request_error","statusCode":400,"extra":{"headers":{}},"errorCode":null,"endpoint":"/api/organizations/XXXXXXXXX/mcp/attach_prompt","method":"POST","name":"Error","message":"Failed to attach prompt","stack":"Error: Failed to attach prompt\n    at ab (https://assets-proxy.anthropic.com/claude-ai/v2/assets/v1/index-BS0g3jI8.js:2:175721)\n    at https://assets-proxy.anthropic.com/claude-ai/v2/assets/v1/index-BS0g3jI8.js:3:73738\n    at async Object.mutationFn (https://assets-proxy.anthropic.com/claude-ai/v2/assets/v1/index-BS0g3jI8.js:3:74463)"}

Steps to Reproduce

1. Start a new conversation
2. Press the + button
3. Select Connectors -> Add from LocalMCP -> (choose a prompt)
4. See error

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.121 (Claude Code) Claude 1.4758.0 (fb266c) 2026-04-24T20:22:30.000Z (Claude for Mac)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

<img width="383" height="70" alt="Image" src="https://github.com/user-attachments/assets/69a3f029-3620-440a-b0a6-0b2675ca3898" />