[BUG] Custom connectors fail with "Couldn't reach the MCP server" error

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 connectors fail with "Couldn't reach the MCP server" error. Issue:
Clicking "Connect" on any connector in Claude.ai immediately shows an error popup. Affects Canva (shows as connected but errors), Gmail, and Instagram connectors.

What Should Happen?

Clicking "Connect" on any connector in Claude.ai immediately shows an error popup. Affects Canva (shows as connected but errors), Gmail, and Instagram connectors.

Error Messages/Logs

couldn't reach the mcp server

Steps to Reproduce

Clicking "Connect" on any connector in Claude.ai immediately shows an error popup. Affects Canva (shows as connected but errors), Gmail, and Instagram connectors.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

ccc

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_