[BUG] Slack MCP server auth broken: "does not support dynamic client registration" (regression in recent update)

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?

Version: 2.1.141
Platform: macOS Darwin 25.2.0, zsh terminal

## What happened
The Slack remote MCP server (https://mcp.slack.com/mcp) stopped authenticating
after a recent Claude Code update. Running /mcp shows "needs authentication" and
attempting to authenticate fails with:

> SDK auth failed: Incompatible auth server: does not support dynamic client registration

What Should Happen?

The Slack MCP server to connect as it did previously (was working ~May 9, 2026).

## Config (.mcp.json)
{
"mcpServers": {
"slack": {
"type": "http",
"url": "https://mcp.slack.com/mcp"
}
}
}

Error Messages/Logs

A recent update added RFC 7591 dynamic client registration to the MCP OAuth flow.
  Slack's OAuth server does not support this, so auth now fails where it previously
  succeeded. Claude Code needs a fallback for OAuth servers that don't support
  dynamic client registration.

Error Message: SDK auth failed: Incompatible auth server: does not support dynamic client registration

Steps to Reproduce

1. Add Slack MCP server to .mcp.json (config above)
2. Run /mcp → shows "needs authentication"
3. Attempt to authenticate → fails with error above

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.141

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_