[BUG] SSE MCP Server fails to connect to Monday.com despite valid endpoint

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?

## Environment

• Claude Code Version: 2.0.75
• OS: Windows 10/11
• Node Version: (run node --version to confirm)
• Install Method: Native

## Description
The Monday.com MCP server (SSE transport) fails to connect even though the endpoint responds correctly when tested directly with curl.

## Configuration
```json
{
"mcpServers": {
"monday-com": {
"type": "sse",
"url": "https://mcp.monday.com/sse",
"headers": {
"Authorization": "<REDACTED>"
}
}
}
}

Observed Behavior

claude mcp list shows:
monday-com: https://mcp.monday.com/sse (SSE) - ✗ Failed to connect

What Should Happen?

Expected Behavior

Connection should succeed - the endpoint is valid and responding.

Error Messages/Logs

Diagnostic Evidence

  Direct curl test succeeds with HTTP 200 and returns valid SSE event:
  curl -H "Authorization: Bearer <REDACTED>" -H "Accept: text/event-stream" "https://mcp.monday.com/sse" --max-time 10

  Response:
  event: endpoint
  data: /sse/message?sessionId=790a59b8b43a489f2798a89fbb2f997f4eb0bfe1ab138dd2b0ef0731de94fe47

  HTTP Status: 200

  The endpoint returns a valid SSE session, but Claude Code's SSE client cannot establish the connection.

  Steps to Reproduce

  1. Get a Monday.com API token from Settings → Developers → API
  2. Add MCP server: claude mcp add monday-com https://mcp.monday.com/sse --transport sse -s user --header "Authorization: <token>"
  3. Run claude mcp list
  4. Observe "Failed to connect" status

Steps to Reproduce

Steps to Reproduce

1. Get a Monday.com API token from Settings → Developers → API
2. Add MCP server: claude mcp add monday-com https://mcp.monday.com/sse --transport sse -s user --header "Authorization: <token>"
3. Run claude mcp list
4. Observe "Failed to connect" status

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.75

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Additional Context

• Token is valid (freshly issued, confirmed via JWT decode)
• Re-adding the server doesn't help
• May be Windows-specific or SSE client timeout issue