Custom MCP connector fails at handshake despite server returning valid 200 response

### MCP server URL or name

https://thisisnotdalishouse.com/wp-json/easy-mcp-ai/v1/mcp

### How are you connecting to this server?

From a custom connector URL I entered manually

### Where does the issue occur?

When pressing the Connect button

### When did you last reproduce this?

2026-07-26 10:05 PM CEST

### Browser and OS

Chrome 150.0.7871.128 , macOS 26.5.2

### Describe the issue

Connecting a custom MCP connector (WordPress site, "Easy MCP AI" plugin v1.7.10) fails with "Connection issue: Could not connect to server" in the Claude.ai UI — using both OAuth and manual token (bearer-in-path) authentication.

However, a direct curl -X POST to the same endpoint with the initialize method returns a clean HTTP/2 200 with a valid MCP handshake response:

json
{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-11-25","capabilities":{"tools":{},"resources":{}},"serverInfo":{"name":"easy-mcp-ai","version":"1.7.10"}}}

including a proper mcp-session-id header.

This confirms the server, authentication, and routing all work correctly outside of Claude.ai. The plugin developer (via WordPress.org support) also confirmed this rules out server-side/hosting issues on their end.

The failure appears to be specific to how Claude.ai's connector negotiates the handshake — possibly related to session header handling, an initial SSE attempt before falling back to POST, or something else in the transport negotiation that a plain curl doesn't replicate.

Environment: Claude.ai web, custom connector, MCP Streamable HTTP transport, protocolVersion 2025-11-25.

Happy to provide the full request/response headers or test further if useful.

### Error details

```shell
"Connection issue: Could not connect to server. Check that the URL points to a valid MCP server."

(No specific error ID was shown in the UI beyond this message.)
```

### Expected behavior

Claude.ai should successfully complete the MCP initialize handshake, since a direct POST request to the same URL with the same credentials returns HTTP 200 with a valid session (see curl output above). The connector should be able to connect and list available tools.

### Additional context

Possibly related to #644 — a different but similarly-shaped symptom where claude.ai's connector fails to properly use the configured auth method despite the server demonstrably working via direct curl (verified 200 + valid mcp-session-id in both cases). Also see similar reports #595 and #594 (part of a known cluster including #479, #505, #582, #593) — same pattern: server verified healthy via curl/external tools, but claude.ai's connect/registration flow fails regardless. #595 was closed as "not planned".