Custom connector fails — OAuth Dynamic Client Registration error (systeme.io MCP server)

### MCP server URL

https://mcp.systeme.io/mcp?mcpKey=YOUR_KEY

### Where does the issue occur?

When pressing the Connect button

### Transport used

streamable-http

### Client registration type

My server does not have auth

### SSE only: Does your server return the expected status code for POST requests?

Not applicable (my server does not use SSE)

### When did you last reproduce this?

2026-08-20 8:55 AM CEST

### Browser and OS

_No response_

### Describe the issue

I'm trying to connect a custom remote MCP connector to the systeme.io MCP server, which authenticates via an API key embedded in the URL (https://mcp.systeme.io/mcp?mcpKey=...) rather than OAuth.

When I click "Connect," Claude attempts an automatic OAuth Dynamic Client Registration flow, which systeme.io's server does not support. This results in the following error:

"Automatic client registration is not supported by systeme.io. Modify the connector and add an OAuth Client ID."

However, systeme.io doesn't use OAuth at all — it only requires the API key already included in the connector URL. There's no OAuth Client ID to provide, so this workaround doesn't apply.

Reference code: ofid_7d12c9664a22212e

Could you look into having the custom connector flow skip OAuth negotiation for servers that authenticate via URL query parameter / API key, rather than always attempting Dynamic Client Registration first?

Thanks!

### Issue details

```shell
I'm trying to connect a custom remote MCP connector to the systeme.io MCP server, which authenticates via an API key embedded in the URL (https://mcp.systeme.io/mcp?mcpKey=...) rather than OAuth.

When I click "Connect," Claude attempts an automatic OAuth Dynamic Client Registration flow, which systeme.io's server does not support. This results in the following error:

"Automatic client registration is not supported by systeme.io. Modify the connector and add an OAuth Client ID."

However, systeme.io doesn't use OAuth at all — it only requires the API key already included in the connector URL. There's no OAuth Client ID to provide, so this workaround doesn't apply.

Reference code: ofid_7d12c9664a22212e

Could you look into having the custom connector flow skip OAuth negotiation for servers that authenticate via URL query parameter / API key, rather than always attempting Dynamic Client Registration first?

Thanks!
```

### Expected behavior

Claude should connect directly to the systeme.io MCP server using the API key already embedded in the connector URL (?mcpKey=...), without attempting an OAuth Dynamic Client Registration flow. Since systeme.io authenticates via this URL-embedded key and does not implement OAuth/DCR, the connector should either skip OAuth negotiation entirely for this server, or offer a way to mark the connector as not requiring OAuth.

### Logs from your server

```shell

```

### Additional context

Server: systeme.io official MCP server (https://mcp.systeme.io/mcp)
Setup followed systeme.io's official documentation for connecting via MCP keys: https://help.systeme.io/article/11230-how-to-connect-your-systemeio-account-with-claude-using-mcp-keys
This appears to be the same root cause as issue #560 (linked below), just affecting a different MCP server (systeme.io instead of magnific.com).