Error when adding my custom MCP connector: ofid_3695c73e7b0401f9

### MCP server URL

http://ui-mcp-urban-insight.pantheonsite.io/mcp

### Where does the issue occur?

When pressing the Connect button

### Transport used

streamable-http

### Client registration type

Dynamic Client Registration (DCR)

### 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?

2026-08-10 6:42am PST

### Browser and OS

All browser Windows and Mac

### Describe the issue

When adding my own connector it fails on Claude Desktop and Claude.ai.

It works on Codex and Claude Code.

### Issue details

```shell
Couldn’t register with My Site’s sign-in service. You can try again, or add an OAuth Client ID in the connector settings. If this persists, share this reference with support: “ofid_3695c73e7b0401f9”
```

### Expected behavior

I can get to the auth screen and register my MCP

### Logs from your server

```shell
Nothing shown, it doesn't reach it.
```

### Additional context

_No response_