Adding a custom MCP connector fails immediately with:

### MCP server URL

https://lhpucplznocvaparkemp.supabase.co/functions/v1/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?

2026-08-07, 13:37:41 UTC

### Browser and OS

MacOS 26.5.2 (25F84) Safari Claude and Claude Desktop App (both)

### Describe the issue

Title: Custom MCP connector: Dynamic Client Registration (POST /register) request never reaches our server

Body:

Summary
Adding a custom MCP connector fails immediately with:

Impossibile registrarsi con il servizio di accesso di . Puoi riprovare oppure aggiungere un OAuth Client ID nelle impostazioni del connettore.

This reproduces identically on two independent, self-hosted MCP OAuth servers (different Supabase projects, unrelated codebases apart from a shared open-source OAuth pattern):

https://lhpucplznocvaparkemp.supabase.co/functions/v1/mcp — ref ofid_e0920cdb3003aae1
https://sqryatvquzfgyccpzyeu.supabase.co/functions/v1/mcp — ref ofid_ffa7af1cbe719ba8 (retried with an active login session already confirmed in the target app — same result)
Evidence the request never leaves Anthropic's infrastructure
Both servers implement RFC 8414 discovery + RFC 7591 Dynamic Client Registration behind Supabase Edge Functions, which log every invocation.

Manually replaying the exact discovery chain Claude follows (GET .well-known/oauth-protected-resource → GET .well-known/oauth-authorization-server → POST /register) succeeds immediately every time (200/200/201) from outside Anthropic's network.
Checking the Edge Function logs right after each failed "Add connector" attempt in the Claude UI shows no corresponding /register request at all — not a 4xx, not a 5xx, nothing. The most recent /register entry is always our own manual test call from minutes earlier.
This points to the request never leaving Anthropic's backend (or being dropped before reaching Supabase), not a bug in the connector's OAuth implementation.

Secondary data point
Using the documented workaround (Advanced settings → manually pre-registered OAuth Client ID) gets one connector past this specific error, but it then immediately shows "La connessione è scaduta. Puoi riconnetterti…" — consistent with the same class of issue recurring one step later in the flow, similar to #754 / #769.

Ask
Could this be a connectivity issue between Anthropic's connector backend and Supabase Edge Functions specifically? Happy to share more detail — reference IDs above.

### Issue details

```shell
It simply does not do what it works in ChatGPT for instance. So it adds the MCP and appears linked with an error before bringing you on the oAuth page and hence it appears in the MCP list but if you press the CONNECT, it does not go through
```

### Expected behavior

I include URL, name and no preferences in terms of Client ID and should redirect me through the url to the oath page where I can access with my credentials (user and pw) and link it... Afterwards, it should show the instruments linked to that connector.

### Logs from your server

```shell

```

### Additional context

_No response_