Custom connector "Couldn't reach" — server healthy, OAuth discovery endpoints all 200, claude.ai never follows up after 401

### MCP server URL

https://brain-dumper-project.vercel.app/api/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 sure

### When did you last reproduce this?

Today, 19 July 2026, ~11:32 UTC

### Browser and OS

Chrome, Windows 10

### Describe the issue

Claude.ai shows "Couldn't reach" / "Connection issue".
Server logs show only POST /api/mcp → 401.
Claude never requests /.well-known/oauth-protected-resource
or any other discovery endpoint after that.
Error ID: ofid_909ab9ae528da8ae

### Issue details

```shell
All discovery endpoints return 200 with valid JSON.
WWW-Authenticate header is present and correct on the 401.
Claude.ai stops after the first 401 and never starts OAuth discovery.
ofid_909ab9ae528da8ae
```

### Expected behavior

After receiving 401 with WWW-Authenticate header, Claude should
fetch /.well-known/oauth-protected-resource and continue the
OAuth flow, redirecting the user to the authorization page.

### Logs from your server

```shell
POST /api/mcp → 401 (WWW-Authenticate header present)
No further requests received from Claude.ai
```

### Additional context

/.well-known/oauth-protected-resource → 200 valid JSON
/.well-known/oauth-protected-resource/api/mcp → 200 valid JSON
/.well-known/openid-configuration → 200 with S256 PKCE support
Stack: Next.js 16, Vercel, oidc-provider v9.9.1