No-auth custom connector: broker completes initialize (200), then aborts after expected OAuth-discovery 404s — never calls tools/list

### MCP server URL

https://obsidian-mcp-debug.jtvps.com/ea2bd8f11c97b15c4e153813b3b1786b/mcp

### 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?

7/2/2026 3:09pm

### Browser and OS

Claude for Windows and Claude.ai in Chrome.

### Describe the issue

Adding a self-hosted no-auth MCP server (FastMCP 2.14.7 / Python, streamable-http) as a custom connector always fails with "Connection issue — Couldn't connect to the server. Check that the URL points to a valid MCP server." The same URL completes full handshakes from Claude Code CLI (✔ Connected, tools callable), ChatGPT custom connectors, and curl.

### Issue details

```shell
Origin nginx logs show the identical sequence from the connector broker (UA python-httpx/0.28.1) on every attempt:

POST //mcp (initialize) → 200 OK, valid JSON-RPC initialize result, <100 ms
GET /.well-known/oauth-protected-resource//mcp → 404 (expected — no-auth server)
GET /.well-known/oauth-protected-resource → 404
GET /.well-known/oauth-authorization-server → 404
Broker aborts. notifications/initialized and tools/list are never sent.
Reproduced with the server in three configurations: stateless + SSE-framed responses, stateless + JSON responses, stateful (Mcp-Session-Id issued) + JSON responses. Server is behind Cloudflare (Full SSL) + nginx; requests demonstrably reach the origin. Protocol versions 2025-03-26 and 2025-06-18 both accepted. Possibly related: #143, #172, #214, #406.
```

### Expected behavior

After a successful initialize and 404s on OAuth discovery, the connector should treat the server as unauthenticated and proceed to notifications/initialized + tools/list, connecting the server (as Claude Code and ChatGPT clients do against this exact URL).

### Logs from your server

```shell

```

### Additional context

_No response_