A client handed https://host/mcp as the server address looks for the
well-known documents below that path instead of asking the origin, and every
one of those probes fell through to the web app: 200 with an HTML body, which
reads to the client as "this server has no OAuth" (Open WebUI reports it as an
unexpected mimetype). The three documents are served under /mcp/.well-known
now, anything else there answers a JSON 404, and the RFC 8414 path-inserted
form /.well-known/oauth-authorization-server/mcp serves the metadata instead
of 404. The wiki says which redirect URI belongs to the client and that a
plain-HTTP APP_URL leaves the discovery document pointing at localhost.
Maurice