The connection-card / manage_connections install of an OAuth catalog entry runs the probe on a
worker thread; `mcp_oauth.start` then rebuilt the worker's error as a bare RuntimeError from its
text, dropping the `failure_class` `_probe_single_server` had tagged. Every pre-authorization
failure (empty or schemeless URL, unreachable host, 404 endpoint, refused client registration)
was recorded as `exception:RuntimeError` -> `exception`.
- run_worker classifies the original exception with probe_failure_class and stores the class on
the flow; start() re-tags the RuntimeError it raises with it.
- probe_failure_class reads InvalidMcpUrlError (not http(s)://host) as config_invalid.
- _require_rendered_remote tags an unset ${VAR} in url/headers as missing_credentials.
- the "server responded but no token" refusal is auth_required.
- a connect_failed message that does not name the server gets a one-line next step
("Could not connect to 'n8n-official' (Not Found). Check its URL ...").
n8n-official is the only catalog entry whose URL is a user-supplied `${VAR}`, so it is the only
one where these user-side errors happen before authorization.
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