It doesn't seem to support the Subsonic API?

I do claim compatibility because I do have compatibility :slight_smile:

Compatibility option is there to address the outdated servers. Vanilla subsonic is not updated since 3 years and have security issues by design. Airsonic is abandoned too. Yet I fully support them, it’s just that the sync is slower because the subsonic API have no end points to get all the songs in an efficient way.
Compatibility option is not a gimmick or a bad thing in the API usage, it’s just that Subsonic API lack things, so servers allowing fast sync are preferred since they offer a better quality of life to the users.

Many bulk-preferring applications have in common that they ignore server-provided sorting and searching and implement their own degrading implementations.

Symfonium is offline first and support many different other servers like Plex, Emby, Jellyfin, Kodi, …, it have it’s own sorting and everything, but sorry it’s actually way more powerful that what API support.

See [Wiki] Smart filters

You should really see a little more what the app can do, to understand the choices.

Whatever changes and API extensions the org will bring in all cases it won’t be added to no more maintained server like Subsonic and Airsonic, so this is not really relevant to the compatibility mode.
Those servers will never offer the features needed to build advanced apps like Symfonium.