Quint
Quint reads the key and its mode, the harmonic code from 1A to 12B, the tempo, the sections and the loudness of a track you own. Import a file, share one in from another app, or hold the phone to a speaker for twenty seconds. The analysis runs on the device through Music Understanding, the framework Apple shipped in iOS 27, and the loudness is measured by Quint's own engine.
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Nothing leaves the phone
Quint makes no network requests at all. There is no account, no sign in, no server and no third party SDK, so the audio you analyze has nowhere to go. Put the phone in airplane mode and every reading still works, because there was never anything to send.
What it will not do
Apple Music streaming tracks are protected and cannot be analyzed. That is the framework's own restriction and Quint says so at the moment it happens. Quint works on files you own, on files shared into it, and on recordings you make with it.