The MusicBrainz software should treat the recording ISRC field as case-insensitive, as per the standard, instead of requiring them to be uppercase only.

When adding ISRCs manually via the editing interface, they are automatically converted uppercase before saving. This doesn't happen for ISRCs submitted via the API, however, such as when using Tatsumo's Spotify ISRC lookup tool.

In most cases, the ISRC coming from Spotify is uppercase already, but not always. A workaround was implemented to convert them to uppercase before submission. This should not be necessary, however, as the software should accept an ISRC in either case, so long as it is valid.