The documentation for the /isrc endpoint states:

isrc lookups return a list of recordings, the 'inc=' arguments supported are identical to a lookup request for a recording.

Unfortunately this doesn't work when fmt=json is added to the query, at least not for artists and releases. Compare the (default fmt=xml response with the JSON response:

• https://musicbrainz.org/ws/2/isrc/USAR19200262?inc=artists+releases
• https://musicbrainz.org/ws/2/isrc/USAR19200262?inc=artists+releases&fmt=json

and, for reference, the recording this ISRC is attached to:

• https://musicbrainz.org/ws/2/recording/7b6873d9-c7c5-4568-884f-96660e8e4528?inc=artists+releases
• https://musicbrainz.org/ws/2/recording/7b6873d9-c7c5-4568-884f-96660e8e4528?inc=artists+releases&fmt=json

In the XML-formatted response, the <recording id="7b6873d9-c7c5-4568-884f-96660e8e4528"> element contains the exact same information in both responses, the only difference being that the /isrc response has wrapped that element inside the <isrc id="USAR19200262"><recording-list count="1">...</recording-list></isrc> structure.

Strangely enough, /isrc/<isrc>?fmt=json&inc=isrcs works (the isrcs array is added to the release object), but not when asking for an XML response. Then the service just.. hangs until it times out. Perhaps a related issue?