Setting my Accept header to application/json (or application/ld+json) produces a JSON response for the following requests:

• lookup: http://musicbrainz.org/ws/2/artist/e1d521ea-5b97-4981-987c-ba988b2a87d7
• browse: http://musicbrainz.org/ws/2/work?artist=e1d521ea-5b97-4981-987c-ba988b2a87d7

Setting my Accept header to application/json (or application/ld+json) produces an XML response for the following request:

• search: http://musicbrainz.org/ws/2/work?query=arid:e1d521ea-5b97-4981-987c-ba988b2a87d7

Using the fmt=json query param works for all 3.