This means that LTS 4 is being discontinued, and LTS 5 is imminent. See the announcement blog post: https://unknownparallel.wordpress.com/2016/01/18/stackage-is-reverting-to-ae... See also related discussion (yesterday): https://www.reddit.com/r/haskell/comments/41gpdk/lts4_with_aeson010_is_being... If there are any questions or concerns, don't hesitate to ping me. -- Dan Burton