Seen that `lsp-types` was released with 2.0 support, but then the revision was made to `< 1.6` https://hackage.haskell.org/package/lsp-types-1.3.0.1/revisions/. So in the future, a new release with working support of `aeson >= 2.0.1` would contribute into HLS having full support for `9.0.1`. Since `brittany` released support of `9.0.1` requiring `aeson >= 2.0.1`: https://hackage.haskell.org/package/brittany That with `aeson < 1.6` here currently makes a restriction that prevents Cabal from solving bounds during HLS build for 9.0.1 with enabled `brittany` plugin: https://github.com/haskell/haskell-language-server/runs/4498074985?check_suite_focus=true