The new recommended toolchain for testing SourceKit-LSP is swift-DEVELOPMENT-SNAPSHOT-2018-12-18-a (https://github.com/apple/sourcekit-lsp#recommended-toolchain ). We generally expect the latest snapshot toolchain to work going forward, but until sourcekit-lsp is tested in Swift CI we will continue to recommend a specific version.
Note: the toolchains between 2018-12-13 and 2018-12-17 cannot build sourcekit-lsp because of the source break with Result . That was fixed by apple/swift#21370 which is in the recommended 2018-12-18-a snapshot.