Swift Packages now support build tool plugins, as defined in SE-0303 and SE-0325. This allows packages to define plugins that can specify tools that should run during a build operation, for example to generate source code. This is supported in both swift package and in Xcode’s support for packages. (79876749) The swift package command now supports command plugins, as defined in SE-0332. This allows Packages to define commands that can be invoked using the swift package command line to perform custom actions on the package. (82895553)

An error emitted by a build tool package plugin doesn’t prevent later build operations that rely on the output from the plugin. This can cause build errors related to missing files that the plugin would have produced. (83715966) Incorrect warnings and errors are sometimes shown in Xcode’s editor while editing the source code of a package plugin. (87610558) Workaround: Turn off Live Issues in Xcode’s preferences while working on the plugin source code.