When running a build in a runtime workbench, you might get the following message:
!MESSAGE Skipping builder com.xyz.builder for project P.
Either the builder is missing from the install, or it
belongs to a project nature that is missing or disabled.
This message means that something is wrong with the builder plug-in or with the attribution of the builder specification in the .project file. The builder plug-in might load fine but still be broken, perhaps because it is missing an ID in the extension point declaration.
If everything else seems right to you, double-check the ID specified in the builder extension point. The ID should not be the plug-in ID of your builder, but rather the concatenation of the plug-in ID and the builder ID. In other words, if the plug-ins ID is org.eclipse.escript.builder, and the ID of the builder is Builder, the builder ID reference in the .project file should be org.eclipse.escript.builder.Builder.