Dependencies of the Spoofax plugin for IntelliJ IDEA are not actually managed in the plugin project itself, but in a separate project: the deps/ project.

Add dependencies to or remove them from the deps/build.gradle file.

If a dependency causes a JAR version conflict with a JAR that’s distributed with IntelliJ IDEA or JPS, see the troubleshooting page for a solution.

Rebuild the dependencies project to use them in the plugin:

./gradlew clean publishToMavenLocal