The JAX-RS Client implementation provided by RESTEasy, when used together with Swarm, stopped working with Swarm 2017.6.0. Worked perfectly well with 2017.5.0. The problem is a bunch of missing dependencies, e.g. Apache HTTP components.
I believe this is caused by some of the dependency handling refactorings that went into 2017.6.0. Here's a standalone test: https://github.com/Ladicek/swarm-tinies/tree/master/testing/jaxrs-client – the POM declares Swarm 2017.5.0, so the test passes, but changing the version makes it fail.
This also broke all Swarm boosters – hence marking as blocker.