I have used Hibernate with Glassfish 3.1.x since "Hib 3.2.5".
Then, I upgraded to "Hib. 4.x" (4.1, 4.2... 4.2.7).
Until 4.2.6, the javassist used was 3.15.0-GA (from "required" subdir).
When I upgraded from 4.2.6 to 4.2.7, I removed the javassist 3.15.0-GA from my project and added 3.18.1-GA.
Today, I also compared the file size of original JAR from "required" subdir to the file in my project: 714.194 bytes (date 2013-10-16).
My project runs fine with 4.2.7, only after migrating to 4.2.8 that it stopped to work to Hibernate.