Hi, I am working on https://jira.jboss.org/jira/browse/GUVNOR-358, please see my comments for details. There are two ways to fix this problem. One is making org.drools.runtime.rule.AccumulateFunction extends from Externalizable interface. Another solution is making it clear both in doc and samples that custom AccumulateFunction needs to implement both AccumulateFunction and Externalizable interfaces. As this is more like a usability issue, I would like to ask your opinions. Which way do you prefer? Thanks. Cheers, Jervis