When loading a knowledge base via a KnowledgeAgent, only the operators defined in org.drools.base.evaluators.Operator (== != >= < <= <) are supported. When loading the same knowledge base via a KnowledgeBuilder, all operators are supported.

Upon loading a knowledge base that contains another operator (e.g. "after", "before"), the following error is printed:

[2011:01:19 14:01:44:exception]
java.lang.RuntimeException: KnowledgeAgent exception while trying to deserialize KnowledgeDefinitionsPackage
at org.drools.agent.impl.KnowledgeAgentImpl.createPackageFromResource(KnowledgeAgentImpl.java:674)
at org.drools.agent.impl.KnowledgeAgentImpl.addResourcesToKnowledgeBase(KnowledgeAgentImpl.java:899)
...
Caused by: org.drools.RuntimeDroolsException: unable to determine operator for symbol [after]
at org.drools.base.evaluators.Operator.determineOperator(Operator.java:87)
at org.drools.base.evaluators.Operator.readResolve(Operator.java:122)

Using Eclipse's debugging feature, I have determined that KnowledgeBuilder loads the rest of the operators through EvaluatorRegistry, while KnowledgeAgent does nothing to load any operators into org.drools.base.evaluators.Operator. I have tried modifying KnowledgeAgent by adding the code below to the constructor of KnowledgeAgentImpl. This resolves the problem: with this addition, the knowledge base with "after"-operator works as intended when loaded via a KnowledgeAgent.

// Load all of Drools' default evaluators instead of just the ones defined
// in org.drools.base.evaluators.Operator
new EvaluatorRegistry();