@Wolfgang: if you are using drools.agent.newInstance=true (the default configuration), then whenever a change-set is applied, either because you explicitly force the agent to do so, or because a monitored resource changes, the kbase inside the agent is thrown away an a new kbase is created. So you need to get the kbase from the agent and retrieve a fresh session from it.
You can confirm this behavior in this set of tests: https://github.com/droolsjbpm/drools/blob/master/drools-compiler/src/test/java/org/drools/agent/KnowledgeAgentIncrementalChangeSetTest.java

But, if you are using drools.agent.newInstance=false the internal kbase (and each stateful session you got from it) is refreshed when a change-set is applied.

Would you please point me to the doc where it says "and each stateful session ... is refreshed"?

If true, this were an extremely risky operation. Consider a stateful session whizzing along and - smack! - rules change right in the middle of a sequence of firings. Shudder.

-W