This (stupid, unit test) rule: rule "Testing contains" dialect "mvel" when Response( $answerSelections : answerSelections ) AnswerSelection( answer.ID in (2, 3) ) then System.out.println("Testing contains activated") end Produces this error: java.lang.AssertionError: KnowledgeBuilder errors: Unable to Analyse Expression answer.ID == 2 || answer.ID == 3: [Error: Failed to compile: 1 compilation error(s): - (1,9) unqualified type in strict mode for: ID] [Near : {... Unknown ....}] ^ [Line: 1, Column: 0] : [Rule name='Testing contains'] Note the implicit rewrite going on. Clearly contains() is getting rewritten internally to a long disjunctive (x == 1 || x == 2 || x == 3 and so on). For whatever reason, in Drools 5.2.0-SNAPSHOT, this runs afoul of MVEL's strict mode, which if I'm understanding things properly shouldn't be visible to the .drl developer. I know that Drools 5.2.0-SNAPSHOT is a mess at the moment but wanted to pass this along. Best, Laird -- View this message in context: http://drools-java-rules-engine.46999.n3.nabble.com/Drools-5-2-0-basic-gr... Sent from the Drools - Dev mailing list archive at Nabble.com.