Define a class public class Assertion { public String keyword; public final List< String > parameters = new ArrayList<>(); public Assertion( String keyword, String ... params) { this .keyword = keyword; for ( String param: params) { parameters.add(param); } } @Override public String toString() { String result = keyword; for ( String param: parameters) { result += " " + param; } return result; } Now define rules rule "Unexpected input type when slide navigation is expected" dialect "mvel" when not(Assertion()) then insert( new Assertion( "UNEXPECTED_CONTENT" , "KW1" , "KW2" )) end rule "Unexpected input type when slide navigation is expected" dialect "mvel" when $ assert : Assertion() then System .out.println($ assert ); retract($ assert ); end Expected output: "UNEXPECTED_CONTENT KW1 KW2" Actual output: "UNEXPECTED_CONTENT KW1" I have walked it through with a debugger, and KW2 is never passed to the constructor.