Using org.drools.verifier found in "drools-verifier-5.1.1.jar" I get an org.drools.RuntimeDroolsException if a pattern in a rule LHS contains a comparison to a literal value using the equals operator. Please consider the following rule I created solely for the purposes of isolating this issue. package com.simple.test.rule import com.p1s.mps.rulevalidator.TestFact; rule "TestRule" when $fact : TestFact(value == 1) then System.out.println($fact.getValue()); end The following class definition for the TestFact. package com.p1s.mps.rulevalidator; public class TestFact { private int value; public TestFact(int value) { this.value = value; } public int getValue() { return value; } public void setValue(int value) { this.value = value; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + value; return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; TestFact other = (TestFact) obj; if (value != other.value) return false; return true; } } The following unit test which attempts to use Verifier. @Test public final void testVerifierOnLiteral() { VerifierBuilder verBuilder = VerifierBuilderFactory.newVerifierBuilder(); Verifier ver = verBuilder.newVerifier(); String ruleText = getRuleFromResource(TEST_RULE); ver.addResourcesToVerify( ResourceFactory.newReaderResource( new StringReader( ruleText ) ), ResourceType.DRL ); Assert.assertTrue(ver.fireAnalysis()); } Running the test with JUnit produces the following exception. java.lang.RuntimeException: org.drools.RuntimeDroolsException: Exception executing ReturnValue constraint org.drools.rule.ReturnValueRestriction@589f81 : [Error: cannot convert <1> to a numeric type: class java.lang.Integer [0]] [Near : {... Unknown ....}] ^ [Line: 1, Column: 0] at org.drools.verifier.builder.VerifierImpl.analyse(VerifierImpl.java:148) at org.drools.verifier.builder.VerifierImpl.fireAnalysis(VerifierImpl.java: 117) at com.p1s.mps.rulevalidator.NumericConversionTest.testVerifierOnLiteral(Nu mericConversionTest.java:64) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown Source) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMet hod.java:44) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallab le.java:15) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMetho d.java:41) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod .java:20) at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.jav a:28) at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java: 31) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner .java:73) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner .java:46) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:180) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:41) at org.junit.runners.ParentRunner$1.evaluate(ParentRunner.java:173) at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.jav a:28) at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java: 31) at org.junit.runners.ParentRunner.run(ParentRunner.java:220) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4Tes tReference.java:46) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.ja va:38) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTe stRunner.java:467) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTe stRunner.java:683) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRun ner.java:390) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRu nner.java:197) Caused by: org.drools.RuntimeDroolsException: Exception executing ReturnValue constraint org.drools.rule.ReturnValueRestriction@589f81 : [Error: cannot convert <1> to a numeric type: class java.lang.Integer [0]] [Near : {... Unknown ....}] ^ [Line: 1, Column: 0] at org.drools.rule.ReturnValueConstraint.isAllowedCachedRight(ReturnValueCo nstraint.java:168) at org.drools.common.TripleBetaConstraints.isAllowedCachedRight(TripleBetaC onstraints.java:211) at org.drools.reteoo.NotNode.assertObject(NotNode.java:140) at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleOb jectSinkAdapter.java:59) at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:145) at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(Com positeObjectSinkAdapter.java:450) at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(Compo siteObjectSinkAdapter.java:378) at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:190) at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:145) at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.jav a:1174) at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.jav a:1123) at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.jav a:917) at org.drools.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSes sionImpl.java:251) at org.drools.verifier.builder.VerifierImpl.analyse(VerifierImpl.java:136) ... 26 more Caused by: org.drools.RuntimeDroolsException: [Error: cannot convert <1> to a numeric type: class java.lang.Integer [0]] [Near : {... Unknown ....}] ^ [Line: 1, Column: 0] at org.drools.rule.ReturnValueRestriction.isAllowed(ReturnValueRestriction. java:236) at org.drools.rule.ReturnValueConstraint.isAllowedCachedRight(ReturnValueCo nstraint.java:162) ... 39 more Caused by: [Error: cannot convert <1> to a numeric type: class java.lang.Integer [0]] [Near : {... Unknown ....}] ^ [Line: 1, Column: 0] at org.mvel2.math.MathProcessor.getInternalNumberFromType(MathProcessor.jav a:577) at org.mvel2.math.MathProcessor._doOperations(MathProcessor.java:155) at org.mvel2.math.MathProcessor.doOperations(MathProcessor.java:79) at org.mvel2.ast.BinaryOperation.getReducedValueAccelerated(BinaryOperation .java:102) at org.mvel2.MVELRuntime.execute(MVELRuntime.java:85) at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:1 04) at org.mvel2.MVEL.executeExpression(MVEL.java:978) at org.drools.base.mvel.MVELReturnValueExpression.evaluate(MVELReturnValueE xpression.java:92) at org.drools.rule.ReturnValueRestriction.isAllowed(ReturnValueRestriction. java:226) ... 40 more However, if I use the following version of the same rule where I have added parentheses it works. package com.simple.test.rule import com.p1s.mps.rulevalidator.TestFact; rule "TestRule" when $fact : TestFact(value == (1)) then System.out.println($fact.getValue()); end Note that both versions of the DRL file compile, run, and produce the correct results. Thank You, Nathan Bell