diff --git a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/cmd/exec/ValidationResult.java b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/cmd/exec/ValidationResult.java
index 94ebc05ea..406ad30eb 100644
--- a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/cmd/exec/ValidationResult.java
+++ b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/cmd/exec/ValidationResult.java
@@ -70,8 +70,11 @@ private <T> void add(Set<ConstraintViolation<T>> violations) {
 			e.setModelAlias(null);
 			e.setMsg(c.getMessage());
 			
-			this.template.add(e);
+			this.addValidationError(e);
 		}
 	}
 	
+	public void addValidationError(ValidationError violation) {
+		this.template.add(violation);
+	}
 }
diff --git a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/defn/extension/NotExists.java b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/defn/extension/NotExists.java
new file mode 100644
index 000000000..00321bac1
--- /dev/null
+++ b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/defn/extension/NotExists.java
@@ -0,0 +1,53 @@
+/**
+ *  Copyright 2016-2019 the original author or authors.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package com.antheminc.oss.nimbus.domain.defn.extension;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Repeatable;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import com.antheminc.oss.nimbus.domain.RepeatContainer;
+import com.antheminc.oss.nimbus.domain.defn.event.StateEvent.OnStateChange;
+import com.antheminc.oss.nimbus.domain.defn.event.StateEvent.OnStateLoad;
+import com.antheminc.oss.nimbus.domain.defn.extension.NotExists.List;
+
+/**
+ * @author Tony Lopez
+ *
+ */
+@Documented
+@Retention(RUNTIME)
+@Target(FIELD)
+@Repeatable(List.class)
+@OnStateChange
+@OnStateLoad
+public @interface NotExists {
+
+	String url();
+	
+	@Documented
+	@Retention(RUNTIME)
+	@Target(FIELD)
+	@RepeatContainer(NotExists.class)
+	@interface List {
+		
+		NotExists[] value();
+	}
+}
diff --git a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/EntityState.java b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/EntityState.java
index fe4f2fce0..9f5ba6c00 100644
--- a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/EntityState.java
+++ b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/EntityState.java
@@ -35,6 +35,7 @@
 
 import com.antheminc.oss.nimbus.domain.cmd.Action;
 import com.antheminc.oss.nimbus.domain.cmd.Command;
+import com.antheminc.oss.nimbus.domain.cmd.exec.ValidationResult;
 import com.antheminc.oss.nimbus.domain.defn.extension.ValidateConditional.ValidationGroup;
 import com.antheminc.oss.nimbus.domain.model.config.EntityConfig;
 import com.antheminc.oss.nimbus.domain.model.config.ModelConfig;
@@ -679,6 +680,8 @@ public int hashCode() {
 		
 		void onStateLoadEvent();
 		void onStateChangeEvent(ExecutionTxnContext txnCtx, Action a);
+		
+		ValidationResult getValidationResult();
 	}
 	
 	public interface LeafParam<T> extends Param<T> {
diff --git a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/extension/NotExistsStateEventHandler.java b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/extension/NotExistsStateEventHandler.java
new file mode 100644
index 000000000..3e2e89411
--- /dev/null
+++ b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/extension/NotExistsStateEventHandler.java
@@ -0,0 +1,56 @@
+/**
+ *  Copyright 2016-2019 the original author or authors.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package com.antheminc.oss.nimbus.domain.model.state.extension;
+
+import java.util.List;
+
+import com.antheminc.oss.nimbus.context.BeanResolverStrategy;
+import com.antheminc.oss.nimbus.domain.cmd.CommandBuilder;
+import com.antheminc.oss.nimbus.domain.cmd.exec.CommandExecution.MultiOutput;
+import com.antheminc.oss.nimbus.domain.defn.extension.NotExists;
+import com.antheminc.oss.nimbus.domain.cmd.exec.CommandExecutorGateway;
+import com.antheminc.oss.nimbus.domain.cmd.exec.ValidationError;
+import com.antheminc.oss.nimbus.domain.model.state.EntityState.Param;
+import com.antheminc.oss.nimbus.support.EnableLoggingInterceptor;
+
+import lombok.AccessLevel;
+import lombok.Getter;
+
+/**
+ * @author Tony Lopez
+ *
+ */
+@EnableLoggingInterceptor
+@Getter(AccessLevel.PROTECTED)
+public class NotExistsStateEventHandler extends AbstractConditionalStateEventHandler<NotExists> {
+
+	private final CommandExecutorGateway commandGateway;
+	
+	public NotExistsStateEventHandler(BeanResolverStrategy beanResolver) {
+		super(beanResolver);
+		this.commandGateway = beanResolver.get(CommandExecutorGateway.class);
+	}
+
+	@Override
+	protected void handleInternal(Param<?> onChangeParam, NotExists configuredAnnotation) {
+		MultiOutput response = getCommandGateway().execute(CommandBuilder.withUri(configuredAnnotation.url()).getCommand(), null);
+		List<?> searchResults = (List<?>) response.getSingleResult();
+		if (!searchResults.isEmpty()) {
+			ValidationError violation = new ValidationError.Param();
+			onChangeParam.getValidationResult().addValidationError(violation);
+		}
+	}
+}
diff --git a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/internal/DefaultParamState.java b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/internal/DefaultParamState.java
index b9884b161..102c249b2 100644
--- a/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/internal/DefaultParamState.java
+++ b/nimbus-core/src/main/java/com/antheminc/oss/nimbus/domain/model/state/internal/DefaultParamState.java
@@ -79,7 +79,7 @@ public class DefaultParamState<T> extends AbstractEntityState<T> implements Para
 
 	private StateType type;
 	
-	private ValidationResult validationResult;
+	private ValidationResult validationResult = new ValidationResult();
 	
 	@JsonIgnore
 	final private Model<?> parentModel;