diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/AbstractOpenApiEndpointVisitor.java b/openapi/src/main/java/io/micronaut/openapi/visitor/AbstractOpenApiEndpointVisitor.java
index bad21c2692..d889bee10b 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/AbstractOpenApiEndpointVisitor.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/AbstractOpenApiEndpointVisitor.java
@@ -1530,7 +1530,9 @@ private boolean isResponseType(ClassElement returnType) {
     }
 
     private boolean isSingleResponseType(ClassElement returnType) {
-        return (returnType.isAssignable("io.reactivex.Single") || returnType.isAssignable("org.reactivestreams.Publisher"))
+        return (returnType.isAssignable("io.reactivex.Single")
+            || returnType.isAssignable("io.reactivex.rxjava3.core.Single")
+            || returnType.isAssignable("org.reactivestreams.Publisher"))
             && returnType.getFirstTypeArgument().isPresent()
             && isResponseType(returnType.getFirstTypeArgument().get());
     }
diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/ContextUtils.java b/openapi/src/main/java/io/micronaut/openapi/visitor/ContextUtils.java
index f274d4a020..e9e010516f 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/ContextUtils.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/ContextUtils.java
@@ -15,6 +15,7 @@
  */
 package io.micronaut.openapi.visitor;
 
+import io.micronaut.core.annotation.Internal;
 import io.micronaut.inject.visitor.VisitorContext;
 
 /**
@@ -22,6 +23,7 @@
  *
  * @since 4.5.0
  */
+@Internal
 public final class ContextUtils {
 
     private ContextUtils() {
diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/ConvertUtils.java b/openapi/src/main/java/io/micronaut/openapi/visitor/ConvertUtils.java
index f34ad995f2..0691804dda 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/ConvertUtils.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/ConvertUtils.java
@@ -40,6 +40,7 @@
 import javax.xml.datatype.XMLGregorianCalendar;
 
 import io.micronaut.core.annotation.AnnotationValue;
+import io.micronaut.core.annotation.Internal;
 import io.micronaut.core.annotation.NonNull;
 import io.micronaut.core.annotation.Nullable;
 import io.micronaut.core.beans.BeanMap;
@@ -68,6 +69,7 @@
  *
  * @since 4.4.1
  */
+@Internal
 public final class ConvertUtils {
 
     private static final TypeReference<Map<String, Object>> MAP_TYPE_REFERENCE = new TypeReference<Map<String, Object>>() {
diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/ElementUtils.java b/openapi/src/main/java/io/micronaut/openapi/visitor/ElementUtils.java
index 5c83c0ddc8..4f68141399 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/ElementUtils.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/ElementUtils.java
@@ -23,6 +23,7 @@
 import java.util.Optional;
 import java.util.concurrent.Future;
 
+import io.micronaut.core.annotation.Internal;
 import io.micronaut.core.annotation.Nullable;
 import io.micronaut.inject.ast.ClassElement;
 import io.micronaut.inject.ast.TypedElement;
@@ -32,6 +33,7 @@
  *
  * @since 4.8.3
  */
+@Internal
 public final class ElementUtils {
 
     public static final List<String> CONTAINER_TYPES = Arrays.asList(
@@ -128,7 +130,9 @@ public static boolean isVoid(ClassElement type) {
      * @return true if this type assignable with known container and type argument is void
      */
     public static boolean isReactiveAndVoid(ClassElement type) {
-        return isContainerType(type) && type.getFirstTypeArgument().isPresent() && isVoid(type.getFirstTypeArgument().get());
+        return type.isAssignable("io.reactivex.Completable")
+            || type.isAssignable("io.reactivex.rxjava3.core.Completable")
+            || (isContainerType(type) && type.getFirstTypeArgument().isPresent() && isVoid(type.getFirstTypeArgument().get()));
     }
 
     private static boolean findAnyAssignable(ClassElement type, List<String> typeNames) {
diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/NumberUtils.java b/openapi/src/main/java/io/micronaut/openapi/visitor/NumberUtils.java
index 0d8402e75d..7ca7865b15 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/NumberUtils.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/NumberUtils.java
@@ -15,12 +15,14 @@
  */
 package io.micronaut.openapi.visitor;
 
+import io.micronaut.core.annotation.Internal;
 import io.micronaut.core.util.StringUtils;
 
 /**
  * Number utils.
  */
-public class NumberUtils {
+@Internal
+public final class NumberUtils {
 
     /**
      * <p>Checks whether the String a valid Java number.</p>
diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/SchemaUtils.java b/openapi/src/main/java/io/micronaut/openapi/visitor/SchemaUtils.java
index 34c5cbe678..b151e5472c 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/SchemaUtils.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/SchemaUtils.java
@@ -18,6 +18,7 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 
+import io.micronaut.core.annotation.Internal;
 import io.swagger.v3.oas.models.Components;
 import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.oas.models.media.ArraySchema;
@@ -32,6 +33,7 @@
  *
  * @since 4.5.0
  */
+@Internal
 public final class SchemaUtils {
 
     public static final Schema<?> EMPTY_SCHEMA = new Schema<>();
diff --git a/openapi/src/main/java/io/micronaut/openapi/visitor/Utils.java b/openapi/src/main/java/io/micronaut/openapi/visitor/Utils.java
index 4fde985100..4359cfbc6a 100644
--- a/openapi/src/main/java/io/micronaut/openapi/visitor/Utils.java
+++ b/openapi/src/main/java/io/micronaut/openapi/visitor/Utils.java
@@ -26,6 +26,7 @@
 
 import io.micronaut.context.env.DefaultPropertyPlaceholderResolver;
 import io.micronaut.context.env.PropertyPlaceholderResolver;
+import io.micronaut.core.annotation.Internal;
 import io.micronaut.core.annotation.NonNull;
 import io.micronaut.core.annotation.Nullable;
 import io.micronaut.core.convert.ArgumentConversionContext;
@@ -42,6 +43,7 @@
  *
  * @since 4.4.0
  */
+@Internal
 public final class Utils {
 
     public static final String PLACEHOLDER_PREFIX = "${";