diff --git a/junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionFailureBuilder.java b/junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionFailureBuilder.java
index d0416c36a167..562cf2857441 100644
--- a/junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionFailureBuilder.java
+++ b/junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionFailureBuilder.java
@@ -30,6 +30,11 @@
  * <p>Using this builder ensures consistency in how failure message are formatted
  * within JUnit Jupiter and for custom user-defined assertions.
  *
+ * <h2>Extensibility</h2>
+ *
+ * <p>Although it is technically possible to extend this class, extension is
+ * strongly discouraged.
+ *
  * @since 5.9
  * @see AssertionFailedError
  */
diff --git a/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentAccessException.java b/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentAccessException.java
index 6fecc8c9fc5a..eb8e296308de 100644
--- a/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentAccessException.java
+++ b/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentAccessException.java
@@ -22,6 +22,10 @@
  * {@link ArgumentsAccessor} if an error occurs while accessing
  * or converting an argument.
  *
+ * <h2>Extensibility</h2>
+ *
+ * <p>This class is designed for extension.
+ *
  * @since 5.2
  * @see ArgumentsAccessor
  */
diff --git a/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentsAggregationException.java b/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentsAggregationException.java
index be7b9019a8d5..3b906b409352 100644
--- a/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentsAggregationException.java
+++ b/junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/ArgumentsAggregationException.java
@@ -22,6 +22,10 @@
  * {@link ArgumentsAggregator} when an error occurs while aggregating
  * arguments.
  *
+ * <h2>Extensibility</h2>
+ *
+ * <p>This class is designed for extension.
+ *
  * @since 5.2
  * @see ArgumentsAggregator
  */
diff --git a/junit-jupiter-params/src/main/java/org/junit/jupiter/params/provider/AnnotationBasedArgumentsProvider.java b/junit-jupiter-params/src/main/java/org/junit/jupiter/params/provider/AnnotationBasedArgumentsProvider.java
index 755dd5d52e40..1c866e401c78 100644
--- a/junit-jupiter-params/src/main/java/org/junit/jupiter/params/provider/AnnotationBasedArgumentsProvider.java
+++ b/junit-jupiter-params/src/main/java/org/junit/jupiter/params/provider/AnnotationBasedArgumentsProvider.java
@@ -30,6 +30,10 @@
  * {@link ArgumentsProvider} implementations that also need to consume an
  * annotation in order to provide the arguments.
  *
+ * <h2>Extensibility</h2>
+ *
+ * <p>This class is designed for extension.
+ *
  * @since 5.10
  * @see org.junit.jupiter.params.ParameterizedClass
  * @see org.junit.jupiter.params.ParameterizedTest