diff --git a/akka-http-core/src/main/scala/akka/http/scaladsl/model/ErrorInfo.scala b/akka-http-core/src/main/scala/akka/http/scaladsl/model/ErrorInfo.scala
index 57b87cee07..5f3f4ba701 100644
--- a/akka-http-core/src/main/scala/akka/http/scaladsl/model/ErrorInfo.scala
+++ b/akka-http-core/src/main/scala/akka/http/scaladsl/model/ErrorInfo.scala
@@ -80,7 +80,10 @@ object EntityStreamException {
  * The limit can also be configured in code, by calling [[HttpEntity#withSizeLimit]]
  * on the entity before materializing its `dataBytes` stream.
  */
-case class EntityStreamSizeException(limit: Long, actualSize: Option[Long] = None) extends RuntimeException {
+final case class EntityStreamSizeException(limit: Long, actualSize: Option[Long] = None) extends RuntimeException {
+
+  override def getMessage = toString
+
   override def toString =
     s"EntityStreamSizeException: actual entity size ($actualSize) exceeded content length limit ($limit bytes)! " +
       s"You can configure this by setting `akka.http.[server|client].parsing.max-content-length` or calling `HttpEntity.withSizeLimit` " +
diff --git a/akka-http-core/src/test/scala/akka/http/impl/engine/server/HttpServerSpec.scala b/akka-http-core/src/test/scala/akka/http/impl/engine/server/HttpServerSpec.scala
index f3cb1224f4..dbb3adc141 100644
--- a/akka-http-core/src/test/scala/akka/http/impl/engine/server/HttpServerSpec.scala
+++ b/akka-http-core/src/test/scala/akka/http/impl/engine/server/HttpServerSpec.scala
@@ -899,6 +899,7 @@ class HttpServerSpec extends AkkaSpec(
                   .thrownBy(entity.dataBytes.runFold(ByteString.empty)(_ ++ _).awaitResult(100.millis))
                   .getCause
                 error shouldEqual EntityStreamSizeException(limit, Some(actualSize))
+                error.getMessage should include ("exceeded content length limit")
 
                 responses.expectRequest()
                 responses.sendError(error.asInstanceOf[Exception])
@@ -921,6 +922,7 @@ class HttpServerSpec extends AkkaSpec(
                   .thrownBy(entity.dataBytes.runFold(ByteString.empty)(_ ++ _).awaitResult(100.millis))
                   .getCause
                 error shouldEqual EntityStreamSizeException(limit, None)
+                error.getMessage should include ("exceeded content length limit")
 
                 responses.expectRequest()
                 responses.sendError(error.asInstanceOf[Exception])
diff --git a/project/MiMa.scala b/project/MiMa.scala
index 289445f619..4d47c4757e 100644
--- a/project/MiMa.scala
+++ b/project/MiMa.scala
@@ -670,7 +670,10 @@ object MiMa extends AutoPlugin {
         // #19983 withoutSizeLimit overrides for Scala API
         ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.http.scaladsl.model.RequestEntity.withoutSizeLimit"),
         ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.http.scaladsl.model.UniversalEntity.withoutSizeLimit"),
-        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.http.scaladsl.model.ResponseEntity.withoutSizeLimit")
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.http.scaladsl.model.ResponseEntity.withoutSizeLimit"),
+
+        // #20014 should have been final always
+        ProblemFilters.exclude[FinalClassProblem]("akka.http.scaladsl.model.EntityStreamSizeException")
       )
     )
   }