diff --git a/panel/chat/message.py b/panel/chat/message.py
index 736f2b9137d..9715e4c6014 100644
--- a/panel/chat/message.py
+++ b/panel/chat/message.py
@@ -479,6 +479,20 @@ def _include_stylesheets_inplace(self, obj):
             for o in obj.objects:
                 self._include_stylesheets_inplace(o)
 
+    def _include_message_css_class_inplace(self, obj):
+        if hasattr(obj, "objects"):
+            for o in obj.objects:
+                self._include_message_css_class_inplace(o)
+        elif isinstance(obj, str):
+            return self._include_message_css_class_inplace(Markdown(obj))
+
+        is_markup = isinstance(obj, HTMLBasePane) and not isinstance(obj, FileBase)
+        if obj.css_classes or not is_markup:
+            return
+        if len(str(obj.object)) > 0:  # only show a background if there is content
+            obj.css_classes = [*(css for css in obj.css_classes if css != "message"), "message"]
+        obj.sizing_mode = None
+
     def _set_params(self, obj, **params):
         """
         Set the sizing mode and height of the object.
@@ -486,9 +500,7 @@ def _set_params(self, obj, **params):
         self._include_stylesheets_inplace(obj)
         is_markup = isinstance(obj, HTMLBasePane) and not isinstance(obj, FileBase)
         if is_markup:
-            if len(str(obj.object)) > 0:  # only show a background if there is content
-                params['css_classes'] = [*(css for css in obj.css_classes if css != "message"), "message"]
-            params['sizing_mode'] = None
+            self._include_message_css_class_inplace(obj)
         else:
             if obj.sizing_mode is None and not obj.width:
                 params['sizing_mode'] = "stretch_width"
@@ -508,6 +520,7 @@ def _create_panel(self, value, old=None):
         if isinstance(value, Viewable):
             self._internal = False
             self._include_stylesheets_inplace(value)
+            self._include_message_css_class_inplace(value)
             return value
 
         renderer = None
diff --git a/panel/tests/chat/test_message.py b/panel/tests/chat/test_message.py
index bb33240f8dc..cb99f46c142 100644
--- a/panel/tests/chat/test_message.py
+++ b/panel/tests/chat/test_message.py
@@ -214,6 +214,27 @@ def test_include_stylesheets_inplace_on_layouts(self):
         assert message.object.stylesheets == ChatMessage._stylesheets + ["chat.css", "row.css"]
         assert message.object.objects[0].stylesheets == ChatMessage._stylesheets + ["chat.css", "row2.css"]
 
+    def test_include_message_css_class_inplace(self):
+        # markdown
+        message = ChatMessage(object=Markdown("hello"))
+        assert message.object.css_classes == ["message"]
+
+        # custom css class; no message appended
+        message = ChatMessage(object=Markdown("hello", css_classes=["custom"]))
+        assert message.object.css_classes == ["custom"]
+
+        # nested in layout; message appended
+        message = ChatMessage(object=Row(Markdown("hello")))
+        assert message.object.objects[0].css_classes == ["message"]
+
+        # nested in layout as a string; message appended
+        message = ChatMessage(object=Row("hello"))
+        assert message.object.objects[0].css_classes == ["message"]
+
+        # nested in layout with custom css; no message appended
+        message = ChatMessage(object=Row(Markdown("hello", css_classes=["custom"])))
+        assert message.object.objects[0].css_classes == ["custom"]
+
     @mpl_available
     def test_can_display_any_python_object_that_panel_can_display(self):
         # For example matplotlib figures