diff --git a/lib/lutaml/xml/transformation/element_builder.rb b/lib/lutaml/xml/transformation/element_builder.rb
index 8373ef20..064b7929 100644
--- a/lib/lutaml/xml/transformation/element_builder.rb
+++ b/lib/lutaml/xml/transformation/element_builder.rb
@@ -140,13 +140,21 @@ def create_nested_model_element(rule, value, options, register,
           actual_class = resolve_polymorphic_class(rule, value, is_polymorphic,
                                                    is_polymorphic_subtype)
 
+          # Dispatch on value's actual class when it is a Serializable that
+          # differs from the declared attribute_type. This covers ad-hoc
+          # polymorphism (caller assigns an unrelated Serializable to a typed
+          # slot) and prevents the wrong transformation being applied
+          # silently. Subsumes is_polymorphic_subtype; preserved here for
+          # the resolve_polymorphic_class call above.
+          dispatch_on_value_class = union || is_polymorphic ||
+            (value.is_a?(Lutaml::Model::Serialize) &&
+             value.class != rule.attribute_type)
+
           # Get transformation for the actual class. Unions resolve the member
           # from the value's own class, like polymorphism.
-          child_transformation = if union
-                                   value.class.transformation_for(:xml, register)
-                                 elsif is_polymorphic || is_polymorphic_subtype
-                                   actual_class.transformation_for(:xml,
-                                                                   register)
+          child_transformation = if dispatch_on_value_class
+                                   value.class.transformation_for(:xml,
+                                                                  register)
                                  else
                                    rule.child_transformation || actual_class.transformation_for(
                                      :xml, register
diff --git a/spec/lutaml/xml/cross_class_dispatch_spec.rb b/spec/lutaml/xml/cross_class_dispatch_spec.rb
new file mode 100644
index 00000000..2336bd2e
--- /dev/null
+++ b/spec/lutaml/xml/cross_class_dispatch_spec.rb
@@ -0,0 +1,79 @@
+# frozen_string_string: true
+
+require "spec_helper"
+
+# When a caller assigns an unrelated Lutaml::Model::Serializable (not a
+# declared subtype) to a typed slot, serialisation must use the value's own
+# class mapping rules. The prior behaviour reused the declared
+# attribute_type's child_transformation regardless of value.class, which
+# silently applied the wrong rules and raised NoMethodError on whichever
+# Ruby attribute name differed.
+#
+# See BUGREPORT.element-builder-dispatch-on-value-class.md for the original
+# analysis and sts-ruby PR #47 for the downstream trigger.
+RSpec.describe "ElementBuilder cross-class dispatch" do
+  let(:declared_type) do
+    Class.new(Lutaml::Model::Serializable) do
+      attribute :name, :string
+
+      xml do
+        root "declared"
+        map_element "name", to: :name
+      end
+    end
+  end
+
+  let(:actual_type) do
+    Class.new(Lutaml::Model::Serializable) do
+      attribute :different_name, :string
+
+      xml do
+        root "actual"
+        map_element "different-name", to: :different_name
+      end
+    end
+  end
+
+  let(:holder_class) do
+    declared = declared_type
+    Class.new(Lutaml::Model::Serializable) do
+      attribute :child, declared
+
+      xml do
+        root "holder"
+        map_element "child", to: :child
+      end
+    end
+  end
+
+  it "serialises an unrelated Serializable using its own mapping" do
+    holder = holder_class.new
+    holder.child = actual_type.new(different_name: "x")
+    xml = holder.to_xml
+
+    parsed = Nokogiri::XML(xml)
+    expect(parsed.at_css("different-name").content).to eq("x")
+  end
+
+  it "preserves the declared-type fast path for matching classes" do
+    holder = holder_class.new
+    holder.child = declared_type.new(name: "y")
+    xml = holder.to_xml
+
+    parsed = Nokogiri::XML(xml)
+    expect(parsed.at_css("name").content).to eq("y")
+  end
+
+  it "round-trips the XML output (parsing yields declared type by design)" do
+    # Deserialisation cannot infer the runtime-assigned class without a
+    # discriminator; it instantiates the declared type. The XML round-trip
+    # itself must still be lossless.
+    holder = holder_class.new
+    holder.child = actual_type.new(different_name: "round-trip")
+    xml = holder.to_xml
+
+    reparsed = holder_class.from_xml(xml)
+    expect(reparsed.child).to be_a(declared_type)
+    expect(xml).to include("<different-name>round-trip</different-name>")
+  end
+end