diff --git a/lib/view_model/active_record/association_manipulation.rb b/lib/view_model/active_record/association_manipulation.rb
index 24e8855c..0b480904 100644
--- a/lib/view_model/active_record/association_manipulation.rb
+++ b/lib/view_model/active_record/association_manipulation.rb
@@ -52,10 +52,9 @@ def load_associated(association_name, scope: nil, eager_include: true, serialize
   def replace_associated(association_name, update_hash, references: {}, deserialize_context: self.class.new_deserialize_context)
     association_data = self.class._association_data(association_name)
 
-    # TODO: do we need this to be a hard constraint? Is it useful?
-    # unless association_data.external?
-    #   raise Viewmodel::DeserializationError::InternalAssociationWrite.new(association_name)
-    # end
+    unless association_data.external?
+      raise ViewModel::DeserializationError::InternalAssociationWrite.new(association_name, self.to_reference)
+    end
 
     if association_data.referenced?
       is_fupdate =
@@ -110,6 +109,10 @@ def append_associated(association_name, subtree_hash_or_hashes, references: {},
     direct_reflection = association_data.direct_reflection
     raise ArgumentError.new("Cannot append to single association '#{association_name}'") unless association_data.collection?
 
+    unless association_data.external?
+      raise ViewModel::DeserializationError::InternalAssociationWrite.new(association_name, self.to_reference)
+    end
+
     ViewModel::Utils.wrap_one_or_many(subtree_hash_or_hashes) do |subtree_hashes|
       model_class.transaction do
         ViewModel::Callbacks.wrap_deserialize(self, deserialize_context: deserialize_context) do |hook_control|
@@ -223,6 +226,10 @@ def delete_associated(association_name, associated_id, type: nil, deserialize_co
     association_data = self.class._association_data(association_name)
     direct_reflection = association_data.direct_reflection
 
+    unless association_data.external?
+      raise ViewModel::DeserializationError::InternalAssociationWrite.new(association_name, self.to_reference)
+    end
+
     unless association_data.collection?
       raise ArgumentError.new("Cannot remove element from single association '#{association_name}'")
     end
diff --git a/lib/view_model/active_record/update_data.rb b/lib/view_model/active_record/update_data.rb
index 4b90ee52..5891d923 100644
--- a/lib/view_model/active_record/update_data.rb
+++ b/lib/view_model/active_record/update_data.rb
@@ -680,11 +680,11 @@ def parse(hash_data, valid_reference_keys)
         when AssociationData
           association_data = member_data
 
-          # TODO: is this a desirable constraint? If so, this implementation
-          # won't work, because replace_associated uses this update machinery
-          # if association_data.external?
-          #   raise ViewModel::DeserializationError::ExternalAssociationWrite.new(name)
-          # end
+          # FIXME: replace_associated uses this update machinery, this won't work
+          # until we mark it.
+          if association_data.external?
+            raise ViewModel::DeserializationError::ExternalAssociationWrite.new(name)
+          end
 
           case
           when value.nil?
diff --git a/lib/view_model/deserialization_error.rb b/lib/view_model/deserialization_error.rb
index f5720639..5360f2bf 100644
--- a/lib/view_model/deserialization_error.rb
+++ b/lib/view_model/deserialization_error.rb
@@ -161,9 +161,6 @@ def meta
       end
     end
 
-    # FIXME: these two errors aren't used, because we haven't decided if it's a
-    # useful constraint. Use or remove before merging.
-
     # Attempted to write to an external association during a deserialization
     class ExternalAssociationWrite < InvalidRequest
       attr_reader :association
diff --git a/test/unit/view_model/active_record/has_many_test.rb b/test/unit/view_model/active_record/has_many_test.rb
index cbe0fce3..bd9c33db 100644
--- a/test/unit/view_model/active_record/has_many_test.rb
+++ b/test/unit/view_model/active_record/has_many_test.rb
@@ -1087,6 +1087,10 @@ def new_model
     end
 
     describe 'with association manipulation' do
+      def subject_association_features
+        super.merge(external: true)
+      end
+
       it 'appends a child' do
         view = create_viewmodel!