diff --git a/app/decorators/models/solidus_friendly_promotions/line_item_decorator.rb b/app/decorators/models/solidus_friendly_promotions/line_item_decorator.rb
index ddaa7292..832981d7 100644
--- a/app/decorators/models/solidus_friendly_promotions/line_item_decorator.rb
+++ b/app/decorators/models/solidus_friendly_promotions/line_item_decorator.rb
@@ -4,7 +4,17 @@ module SolidusFriendlyPromotions
   module LineItemDecorator
     def self.prepended(base)
       base.belongs_to :managed_by_order_action, class_name: "SolidusFriendlyPromotions::PromotionAction", optional: true
+      base.validate :validate_managed_quantity_same, on: :update
     end
+
+    private
+
+    def validate_managed_quantity_same
+      if managed_by_order_action && quantity_changed?
+        errors.add(:quantity, :cannot_be_changed_for_automated_items)
+      end
+    end
+
     Spree::LineItem.prepend self
     Spree::LineItem.prepend SolidusFriendlyPromotions::DiscountableAmount
   end
diff --git a/app/decorators/models/solidus_friendly_promotions/order_decorator.rb b/app/decorators/models/solidus_friendly_promotions/order_decorator.rb
index 87e69f4f..53128bae 100644
--- a/app/decorators/models/solidus_friendly_promotions/order_decorator.rb
+++ b/app/decorators/models/solidus_friendly_promotions/order_decorator.rb
@@ -29,6 +29,13 @@ def reset_current_discounts
       shipments.each(&:reset_current_discounts)
     end
 
+    # This helper method excludes line items that are managed by an order action for the benefit
+    # of calculators and actions that discount normal line items. Line items that are managed by an
+    # order actions handle their discounts themselves.
+    def discountable_line_items
+      line_items.reject(&:managed_by_order_action)
+    end
+
     def apply_shipping_promotions
       if Spree::Config.promotion_adjuster_class <= SolidusFriendlyPromotions::FriendlyPromotionAdjuster
         recalculate
diff --git a/app/models/solidus_friendly_promotions/actions/adjust_line_item_quantity_groups.rb b/app/models/solidus_friendly_promotions/actions/adjust_line_item_quantity_groups.rb
index 7dc86e5e..777bfd5e 100644
--- a/app/models/solidus_friendly_promotions/actions/adjust_line_item_quantity_groups.rb
+++ b/app/models/solidus_friendly_promotions/actions/adjust_line_item_quantity_groups.rb
@@ -79,7 +79,7 @@ def compute_amount(line_item)
       private
 
       def actionable_line_items(order)
-        order.line_items.select do |item|
+        order.discountable_line_items.select do |item|
           promotion.rules.select do |rule|
             rule.applicable?(item)
           end.all? { |rule| rule.eligible?(item) }
diff --git a/app/models/solidus_friendly_promotions/calculators/distributed_amount.rb b/app/models/solidus_friendly_promotions/calculators/distributed_amount.rb
index 5bb5f80a..15451ee2 100644
--- a/app/models/solidus_friendly_promotions/calculators/distributed_amount.rb
+++ b/app/models/solidus_friendly_promotions/calculators/distributed_amount.rb
@@ -28,7 +28,7 @@ def compute_line_item(line_item)
       private
 
       def eligible_line_items(order)
-        order.line_items.select do |line_item|
+        order.discountable_line_items.select do |line_item|
           calculable.promotion.eligible_by_applicable_rules?(line_item)
         end
       end
diff --git a/app/models/solidus_friendly_promotions/friendly_promotion_adjuster/discount_order.rb b/app/models/solidus_friendly_promotions/friendly_promotion_adjuster/discount_order.rb
index e8cdbed5..d96daa18 100644
--- a/app/models/solidus_friendly_promotions/friendly_promotion_adjuster/discount_order.rb
+++ b/app/models/solidus_friendly_promotions/friendly_promotion_adjuster/discount_order.rb
@@ -48,8 +48,8 @@ def perform_order_actions(lane_promotions, lane)
       end
 
       def adjust_line_items(promotions)
-        order.line_items.select do |line_item|
-          line_item.variant.product.promotionable? && !line_item.managed_by_order_action
+        order.discountable_line_items.select do |line_item|
+          line_item.variant.product.promotionable?
         end.map do |line_item|
           discounts = generate_discounts(promotions, line_item)
           chosen_item_discounts = SolidusFriendlyPromotions.config.discount_chooser_class.new(discounts).call
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 8235ab1b..b6312a99 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -257,3 +257,7 @@ en:
           attributes:
             base:
               disallowed_with_apply_automatically: Could not create promotion code on promotion that apply automatically
+        spree/line_item:
+          attributes:
+            quantity:
+              cannot_be_changed_for_automated_items: cannot be changed on a line item managed by a promotion action
diff --git a/spec/models/spree/line_item_spec.rb b/spec/models/spree/line_item_spec.rb
index d2d50e04..8b7d2a06 100644
--- a/spec/models/spree/line_item_spec.rb
+++ b/spec/models/spree/line_item_spec.rb
@@ -36,4 +36,25 @@
       expect { subject }.to change { line_item.current_discounts.length }.from(1).to(0)
     end
   end
+
+  describe "changing quantities" do
+    context "when line item is managed by an automation" do
+      let(:order) { create(:order) }
+      let(:variant) { create(:variant) }
+      let(:promotion) { create(:friendly_promotion, apply_automatically: true) }
+      let(:promotion_action) { SolidusFriendlyPromotions::Actions::CreateDiscountedItem.create!(calculator: hundred_percent, preferred_variant_id: variant.id, promotion: promotion) }
+      let(:hundred_percent) { SolidusFriendlyPromotions::Calculators::Percent.new(preferred_percent: 100) }
+
+      before do
+        order.line_items.create!(variant: variant, managed_by_order_action: promotion_action, quantity: 1)
+      end
+
+      it "makes the line item invalid" do
+        line_item = order.line_items.first
+        line_item.quantity = 2
+        expect { line_item.save! }.to raise_exception(ActiveRecord::RecordInvalid)
+        expect(line_item.errors.full_messages.first).to eq("Quantity cannot be changed on a line item managed by a promotion action")
+      end
+    end
+  end
 end