diff --git a/app/javascript/controllers/rule/conditions_controller.js b/app/javascript/controllers/rule/conditions_controller.js
index ad23c862c..766c40e05 100644
--- a/app/javascript/controllers/rule/conditions_controller.js
+++ b/app/javascript/controllers/rule/conditions_controller.js
@@ -123,7 +123,8 @@ export default class extends Controller {
   }
 
   #uniqueKey() {
-    return Date.now();
+    this.keySequence = (this.keySequence ?? 0) + 1;
+    return Date.now() * 1000 + this.keySequence;
   }
 
   #toggleValueFieldVisibility() {
diff --git a/app/javascript/controllers/rules_controller.js b/app/javascript/controllers/rules_controller.js
index 5466a0a62..77c40d4b9 100644
--- a/app/javascript/controllers/rules_controller.js
+++ b/app/javascript/controllers/rules_controller.js
@@ -50,12 +50,8 @@ export default class extends Controller {
   }
 
   #uniqueKey() {
-    // Prefixed so it can never collide with the numeric indexes Rails
-    // assigns to already-persisted conditions/actions when rendering an
-    // edit form (0, 1, 2, ...). A plain monotonic counter starting at 1
-    // would otherwise reuse index 1 and clobber an existing nested record.
     this.keySequence = (this.keySequence ?? 0) + 1;
-    return `new_${this.keySequence}`;
+    return Date.now() * 1000 + this.keySequence;
   }
 
   // Updates the prefix visibility of all conditions and condition groups
diff --git a/test/system/rules_test.rb b/test/system/rules_test.rb
index 6de961284..2e54c814b 100644
--- a/test/system/rules_test.rb
+++ b/test/system/rules_test.rb
@@ -56,4 +56,26 @@ class RulesTest < ApplicationSystemTestCase
     assert_selector "h3", text: "Legacy bad rule"
     assert_text "Nicht unterstützt (name)"
   end
+
+  test "creates a transaction rule through the modal with dynamically added condition and action" do
+    visit new_rule_path(resource_type: "transaction")
+
+    within "dialog" do
+      click_on "Add condition"
+      find("[data-rules-target='conditionsList'] input[name$='[value]']").fill_in(with: "Coffee")
+      click_on "Add action"
+      click_on "Create Rule"
+    end
+
+    # A successful create lands on the confirmation dialog; before the
+    # nested-attribute keys were numeric, the submit failed validation with
+    # "must have at least one action" because strong params dropped the rows.
+    assert_text "Confirm changes"
+
+    rule = Rule.order(:created_at).last
+    assert_equal "transaction_name", rule.conditions.first.condition_type
+    assert_equal "Coffee", rule.conditions.first.value
+    assert_equal "set_transaction_category", rule.actions.first.action_type
+    assert rule.actions.first.value.present?
+  end
 end