Translate ValidationErrors#full_messages once instead of on every call

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#2918](https://github.com/ruby-grape/grape/pull/2918): Skip the dry-types round trip when a value already is the declared type - [@ericproulx](https://github.com/ericproulx).
 * [#2917](https://github.com/ruby-grape/grape/pull/2917): Read path captures out of the router's union match instead of re-running the route's pattern - [@ericproulx](https://github.com/ericproulx).
 * [#2921](https://github.com/ruby-grape/grape/pull/2921): Pin the router's request-time isolation regressions through requests instead of its instance variables - [@ericproulx](https://github.com/ericproulx).
+* [#2934](https://github.com/ruby-grape/grape/pull/2934): Translate `ValidationErrors#full_messages` once instead of on every call - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 4.0.0 (2026-09-07)

lib/grape/exceptions/validation_errors.rb

@@ -23,7 +23,19 @@ def to_json(*_opts)
         as_json.to_json
       end
 
+      # Translated once, when the error is built for its #message, and handed
+      # out as a copy from then on. Every lookup here is an I18n call of a few
+      # microseconds, and the README's recipe for answering with the list,
+      # +error!({ messages: e.full_messages }, 400)+, asked for all of them a
+      # second time. It also keeps the list the same as #message, which was
+      # already fixed at that point, if the locale changes in between.
       def full_messages
+        (@full_messages ||= translate_full_messages).dup
+      end
+
+      private
+
+      def translate_full_messages
         messages = errors.flat_map do |attributes, errs|
           errs.map do |error|
             translate(
@@ -39,8 +51,6 @@ def full_messages
         messages
       end
 
-      private
-
       def translate_attributes(keys)
         keys.map do |key|
           translate(key, scope: 'grape.errors.attributes', default: key.to_s)

spec/grape/exceptions/validation_errors_spec.rb

@@ -65,6 +65,15 @@
         expect(subject.first).to eq('admin_field Can not set admin-only field')
       end
     end
+
+    context 'when the caller changes the array it was given' do
+      subject(:error) { described_class.new(exceptions: [Grape::Exceptions::Validation.new(params: ['id'], message: :presence)]) }
+
+      it 'returns the same messages the next time' do
+        error.full_messages << 'name is missing'
+        expect(error.full_messages).to eq(['id is missing'])
+      end
+    end
   end
 
   context 'api' do