Add `api_versions` to `/api/v2/instance` (#31354)

app/serializers/rest/instance_serializer.rb

@@ -11,7 +11,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
 
   attributes :domain, :title, :version, :source_url, :description,
              :usage, :thumbnail, :languages, :configuration,
-             :registrations
+             :registrations, :api_versions
 
   has_one :contact, serializer: ContactSerializer
   has_many :rules, serializer: REST::RuleSerializer
@@ -94,6 +94,12 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     }
   end
 
+  def api_versions
+    {
+      mastodon: 1,
+    }
+  end
+
   private
 
   def registrations_enabled?

spec/requests/api/v2/instance_spec.rb

@@ -18,6 +18,7 @@ describe 'Instances' do
         expect(body_as_json)
           .to be_present
           .and include(title: 'Mastodon')
+          .and include_api_versions
           .and include_configuration_limits
       end
     end
@@ -32,6 +33,7 @@ describe 'Instances' do
         expect(body_as_json)
           .to be_present
           .and include(title: 'Mastodon')
+          .and include_api_versions
           .and include_configuration_limits
       end
     end
@@ -53,5 +55,13 @@ describe 'Instances' do
         )
       )
     end
+
+    def include_api_versions
+      include(
+        api_versions: include(
+          mastodon: anything
+        )
+      )
+    end
   end
 end