Add coercions for backwards compatibility

app/graphql/mutations/merge_requests/set_labels.rb

@@ -23,6 +23,9 @@ module Mutations
         merge_request = authorized_find!(project_path: project_path, iid: iid)
         project = merge_request.project
 
+        # TODO: remove this line when the compatibility layer is removed:
+        # See: https://gitlab.com/gitlab-org/gitlab/-/issues/257883
+        label_ids = label_ids.map { |id| ::Types::GlobalIDType[::Label].coerce_isolated_input(id) }
         # MergeRequests::UpdateService expects integers
         label_ids = label_ids.compact.map(&:model_id)
 

app/graphql/resolvers/concerns/resolves_project.rb

 # frozen_string_literal: true
 
 module ResolvesProject
+  # Accepts EITHER one of
+  #  - full_path: String (see Project#full_path)
+  #  - project_id: GlobalID. Arguments should be typed as: `::Types::GlobalIDType[Project]`
   def resolve_project(full_path: nil, project_id: nil)
     unless full_path.present? ^ project_id.present?
       raise ::Gitlab::Graphql::Errors::ArgumentError, 'Incompatible arguments: projectId, projectPath.'

app/graphql/resolvers/user_merge_requests_resolver.rb

@@ -8,7 +8,7 @@ module Resolvers
           required: false,
           description: 'The full-path of the project the authored merge requests should be in. Incompatible with projectId.'
 
-    argument :project_id, GraphQL::ID_TYPE,
+    argument :project_id, ::Types::GlobalIDType[::Project],
           required: false,
           description: 'The global ID of the project the authored merge requests should be in. Incompatible with projectPath.'
 
@@ -50,8 +50,10 @@ module Resolvers
     end
 
     def load_project(project_path, project_id)
-      @project = resolve_project(full_path: project_path, project_id: project_id)
-      @project = @project.sync if @project.respond_to?(:sync)
+      # TODO: remove this line when the compatibility layer is removed
+      # See: https://gitlab.com/gitlab-org/gitlab/-/issues/257883
+      project_id &&= ::Types::GlobalIDType[::Project].coerce_isolated_input(project_id)
+      @project = ::Gitlab::Graphql::Lazy.force(resolve_project(full_path: project_path, project_id: project_id))
     end
 
     def no_results_possible?(args)