diff --git a/lib/selective/ruby/core/controller.rb b/lib/selective/ruby/core/controller.rb
index 30d3e8e..3514e90 100644
--- a/lib/selective/ruby/core/controller.rb
+++ b/lib/selective/ruby/core/controller.rb
@@ -121,8 +121,10 @@ def transport_url
         end
 
         def build_env
-          result = `#{Pathname.new(__dir__) + BUILD_ENV_SCRIPT_PATH}`
-          JSON.parse(result)
+          @build_env ||= begin
+            result = `#{Pathname.new(__dir__) + BUILD_ENV_SCRIPT_PATH}`
+            JSON.parse(result)
+          end
         end
 
         def spawn_transport_process(url)
@@ -237,8 +239,10 @@ def handle_remove_failed_test_case_result(test_case_id)
         end
 
         def modified_test_files
-          # Todo: This should find files changed in the current branch
-          `git diff --name-only`.split("\n").filter do |f|
+          target_branch = build_env["target_branch"]
+          return [] if target_branch.nil? || target_branch.empty?
+          
+          `git diff #{target_branch} --name-only`.split("\n").filter do |f|
             f.match?(/^#{runner.base_test_path}/)
           end
         end