diff --git a/tool/lib/core_assertions.rb b/tool/lib/core_assertions.rb
index ede490576c..1900b7088d 100644
--- a/tool/lib/core_assertions.rb
+++ b/tool/lib/core_assertions.rb
@@ -371,6 +371,10 @@ eom
       def assert_ractor(src, args: [], require: nil, require_relative: nil, file: nil, line: nil, ignore_stderr: nil, **opt)
         return unless defined?(Ractor)
 
+        # https://bugs.ruby-lang.org/issues/21262
+        shim_value = "class Ractor; alias value take; end" unless Ractor.method_defined?(:value)
+        shim_join = "class Ractor; alias join take; end" unless Ractor.method_defined?(:join)
+
         require = "require #{require.inspect}" if require
         if require_relative
           dir = File.dirname(caller_locations[0,1][0].absolute_path)
@@ -379,6 +383,8 @@ eom
         end
 
         assert_separately(args, file, line, <<~RUBY, ignore_stderr: ignore_stderr, **opt)
+          #{shim_value}
+          #{shim_join}
           #{require}
           previous_verbose = $VERBOSE
           $VERBOSE = nil