diff --git a/spec/default.mspec b/spec/default.mspec
index 4f2c062386..082d432c7f 100644
--- a/spec/default.mspec
+++ b/spec/default.mspec
@@ -24,3 +24,28 @@ class MSpecScript
     --
   ]
 end
+
+class MSpecScript
+  if /(?:\A|\s)--jobserver-(?:auth|fds)=(\d+),(\d+)/ =~ ENV["MAKEFLAGS"]
+    begin
+      r = IO.for_fd($1.to_i(10), "rb", autoclose: false)
+      w = IO.for_fd($2.to_i(10), "wb", autoclose: false)
+    rescue
+      r.close if r
+    else
+      jobtokens = r.read_nonblock(1024)
+      cores = jobtokens.size
+      if cores > 0
+        jobserver = w
+        at_exit {
+          jobserver.print(jobtokens)
+          jobserver.close
+        }
+      end
+      remove_method :cores
+      define_method(:cores) do
+        cores
+      end
+    end
+  end
+end