--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -240,16 +240,17 @@ class TupOnly(CommonBackend, PartialBack
             cmd.extend([
                 obj.script,
                 obj.method,
                 obj.outputs[0],
                 '%s.pp' % obj.outputs[0], # deps file required
             ])
             full_inputs = [f.full_path for f in obj.inputs]
             cmd.extend(full_inputs)
+            cmd.extend(shell_quote(f) for f in obj.flags)
 
             outputs = []
             outputs.extend(obj.outputs)
             outputs.append('%s.pp' % obj.outputs[0])
 
             backend_file.rule(
                 display='python {script}:{method} -> [%o]'.format(script=obj.script, method=obj.method),
                 cmd=cmd,