diff --git a/tox.ini b/tox.ini
index 0eb5b23..33c4117 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,7 +28,8 @@ commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-commands = oslo_debug_helper {posargs}
+whitelist_externals = oslo_debug_helper
+commands = oslo_debug_helper -t mistral_lib/tests {posargs}
 
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.