diff --git a/tox.ini b/tox.ini
index f64afc38..20c42d06 100644
--- a/tox.ini
+++ b/tox.ini
@@ -72,6 +72,7 @@ commands =
 [testenv:unit]
 description = Run unit tests
 deps =
+    cosl
     coverage[toml]
     pytest
     -r{toxinidir}/requirements.txt