diff --git a/dev-requirements.txt b/dev-requirements.txt
index c834bfe..079010a 100644
--- a/dev-requirements.txt
+++ b/dev-requirements.txt
@@ -1,4 +1,4 @@
-black<=25.11.0
+black
 mypy<=1.18.2
 numpydoc<=1.9.0
 pylint<=4.0.3
diff --git a/pyproject.toml b/pyproject.toml
index 51cfce4..35bbed7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -24,6 +24,7 @@ classifiers = [
 dependencies = [
     "alive-progress<=3.3.0",
     "ert<=17.0.0",
+    "iterative_ensemble_smoother<=0.6.0",
     "mako<=1.3.10",
     "matplotlib<=3.10.7",
     "opencv-python<=4.12.0.88",