diff --git a/pyproject.toml b/pyproject.toml
index 7039f51..cd847f6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -78,7 +78,11 @@ bounce = [
     "pytest>=7.3.1,<7.4.0",
     "bounce @ git+https://github.com/miguelgondu/bounce.git@main"
 ]
-lambo2 = ["pytorch-cortex", "beignet"]
+lambo2 = [
+    "pytorch-cortex",
+    "beignet",
+    "pydantic<2.0.0",
+]
 
 [project.urls]
 Homepage = "https://github.com/MachineLearningLifeScience/poli-baselines"