diff --git a/pyproject.toml b/pyproject.toml
index 2e2c32ff73..01eece8bac 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -39,6 +39,7 @@ dependencies = [
   "rtree>=1.2",
   "scipy",
   "sympy",
+  "islpy<2026.2",
   "islpy>=2025.1.5; sys_platform == 'darwin'",
 ]
 classifiers = [