diff --git a/service_capacity_modeling/interface.py b/service_capacity_modeling/interface.py
index f4d974c..1dd5e7b 100644
--- a/service_capacity_modeling/interface.py
+++ b/service_capacity_modeling/interface.py
@@ -41,6 +41,7 @@ class Interval(BaseModel):
 
     class Config:
         allow_mutation = False
+        frozen = True
 
     @property
     def can_simulate(self):