diff --git a/src/mistralai/models/batchjobin.py b/src/mistralai/models/batchjobin.py
index aa0bb5b..61e4cdb 100644
--- a/src/mistralai/models/batchjobin.py
+++ b/src/mistralai/models/batchjobin.py
@@ -34,9 +34,9 @@ class BatchJobIn(BaseModel):
 
     @model_serializer(mode="wrap")
     def serialize_model(self, handler):
-        optional_fields = ["model", "agent_id", "metadata", "timeout_hours"]
-        nullable_fields = ["model", "agent_id", "metadata"]
-        null_default_fields = []
+        optional_fields = {"model", "agent_id", "metadata", "timeout_hours"}
+        nullable_fields = {"model", "agent_id", "metadata"}
+        null_default_fields = set()
 
         serialized = handler(self)
 
@@ -48,15 +48,12 @@ def serialize_model(self, handler):
             serialized.pop(k, None)
 
             optional_nullable = k in optional_fields and k in nullable_fields
-            is_set = (
-                self.__pydantic_fields_set__.intersection({n})
-                or k in null_default_fields
-            )  # pylint: disable=no-member
+            is_set = n in self.__pydantic_fields_set__ or k in null_default_fields  # pylint: disable=no-member
 
             if val is not None and val != UNSET_SENTINEL:
                 m[k] = val
             elif val != UNSET_SENTINEL and (
-                not k in optional_fields or (optional_nullable and is_set)
+                k not in optional_fields or (optional_nullable and is_set)
             ):
                 m[k] = val