diff --git a/src/mistralai/utils/serializers.py b/src/mistralai/utils/serializers.py
index 378a14c..ed9dfed 100644
--- a/src/mistralai/utils/serializers.py
+++ b/src/mistralai/utils/serializers.py
@@ -13,6 +13,7 @@
 from pydantic_core import from_json
 
 from ..types.basemodel import BaseModel, Nullable, OptionalNullable, Unset
+from functools import lru_cache
 
 
 def serialize_decimal(as_str: bool):
@@ -141,14 +142,8 @@ def unmarshal_json(raw, typ: Any) -> Any:
 
 
 def unmarshal(val, typ: Any) -> Any:
-    unmarshaller = create_model(
-        "Unmarshaller",
-        body=(typ, ...),
-        __config__=ConfigDict(populate_by_name=True, arbitrary_types_allowed=True),
-    )
-
+    unmarshaller = _get_unmarshaller(typ)
     m = unmarshaller(body=val)
-
     # pyright: ignore[reportAttributeAccessIssue]
     return m.body  # type: ignore
 
@@ -178,7 +173,7 @@ def is_nullable(field):
     if origin is Nullable or origin is OptionalNullable:
         return True
 
-    if not origin is Union or type(None) not in get_args(field):
+    if origin is not Union or type(None) not in get_args(field):
         return False
 
     for arg in get_args(field):
@@ -247,3 +242,12 @@ def _get_typing_objects_by_name_of(name: str) -> Tuple[Any, ...]:
             f"Neither typing nor typing_extensions has an object called {name!r}"
         )
     return result
+
+
+@lru_cache(maxsize=64)
+def _get_unmarshaller(typ: Any):
+    return create_model(
+        "Unmarshaller",
+        body=(typ, ...),
+        __config__=ConfigDict(populate_by_name=True, arbitrary_types_allowed=True),
+    )