diff --git a/src/mistralai/basesdk.py b/src/mistralai/basesdk.py
index 6b62dda..9f037be 100644
--- a/src/mistralai/basesdk.py
+++ b/src/mistralai/basesdk.py
@@ -30,15 +30,25 @@ def __init__(
         self.parent_ref = parent_ref
 
     def _get_url(self, base_url, url_variables):
-        sdk_url, sdk_variables = self.sdk_configuration.get_server_details()
-
-        if base_url is None:
-            base_url = sdk_url
-
-        if url_variables is None:
-            url_variables = sdk_variables
-
-        return utils.template_url(base_url, url_variables)
+        sdk_configuration = (
+            self.sdk_configuration
+        )  # local var for attribute lookup speed
+        sdk_url, sdk_variables = sdk_configuration.get_server_details()
+
+        base = base_url if base_url is not None else sdk_url
+        vars_ = url_variables if url_variables is not None else sdk_variables
+
+        # FAST template_url optimization: use str.format for variable substitution in one pass
+        # This is a safe rewrite assuming only keys in vars_ are expected in base.
+        # Fall back to the original if values are not string (shouldn't happen for URLs)
+        if vars_:
+            try:
+                # Replace {key} with value for each key via format mapping
+                base = base.format(**{k: str(v) for k, v in vars_.items()})
+            except KeyError:
+                # If the url template has keys not supplied, fallback to original method.
+                return utils.template_url(base, vars_)
+        return base
 
     def _build_request_async(
         self,
@@ -225,10 +235,10 @@ def do_request(
         stream=False,
         retry_config: Optional[Tuple[RetryConfig, List[str]]] = None,
     ) -> httpx.Response:
-        client = self.sdk_configuration.client
-        logger = self.sdk_configuration.debug_logger
-
-        hooks = self.sdk_configuration.__dict__["_hooks"]
+        sdk_configuration = self.sdk_configuration
+        client = sdk_configuration.client
+        logger = sdk_configuration.debug_logger
+        hooks = sdk_configuration.__dict__.get("_hooks")
 
         def do():
             http_res = None
diff --git a/src/mistralai/utils/serializers.py b/src/mistralai/utils/serializers.py
index 378a14c..2195a80 100644
--- a/src/mistralai/utils/serializers.py
+++ b/src/mistralai/utils/serializers.py
@@ -178,7 +178,7 @@ 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):
diff --git a/src/mistralai/utils/values.py b/src/mistralai/utils/values.py
index dae01a4..ac217f4 100644
--- a/src/mistralai/utils/values.py
+++ b/src/mistralai/utils/values.py
@@ -52,9 +52,11 @@ def match_status_codes(status_codes: List[str], status_code: int) -> bool:
 
 T = TypeVar("T")
 
+
 def cast_partial(typ):
     return partial(cast, typ)
 
+
 def get_global_from_env(
     value: Optional[T], env_key: str, type_cast: Callable[[str], T]
 ) -> Optional[T]: