diff --git a/sdk/evaluation/azure-ai-evaluation/azure/ai/evaluation/_evaluators/_tool_output_utilization/_tool_output_utilization.py b/sdk/evaluation/azure-ai-evaluation/azure/ai/evaluation/_evaluators/_tool_output_utilization/_tool_output_utilization.py
index c85107df457a..cf9f11512d30 100644
--- a/sdk/evaluation/azure-ai-evaluation/azure/ai/evaluation/_evaluators/_tool_output_utilization/_tool_output_utilization.py
+++ b/sdk/evaluation/azure-ai-evaluation/azure/ai/evaluation/_evaluators/_tool_output_utilization/_tool_output_utilization.py
@@ -86,8 +86,7 @@ def __init__(
 
         # Initialize input validator
         self._validator = ToolDefinitionsValidator(
-            error_target=ErrorTarget.TOOL_OUTPUT_UTILIZATION_EVALUATOR,
-            check_for_unsupported_tools=True,
+            error_target=ErrorTarget.TOOL_OUTPUT_UTILIZATION_EVALUATOR, optional_tool_definitions=False
         )
 
         super().__init__(
@@ -143,7 +142,7 @@ def __call__(
         :paramtype query: Union[str, List[dict]]
         :keyword response: The response being evaluated, either a string or a list of messages (full agent response potentially including tool calls)
         :paramtype response: Union[str, List[dict]]
-        :keyword tool_definitions: An optional list of messages containing the tool definitions the agent is aware of.
+        :keyword tool_definitions: A list of messages containing the tool definitions the agent is aware of.
         :paramtype tool_definitions: Union[dict, List[dict]]
         :return: A dictionary with the tool output utilization evaluation results.
         :rtype: Dict[str, Union[str, float]]