diff --git a/pydantic_ai_slim/pydantic_ai/models/openai.py b/pydantic_ai_slim/pydantic_ai/models/openai.py
index 51b61c5d7b..f8932485fa 100644
--- a/pydantic_ai_slim/pydantic_ai/models/openai.py
+++ b/pydantic_ai_slim/pydantic_ai/models/openai.py
@@ -630,7 +630,8 @@ def _map_tool_definition(f: ToolDefinition) -> responses.FunctionToolParam:
             'parameters': f.parameters_json_schema,
             'type': 'function',
             'description': f.description,
-            'strict': True,
+            # TODO(Marcelo): We should make this configurable, and if True, set `additionalProperties` to False.
+            'strict': False,
         }
 
     async def _map_message(self, messages: list[ModelMessage]) -> tuple[str, list[responses.ResponseInputItemParam]]: