diff --git a/temporalio/contrib/openai_agents/_model_parameters.py b/temporalio/contrib/openai_agents/_model_parameters.py
index c156c9be5..8c5c7cb3d 100644
--- a/temporalio/contrib/openai_agents/_model_parameters.py
+++ b/temporalio/contrib/openai_agents/_model_parameters.py
@@ -20,13 +20,13 @@ class ModelActivityParameters:
     task_queue: Optional[str] = None
     """Specific task queue to use for model activities."""
 
-    schedule_to_close_timeout: Optional[timedelta] = timedelta(seconds=60)
+    schedule_to_close_timeout: Optional[timedelta] = None
     """Maximum time from scheduling to completion."""
 
     schedule_to_start_timeout: Optional[timedelta] = None
     """Maximum time from scheduling to starting."""
 
-    start_to_close_timeout: Optional[timedelta] = None
+    start_to_close_timeout: Optional[timedelta] = timedelta(seconds=60)
     """Maximum time for the activity to complete."""
 
     heartbeat_timeout: Optional[timedelta] = None