diff --git a/python/copilot/session.py b/python/copilot/session.py
index b4ae210df..f15195c55 100644
--- a/python/copilot/session.py
+++ b/python/copilot/session.py
@@ -744,7 +744,7 @@ async def log(
         message: str,
         *,
         level: str | None = None,
-        ephemeral: bool | None = None,
+        ephemeral: bool = False,
     ) -> None:
         """
         Log a message to the session timeline.
@@ -756,6 +756,7 @@ async def log(
             message: The human-readable message to log.
             level: Log severity level ("info", "warning", "error"). Defaults to "info".
             ephemeral: When True, the message is transient and not persisted to disk.
+                Defaults to False.
 
         Raises:
             Exception: If the session has been destroyed or the connection fails.
@@ -769,6 +770,6 @@ async def log(
         params = SessionLogParams(
             message=message,
             level=Level(level) if level is not None else None,
-            ephemeral=ephemeral,
+            ephemeral=ephemeral or None,
         )
         await self.rpc.log(params)