diff --git a/langchain/memory/chat_message_histories/cosmos_db.py b/langchain/memory/chat_message_histories/cosmos_db.py
index 6da81570..8030907c 100644
--- a/langchain/memory/chat_message_histories/cosmos_db.py
+++ b/langchain/memory/chat_message_histories/cosmos_db.py
@@ -17,7 +17,7 @@ from langchain.schema import (
 logger = logging.getLogger(__name__)
 
 if TYPE_CHECKING:
-    from azure.cosmos import ContainerProxy, CosmosClient
+    from azure.cosmos import ContainerProxy
 
 
 class CosmosDBChatMessageHistory(BaseChatMessageHistory):
@@ -60,19 +60,10 @@ class CosmosDBChatMessageHistory(BaseChatMessageHistory):
         self.user_id = user_id
         self.ttl = ttl
 
-        self._client: Optional[CosmosClient] = None
-        self._container: Optional[ContainerProxy] = None
         self.messages: List[BaseMessage] = []
-
-    def prepare_cosmos(self) -> None:
-        """Prepare the CosmosDB client.
-
-        Use this function or the context manager to make sure your database is ready.
-        """
         try:
             from azure.cosmos import (  # pylint: disable=import-outside-toplevel # noqa: E501
                 CosmosClient,
-                PartitionKey,
             )
         except ImportError as exc:
             raise ImportError(
@@ -88,6 +79,21 @@ class CosmosDBChatMessageHistory(BaseChatMessageHistory):
             )
         else:
             raise ValueError("Either a connection string or a credential must be set.")
+        self._container: Optional[ContainerProxy] = None
+
+    def prepare_cosmos(self) -> None:
+        """Prepare the CosmosDB client.
+
+        Use this function or the context manager to make sure your database is ready.
+        """
+        try:
+            from azure.cosmos import (  # pylint: disable=import-outside-toplevel # noqa: E501
+                PartitionKey,
+            )
+        except ImportError as exc:
+            raise ImportError(
+                "You must install the azure-cosmos package to use the CosmosDBChatMessageHistory."  # noqa: E501
+            ) from exc
         database = self._client.create_database_if_not_exists(self.cosmos_database)
         self._container = database.create_container_if_not_exists(
             self.cosmos_container,
@@ -98,11 +104,9 @@ class CosmosDBChatMessageHistory(BaseChatMessageHistory):
 
     def __enter__(self) -> "CosmosDBChatMessageHistory":
         """Context manager entry point."""
-        if self._client:
-            self._client.__enter__()
-            self.prepare_cosmos()
-            return self
-        raise ValueError("Client not initialized")
+        self._client.__enter__()
+        self.prepare_cosmos()
+        return self
 
     def __exit__(
         self,
@@ -112,8 +116,7 @@ class CosmosDBChatMessageHistory(BaseChatMessageHistory):
     ) -> None:
         """Context manager exit"""
         self.upsert_messages()
-        if self._client:
-            self._client.__exit__(exc_type, exc_val, traceback)
+        self._client.__exit__(exc_type, exc_val, traceback)
 
     def load_messages(self) -> None:
         """Retrieve the messages from Cosmos"""
@@ -134,11 +137,7 @@ class CosmosDBChatMessageHistory(BaseChatMessageHistory):
         except CosmosHttpResponseError:
             logger.info("no session found")
             return
-        if (
-            "messages" in item
-            and len(item["messages"]) > 0
-            and isinstance(item["messages"][0], list)
-        ):
+        if "messages" in item and len(item["messages"]) > 0:
             self.messages = messages_from_dict(item["messages"])
 
     def add_user_message(self, message: str) -> None: