diff --git a/core/session_manager.py b/core/session_manager.py
index 699a59b..e9a2740 100644
--- a/core/session_manager.py
+++ b/core/session_manager.py
@@ -20,6 +20,15 @@ from .models import Session, ChatMessage
 logger = logging.getLogger(__name__)
 
 
+def _message_timestamp_iso(value: Optional[datetime]) -> Optional[str]:
+    """Return a stable ISO timestamp for chat message metadata."""
+    if not value:
+        return None
+    if value.tzinfo is None:
+        value = value.replace(tzinfo=timezone.utc)
+    return value.isoformat().replace("+00:00", "Z")
+
+
 class SessionManager:
     """
     Manages chat sessions with database persistence.
@@ -107,6 +116,7 @@ class SessionManager:
                 meta = json.loads(db_msg.meta_data) if db_msg.meta_data else {}
                 if meta is None: meta = {}
                 meta['_db_id'] = db_msg.id
+                meta.setdefault('timestamp', _message_timestamp_iso(db_msg.timestamp))
                 history.append(ChatMessage(
                     role=db_msg.role,
                     content=db_msg.content,
@@ -121,6 +131,7 @@ class SessionManager:
                 meta = json.loads(db_msg.meta_data) if db_msg.meta_data else {}
                 if meta is None: meta = {}
                 meta['_db_id'] = db_msg.id
+                meta.setdefault('timestamp', _message_timestamp_iso(db_msg.timestamp))
                 history.append(ChatMessage(
                     role=db_msg.role,
                     content=db_msg.content,
@@ -177,12 +188,17 @@ class SessionManager:
         db = SessionLocal()
         try:
             msg_id = str(uuid.uuid4())
+            msg_time = datetime.utcnow()
+            if message.metadata is None:
+                message.metadata = {}
+            message.metadata.setdefault('timestamp', _message_timestamp_iso(msg_time))
             db_message = DbChatMessage(
                 id=msg_id,
                 session_id=session_id,
                 role=message.role,
                 content=message.content,
-                meta_data=json.dumps(message.metadata) if message.metadata else None
+                meta_data=json.dumps(message.metadata) if message.metadata else None,
+                timestamp=msg_time,
             )
             db.add(db_message)
 
@@ -199,8 +215,6 @@ class SessionManager:
             db.commit()
 
             # Store DB ID on the in-memory message for edit/delete by ID
-            if message.metadata is None:
-                message.metadata = {}
             message.metadata['_db_id'] = msg_id
 
             logger.debug(f"Persisted message to session {session_id}")