diff --git a/gateway-bot/services/doc_service.py b/gateway-bot/services/doc_service.py
index 3ae10754..819de4f2 100644
--- a/gateway-bot/services/doc_service.py
+++ b/gateway-bot/services/doc_service.py
@@ -105,10 +105,9 @@ class DocumentService:
             True if saved successfully
         """
         try:
-            # Extract fallback user_id from session_id if not provided.
-            # Format: "channel:identifier" or "channel:user_id"
-            parts = session_id.split(":", 1)
-            fact_user_id = user_id or (parts[1] if len(parts) > 1 else session_id)
+            # Use stable synthetic user key per session, so context can be
+            # retrieved later using only session_id (without caller user_id).
+            fact_user_id = f"session:{session_id}"
             
             # Save as fact in Memory Service
             fact_key = f"doc_context:{session_id}"
@@ -148,8 +147,7 @@ class DocumentService:
             DocContext or None
         """
         try:
-            parts = session_id.split(":", 1)
-            user_id = parts[1] if len(parts) > 1 else session_id
+            user_id = f"session:{session_id}"
             
             fact_key = f"doc_context:{session_id}"