diff --git a/docs/docs/expression_language/cookbook/retrieval.ipynb b/docs/docs/expression_language/cookbook/retrieval.ipynb
index 2689ffcd96..27470a4a56 100644
--- a/docs/docs/expression_language/cookbook/retrieval.ipynb
+++ b/docs/docs/expression_language/cookbook/retrieval.ipynb
@@ -234,7 +234,13 @@
     "from typing import List, Tuple\n",
     "\n",
     "\n",
-    "def _format_chat_history(chat_history: List[Tuple]) -> str:\n",
+    "def _format_chat_history(chat_history: List[Tuple[str, str]]) -> str:\n",
+    "    # chat history is of format:\n",
+    "    # [\n",
+    "    #   (human_message_str, ai_message_str),\n",
+    "    #   ...\n",
+    "    # ]\n",
+    "    # see below for an example of how it's invoked\n",
     "    buffer = \"\"\n",
     "    for dialogue_turn in chat_history:\n",
     "        human = \"Human: \" + dialogue_turn[0]\n",