x

libs/core/langchain_core/prompts/chat.py

@@ -949,6 +949,13 @@ def _create_template_from_message_type(
         message = AIMessagePromptTemplate.from_template(cast(str, template))
     elif message_type == "system":
         message = SystemMessagePromptTemplate.from_template(cast(str, template))
+    elif message_type == "placeholder":
+        if not isinstance(template, str) or template[0] != "{" or template[-1] != "}":
+            raise ValueError(
+                r"Expected a placeholder variable of the format {variable_name},"
+                f" got {template}."
+            )
+        message = MessagesPlaceholder(variable_name=template[1:-1])
     else:
         raise ValueError(
             f"Unexpected message type: {message_type}. Use one of 'human',"

libs/core/tests/unit_tests/prompts/test_chat.py

@@ -151,11 +151,14 @@ def test_chat_prompt_template_from_messages_using_role_strings() -> None:
             ("system", "You are a helpful AI bot. Your name is {name}."),
             ("human", "Hello, how are you doing?"),
             ("ai", "I'm doing well, thanks!"),
+            ("placeholder", "{chat_history}"),
             ("human", "{user_input}"),
         ]
     )
 
-    messages = template.format_messages(name="Bob", user_input="What is your name?")
+    messages = template.format_messages(
+        name="Bob", user_input="What is your name?", chat_history=[]
+    )
 
     assert messages == [
         SystemMessage(
@@ -170,6 +173,29 @@ def test_chat_prompt_template_from_messages_using_role_strings() -> None:
         HumanMessage(content="What is your name?", additional_kwargs={}, example=False),
     ]
 
+    messages = template.format_messages(
+        name="Bob",
+        user_input="What is your name?",
+        chat_history=[("human", "That's great to hear"), ("ai", "Thanks!")],
+    )
+
+    assert messages == [
+        SystemMessage(
+            content="You are a helpful AI bot. Your name is Bob.", additional_kwargs={}
+        ),
+        HumanMessage(
+            content="Hello, how are you doing?", additional_kwargs={}, example=False
+        ),
+        AIMessage(
+            content="I'm doing well, thanks!", additional_kwargs={}, example=False
+        ),
+        HumanMessage(
+            content="That's great to hear", additional_kwargs={}, example=False
+        ),
+        AIMessage(content="Thanks!", additional_kwargs={}, example=False),
+        HumanMessage(content="What is your name?", additional_kwargs={}, example=False),
+    ]
+
 
 def test_chat_prompt_template_with_messages() -> None:
     messages: List[Union[BaseMessagePromptTemplate, BaseMessage]] = (