add_messages

Basic usage:

from langchain_core.messages import AIMessage, HumanMessage

msgs1 = [HumanMessage(content="Hello", id="1")]
msgs2 = [AIMessage(content="Hi there!", id="2")]
add_messages(msgs1, msgs2)
# [HumanMessage(content='Hello', id='1'), AIMessage(content='Hi there!', id='2')]

Overwrite existing message:

msgs1 = [HumanMessage(content="Hello", id="1")]
msgs2 = [HumanMessage(content="Hello again", id="1")]
add_messages(msgs1, msgs2)
# [HumanMessage(content='Hello again', id='1')]

Use in a StateGraph:

from typing import Annotated
from typing_extensions import TypedDict
from langgraph.graph import StateGraph

class State(TypedDict):
    messages: Annotated[list, add_messages]

builder = StateGraph(State)
builder.add_node("chatbot", lambda state: {"messages": [("assistant", "Hello")]})
builder.set_entry_point("chatbot")
builder.set_finish_point("chatbot")
graph = builder.compile()
graph.invoke({})
# {'messages': [AIMessage(content='Hello', id=...)]}

Use OpenAI message format:

from typing import Annotated
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, add_messages

class State(TypedDict):
    messages: Annotated[list, add_messages(format="langchain-openai")]

def chatbot_node(state: State) -> list:
    return {
        "messages": [
            {
                "role": "user",
                "content": [
                    {
                        "type": "text",
                        "text": "Here's an image:",
                        "cache_control": {"type": "ephemeral"},
                    },
                    {
                        "type": "image",
                        "source": {
                            "type": "base64",
                            "media_type": "image/jpeg",
                            "data": "1234",
                        },
                    },
                ],
            },
        ]
    }

builder = StateGraph(State)
builder.add_node("chatbot", chatbot_node)
builder.set_entry_point("chatbot")
builder.set_finish_point("chatbot")
graph = builder.compile()
graph.invoke({"messages": []})
# {
#     'messages': [
#         HumanMessage(
#             content=[
#                 {"type": "text", "text": "Here's an image:"},
#                 {
#                     "type": "image_url",
#                     "image_url": {"url": "data:image/jpeg;base64,1234"},
#                 },
#             ],
#         ),
#     ]
# }