[Python] Langgraph: How to retrieve and store chat history throughout the session

I am new to the Langgraph. I am trying to retrieve chathistory from state into a variable untill the session close.

below what i have tried

llm = ChatOpenAI(temperature=1.0)

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

graph_builder = StateGraph(State)

print(graph_builder)

def chatbot(state:State):
user_message = state["messages"][-1].content
ai_response = llm.invoke(user_message)
return {"messages": [AIMessage(content=ai_response)]}

def past_message_node(state: State):
print("Conversation history")
past_message = state['messages']
contents = [msg.content for msg in past_message]
print("Message History:", ", ".join(contents))
return state

raph_builder.add_node("chatbot", chatbot)
graph_builder.add_edge(START, "chatbot")
graph_builder.add_node("past_message_node", past_message_node)
graph_builder.add_edge("chatbot", "past_message_node")
graph_builder.add_edge("past_message_node", END)
graph = graph_builder.compile()
while True:
user_input = input("User: ")

if user_input.lower() in ['quit', 'q']:
print("Chat Ended")
break
for event in graph.stream({"messages": ("user", user_input)}):
for value in event.values():
messages = value["messages"]
if (isinstance(messages, list)):
last_message = messages[-1]
else:
last_message = messages
print("Assistent: ", last_message.content)


#Expected: Along with AI_message, i want to print the chathistory from begining

For eg:
input1: Hi
Ai_message: How can i help you
Message History: Hi, How can i help you

input2: What is the capital of UK?
Ai_message: London
Message History: Hi, How can i help you, What is the capital of UK?, London



ideally i am expecting the chat_hisitory from the state["message"] appended each time, so that i can retrieve the chat_hisitory

#Actual

Getting multiple error
pydantic_core._pydantic_core.ValidationError: 21 validation errors for AIMessage
content.str
Input should be a valid string [type=string_type, input_value=AIMessage(content='Hello!...o': 0, 'reasoning': 0}}), input_type=AIMessage]


so what is the best solution to retrive and store chat_history in one variable and print through out the session?

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