main_basic_example.py

from dotenv import load_dotenv
from typing import Annotated, Literal
from langgraph.graph import StateGraph, START, END
from langgraph.graph.message import add_messages
from langchain.chat_models import init_chat_model
from pydantic import BaseModel, Field
from typing_extensions import TypedDict

load_dotenv()

llm = init_chat_model("anthropic:claude-sonnet-4-5-20250929")
#"anthropic:claude-sonnet-4-5-20250929" also possible as modern alternative

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

graph_builder = StateGraph(State)

def chatbot(state:State):
    return {"messages": [llm.invoke(state["messages"])]}

graph_builder.add_node("chatbot", chatbot)

graph_builder.add_edge(START,"chatbot")

graph_builder.add_edge("chatbot", END)

graph = graph_builder.compile()

user_input = input("Enter a message: ")

state = graph.invoke({"messages": [{"role": "user", "content": user_input}]})

print(state["messages"][-1])