Tool calling in LangGraph and how to update the Graph-State with Tools output

[dominik-roemer]

Hey,
atm I'm looking into LangGraph and the agents ability to call tools. I was able to build an agent which has this architecture. The idea is taken from: https://langchain-ai.github.io/langgraph/tutorials/introduction/#part-2-enhancing-the-chatbot-with-tools

When a tool is called the Chatbot receives a Toolmesage with the output of the tool. Now my question is, how to take this output and write it into the state. Let's say my state looks something like this:

messages: Annotated[list[AnyMessage], add_messages]
   first_name: str
   last_name: str
   date_of_birth: str

And I have a very simple tool which is called when the user provides his name in the query, to find the date_of_birth in the database.

find_user_date_of_birth(user_name: str):
"""This tool is used to search for the users date of birth in the customer DB if the user name is given in the query."""
  date_of_birth = "01.01.1990" # this is just a mock tool
  return f"The user {username} birthday is {date_of_birth}"

How am I now able to update the GraphState in a way that date_of_birth is set in the State? The tool node is returning {"messages": output} which will update the state. But what if I want to return "date_of_birth"? Is there an easy way to solve this dynamically or is there even a better way?

[isahers1]

The prebuilt ToolNode will only append to the messages key in your state, but you can define a custom node like so:

def custom_tool_node(state):
    return {"date_of_birth": find_user_date_of_birth(state['first_name']+state['last_name'])}

This node will update the "date_of_birth" key. You may also wish to append the ToolMessage to the list of messages, so be careful of that as well. Let me know if this helps!

[thoffmann-artidis]

Is there any reason why ToolNode doesn't write to any other state variable than messages? It's a widespread use case where you would like to retrieve some data through a tool call and pass it to another agent.