So you want access to first human message all the time? Why not modify your AgentState and add a node to your graph, so the subsequent nodes can also access it.

Something like:

def get_initial_human_content(state):
    for msg in state['messages']:
        if isinstance(msg, HumanMessage):
            return msg.content
    return None

class AgentState(TypedDict):
    # The add_messages function defines how an update should be processed
    # Default is to replace. add_messages says "append"
    messages: Annotated[Sequence[BaseMessage], add_messages]
    initial_human_content: str