How can I customize the agent’s response when a required parameter is missing?

[bhavan-kaya]

Please refer to the code and the output below.
The agent detects that the leave_id parameter is missing and responds to the user with a default message.
I would like to customize this message to send a different response. How can I achieve this?

Example Code:

from typing import Literal

from langchain_core.messages import HumanMessage
from langchain_core.tools import tool
from langchain_openai import ChatOpenAI
from langgraph.checkpoint.memory import MemorySaver
from langgraph.graph import END, START, MessagesState, StateGraph
from langgraph.prebuilt import ToolNode

@tool
def apply_leave(leave_id: str):
    """Apply for leave based on leave id."""
    print("Applying for leave with id:", leave_id)

    return "It's 90 degrees and sunny."

tools = [apply_leave]
tool_node = ToolNode(tools)
model = ChatOpenAI(model="gpt-4o-mini", temperature=0).bind_tools(tools)

def should_continue(state: MessagesState) -> Literal["tools", END]:
    messages = state["messages"]
    last_message = messages[-1]
    if last_message.tool_calls:
        return "tools"
    return END

def call_model(state: MessagesState):
    messages = state["messages"]
    response = model.invoke(messages)
    return {"messages": [response]}

workflow = StateGraph(MessagesState)

workflow.add_node("agent", call_model)
workflow.add_node("tools", tool_node)

workflow.add_edge(START, "agent")
workflow.add_conditional_edges("agent", should_continue)
workflow.add_edge("tools", "agent")

checkpointer = MemorySaver()
app = workflow.compile(checkpointer=checkpointer)

# Run
final_state = app.invoke(
    {"messages": [HumanMessage(content="I need to apply for a leave")]},
    config={"configurable": {"thread_id": 1}},
)

final_state["messages"][-1].content

Output:

Could you please provide me with the leave ID that you would like to use to apply for your leave?