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How to add a custom system prompt to the prebuilt ReAct agent

Prerequisites

This guide assumes familiarity with the following:

This tutorial will show how to add a custom system prompt to the prebuilt ReAct agent. Please see this tutorial for how to get started with the prebuilt ReAct agent

You can add a custom system prompt by passing a string to the state_modifier param.

Setup

First, let's install the required packages and set our API keys

%%capture --no-stderr
%pip install -U langgraph langchain-openai

import getpass
import os


def _set_env(var: str):
    if not os.environ.get(var):
        os.environ[var] = getpass.getpass(f"{var}: ")


_set_env("OPENAI_API_KEY")

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Code

# First we initialize the model we want to use.
from langchain_openai import ChatOpenAI

model = ChatOpenAI(model="gpt-4o", temperature=0)


# For this tutorial we will use custom tool that returns pre-defined values for weather in two cities (NYC & SF)

from typing import Literal

from langchain_core.tools import tool


@tool
def get_weather(city: Literal["nyc", "sf"]):
    """Use this to get weather information."""
    if city == "nyc":
        return "It might be cloudy in nyc"
    elif city == "sf":
        return "It's always sunny in sf"
    else:
        raise AssertionError("Unknown city")


tools = [get_weather]

# We can add our system prompt here

prompt = "Respond in Italian"

# Define the graph

from langgraph.prebuilt import create_react_agent

graph = create_react_agent(model, tools=tools, state_modifier=prompt)

API Reference: ChatOpenAI | tool | create_react_agent

Usage

def print_stream(stream):
    for s in stream:
        message = s["messages"][-1]
        if isinstance(message, tuple):
            print(message)
        else:
            message.pretty_print()


inputs = {"messages": [("user", "What's the weather in NYC?")]}

print_stream(graph.stream(inputs, stream_mode="values"))

================================ Human Message =================================

What's the weather in NYC?
================================== Ai Message ==================================
Tool Calls:
  get_weather (call_b02uzBRrIm2uciJa8zDXCDxT)
 Call ID: call_b02uzBRrIm2uciJa8zDXCDxT
  Args:
    city: nyc
================================= Tool Message =================================
Name: get_weather

It might be cloudy in nyc
================================== Ai Message ==================================

A New York potrebbe essere nuvoloso.

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