How to create a ReAct agent from scratch (Functional API)¶

Prerequisites

This guide assumes familiarity with the following:

This guide demonstrates how to implement a ReAct agent using the LangGraph Functional API.

The ReAct agent is a tool-calling agent that operates as follows:

Queries are issued to a chat model;
If the model generates no tool calls, we return the model response.
If the model generates tool calls, we execute the tool calls with available tools, append them as tool messages to our message list, and repeat the process.

This is a simple and versatile set-up that can be extended with memory, human-in-the-loop capabilities, and other features. See the dedicated how-to guides for examples.

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")

Set up LangSmith for better debugging

Sign up for LangSmith to quickly spot issues and improve the performance of your LangGraph projects. LangSmith lets you use trace data to debug, test, and monitor your LLM aps built with LangGraph — read more about how to get started in the docs.

Create ReAct agent¶

Now that you have installed the required packages and set your environment variables, we can create our agent.

Define model and tools¶

Let's first define the tools and model we will use for our example. Here we will use a single place-holder tool that gets a description of the weather for a location.

We will use an OpenAI chat model for this example, but any model supporting tool-calling will suffice.

from langchain_openai import ChatOpenAI
from langchain_core.tools import tool

model = ChatOpenAI(model="gpt-4o-mini")


@tool
def get_weather(location: str):
    """Call to get the weather from a specific location."""
    # This is a placeholder for the actual implementation
    if any([city in location.lower() for city in ["sf", "san francisco"]]):
        return "It's sunny!"
    elif "boston" in location.lower():
        return "It's rainy!"
    else:
        return f"I am not sure what the weather is in {location}"


tools = [get_weather]

API Reference: ChatOpenAI | tool

Define tasks¶

We next define the tasks we will execute. Here there are two different tasks:

Call model: We want to query our chat model with a list of messages.
Call tool: If our model generates tool calls, we want to execute them.

from langchain_core.messages import ToolMessage
from langgraph.func import entrypoint, task

tools_by_name = {tool.name: tool for tool in tools}


@task
def call_model(messages):
    """Call model with a sequence of messages."""
    response = model.bind_tools(tools).invoke(messages)
    return response


@task
def call_tool(tool_call):
    tool = tools_by_name[tool_call["name"]]
    observation = tool.invoke(tool_call["args"])
    return ToolMessage(content=observation, tool_call_id=tool_call["id"])

API Reference: ToolMessage | entrypoint | task

Define entrypoint¶

Our entrypoint will handle the orchestration of these two tasks. As described above, when our call_model task generates tool calls, the call_tool task will generate responses for each. We append all messages to a single messages list.

Tip

Note that because tasks return future-like objects, the below implementation executes tools in parallel.

from langgraph.graph.message import add_messages


@entrypoint()
def agent(messages):
    llm_response = call_model(messages).result()
    while True:
        if not llm_response.tool_calls:
            break

        # Execute tools
        tool_result_futures = [
            call_tool(tool_call) for tool_call in llm_response.tool_calls
        ]
        tool_results = [fut.result() for fut in tool_result_futures]

        # Append to message list
        messages = add_messages(messages, [llm_response, *tool_results])

        # Call model again
        llm_response = call_model(messages).result()

    return llm_response

API Reference: add_messages

Usage¶

To use our agent, we invoke it with a messages list. Based on our implementation, these can be LangChain message objects or OpenAI-style dicts:

user_message = {"role": "user", "content": "What's the weather in san francisco?"}
print(user_message)

for step in agent.stream([user_message]):
    for task_name, message in step.items():
        if task_name == "agent":
            continue  # Just print task updates
        print(f"\n{task_name}:")
        message.pretty_print()

{'role': 'user', 'content': "What's the weather in san francisco?"}

call_model:
==================================[1m Ai Message [0m==================================
Tool Calls:
  get_weather (call_tNnkrjnoz6MNfCHJpwfuEQ0v)
 Call ID: call_tNnkrjnoz6MNfCHJpwfuEQ0v
  Args:
    location: san francisco

call_tool:
=================================[1m Tool Message [0m=================================

It's sunny!

call_model:
==================================[1m Ai Message [0m==================================

The weather in San Francisco is sunny!

Perfect! The graph correctly calls the get_weather tool and responds to the user after receiving the information from the tool. Check out the LangSmith trace here.

Add thread-level persistence¶

Adding thread-level persistence lets us support conversational experiences with our agent: subsequent invocations will append to the prior messages list, retaining the full conversational context.

To add thread-level persistence to our agent:

Select a checkpointer: here we will use MemorySaver, a simple in-memory checkpointer.
Update our entrypoint to accept the previous messages state as a second argument. Here, we simply append the message updates to the previous sequence of messages.
Choose which values will be returned from the workflow and which will be saved by the checkpointer as previous using entrypoint.final (optional)

from langgraph.checkpoint.memory import MemorySaver

checkpointer = MemorySaver()


@entrypoint(checkpointer=checkpointer)
def agent(messages, previous):
    if previous is not None:
        messages = add_messages(previous, messages)

    llm_response = call_model(messages).result()
    while True:
        if not llm_response.tool_calls:
            break

        # Execute tools
        tool_result_futures = [
            call_tool(tool_call) for tool_call in llm_response.tool_calls
        ]
        tool_results = [fut.result() for fut in tool_result_futures]

        # Append to message list
        messages = add_messages(messages, [llm_response, *tool_results])

        # Call model again
        llm_response = call_model(messages).result()

    # Generate final response
    messages = add_messages(messages, llm_response)
    return entrypoint.final(value=llm_response, save=messages)

API Reference: MemorySaver

We will now need to pass in a config when running our application. The config will specify an identifier for the conversational thread.

Tip

Read more about thread-level persistence in our concepts page and how-to guides.

config = {"configurable": {"thread_id": "1"}}

We start a thread the same way as before, this time passing in the config:

user_message = {"role": "user", "content": "What's the weather in san francisco?"}
print(user_message)

for step in agent.stream([user_message], config):
    for task_name, message in step.items():
        if task_name == "agent":
            continue  # Just print task updates
        print(f"\n{task_name}:")
        message.pretty_print()

{'role': 'user', 'content': "What's the weather in san francisco?"}

call_model:
==================================[1m Ai Message [0m==================================
Tool Calls:
  get_weather (call_lubbUSdDofmOhFunPEZLBz3g)
 Call ID: call_lubbUSdDofmOhFunPEZLBz3g
  Args:
    location: San Francisco

call_tool:
=================================[1m Tool Message [0m=================================

It's sunny!

call_model:
==================================[1m Ai Message [0m==================================

The weather in San Francisco is sunny!

When we ask a follow-up conversation, the model uses the prior context to infer that we are asking about the weather:

user_message = {"role": "user", "content": "How does it compare to Boston, MA?"}
print(user_message)

for step in agent.stream([user_message], config):
    for task_name, message in step.items():
        if task_name == "agent":
            continue  # Just print task updates
        print(f"\n{task_name}:")
        message.pretty_print()

{'role': 'user', 'content': 'How does it compare to Boston, MA?'}

call_model:
==================================[1m Ai Message [0m==================================
Tool Calls:
  get_weather (call_8sTKYAhSIHOdjLD5d6gaswuV)
 Call ID: call_8sTKYAhSIHOdjLD5d6gaswuV
  Args:
    location: Boston, MA

call_tool:
=================================[1m Tool Message [0m=================================

It's rainy!

call_model:
==================================[1m Ai Message [0m==================================

Compared to San Francisco, which is sunny, Boston, MA is experiencing rainy weather.

In the LangSmith trace, we can see that the full conversational context is retained in each model call.