Day 12: Enter LangGraph — Moving from Chains to Cyclic Graphs 🕸️

Today, we leave the world of linear "Chains" and enter the most powerful evolution of the LangChain ecosystem: LangGraph.

If you've been following along, you've noticed that Chains always go forward: A -> B -> C. But real intelligence requires loops. Think about how you work: you write code, you run it, it fails, so you go back and fix it. That's a cycle. LangGraph is designed to let AI agents do exactly that.

🔄 Why LangGraph?

Standard LangChain "Chains" are Directed Acyclic Graphs (DAGs). They can't loop.
LangGraph allows for cycles, which are essential for:

Self-Correction: "I tried to search Google, but got no results. I'll try a different keyword."
Multi-Agent Collaboration: "Agent A writes the code, Agent B reviews it and sends it back for edits."
Persistence: Saving the state of a conversation so you can pause and resume it days later.

🏗️ The Core Concepts

To build with LangGraph, you need to understand three things:

State: A shared "notebook" (usually a Python dictionary) that all parts of your agent can read and write to.

Nodes: Simple Python functions that take the current State, do some work, and return an update.

Edges: The "roads" between nodes. They decide which node to go to next.

🛠️ Building a Basic "Smart Assistant" Graph

Let's build a graph where the AI decides whether to use a tool or just reply.

import operator
from typing import Annotated, TypedDict, Union
from langgraph.graph import StateGraph, START, END
from langchain_openai import ChatOpenAI

# 1. Define the State
class AgentState(TypedDict):
    # This stores our message history
    messages: Annotated[list, operator.add]

# 2. Define a Node (The Brain)
model = ChatOpenAI(model="gpt-4o")

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

# 3. Build the Graph
workflow = StateGraph(AgentState)

# Add our node
workflow.add_node("agent", call_model)

# Define the flow
workflow.add_edge(START, "agent")
workflow.add_edge("agent", END)

# 4. Compile it
app = workflow.compile()

# Run it!
input_state = {"messages": [("user", "Explain LangGraph in 10 words.")]}
for event in app.stream(input_state):
    print(event)