⚡ Building language agents as graphs ⚡

LangGraph.js is a library for building stateful, multi-actor applications with LLMs, used to create agent and multi-agent workflows. Built on top of LangChain.js, it offers these core benefits compared to other LLM frameworks: cycles, controllability, and persistence. LangGraph allows you to define flows that involve cycles, essential for most agentic architectures, differentiating it from DAG-based solutions. As a very low-level framework, it provides fine-grained control over both the flow and state of your application, crucial for creating reliable agents. Additionally, LangGraph includes built-in persistence, enabling advanced human-in-the-loop and memory features.

LangGraph is inspired by Pregel and Apache Beam. The public interface draws inspiration from NetworkX. LangGraph is built by LangChain Inc, the creators of LangChain, but can be used without LangChain.

• Cycles and Branching: Implement loops and conditionals in your apps.
• Persistence: Automatically save state after each step in the graph. Pause and resume the graph execution at any point to support error recovery, human-in-the-loop workflows, time travel and more.
• Human-in-the-Loop: Interrupt graph execution to approve or edit next action planned by the agent.
• Streaming Support: Stream outputs as they are produced by each node (including token streaming).
• Integration with LangChain: LangGraph integrates seamlessly with LangChain and LangSmith (but does not require them).

npm install @langchain/langgraph

One of the central concepts of LangGraph is state. Each graph execution creates a state that is passed between nodes in the graph as they execute, and each node updates this internal state with its return value after it executes. The way that the graph updates its internal state is defined by either the type of graph chosen or a custom function.

Let's take a look at a simple example of an agent that can use a search tool.

First install the required dependencies:

npm install @langchain/anthropic

Then set the required environment variables:

export ANTHROPIC_API_KEY=sk-...

Optionally, set up LangSmith for best-in-class observability:

export LANGCHAIN_TRACING_V2=true
export LANGCHAIN_API_KEY=ls__...

Now let's define our agent:

import { HumanMessage, AIMessage } from "@langchain/core/messages";
import { DynamicStructuredTool } from "@langchain/core/tools";
import { z } from "zod";
import { ChatAnthropic } from "@langchain/anthropic";
import { END, START, StateGraph, StateGraphArgs } from "@langchain/langgraph";
import { MemorySaver } from "@langchain/langgraph";
import { ToolNode } from "@langchain/langgraph/prebuilt";

// Define the state interface
interface AgentState {
  messages: HumanMessage[];
}

// Define the graph state
const graphState: StateGraphArgs<AgentState>["channels"] = {
  messages: {
    value: (x: HumanMessage[], y: HumanMessage[]) => x.concat(y),
    default: () => [],
  },
};

// Define the tools for the agent to use

const searchTool = new DynamicStructuredTool({
  name: "search",
  description:
    "Call to surf the web.",
  schema: z.object({
    query: z.string().describe("The query to use in your search."),
  }),
  func: async ({ query }: { query: string }) => {
    // This is a placeholder for the actual implementation
    if (query.toLowerCase().includes("sf") || query.toLowerCase().includes("san francisco")) {
      return "It's 60 degrees and foggy."
    }
    return "It's 90 degrees and sunny."
  },
});

const tools = [searchTool];
const toolNode = new ToolNode<AgentState>(tools);

const model = new ChatAnthropic({
  model: "claude-3-sonnet-20240229",
  temperature: 0,
}).bindTools(tools);

// Define the function that determines whether to continue or not
function shouldContinue(state: AgentState): "tools" | typeof END {
  const messages = state.messages;
  const lastMessage = messages[messages.length - 1] as AIMessage;

  // If the LLM makes a tool call, then we route to the "tools" node
  if (lastMessage.tool_calls?.length) {
    return "tools";
  }
  // Otherwise, we stop (reply to the user)
  return END;
}

// Define the function that calls the model
async function callModel(state: AgentState) {
  const messages = state.messages;
  const response = await model.invoke(messages);

  // We return a list, because this will get added to the existing list
  return { messages: [response] };
}

// Define a new graph
const workflow = new StateGraph<AgentState>({ channels: graphState })
  .addNode("agent", callModel)
  .addNode("tools", toolNode)
  .addEdge(START, "agent")
  .addConditionalEdges("agent", shouldContinue)
  .addEdge("tools", "agent");

// Initialize memory to persist state between graph runs
const checkpointer = new MemorySaver();

// Finally, we compile it!
// This compiles it into a LangChain Runnable.
// Note that we're (optionally) passing the memory when compiling the graph
const app = workflow.compile({ checkpointer });

// Use the Runnable
const finalState = await app.invoke(
  { messages: [new HumanMessage("what is the weather in sf")] },
  { configurable: { thread_id: "42" } }
);
console.log(finalState.messages[finalState.messages.length - 1].content);

This will output:

Based on the search results, I can tell you that the current weather in San Francisco is:\n\nTemperature: 60 degrees Fahrenheit\nConditions: Foggy\n\nSan Francisco is known for its microclimates and frequent fog, especially during the summer months. The temperature of 60°F (about 15.5°C) is quite typical for the city, which tends to have mild temperatures year-round. The fog, often referred to as "Karl the Fog" by locals, is a characteristic feature of San Francisco\'s weather, particularly in the mornings and evenings.\n\nIs there anything else you\'d like to know about the weather in San Francisco or any other location?

Now when we pass the same "thread_id", the conversation context is retained via the saved state (i.e. stored list of messages):

const nextState = await app.invoke(
  { messages: [new HumanMessage("what about ny")] },
  { configurable: { thread_id: "42" } }
);
console.log(nextState.messages[nextState.messages.length - 1].content);

Based on the search results, I can tell you that the current weather in New York City is:\n\nTemperature: 90 degrees Fahrenheit (approximately 32.2 degrees Celsius)\nConditions: Sunny\n\nThis weather is quite different from what we just saw in San Francisco. New York is experiencing much warmer temperatures right now. Here are a few points to note:\n\n1. The temperature of 90°F is quite hot, typical of summer weather in New York City.\n2. The sunny conditions suggest clear skies, which is great for outdoor activities but also means it might feel even hotter due to direct sunlight.\n3. This kind of weather in New York often comes with high humidity, which can make it feel even warmer than the actual temperature suggests.\n\nIt's interesting to see the stark contrast between San Francisco's mild, foggy weather and New York's hot, sunny conditions. This difference illustrates how varied weather can be across different parts of the United States, even on the same day.\n\nIs there anything else you'd like to know about the weather in New York or any other location?

1. Initialize the model and tools.

   • We use ChatAnthropic as our LLM. NOTE: We need make sure the model knows that it has these tools available to call. We can do this by converting the LangChain tools into the format for Anthropic tool calling using the .bindTools() method.
   • We define the tools we want to use -- a search tool in our case. It is really easy to create your own tools - see documentation here on how to do that.
2. Initialize graph with state.

   • We initialize the graph (StateGraph) by passing the state interface (AgentState).
   • The graphState object defines how updates from each node should be merged into the graph's state.
3. Define graph nodes.

   There are two main nodes we need:

   • The agent node: responsible for deciding what (if any) actions to take.
   • The tools node that invokes tools: if the agent decides to take an action, this node will then execute that action.
4. Define entry point and graph edges.

   First, we need to set the entry point for graph execution - agent node.

   Then we define one normal and one conditional edge. A conditional edge means that the destination depends on the contents of the graph's state (AgentState). In our case, the destination is not known until the agent (LLM) decides.

   • Conditional edge: after the agent is called, we should either:
     • a. Run tools if the agent said to take an action, OR
     • b. Finish (respond to the user) if the agent did not ask to run tools
   • Normal edge: after the tools are invoked, the graph should always return to the agent to decide what to do next
5. Compile the graph.

   • When we compile the graph, we turn it into a LangChain Runnable, which automatically enables calling .invoke(), .stream() and .batch() with your inputs.
   • We can also optionally pass a checkpointer object for persisting state between graph runs, enabling memory, human-in-the-loop workflows, time travel and more. In our case we use MemorySaver - a simple in-memory checkpointer.
6. Execute the graph.

   1. LangGraph adds the input message to the internal state, then passes the state to the entrypoint node, "agent".

   2. The "agent" node executes, invoking the chat model.

   3. The chat model returns an AIMessage. LangGraph adds this to the state.

   4. The graph cycles through the following steps until there are no more tool_calls on the AIMessage:

      • If AIMessage has tool_calls, the "tools" node executes.
      • The "agent" node executes again and returns an AIMessage.

   5. Execution progresses to the special END value and outputs the final state. As a result, we get a list of all our chat messages as output.

• Tutorials: Learn to build with LangGraph through guided examples.
• How-to Guides: Accomplish specific things within LangGraph, from streaming, to adding memory & persistence, to common design patterns (branching, subgraphs, etc.). These are the place to go if you want to copy and run a specific code snippet.
• Conceptual Guides: In-depth explanations of the key concepts and principles behind LangGraph, such as nodes, edges, state and more.
• API Reference: Review important classes and methods, simple examples of how to use the graph and checkpointing APIs, higher-level prebuilt components and more.