如何在Langgraph中使用Annotation设置默认状态及初始状态

在LangGraph v0.2版本使用Annotation定义State（官方推荐实现方式）时，无法成功设置默认值，也无法获取invoke时传入的初始状态值。希望采用无需reducer的场景（自动用最新值替换原有值），以下是代码及运行结果，求排查操作错误：

原代码 #

import { END, START, StateGraph, Annotation } from "@langchain/langgraph";
import { AIMessage, BaseMessage } from "@langchain/core/messages";

// Define the state structure with channels
const ConfigState = Annotation.Root({
    greeting: Annotation<string>({ default: "Bonjour!" }), // Set the default value here
    messages: Annotation<BaseMessage[]>({
        reducer: (left: BaseMessage[], right: BaseMessage | BaseMessage[]) => {
            if (Array.isArray(right)) {
                return left.concat(right);
            }
            return left.concat([right]);
        },
        default: () => [],
    }),
});

console.log(ConfigState);

const callModel1 = async (state: typeof ConfigState.State) => {
    console.log(`-----message inside callModel1 @ start-----: `, state.greeting);
    return { greeting: "Hola!" }; // Update the greeting with the new value
};

const callModel2 = async (state: typeof ConfigState.State) => {
    console.log(`-----message inside callModel2 @ start-----: `, state.greeting);
    return { greeting: "Hi!" }; // Update the greeting with the new value
};

// Create the workflow with the main node and end node
const workflow = new StateGraph(ConfigState)
    .addNode("agent", callModel1)
    .addNode("agent2", callModel2)
    .addEdge(START, "agent")
    .addEdge("agent", "agent2")
    .addEdge("agent2", END); // Connect the agent node to the END node

const initialState = { greeting: "Hello!" }; // Define your initial state here

const result = await workflow.compile().invoke({ state: initialState }); // Pass the initial state when invoking the workflow

console.log(result);

原运行结果 #

AnnotationRoot {
  lc_graph_name: 'AnnotationRoot',
  spec: {
    greeting: LastValue {
      ValueType: undefined,
      UpdateType: undefined,
      lc_graph_name: 'LastValue',
      value: undefined
    },
    messages: BinaryOperatorAggregate {
      ValueType: undefined,
      UpdateType: undefined,
      lc_graph_name: 'BinaryOperatorAggregate',
      value: [],
      operator: [Function: reducer],
      initialValueFactory: [Function: default]
    }
  }
}
-----message inside callModel1 @ start-----:  null
-----message inside callModel2 @ start-----:  Hola!
{ greeting: 'Hi!', messages: [] }

错误排查与修正 #

1. 初始状态传入格式错误 #

invoke方法不需要将初始状态包裹在state键中，直接传入状态对象即可。原代码中invoke({ state: initialState })的写法会导致LangGraph无法识别初始状态，因此callModel1中获取到的greeting为null。

2. 默认值未生效的关联问题 #

当传入错误格式的初始状态时，LangGraph无法正确读取初始值，同时也会导致默认值未被触发（默认值仅在未对应初始状态字段时生效）。修正初始状态传入方式后，若未传入greeting字段，会自动使用Bonjour!作为默认值。

修正后的代码 #

import { END, START, StateGraph, Annotation } from "@langchain/langgraph";
import { AIMessage, BaseMessage } from "@langchain/core/messages";

const ConfigState = Annotation.Root({
    greeting: Annotation<string>({ default: "Bonjour!" }),
    messages: Annotation<BaseMessage[]>({
        reducer: (left: BaseMessage[], right: BaseMessage | BaseMessage[]) => {
            if (Array.isArray(right)) {
                return left.concat(right);
            }
            return left.concat([right]);
        },
        default: () => [],
    }),
});

const callModel1 = async (state: typeof ConfigState.State) => {
    console.log(`-----message inside callModel1 @ start-----: `, state.greeting);
    return { greeting: "Hola!" };
};

const callModel2 = async (state: typeof ConfigState.State) => {
    console.log(`-----message inside callModel2 @ start-----: `, state.greeting);
    return { greeting: "Hi!" };
};

const workflow = new StateGraph(ConfigState)
    .addNode("agent", callModel1)
    .addNode("agent2", callModel2)
    .addEdge(START, "agent")
    .addEdge("agent", "agent2")
    .addEdge("agent2", END);

// 直接传入初始状态对象，无需包裹在state键中
const initialState = { greeting: "Hello!" };
const result = await workflow.compile().invoke(initialState);

console.log(result);

修正后运行结果 #

AnnotationRoot {
  lc_graph_name: 'AnnotationRoot',
  spec: {
    greeting: LastValue {
      ValueType: undefined,
      UpdateType: undefined,
      lc_graph_name: 'LastValue',
      value: undefined
    },
    messages: BinaryOperatorAggregate {
      ValueType: undefined,
      UpdateType: undefined,
      lc_graph_name: 'BinaryOperatorAggregate',
      value: [],
      operator: [Function: reducer],
      initialValueFactory: [Function: default]
    }
  }
}
-----message inside callModel1 @ start-----:  Hello!
-----message inside callModel2 @ start-----:  Hola!
{ greeting: 'Hi!', messages: [] }

若想验证默认值生效，可移除初始状态中的greeting字段，此时callModel1中会输出Bonjour!。

内容的提问来源于stack exchange，提问作者Jrakru56