在之前的文章中，我们深入讲解了如何使用 LangChain + Ollama 构建本地大模型调用方案。
但是，随着业务需求不断增长，我们发现仅仅调用模型已经远远不够——我们希望构建具备条件判断、流程控制、工具调用以及状态记忆的智能 Agent。

这时候，LangChain 的 create_agent + LangGraph 的 StateGraph 就成为了真正面向工程的利器。

今天，我们就来讲清楚：

什么是 LangGraph？为什么它是构建可控 Agent 的未来？如何在最新 API 下用 create_agent 和 StateGraph 构建有状态智能体？

一、为什么要用 LangGraph 构建 Agent？

在 LangChain 最新版本中，Agent API 已经全面升级，官方推荐使用 create_agent 构建生产级智能体，并基于 LangGraph 对内部流程进行图结构编排。

create_agent
是一个高阶接口，用于构建图式 Agent。它内部依赖 LangGraph 执行器，在一个状态图中逐步完成模型推理、工具调用、决策流跳转等逻辑。

过去我们可能使用 Chain + Logic 组合来处理流程，但随着逻辑复杂度增加，线性写法很难维护、扩展和调试。
而 LangGraph 的图结构 可以让我们：

• 用 状态（state） 表达全局对话或任务信息
• 用 节点（nodes） 表达流程逻辑
• 用 边（edges） 表达不同分支与条件
• 用 记忆插件 实现短期和长期记忆

这组合起来，就形成了一个可控、有状态流程的智能 Agent。

二、什么是 StateGraph？

StateGraph 是 LangGraph 的核心抽象，它表示一个具有全局状态和节点流转逻辑的图。
每个节点本质上是一个函数，这个函数：

• 接收当前全局状态
• 返回修改后的状态或跳转指令

它非常适合把“复杂流程问题”映射为“图状态机”，无论是对话、工具调用还是多步骤任务。

简化后的 StateGraph 工作流程如下：

StateGraph(StateType)
    ├── add_node(name, function)
    ├── add_edge(source, target)
    └── compile()
        → graph.invoke({state input})

解释一下：

• StateType：定义全局状态结构
• add_node：定义节点行为逻辑
• add_edge：定义节点间的流程跳转关系

三、新 API：create_agent 如何使用？

从 LangChain 最新版本开始，旧的 create_react_agent 已被废弃，统一使用 create_agent。

一个最简单的示例：

from langchain.agents import create_agent
from langchain_openai import ChatOpenAI

model = ChatOpenAI(
    model="qwen3:8b",
    base_url="http://localhost:11434/v1",
    api_key="your api key",
)

agent = create_agent(
    model=model,
    tools=[],
    system_prompt="你是一个智能助手，负责处理用户请求。",
)

response = agent.invoke({
    "messages": [{"role": "user", "content": "什么是 LangGraph？"}]
})
print(response)

📌 重点说明：

• model 可以是任何支持工具调用的聊天模型
• tools 是 Agent 可调用的外部能力（如检索、代码执行等）
• system_prompt 是 Agent 的基础角色指令

实际上，create_agent 内部会构建一个 StateGraph，并把模型 + 工具节点组合成可执行流程。

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