单个Agent使用少量的工具通常是高效的，但是即便强大如gpt-4，在使用多个工具的时候，效果也会大大折扣。

“分而治之”的方式可以更容易完成任务。为每个任务或者每个领域单独创建一个Agent，然后将任务分发给给正确的“专家”(处理特定任务的Agent)。

下面的nootbook展示了如何使用LangGraph实现多Agent方式，多Agent组成的最图结果大概是下面这样的：


在正式开始介绍之前，多说一句：这里主要介绍如何使用LangGraph实现多Agent的某些设计模式，如果这个设计模式恰好是你需要的，我们推荐你将它和本文档介绍的其他模式结合起来是使用，这个可以获得更好的性能。

## 准备基础环境

```Bash
%%capture --no-stderr
%pip install -U langchain langchain_openai langsmith pandas langchain_experimental matplotlib langgraph langchain_core
```

代码中引入环境变量

```Bash
import getpass
import os


def _set_if_undefined(var: str):
if not os.environ.get(var):
os.environ[var] = getpass.getpass(f"Please provide your {var}")


_set_if_undefined("OPENAI_API_KEY")
_set_if_undefined("LANGCHAIN_API_KEY")
_set_if_undefined("TAVILY_API_KEY")

# Optional, add tracing in LangSmith
os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_PROJECT"] = "Multi-agent Collaboration"
```

## 创建Agent

下面的辅助函数，可以帮助创建Agent。创建好的Agent会成为Graph中的Node节点

```Bash
from langchain_core.messages import (
BaseMessage,
HumanMessage,
ToolMessage,
)
from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder

from langgraph.graph import END, StateGraph, START


def create_agent(llm, tools, system_message: str):
"""Create an agent."""
prompt = ChatPromptTemplate.from_messages(
[
(
"system",
"You are a helpful AI assistant, collaborating with other assistants."
" Use the provided tools to progress towards answering the question."
" If you are unable to fully answer, that's OK, another assistant with different tools "
" will help where you left off. Execute what you can to make progress."
" If you or any of the other assistants have the final answer or deliverable,"
" prefix your response with FINAL ANSWER so the team knows to stop."
" You have access to the following tools: {tool_names}.\n{system_message}",
),
MessagesPlaceholder(variable_name="messages"),
]
)
prompt = prompt.partial(system_message=system_message)
prompt = prompt.partial(tool_names=", ".join([tool.name for tool in tools]))
return prompt | llm.bind_tools(tools)

```

## 定义工具

我们也可以定义一些未来Agent会用到的工具

```Bash
from typing import Annotated

from langchain_community.tools.tavily_search import TavilySearchResults
from langchain_core.tools import tool
from langchain_experimental.utilities import PythonREPL

tavily_tool = TavilySearchResults(max_results=5)

# Warning: This executes code locally, which can be unsafe when not sandboxed

repl = PythonREPL()


@tool
def python_repl(
code: Annotated[str, "The python code to execute to generate your chart."],
):
"""Use this to execute python code. If you want to see the output of a value,
you should print it out with `print(...)`. This is visible to the user."""
try:
result = repl.run(code)
except BaseException as e:
return f"Failed to execute. Error: {repr(e)}"
result_str = f"Successfully executed:\n```python\n{code}\n```\nStdout: {result}"
return (
result_str + "\n\nIf you have completed all tasks, respond with FINAL ANSWER."
)
```

## 创建graph

我们已经创建了工具和辅助函数，接下里，会使用LangGraph创建一些独立的Agent，然后告诉它们之间如何交互。

### 定义状态State

首先创建图的状态State。状态的内容结构是带有Key的消息List，用来追踪最新的sender。

```Bash
import operator
from typing import Annotated, Sequence, TypedDict

from langchain_openai import ChatOpenAI


# This defines the object that is passed between each node
# in the graph. We will create different nodes for each agent and tool
class AgentState(TypedDict):
messages: Annotated[Sequence[BaseMessage], operator.add]
sender: str
```

### 定义Agent Nodes

接下来为Agent定义一个Node节点

```Bash
import functools

from langchain_core.messages import AIMessage


# Helper function to create a node for a given agent
def agent_node(state, agent, name):
result = agent.invoke(state)
# We convert the agent output into a format that is suitable to append to the global state
if isinstance(result, ToolMessage):
pass
else:
result = AIMessage(**result.dict(exclude={"type", "name"}), name=name)
return {
"messages": [result],
# Since we have a strict workflow, we can
# track the sender so we know who to pass to next.
"sender": name,
}


llm = ChatOpenAI(model="gpt-4-1106-preview")

# Research agent and node
research_agent = create_agent(
llm,
[tavily_tool],
system_message="You should provide accurate data for the chart_generator to use.",
)
research_node = functools.partial(agent_node, agent=research_agent, name="Researcher")

# chart_generator
chart_agent = create_agent(
llm,
[python_repl],
system_message="Any charts you display will be visible by the user.",
)
chart_node = functools.partial(agent_node, agent=chart_agent, name="chart_generator")
```

### 定义工具节点

定义一个工具节点来运行工具函数

```Bash
from langgraph.prebuilt import ToolNode

tools = [tavily_tool, python_repl]
tool_node = ToolNode(tools)
```

### 定义边的逻辑

接下来定义逻辑边，它可以根据上游Agent的结果决定接下来，路由到哪里

```Bash
# Either agent can decide to end
from typing import Literal


def router(state) -> Literal["call_tool", "__end__", "continue"]:
# This is the router
messages = state["messages"]
last_message = messages[-1]
if last_message.tool_calls:
# The previous agent is invoking a tool
return "call_tool"
if "FINAL ANSWER" in last_message.content:
# Any agent decided the work is done
return "__end__"
return "continue"
```

### 定义Graph

接下来，把上面定义的东西聚合在一起，组成一个图

```Bash
workflow = StateGraph(AgentState)

workflow.add_node("Researcher", research_node)
workflow.add_node("chart_generator", chart_node)
workflow.add_node("call_tool", tool_node)

workflow.add_conditional_edges(
"Researcher",
router,
{"continue": "chart_generator", "call_tool": "call_tool", "__end__": END},
)
workflow.add_conditional_edges(
"chart_generator",
router,
{"continue": "Researcher", "call_tool": "call_tool", "__end__": END},
)

workflow.add_conditional_edges(
"call_tool",
# Each agent node updates the 'sender' field
# the tool calling node does not, meaning
# this edge will route back to the original agent
# who invoked the tool
lambda x: x["sender"],
{
"Researcher": "Researcher",
"chart_generator": "chart_generator",
},
)
workflow.add_edge(START, "Researcher")
graph = workflow.compile()
```

可视化一下图

```Bash
from IPython.display import Image, display

try:
display(Image(graph.get_graph(xray=True).draw_mermaid_png()))
except Exception:
# This requires some extra dependencies and is optional
pass
```


### 调用

创建好图之后，就可以调用了

```Bash
events = graph.stream(
{
"messages": [
HumanMessage(
content="Fetch the UK's GDP over the past 5 years,"
" then draw a line graph of it."
" Once you code it up, finish."
)
],
},
# Maximum number of steps to take in the graph
{"recursion_limit": 150},
)
for s in events:
print(s)
print("----")
```