Force Calling a Tool First¶

In this example we will build a ReAct agent that always calls a certain tool first, before making any plans. In this example, we will create an agent with a search tool. However, at the start we will force the agent to call the search tool (and then let it do whatever it wants after). This is useful when you know you want to execute specific actions in your application but also want the flexibility of letting the LLM follow up on the user's query after going through that fixed sequence.

Setup¶

First we need to install the packages required

In [1]:

%%capture --no-stderr
%pip install --quiet -U langchain langchain_openai tavily-python

%%capture --no-stderr %pip install --quiet -U langchain langchain_openai tavily-python

Next, we need to set API keys for OpenAI (the LLM we will use) and Tavily (the search tool we will use)

In [ ]:

import os
import getpass

os.environ["OPENAI_API_KEY"] = getpass.getpass("OpenAI API Key:")
os.environ["TAVILY_API_KEY"] = getpass.getpass("Tavily API Key:")

import os import getpass os.environ["OPENAI_API_KEY"] = getpass.getpass("OpenAI API Key:") os.environ["TAVILY_API_KEY"] = getpass.getpass("Tavily API Key:")

Optionally, we can set API key for LangSmith tracing, which will give us best-in-class observability.

In [ ]:

os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_API_KEY"] = getpass.getpass("LangSmith API Key:")

os.environ["LANGCHAIN_TRACING_V2"] = "true" os.environ["LANGCHAIN_API_KEY"] = getpass.getpass("LangSmith API Key:")

Set up the tools¶

We will first define the tools we want to use. For this simple example, we will use a built-in search tool via Tavily. However, it is really easy to create your own tools - see documentation here on how to do that.

In [1]:

from langchain_community.tools.tavily_search import TavilySearchResults

tools = [TavilySearchResults(max_results=1)]

from langchain_community.tools.tavily_search import TavilySearchResults tools = [TavilySearchResults(max_results=1)]

We can now wrap these tools in a simple ToolExecutor. This is a real simple class that takes in a ToolInvocation and calls that tool, returning the output. A ToolInvocation is any class with tool and tool_input attribute.

In [2]:

from langgraph.prebuilt import ToolExecutor

tool_executor = ToolExecutor(tools)

from langgraph.prebuilt import ToolExecutor tool_executor = ToolExecutor(tools)

Set up the model¶

Now we need to load the chat model we want to use. Importantly, this should satisfy two criteria:

1. It should work with messages. We will represent all agent state in the form of messages, so it needs to be able to work well with them.
2. It should work with OpenAI function calling. This means it should either be an OpenAI model or a model that exposes a similar interface.

Note: these model requirements are not requirements for using LangGraph - they are just requirements for this one example.

In [4]:

from langchain_openai import ChatOpenAI

# We will set streaming=True so that we can stream tokens
# See the streaming section for more information on this.
model = ChatOpenAI(temperature=0, streaming=True)

from langchain_openai import ChatOpenAI # We will set streaming=True so that we can stream tokens # See the streaming section for more information on this. model = ChatOpenAI(temperature=0, streaming=True)

After we've done this, we should 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 OpenAI function calling, and then bind them to the model class.

In [6]:

model = model.bind_tools(tools)

model = model.bind_tools(tools)

Define the agent state¶

The main type of graph in langgraph is the StatefulGraph. This graph is parameterized by a state object that it passes around to each node. Each node then returns operations to update that state. These operations can either SET specific attributes on the state (e.g. overwrite the existing values) or ADD to the existing attribute. Whether to set or add is denoted by annotating the state object you construct the graph with.

For this example, the state we will track will just be a list of messages. We want each node to just add messages to that list. Therefore, we will use a TypedDict with one key (messages) and annotate it so that the messages attribute is always added to.

In [10]:

from typing import TypedDict, Annotated, Sequence
import operator
from langchain_core.messages import BaseMessage


class AgentState(TypedDict):
    messages: Annotated[Sequence[BaseMessage], operator.add]

from typing import TypedDict, Annotated, Sequence import operator from langchain_core.messages import BaseMessage class AgentState(TypedDict): messages: Annotated[Sequence[BaseMessage], operator.add]

Define the nodes¶

We now need to define a few different nodes in our graph. In langgraph, a node can be either a function or a runnable. There are two main nodes we need for this:

1. The agent: responsible for deciding what (if any) actions to take.
2. A function to invoke tools: if the agent decides to take an action, this node will then execute that action.

We will also need to define some edges. Some of these edges may be conditional. The reason they are conditional is that based on the output of a node, one of several paths may be taken. The path that is taken is not known until that node is run (the LLM decides).

1. Conditional Edge: after the agent is called, we should either: a. If the agent said to take an action, then the function to invoke tools should be called b. If the agent said that it was finished, then it should finish
2. Normal Edge: after the tools are invoked, it should always go back to the agent to decide what to do next

Let's define the nodes, as well as a function to decide how what conditional edge to take.

In [7]:

from langgraph.prebuilt import ToolInvocation
from langchain_core.messages import ToolMessage


# Define the function that determines whether to continue or not
def should_continue(state):
    messages = state["messages"]
    last_message = messages[-1]
    # If there is no function call, then we finish
    if not last_message.tool_calls:
        return "end"
    # Otherwise if there is, we continue
    else:
        return "continue"


# Define the function that calls the model
def call_model(state):
    messages = state["messages"]
    response = model.invoke(messages)
    # We return a list, because this will get added to the existing list
    return {"messages": [response]}


# Define the function to execute tools
def call_tool(state):
    messages = state["messages"]
    # Based on the continue condition
    # we know the last message involves a function call
    last_message = messages[-1]
    # We construct an ToolInvocation for each tool call
    tool_invocations = []
    for tool_call in last_message.tool_calls:
        action = ToolInvocation(
            tool=tool_call["name"],
            tool_input=tool_call["args"],
        )
        tool_invocations.append(action)

    action = ToolInvocation(
        tool=tool_call["name"],
        tool_input=tool_call["args"],
    )
    # We call the tool_executor and get back a response
    responses = tool_executor.batch(tool_invocations, return_exceptions=True)
    # We use the response to create tool messages
    tool_messages = [
        ToolMessage(
            content=str(response),
            name=tc["name"],
            tool_call_id=tc["id"],
        )
        for tc, response in zip(last_message.tool_calls, responses)
    ]

    # We return a list, because this will get added to the existing list
    return {"messages": tool_messages}

from langgraph.prebuilt import ToolInvocation from langchain_core.messages import ToolMessage # Define the function that determines whether to continue or not def should_continue(state): messages = state["messages"] last_message = messages[-1] # If there is no function call, then we finish if not last_message.tool_calls: return "end" # Otherwise if there is, we continue else: return "continue" # Define the function that calls the model def call_model(state): messages = state["messages"] response = model.invoke(messages) # We return a list, because this will get added to the existing list return {"messages": [response]} # Define the function to execute tools def call_tool(state): messages = state["messages"] # Based on the continue condition # we know the last message involves a function call last_message = messages[-1] # We construct an ToolInvocation for each tool call tool_invocations = [] for tool_call in last_message.tool_calls: action = ToolInvocation( tool=tool_call["name"], tool_input=tool_call["args"], ) tool_invocations.append(action) action = ToolInvocation( tool=tool_call["name"], tool_input=tool_call["args"], ) # We call the tool_executor and get back a response responses = tool_executor.batch(tool_invocations, return_exceptions=True) # We use the response to create tool messages tool_messages = [ ToolMessage( content=str(response), name=tc["name"], tool_call_id=tc["id"], ) for tc, response in zip(last_message.tool_calls, responses) ] # We return a list, because this will get added to the existing list return {"messages": tool_messages}

MODIFICATION

Here we create a node that returns an AIMessage with a tool call - we will use this at the start to force it call a tool

In [16]:

# This is the new first - the first call of the model we want to explicitly hard-code some action
from langchain_core.messages import AIMessage


def first_model(state):
    human_input = state["messages"][-1].content
    return {
        "messages": [
            AIMessage(
                content="",
                tool_calls=[
                    {
                        "name": "tavily_search_results_json",
                        "args": {
                            "query": human_input,
                        },
                        "id": "tool_abcd123",
                    }
                ],
            )
        ]
    }

# This is the new first - the first call of the model we want to explicitly hard-code some action from langchain_core.messages import AIMessage def first_model(state): human_input = state["messages"][-1].content return { "messages": [ AIMessage( content="", tool_calls=[ { "name": "tavily_search_results_json", "args": { "query": human_input, }, "id": "tool_abcd123", } ], ) ] }

Define the graph¶

We can now put it all together and define the graph!

MODIFICATION

We will define a first_agent node which we will set as the entrypoint.

In [17]:

from langgraph.graph import StateGraph, END

# Define a new graph
workflow = StateGraph(AgentState)

# Define the new entrypoint
workflow.add_node("first_agent", first_model)

# Define the two nodes we will cycle between
workflow.add_node("agent", call_model)
workflow.add_node("action", call_tool)

# Set the entrypoint as `agent`
# This means that this node is the first one called
workflow.set_entry_point("first_agent")

# We now add a conditional edge
workflow.add_conditional_edges(
    # First, we define the start node. We use `agent`.
    # This means these are the edges taken after the `agent` node is called.
    "agent",
    # Next, we pass in the function that will determine which node is called next.
    should_continue,
    # Finally we pass in a mapping.
    # The keys are strings, and the values are other nodes.
    # END is a special node marking that the graph should finish.
    # What will happen is we will call `should_continue`, and then the output of that
    # will be matched against the keys in this mapping.
    # Based on which one it matches, that node will then be called.
    {
        # If `tools`, then we call the tool node.
        "continue": "action",
        # Otherwise we finish.
        "end": END,
    },
)

# We now add a normal edge from `tools` to `agent`.
# This means that after `tools` is called, `agent` node is called next.
workflow.add_edge("action", "agent")

# After we call the first agent, we know we want to go to action
workflow.add_edge("first_agent", "action")

# Finally, we compile it!
# This compiles it into a LangChain Runnable,
# meaning you can use it as you would any other runnable
app = workflow.compile()

from langgraph.graph import StateGraph, END # Define a new graph workflow = StateGraph(AgentState) # Define the new entrypoint workflow.add_node("first_agent", first_model) # Define the two nodes we will cycle between workflow.add_node("agent", call_model) workflow.add_node("action", call_tool) # Set the entrypoint as `agent` # This means that this node is the first one called workflow.set_entry_point("first_agent") # We now add a conditional edge workflow.add_conditional_edges( # First, we define the start node. We use `agent`. # This means these are the edges taken after the `agent` node is called. "agent", # Next, we pass in the function that will determine which node is called next. should_continue, # Finally we pass in a mapping. # The keys are strings, and the values are other nodes. # END is a special node marking that the graph should finish. # What will happen is we will call `should_continue`, and then the output of that # will be matched against the keys in this mapping. # Based on which one it matches, that node will then be called. { # If `tools`, then we call the tool node. "continue": "action", # Otherwise we finish. "end": END, }, ) # We now add a normal edge from `tools` to `agent`. # This means that after `tools` is called, `agent` node is called next. workflow.add_edge("action", "agent") # After we call the first agent, we know we want to go to action workflow.add_edge("first_agent", "action") # Finally, we compile it! # This compiles it into a LangChain Runnable, # meaning you can use it as you would any other runnable app = workflow.compile()

In [18]:

from IPython.display import Image, display

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

from IPython.display import Image, display try: display(Image(app.get_graph(xray=True).draw_mermaid_png())) except: # This requires some extra dependencies and is optional pass

Use it!¶

We can now use it! This now exposes the same interface as all other LangChain runnables.

In [19]:

from langchain_core.messages import HumanMessage

inputs = {"messages": [HumanMessage(content="what is the weather in sf")]}
for output in app.stream(inputs):
    # stream() yields dictionaries with output keyed by node name
    for key, value in output.items():
        print(f"Output from node '{key}':")
        print("---")
        print(value)
    print("\n---\n")

from langchain_core.messages import HumanMessage inputs = {"messages": [HumanMessage(content="what is the weather in sf")]} for output in app.stream(inputs): # stream() yields dictionaries with output keyed by node name for key, value in output.items(): print(f"Output from node '{key}':") print("---") print(value) print("\n---\n")

Output from node 'first_agent':
---
{'messages': [AIMessage(content='', tool_calls=[{'name': 'tavily_search_results_json', 'args': {'query': 'what is the weather in sf'}, 'id': 'tool_abcd123'}])]}

---

Output from node 'action':
---
{'messages': [ToolMessage(content='[{\'url\': \'https://www.weatherapi.com/\', \'content\': "{\'location\': {\'name\': \'San Francisco\', \'region\': \'California\', \'country\': \'United States of America\', \'lat\': 37.78, \'lon\': -122.42, \'tz_id\': \'America/Los_Angeles\', \'localtime_epoch\': 1714808650, \'localtime\': \'2024-05-04 0:44\'}, \'current\': {\'last_updated_epoch\': 1714807800, \'last_updated\': \'2024-05-04 00:30\', \'temp_c\': 12.8, \'temp_f\': 55.0, \'is_day\': 0, \'condition\': {\'text\': \'Overcast\', \'icon\': \'//cdn.weatherapi.com/weather/64x64/night/122.png\', \'code\': 1009}, \'wind_mph\': 11.9, \'wind_kph\': 19.1, \'wind_degree\': 240, \'wind_dir\': \'WSW\', \'pressure_mb\': 1013.0, \'pressure_in\': 29.9, \'precip_mm\': 0.0, \'precip_in\': 0.0, \'humidity\': 96, \'cloud\': 100, \'feelslike_c\': 11.4, \'feelslike_f\': 52.4, \'vis_km\': 16.0, \'vis_miles\': 9.0, \'uv\': 1.0, \'gust_mph\': 14.9, \'gust_kph\': 23.9}}"}]', name='tavily_search_results_json', tool_call_id='tool_abcd123')]}

---

Output from node 'agent':
---
{'messages': [AIMessage(content='The current weather in San Francisco is as follows:\n- Temperature: 12.8°C (55.0°F)\n- Condition: Overcast\n- Wind: 11.9 mph from WSW\n- Humidity: 96%\n- Cloud Cover: 100%\n- Visibility: 16.0 km (9.0 miles)\n- UV Index: 1.0\n\nFor more details, you can visit [Weather API](https://www.weatherapi.com/).', response_metadata={'finish_reason': 'stop'}, id='run-57b5d14c-08c3-481d-9875-fc3a9472475c-0')]}

---

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