Develop a LangChain agent

This page shows you how to develop an agent by using the framework-specific LangChain template (the LangchainAgent class in the Vertex AI SDK for Python). The agent returns the exchange rate between two currencies on a specified date. Here are the steps:

Define and configure a model
Define and use a tool
(Optional) Store chat history
(Optional) Customize the prompt template
(Optional) Customize the orchestration

Before you begin

Make sure your environment is set up by following the steps in Set up your environment .

Step 1. Define and configure a model

Define the Model version to use.

  model 
 = 
 "gemini-2.0-flash"

(Optional) Configure the safety settings of the model. To learn more about the options available for safety settings in Gemini, see Configure safety attributes . The following is an example of how you can configure the safety settings:

  from 
  
 langchain_google_vertexai 
  
 import 
 HarmBlockThreshold 
 , 
 HarmCategory 
 safety_settings 
 = 
 { 
 HarmCategory 
 . 
 HARM_CATEGORY_UNSPECIFIED 
 : 
 HarmBlockThreshold 
 . 
 BLOCK_NONE 
 , 
 HarmCategory 
 . 
 HARM_CATEGORY_DANGEROUS_CONTENT 
 : 
 HarmBlockThreshold 
 . 
 BLOCK_MEDIUM_AND_ABOVE 
 , 
 HarmCategory 
 . 
 HARM_CATEGORY_HATE_SPEECH 
 : 
 HarmBlockThreshold 
 . 
 BLOCK_ONLY_HIGH 
 , 
 HarmCategory 
 . 
 HARM_CATEGORY_HARASSMENT 
 : 
 HarmBlockThreshold 
 . 
 BLOCK_LOW_AND_ABOVE 
 , 
 HarmCategory 
 . 
 HARM_CATEGORY_SEXUALLY_EXPLICIT 
 : 
 HarmBlockThreshold 
 . 
 BLOCK_NONE 
 , 
 }

(Optional) Specify model parameters in the following way:

  model_kwargs 
 = 
 { 
 # temperature (float): The sampling temperature controls the degree of 
 # randomness in token selection. 
 "temperature" 
 : 
 0.28 
 , 
 # max_output_tokens (int): The token limit determines the maximum amount of 
 # text output from one prompt. 
 "max_output_tokens" 
 : 
 1000 
 , 
 # top_p (float): Tokens are selected from most probable to least until 
 # the sum of their probabilities equals the top-p value. 
 "top_p" 
 : 
 0.95 
 , 
 # top_k (int): The next token is selected from among the top-k most 
 # probable tokens. This is not supported by all model versions. See 
 # https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/image-understanding#valid_parameter_values 
 # for details. 
 "top_k" 
 : 
 None 
 , 
 # safety_settings (Dict[HarmCategory, HarmBlockThreshold]): The safety 
 # settings to use for generating content. 
 # (you must create your safety settings using the previous step first). 
 "safety_settings" 
 : 
 safety_settings 
 , 
 }

Create a LangchainAgent using the model configurations:

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 # Required. 
 model_kwargs 
 = 
 model_kwargs 
 , 
 # Optional. 
 )

If you are running in an interactive environment (e.g. terminal or Colab notebook), you can run a query as an intermediate testing step:

  response 
 = 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "What is the exchange rate from US dollars to SEK today?" 
 ) 
 print 
 ( 
 response 
 )

The response is a Python dictionary similar to the following example:

  { 
 "input" 
 : 
 "What is the exchange rate from US dollars to Swedish currency?" 
 , 
 "output" 
 : 
 """I cannot provide the live exchange rate from US dollars to Swedish currency (Swedish krona, SEK). 
 **Here's why:** 
 * **Exchange rates constantly fluctuate.** Factors like global economics, interest rates, and political events cause 
 these changes throughout the day. 
 * **Providing inaccurate information would be misleading.** 
 **How to find the current exchange rate:** 
 1. **Use a reliable online converter:** Many websites specialize in live exchange rates. Some popular options include: 
 * Google Finance (google.com/finance) 
 * XE.com 
 * Bank websites (like Bank of America, Chase, etc.) 
 2. **Contact your bank or financial institution:** They can give you the exact exchange rate they are using. 
 Remember to factor in any fees or commissions when exchanging currency. 
 """ 
 }

(Optional) Advanced customization

The LangchainAgent template uses ChatVertexAI by default, because it provides access to all foundational models available in Google Cloud. To use a model that is not available through ChatVertexAI , you can specify the model_builder= argument, with a Python function of the following signature:

  from 
  
 typing 
  
 import 
 Optional 
 def 
  
 model_builder 
 ( 
 * 
 , 
 model_name 
 : 
 str 
 , 
 # Required. The name of the model 
 model_kwargs 
 : 
 Optional 
 [ 
 dict 
 ] 
 = 
 None 
 , 
 # Optional. The model keyword arguments. 
 ** 
 kwargs 
 , 
 # Optional. The remaining keyword arguments to be ignored. 
 ):

For a list of the chat models supported in LangChain and their capabilities, see Chat Models . The set of supported values for model= and model_kwargs= are specific to each chat model, so you have to refer to their corresponding documentation for details.

ChatVertexAI

Installed by default.

It is used in LangchainAgent when you omit the model_builder argument, for example

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 # Required. 
 model_kwargs 
 = 
 model_kwargs 
 , 
 # Optional. 
 )

ChatAnthropic

First, follow their documentation to set up an account and install the package.

Next, define a model_builder that returns ChatAnthropic :

  def 
  
 model_builder 
 ( 
 * 
 , 
 model_name 
 : 
 str 
 , 
 model_kwargs 
 = 
 None 
 , 
 ** 
 kwargs 
 ): 
 from 
  
 langchain_anthropic 
  
 import 
 ChatAnthropic 
 return 
 ChatAnthropic 
 ( 
 model_name 
 = 
 model_name 
 , 
 ** 
 model_kwargs 
 )

Finally, use it in LangchainAgent with the following code:

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 "claude-3-opus-20240229" 
 , 
 # Required. 
 model_builder 
 = 
 model_builder 
 , 
 # Required. 
 model_kwargs 
 = 
 { 
 "api_key" 
 : 
 " ANTHROPIC_API_KEY 
" 
 , 
 # Required. 
 "temperature" 
 : 
 0.28 
 , 
 # Optional. 
 "max_tokens" 
 : 
 1000 
 , 
 # Optional. 
 }, 
 )

ChatOpenAI

You can use ChatOpenAI in conjunction with Gemini's ChatCompletions API .

First, follow their documentation to install the package.

Next, define a model_builder that returns ChatOpenAI :

  def 
  
 model_builder 
 ( 
 * 
 , 
 model_name 
 : 
 str 
 , 
 model_kwargs 
 = 
 None 
 , 
 project 
 : 
 str 
 , 
 # Specified via vertexai.init 
 location 
 : 
 str 
 , 
 # Specified via vertexai.init 
 ** 
 kwargs 
 , 
 ): 
 import 
  
 google.auth 
 from 
  
 langchain_openai 
  
 import 
 ChatOpenAI 
 # Note: the credential lives for 1 hour by default. 
 # After expiration, it must be refreshed. 
 creds 
 , 
 _ 
 = 
 google 
 . 
 auth 
 . 
 default 
 ( 
 scopes 
 = 
 [ 
 "https://www.googleapis.com/auth/cloud-platform" 
 ]) 
 auth_req 
 = 
 google 
 . 
 auth 
 . 
 transport 
 . 
 requests 
 . 
 Request 
 () 
 creds 
 . 
 refresh 
 ( 
 auth_req 
 ) 
 if 
 model_kwargs 
 is 
 None 
 : 
 model_kwargs 
 = 
 {} 
 endpoint 
 = 
 f 
 "https:// 
 { 
 location 
 } 
 -aiplatform.googleapis.com" 
 base_url 
 = 
 f 
 ' 
 { 
 endpoint 
 } 
 /v1beta1/projects/ 
 { 
 project 
 } 
 /locations/ 
 { 
 location 
 } 
 /endpoints/openapi' 
 return 
 ChatOpenAI 
 ( 
 model 
 = 
 model_name 
 , 
 base_url 
 = 
 base_url 
 , 
 api_key 
 = 
 creds 
 . 
 token 
 , 
 ** 
 model_kwargs 
 , 
 )

Finally, use it in LangchainAgent with the following code:

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 "google/gemini-2.0-flash" 
 , 
 # Or "meta/llama3-405b-instruct-maas" 
 model_builder 
 = 
 model_builder 
 , 
 # Required. 
 model_kwargs 
 = 
 { 
 "temperature" 
 : 
 0 
 , 
 # Optional. 
 "max_retries" 
 : 
 2 
 , 
 # Optional. 
 }, 
 )

Step 2. Define and use a tool

After you define your model, the next step is to define the tools that your model uses for reasoning. A tool can be a LangChain tool or a Python function. You can also convert a defined Python function to a LangChain Tool .

When you define your function, it's important to include comments that fully and clearly describe the function's parameters, what the function does, and what the function returns. This information is used by the model to determine which function to use. You must also test your function locally to confirm that it works.

Use the following code to define a function that returns an exchange rate:

  def 
  
 get_exchange_rate 
 ( 
 currency_from 
 : 
 str 
 = 
 "USD" 
 , 
 currency_to 
 : 
 str 
 = 
 "EUR" 
 , 
 currency_date 
 : 
 str 
 = 
 "latest" 
 , 
 ): 
  
 """Retrieves the exchange rate between two currencies on a specified date. 
 Uses the Frankfurter API (https://api.frankfurter.app/) to obtain 
 exchange rate data. 
 Args: 
 currency_from: The base currency (3-letter currency code). 
 Defaults to "USD" (US Dollar). 
 currency_to: The target currency (3-letter currency code). 
 Defaults to "EUR" (Euro). 
 currency_date: The date for which to retrieve the exchange rate. 
 Defaults to "latest" for the most recent exchange rate data. 
 Can be specified in YYYY-MM-DD format for historical rates. 
 Returns: 
 dict: A dictionary containing the exchange rate information. 
 Example: {"amount": 1.0, "base": "USD", "date": "2023-11-24", 
 "rates": {"EUR": 0.95534}} 
 """ 
 import 
  
 requests 
 response 
 = 
 requests 
 . 
 get 
 ( 
 f 
 "https://api.frankfurter.app/ 
 { 
 currency_date 
 } 
 " 
 , 
 params 
 = 
 { 
 "from" 
 : 
 currency_from 
 , 
 "to" 
 : 
 currency_to 
 }, 
 ) 
 return 
 response 
 . 
 json 
 ()

To test the function before you use it in your agent, run the following:

  get_exchange_rate 
 ( 
 currency_from 
 = 
 "USD" 
 , 
 currency_to 
 = 
 "SEK" 
 )

The response should be similar to the following:

  { 
 'amount' 
 : 
 1.0 
 , 
 'base' 
 : 
 'USD' 
 , 
 'date' 
 : 
 '2024-02-22' 
 , 
 'rates' 
 : 
 { 
 'SEK' 
 : 
 10.3043 
 }}

To use the tool inside LangchainAgent , you will add it to the list of tools under the tools= argument:

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 # Required. 
 tools 
 = 
 [ 
 get_exchange_rate 
 ], 
 # Optional. 
 model_kwargs 
 = 
 model_kwargs 
 , 
 # Optional. 
 )

You can test the agent locally by performing test queries against it. Run the following command to test the agent locally using US dollars and Swedish Krona:

  response 
 = 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "What is the exchange rate from US dollars to Swedish currency?" 
 )

The response is a dictionary that's similar to the following:

  { 
 "input" 
 : 
 "What is the exchange rate from US dollars to Swedish currency?" 
 , 
 "output" 
 : 
 "For 1 US dollar you will get 10.7345 Swedish Krona." 
 }

(Optional) Multiple tools

Tools for LangchainAgent can be defined and instantiated in other ways.

Grounding Tool

First, import the generate_models package and create the tool

  from 
  
 vertexai.generative_models 
  
 import 
  grounding 
 
 , 
 Tool 
 grounded_search_tool 
 = 
 Tool 
 . 
 from_google_search_retrieval 
 ( 
  grounding 
 
 . 
  GoogleSearchRetrieval 
 
 () 
 )

Next, use the tool inside LangchainAgent :

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 tools 
 = 
 [ 
 grounded_search_tool 
 ], 
 ) 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "When is the next total solar eclipse in US?" 
 )

The response is a dictionary that is similar to the following:

  { 
 "input" 
 : 
 "When is the next total solar eclipse in US?" 
 , 
 "output" 
 : 
 """The next total solar eclipse in the U.S. will be on August 23, 2044. 
 This eclipse will be visible from three states: Montana, North Dakota, and 
 South Dakota. The path of totality will begin in Greenland, travel through 
 Canada, and end around sunset in the United States.""" 
 }

For details, visit Grounding .

LangChain Tool

First, install the package that defines the tool.

 pip  
install  
langchain-google-community

Next, import the package and create the tool.

  from 
  
 langchain_google_community 
  
 import 
 VertexAISearchRetriever 
 from 
  
 langchain.tools.retriever 
  
 import 
 create_retriever_tool 
 retriever 
 = 
 VertexAISearchRetriever 
 ( 
 project_id 
 = 
 " PROJECT_ID 
" 
 , 
 data_store_id 
 = 
 " DATA_STORE_ID 
" 
 , 
 location_id 
 = 
 " DATA_STORE_LOCATION_ID 
" 
 , 
 engine_data_type 
 = 
 1 
 , 
 max_documents 
 = 
 10 
 , 
 ) 
 movie_search_tool 
 = 
 create_retriever_tool 
 ( 
 retriever 
 = 
 retriever 
 , 
 name 
 = 
 "search_movies" 
 , 
 description 
 = 
 "Searches information about movies." 
 , 
 )

Finally, use the tool inside LangchainAgent :

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 tools 
 = 
 [ 
 movie_search_tool 
 ], 
 ) 
 response 
 = 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "List some sci-fi movies from the 1990s" 
 , 
 )

It should return a response such as

  { 
 "input" 
 : 
 "List some sci-fi movies from the 1990s" 
 , 
 "output" 
 : 
 """Here are some sci-fi movies from the 1990s: 
 * The Matrix (1999): A computer hacker learns from mysterious rebels about the true nature of his reality and his role in the war against its controllers. 
 * Star Wars: Episode I - The Phantom Menace (1999): Two Jedi Knights escape a hostile blockade to find a queen and her protector, and come across a young boy [...] 
 * Men in Black (1997): A police officer joins a secret organization that monitors extraterrestrial interactions on Earth. 
 [...] 
 """ 
 }

To see the full example, visit the notebook .

For more examples of tools available in LangChain, visit Google Tools .

Vertex AI Extension

First, import the extensions package and create the tool

  from 
  
 typing 
  
 import 
 Optional 
 def 
  
 generate_and_execute_code 
 ( 
 query 
 : 
 str 
 , 
 files 
 : 
 Optional 
 [ 
 list 
 [ 
 str 
 ]] 
 = 
 None 
 , 
 file_gcs_uris 
 : 
 Optional 
 [ 
 list 
 [ 
 str 
 ]] 
 = 
 None 
 , 
 ) 
 - 
> str 
 : 
  
 """Get the results of a natural language query by generating and executing 
 a code snippet. 
 Example queries: "Find the max in [1, 2, 5]" or "Plot average sales by 
 year (from data.csv)". Only one of `file_gcs_uris` and `files` field 
 should be provided. 
 Args: 
 query: 
 The natural language query to generate and execute. 
 file_gcs_uris: 
 Optional. URIs of input files to use when executing the code 
 snippet. For example, ["gs://input-bucket/data.csv"]. 
 files: 
 Optional. Input files to use when executing the generated code. 
 If specified, the file contents are expected be base64-encoded. 
 For example: [{"name": "data.csv", "contents": "aXRlbTEsaXRlbTI="}]. 
 Returns: 
 The results of the query. 
 """ 
 operation_params 
 = 
 { 
 "query" 
 : 
 query 
 } 
 if 
 files 
 : 
 operation_params 
 [ 
 "files" 
 ] 
 = 
 files 
 if 
 file_gcs_uris 
 : 
 operation_params 
 [ 
 "file_gcs_uris" 
 ] 
 = 
 file_gcs_uris 
 from 
  
 vertexai.preview 
  
 import 
 extensions 
 # If you have an existing extension instance, you can get it here 
 # i.e. code_interpreter = extensions.Extension(resource_name). 
 code_interpreter 
 = 
 extensions 
 . 
 Extension 
 . 
 from_hub 
 ( 
 "code_interpreter" 
 ) 
 return 
 extensions 
 . 
 Extension 
 . 
 from_hub 
 ( 
 "code_interpreter" 
 ) 
 . 
 execute 
 ( 
 operation_id 
 = 
 "generate_and_execute" 
 , 
 operation_params 
 = 
 operation_params 
 , 
 )

Next, use the tool inside LangchainAgent :

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 tools 
 = 
 [ 
 generate_and_execute_code 
 ], 
 ) 
 agent 
 . 
 query 
 ( 
 input 
 = 
 """Using the data below, construct a bar chart that includes only the height values with different colors for the bars: 
 tree_heights_prices = { 
  
 \" 
 Pine 
 \" 
 : { 
 \" 
 height 
 \" 
 : 100, 
 \" 
 price 
 \" 
 : 100}, 
  
 \" 
 Oak 
 \" 
 : { 
 \" 
 height 
 \" 
 : 65, 
 \" 
 price 
 \" 
 : 135}, 
  
 \" 
 Birch 
 \" 
 : { 
 \" 
 height 
 \" 
 : 45, 
 \" 
 price 
 \" 
 : 80}, 
  
 \" 
 Redwood 
 \" 
 : { 
 \" 
 height 
 \" 
 : 200, 
 \" 
 price 
 \" 
 : 200}, 
  
 \" 
 Fir 
 \" 
 : { 
 \" 
 height 
 \" 
 : 180, 
 \" 
 price 
 \" 
 : 162}, 
 } 
 """ 
 )

It should return a response such as

  { 
 "input" 
 : 
 """Using the data below, construct a bar chart that includes only the height values with different colors for the bars: 
 tree_heights_prices = { 
  
 \" 
 Pine 
 \" 
 : { 
 \" 
 height 
 \" 
 : 100, 
 \" 
 price 
 \" 
 : 100}, 
  
 \" 
 Oak 
 \" 
 : { 
 \" 
 height 
 \" 
 : 65, 
 \" 
 price 
 \" 
 : 135}, 
  
 \" 
 Birch 
 \" 
 : { 
 \" 
 height 
 \" 
 : 45, 
 \" 
 price 
 \" 
 : 80}, 
  
 \" 
 Redwood 
 \" 
 : { 
 \" 
 height 
 \" 
 : 200, 
 \" 
 price 
 \" 
 : 200}, 
  
 \" 
 Fir 
 \" 
 : { 
 \" 
 height 
 \" 
 : 180, 
 \" 
 price 
 \" 
 : 162}, 
 } 
 """ 
 , 
 "output" 
 : 
 """Here's the generated bar chart: 
 ```python 
 import matplotlib.pyplot as plt 
 tree_heights_prices = { 
 "Pine": {"height": 100, "price": 100}, 
 "Oak": {"height": 65, "price": 135}, 
 "Birch": {"height": 45, "price": 80}, 
 "Redwood": {"height": 200, "price": 200}, 
 "Fir": {"height": 180, "price": 162}, 
 } 
 heights = [tree["height"] for tree in tree_heights_prices.values()] 
 names = list(tree_heights_prices.keys()) 
 plt.bar(names, heights, color=['red', 'green', 'blue', 'purple', 'orange']) 
 plt.xlabel('Tree Species') 
 plt.ylabel('Height') 
 plt.title('Tree Heights') 
 plt.show() 
 ``` 
 """ 
 }

For your deployed agent to access the Code Interpreter extension, you must add the Vertex AI User role ( roles/aiplatform.user ) to the AI Platform Reasoning Engine Service Agent service account. For more information, see Managing access .

For details, visit Vertex AI Extensions .

You can use all (or a subset) of the tools you've created in LangchainAgent :

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 tools 
 = 
 [ 
 get_exchange_rate 
 , 
 # Optional (Python function) 
 grounded_search_tool 
 , 
 # Optional (Grounding Tool) 
 movie_search_tool 
 , 
 # Optional (Langchain Tool) 
 generate_and_execute_code 
 , 
 # Optional (Vertex Extension) 
 ], 
 ) 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "When is the next total solar eclipse in US?" 
 )

(Optional) Tool configuration

With Gemini, you can place constraints on tool usage. For example, instead of allowing the model to generate natural language responses, you can force it to only generate function calls ("forced function calling").

  from 
  
 vertexai 
  
 import 
 agent_engines 
 from 
  
 vertexai.preview.generative_models 
  
 import 
 ToolConfig 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 "gemini-2.0-flash" 
 , 
 tools 
 = 
 [ 
 search_arxiv 
 , 
 get_exchange_rate 
 ], 
 model_tool_kwargs 
 = 
 { 
 "tool_config" 
 : 
 { 
 # Specify the tool configuration here. 
 "function_calling_config" 
 : 
 { 
 "mode" 
 : 
 ToolConfig 
 . 
 FunctionCallingConfig 
 . 
 Mode 
 . 
 ANY 
 , 
 "allowed_function_names" 
 : 
 [ 
 "search_arxiv" 
 , 
 "get_exchange_rate" 
 ], 
 }, 
 }, 
 }, 
 ) 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "Explain the Schrodinger equation in a few sentences" 
 , 
 )

For details, visit Tool Configuration .

Step 3. Store chat history

To track chat messages and append them to a database, define a get_session_history function and pass it in when you create the agent. This function should take in a session_id and return a BaseChatMessageHistory object.

session_id is an identifier for the session that these input messages belong to. This lets you maintain several conversations at the same time.
BaseChatMessageHistory is the interface for classes that can load and save message objects.

Set up a database

For a list of the ChatMessageHistory providers from Google that are supported in LangChain, see Memory .

First, follow LangChain's documentation to install and use the relevant package to set up a database of your choice (e.g. Firestore, Bigtable, or Spanner):

Next, define a get_session_history function as follows:

Firestore (Native Mode)

  def 
  
 get_session_history 
 ( 
 session_id 
 : 
 str 
 ): 
 from 
  
 langchain_google_firestore 
  
 import 
 FirestoreChatMessageHistory 
 from 
  
 google.cloud 
  
 import 
 firestore 
 client 
 = 
 firestore 
 . 
  Client 
 
 ( 
 project 
 = 
 " PROJECT_ID 
" 
 ) 
 return 
 FirestoreChatMessageHistory 
 ( 
 client 
 = 
 client 
 , 
 session_id 
 = 
 session_id 
 , 
 collection 
 = 
 " TABLE_NAME 
" 
 , 
 encode_message 
 = 
 False 
 , 
 )

Bigtable

  def 
  
 get_session_history 
 ( 
 session_id 
 : 
 str 
 ): 
 from 
  
 langchain_google_bigtable 
  
 import 
 BigtableChatMessageHistory 
 return 
 BigtableChatMessageHistory 
 ( 
 instance_id 
 = 
 " INSTANCE_ID 
" 
 , 
 table_id 
 = 
 " TABLE_NAME 
" 
 , 
 session_id 
 = 
 session_id 
 , 
 )

Spanner

  def 
  
 get_session_history 
 ( 
 session_id 
 : 
 str 
 ): 
 from 
  
 langchain_google_spanner 
  
 import 
 SpannerChatMessageHistory 
 return 
 SpannerChatMessageHistory 
 ( 
 instance_id 
 = 
 " INSTANCE_ID 
" 
 , 
 database_id 
 = 
 " DATABASE_ID 
" 
 , 
 table_name 
 = 
 " TABLE_NAME 
" 
 , 
 session_id 
 = 
 session_id 
 , 
 )

Finally, create the agent and pass it in as chat_history :

  from 
  
 vertexai 
  
 import 
 agent_engines 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 chat_history 
 = 
 get_session_history 
 , 
 # <- new 
 )

When querying the agent, make sure you pass in the session_id so that the agent has "memory" of past questions and answers:

  agent 
 . 
 query 
 ( 
 input 
 = 
 "What is the exchange rate from US dollars to Swedish currency?" 
 , 
 config 
 = 
 { 
 "configurable" 
 : 
 { 
 "session_id" 
 : 
 " SESSION_ID 
" 
 }}, 
 )

You can check that subsequent queries will retain memory of the session:

  response 
 = 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "How much is 100 USD?" 
 , 
 config 
 = 
 { 
 "configurable" 
 : 
 { 
 "session_id" 
 : 
 " SESSION_ID 
" 
 }}, 
 ) 
 print 
 ( 
 response 
 )

Step 4. Customize the prompt template

Prompt templates help to translate user input into instructions for a model, and are used to guide a model's response, helping it understand the context and generate relevant and coherent language-based output. For details, visit ChatPromptTemplates .

The default prompt template is organized sequentially into sections.

Section	Description
(Optional) System instruction	Instructions for the agent to be applied across all queries.
(Optional) Chat history	Messages corresponding to the chat history from a past session.
User input	The query from the user for the agent to respond to.
Agent Scratchpad	Messages created by the agent (e.g. with function calling) as it performs uses its tools and performs reasoning to formulate a response to the user.

The default prompt template is generated if you create the agent without specifying your own prompt template, and will look like the following in full:

  from 
  
 langchain_core.prompts 
  
 import 
 ChatPromptTemplate 
 from 
  
 langchain.agents.format_scratchpad.tools 
  
 import 
 format_to_tool_messages 
 prompt_template 
 = 
 { 
 "user_input" 
 : 
 lambda 
 x 
 : 
 x 
 [ 
 "input" 
 ], 
 "history" 
 : 
 lambda 
 x 
 : 
 x 
 [ 
 "history" 
 ], 
 "agent_scratchpad" 
 : 
 lambda 
 x 
 : 
 format_to_tool_messages 
 ( 
 x 
 [ 
 "intermediate_steps" 
 ]), 
 } 
 | 
 ChatPromptTemplate 
 . 
 from_messages 
 ([ 
 ( 
 "system" 
 , 
 " 
 {system_instruction} 
 " 
 ), 
 ( 
 "placeholder" 
 , 
 " 
 {history} 
 " 
 ), 
 ( 
 "user" 
 , 
 " 
 {user_input} 
 " 
 ), 
 ( 
 "placeholder" 
 , 
 " 
 {agent_scratchpad} 
 " 
 ), 
 ])

You are implicitly using the full prompt template when you instantiate the agent in the following example:

  from 
  
 vertexai 
  
 import 
 agent_engines 
 system_instruction 
 = 
 "I help look up the rate between currencies" 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 system_instruction 
 = 
 system_instruction 
 , 
 chat_history 
 = 
 get_session_history 
 , 
 tools 
 = 
 [ 
 get_exchange_rate 
 ], 
 )

You can override the default prompt template with your own prompt template, and use it when constructing the agent, for example:

  from 
  
 vertexai 
  
 import 
 agent_engines 
 custom_prompt_template 
 = 
 { 
 "user_input" 
 : 
 lambda 
 x 
 : 
 x 
 [ 
 "input" 
 ], 
 "history" 
 : 
 lambda 
 x 
 : 
 x 
 [ 
 "history" 
 ], 
 "agent_scratchpad" 
 : 
 lambda 
 x 
 : 
 format_to_tool_messages 
 ( 
 x 
 [ 
 "intermediate_steps" 
 ]), 
 } 
 | 
 ChatPromptTemplate 
 . 
 from_messages 
 ([ 
 ( 
 "placeholder" 
 , 
 " 
 {history} 
 " 
 ), 
 ( 
 "user" 
 , 
 " 
 {user_input} 
 " 
 ), 
 ( 
 "placeholder" 
 , 
 " 
 {agent_scratchpad} 
 " 
 ), 
 ]) 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 prompt 
 = 
 custom_prompt_template 
 , 
 chat_history 
 = 
 get_session_history 
 , 
 tools 
 = 
 [ 
 get_exchange_rate 
 ], 
 ) 
 agent 
 . 
 query 
 ( 
 input 
 = 
 "What is the exchange rate from US dollars to Swedish currency?" 
 , 
 config 
 = 
 { 
 "configurable" 
 : 
 { 
 "session_id" 
 : 
 " SESSION_ID 
" 
 }}, 
 )

Step 5. Customize the orchestration

All LangChain components implement the Runnable interface , which provide input and output schemas for orchestration. The LangchainAgent class requires a runnable to be built for it to respond to queries. By default, LangchainAgent will build such a runnable by binding the model with tools and use an AgentExecutor that is wrapped into a RunnableWithMessageHistory if chat history is enabled .

You might want to customize the orchestration if you intend to (i) implement an agent that performs a deterministic set of steps (rather than to perform open-ended reasoning), or (ii) prompt the Agent in a ReAct-like fashion to annotate each step with thoughts for why it performed that step. To do so, you have to override the default runnable when creating LangchainAgent by specifying the runnable_builder= argument with a Python function of the following signature:

  from 
  
 typing 
  
 import 
 Optional 
 from 
  
 langchain_core.language_models 
  
 import 
 BaseLanguageModel 
 def 
  
 runnable_builder 
 ( 
 model 
 : 
 BaseLanguageModel 
 , 
 * 
 , 
 system_instruction 
 : 
 Optional 
 [ 
 str 
 ] 
 = 
 None 
 , 
 prompt 
 : 
 Optional 
 [ 
 "RunnableSerializable" 
 ] 
 = 
 None 
 , 
 tools 
 : 
 Optional 
 [ 
 Sequence 
 [ 
 "_ToolLike" 
 ]] 
 = 
 None 
 , 
 chat_history 
 : 
 Optional 
 [ 
 "GetSessionHistoryCallable" 
 ] 
 = 
 None 
 , 
 model_tool_kwargs 
 : 
 Optional 
 [ 
 Mapping 
 [ 
 str 
 , 
 Any 
 ]] 
 = 
 None 
 , 
 agent_executor_kwargs 
 : 
 Optional 
 [ 
 Mapping 
 [ 
 str 
 , 
 Any 
 ]] 
 = 
 None 
 , 
 runnable_kwargs 
 : 
 Optional 
 [ 
 Mapping 
 [ 
 str 
 , 
 Any 
 ]] 
 = 
 None 
 , 
 ** 
 kwargs 
 , 
 ):

where

model corresponds to the chat model being returned from the model_builder (see Define and configure a model ),
tools and model_tool_kwargs corresponds to the tools and configurations to be used (see Define and use a tool ),
chat_history corresponds to the database for storing chat messages (see Store chat history ),
system_instruction and prompt corresponds to the prompt configuration (see Customize the prompt template ),
agent_executor_kwargs and runnable_kwargs are the keyword arguments you can use for customizing the runnable to be built.

This gives different options for customizing the orchestration logic.

ChatModel

In the simplest case, to create an agent without orchestration, you can override the runnable_builder for LangchainAgent to return the model directly.

  from 
  
 vertexai 
  
 import 
 agent_engines 
 from 
  
 langchain_core.language_models 
  
 import 
 BaseLanguageModel 
 def 
  
 llm_builder 
 ( 
 model 
 : 
 BaseLanguageModel 
 , 
 ** 
 kwargs 
 ): 
 return 
 model 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 runnable_builder 
 = 
 llm_builder 
 , 
 )

ReAct

To override the default tool-calling behavior with your own ReAct agent based on your own prompt (see Customize the Prompt Template ), you need to override the runnable_builder for LangchainAgent .

  from 
  
 typing 
  
 import 
 Sequence 
 from 
  
 langchain_core.language_models 
  
 import 
 BaseLanguageModel 
 from 
  
 langchain_core.prompts 
  
 import 
 BasePromptTemplate 
 from 
  
 langchain_core.tools 
  
 import 
 BaseTool 
 from 
  
 langchain 
  
 import 
 hub 
 from 
  
 vertexai 
  
 import 
 agent_engines 
 def 
  
 react_builder 
 ( 
 model 
 : 
 BaseLanguageModel 
 , 
 * 
 , 
 tools 
 : 
 Sequence 
 [ 
 BaseTool 
 ], 
 prompt 
 : 
 BasePromptTemplate 
 , 
 agent_executor_kwargs 
 = 
 None 
 , 
 ** 
 kwargs 
 , 
 ): 
 from 
  
 langchain.agents.react.agent 
  
 import 
 create_react_agent 
 from 
  
 langchain.agents 
  
 import 
 AgentExecutor 
 agent 
 = 
 create_react_agent 
 ( 
 model 
 , 
 tools 
 , 
 prompt 
 ) 
 return 
 AgentExecutor 
 ( 
 agent 
 = 
 agent 
 , 
 tools 
 = 
 tools 
 , 
 ** 
 agent_executor_kwargs 
 ) 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 tools 
 = 
 [ 
 get_exchange_rate 
 ], 
 prompt 
 = 
 hub 
 . 
 pull 
 ( 
 "hwchase17/react" 
 ), 
 agent_executor_kwargs 
 = 
 { 
 "verbose" 
 : 
 True 
 }, 
 # Optional. For illustration. 
 runnable_builder 
 = 
 react_builder 
 , 
 )

LCEL Syntax

To construct the following graph using LangChain Expression Language (LCEL),

 Input
   /   \
 Pros  Cons
   \   /
  Summary

you need to override the runnable_builder for LangchainAgent :

  from 
  
 vertexai 
  
 import 
 agent_engines 
 def 
  
 lcel_builder 
 ( 
 * 
 , 
 model 
 , 
 ** 
 kwargs 
 ): 
 from 
  
 operator 
  
 import 
 itemgetter 
 from 
  
 langchain_core.prompts 
  
 import 
 ChatPromptTemplate 
 from 
  
 langchain_core.runnables 
  
 import 
 RunnablePassthrough 
 from 
  
 langchain_core.output_parsers 
  
 import 
 StrOutputParser 
 output_parser 
 = 
 StrOutputParser 
 () 
 planner 
 = 
 ChatPromptTemplate 
 . 
 from_template 
 ( 
 "Generate an argument about: 
 {input} 
 " 
 ) 
 | 
 model 
 | 
 output_parser 
 | 
 { 
 "argument" 
 : 
 RunnablePassthrough 
 ()} 
 pros 
 = 
 ChatPromptTemplate 
 . 
 from_template 
 ( 
 "List the positive aspects of 
 {argument} 
 " 
 ) 
 | 
 model 
 | 
 output_parser 
 cons 
 = 
 ChatPromptTemplate 
 . 
 from_template 
 ( 
 "List the negative aspects of 
 {argument} 
 " 
 ) 
 | 
 model 
 | 
 output_parser 
 final_responder 
 = 
 ChatPromptTemplate 
 . 
 from_template 
 ( 
 "Argument: 
 {argument} 
 \n 
 Pros: 
 \n 
 {pros} 
 \n\n 
 Cons: 
 \n 
 {cons} 
 \n 
 " 
 "Generate a final response given the critique" 
 , 
 ) 
 | 
 model 
 | 
 output_parser 
 return 
 planner 
 | 
 { 
 "pros" 
 : 
 pros 
 , 
 "cons" 
 : 
 cons 
 , 
 "argument" 
 : 
 itemgetter 
 ( 
 "argument" 
 ), 
 } 
 | 
 final_responder 
 agent 
 = 
 agent_engines 
 . 
 LangchainAgent 
 ( 
 model 
 = 
 model 
 , 
 runnable_builder 
 = 
 lcel_builder 
 , 
 )

LangGraph

To construct the following graph using LangGraph ,

 Input
   /   \
 Pros  Cons
   \   /
  Summary