Using Gemini API in LangChain: Step-by-Step Tutorial

What is LangChain and Why Use It?

LangChain is an open-source framework that simplifies the use of Large Language Models (LLMs) like OpenAI, Gemini (Google), and others by adding structure, tools, and memory to help build real-world applications such as chatbots, assistants, agents, or AI-enhanced software.

Why Use LangChain for LLM Projects?

Chainable Components: Easily build pipelines combining prompts, LLMs, tools, and memory.
Multi-Model Support: Work with Gemini, OpenAI, Anthropic, Hugging Face, etc.
Built-in Templates: Manage prompts more effectively.
Supports Multi-Turn Chat: Manage complex interactions with memory and roles.
Tool and API Integration: Let the model interact with external APIs or functions.

Let's Walk Through the Code: Gemini + LangChain

I will break the code into 4 main parts, each showcasing different features of LangChain and Gemini API.

Part 1: Basic Gemini API Call Using LangChain

import os
from dotenv import load_dotenv
load_dotenv(".env")  # Load environment variables from .env file
api_key = os.getenv("GOOGLE_API_KEY")  # Get Google API Key securely

# Import Gemini LLM wrapper from LangChain
from langchain_google_genai import ChatGoogleGenerativeAI

# Initialize the Gemini model with specific configuration
llm = ChatGoogleGenerativeAI(model='gemini-2.0-flash', temperature=0.9)

# Send a prompt to the model and get a response
response = llm.invoke('Explain what ChatGoogleGenerativeAI function do in langchain_google_genai.')
print(response.content)

Function Explanation:

load_dotenv(".env"): Loads your .env file which stores your API key.
os.getenv(): Retrieves your Gemini API key without hardcoding it in the script.
ChatGoogleGenerativeAI(): Initializes the Gemini model with the name and temperature (creativity level).
invoke(): Sends a prompt to the LLM and returns a response object.

This is a basic setup that demonstrates how to call Gemini via LangChain and print the result.

Part 2: Using PromptTemplate and Chain Composition

from langchain.prompts import PromptTemplate

llm = ChatGoogleGenerativeAI(model='gemini-2.0-flash', temperature=0.9)

# Define a reusable prompt structure with a placeholder
prompt = PromptTemplate.from_template('You are python coding agent. Explain me how to use {message}.')

# Chain the prompt template with the model using the pipe operator
chain = prompt | llm

# Provide the actual input for the placeholder
message = 'heapq'
response = chain.invoke(input=message)
print(response.content)

Function Explanation:

PromptTemplate.from_template(): Creates a parameterized prompt where {message} is replaced dynamically.
| (pipe operator): Chains the prompt and the LLM, forming a pipeline: prompt → LLM.
invoke(input=message): Passes the actual user input ('heapq') to the chain and gets the final output.

This part shows how to modularize your prompts and chain components together, which is a powerful pattern in LangChain.

Part 3: Using System and Human Messages for Role-Based Input

from langchain_core.messages import HumanMessage, SystemMessage

llm_pro = ChatGoogleGenerativeAI(model='gemini-2.5-pro-exp-03-25', temperature=0.9)

# Create structured messages indicating who is speaking (system vs human)
output = llm_pro.invoke([
    SystemMessage(content='Answer within 100 charicter in Korean.'),
    HumanMessage(content='Forcasting Amazon stock price for next week.')
])
print(output.content)

Function Explanation:

SystemMessage(): Gives the model background behavior (e.g., how to answer).
HumanMessage(): Simulates the user’s actual question or prompt.
Passing a list of messages: Supports multi-turn conversation or giving context with different roles.

This part introduces the chat-based message structure—important for building assistants with clear roles and behavior instructions.

Part 4: Using ChatPromptTemplate with Role Keywords

from langchain_core.prompts import ChatPromptTemplate

llm_pro = ChatGoogleGenerativeAI(model='gemini-2.5-pro-exp-03-25', temperature=0.9)

# Define a chat-style prompt template with role-based content
prompt = ChatPromptTemplate([
    ('system', 'Answer within 100 charicter in Korean.'),
    ('user', 'Forcasting {message} stock price for next week.')
])

# Create a chain from the prompt to the model
chain = prompt | llm_pro

# Replace the variable in the prompt template
message = 'Amazon'
response = chain.invoke(input=message)
print(response.content)

Function Explanation:

ChatPromptTemplate(): Provides a flexible way to define role-specific messages using keywords like 'system' and 'user'.
Templates with variables: {message} is dynamically filled in at runtime.
Prompt Chaining: Works just like before—template flows into the LLM.

This version is cleaner and more scalable than manually defining SystemMessage and HumanMessage.

2025.04.23 - [AI] - Building a Simple LLM Chatbot Server Using Google Gemini 2.5 Pro and LangChain

References

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