Skip to content

Mastering LLM-Powered Applications with LangChain.dart

In the rapidly evolving landscape of artificial intelligence and natural language processing, Large Language Models (LLMs) have become a game-changer for developers. For Flutter and Dart enthusiasts looking to harness the power of LLMs, LangChain.dart emerges as a robust solution. In this comprehensive guide, we’ll explore how to leverage LangChain.dart to build sophisticated, LLM-powered applications.

What is LangChain.dart?

LangChain.dart is an unofficial Dart port of the popular LangChain Python framework. It provides a set of ready-to-use components for working with language models and a standard interface for chaining them together. This allows developers to create advanced use cases such as chatbots, question-answering systems with Retrieval-Augmented Generation (RAG), agents, summarization tools, and more.

Key Features of LangChain.dart

  1. Model I/O: Unified API for interacting with various LLM providers (e.g., OpenAI, Google, Mistral, Ollama).
  2. Retrieval: Tools for loading, transforming, and retrieving user data to ground model responses.
  3. Agents: LLM-powered "bots" that can make informed decisions about using available tools to accomplish tasks.

Getting Started with LangChain.dart

To begin using LangChain.dart in your Flutter project, add the following dependencies to your pubspec.yaml file:

  langchain: ^0.0.1
  langchain_openai: ^0.0.1
  # Add other integration-specific packages as needed

Building a Simple LLM-Powered Application

Let’s create a basic example using LangChain.dart to interact with an LLM. We’ll use the ChatGoogleGenerativeAI model for this demonstration.

import 'package:langchain/langchain.dart';
import 'package:langchain_google/langchain_google.dart';

Future<void> main() async {
  final model = ChatGoogleGenerativeAI(apiKey: 'YOUR_GOOGLE_API_KEY');
  final prompt = PromptValue.string('Explain the concept of LangChain in simple terms.');

  try {
    final result = await model.invoke(prompt);
    print('LLM Response: ${result.firstOutputAsString}');
  } catch (e) {
    print('Error: $e');

In this example, we’re using the ChatGoogleGenerativeAI model to generate an explanation of LangChain. Make sure to replace ‘YOUR_GOOGLE_API_KEY’ with your actual Google API key.

Advanced Usage: Implementing a RAG Pipeline

Now, let’s explore a more advanced use case: implementing a Retrieval-Augmented Generation (RAG) pipeline. This example demonstrates how to combine multiple LangChain.dart components to create a powerful question-answering system.

import 'package:langchain/langchain.dart';
import 'package:langchain_openai/langchain_openai.dart';

Future<void> main() async {
  // Set up the vector store
  final vectorStore = MemoryVectorStore(
    embeddings: OpenAIEmbeddings(apiKey: 'YOUR_OPENAI_API_KEY'),

  // Add documents to the vector store
  await vectorStore.addDocuments(
    documents: [
      Document(pageContent: 'LangChain.dart is a powerful library for building LLM applications.'),
      Document(pageContent: 'Flutter developers can use LangChain.dart to create AI-powered apps.'),

  // Define the retrieval chain
  final retriever = vectorStore.asRetriever();
  final setupAndRetrieval = Runnable.fromMap<String>({
    'context': retriever.pipe(
      Runnable.mapInput((docs) => => d.pageContent).join('\n')),
    'question': Runnable.passthrough(),

  // Create a RAG prompt template
  final promptTemplate = ChatPromptTemplate.fromTemplates([
    (ChatMessageType.system, 'Answer the question based only on this context:\n{context}'),
    (ChatMessageType.human, '{question}'),

  // Set up the final chain
  final model = ChatOpenAI(apiKey: 'YOUR_OPENAI_API_KEY');
  const outputParser = StringOutputParser<ChatResult>();
  final chain = setupAndRetrieval

  // Run the RAG pipeline
  final question = 'What can Flutter developers use LangChain.dart for?';
  final answer = await chain.invoke(question);
  print('Question: $question');
  print('Answer: $answer');

This advanced example demonstrates how to:

  1. Set up a vector store with OpenAI embeddings
  2. Add documents to the vector store
  3. Create a retrieval chain
  4. Define a RAG prompt template
  5. Construct the final processing chain
  6. Run the pipeline to answer a question based on the stored context


LangChain.dart opens up a world of possibilities for Flutter and Dart developers looking to integrate LLM capabilities into their applications. From simple text generation to complex RAG systems, this library provides the tools and abstractions needed to build sophisticated AI-powered features.

As you continue exploring LangChain.dart, consider diving into its other components such as agents, tool integrations, and various LLM providers. The modular design of LangChain.dart allows you to mix and match components to create tailored solutions for your specific use cases.

For more information and advanced usage, check out the official LangChain.dart documentation and join the LangChain.dart Discord community to connect with other developers and get support.

Happy coding, and may your LLM-powered Flutter apps revolutionize the way we interact with AI!

Leave a Reply

Your email address will not be published. Required fields are marked *