Langchain

LangChain - Build LLM Applications with Agents & RAG

LangChain v1 is the fast way to build provider-agnostic agents and LLM-powered applications. LangChain agents run on top of LangGraph, so you can start high-level with create_agent(...) and drop to LangGraph when you need more explicit control.

When to use LangChain

Use LangChain when you want to:

• build agents quickly with create_agent(...)
• connect to OpenAI, Anthropic, Google, and other providers through dedicated integration packages
• add tools, structured output, and retrieval without hand-writing graph orchestration
• prototype RAG workflows before dropping into LangGraph for more control

Use LangGraph instead when you need:

• explicit stateful workflows with loops, Command, and Send
• persistence, interrupts, or custom orchestration logic as first-class concerns
• deeper control over node boundaries and execution flow

Quick start

Installation

# Core framework
pip install -U langchain

# Provider integrations
pip install -U langchain-anthropic
pip install -U langchain-openai

# Common RAG extras
pip install -U langchain-community langchain-chroma

# Text splitters package required for current RAG examples
pip install -U langchain-text-splitters

Official install docs:

• https://docs.langchain.com/oss/python/langchain/install
• https://docs.langchain.com/oss/python/integrations/providers/overview

Basic model usage

from langchain_anthropic import ChatAnthropic

llm = ChatAnthropic(model="claude-sonnet-4-5-20250929")
response = llm.invoke("Explain quantum computing in 2 sentences")
print(response.content)

Create an agent

from langchain.agents import create_agent
from langchain_anthropic import ChatAnthropic


def get_weather(city: str) -> str:
    """Get weather information for a city."""
    return f"It's always sunny in {city}!"


agent = create_agent(
    model=ChatAnthropic(model="claude-sonnet-4-5-20250929"),
    tools=[get_weather],
    system_prompt="You are a helpful assistant. Use tools when needed.",
)

result = agent.invoke(
    {"messages": [{"role": "user", "content": "What's the weather in Paris?"}]}
)
print(result["messages"][-1].content)

create_agent(...) is the current LangChain v1 entry point. Older helper APIs such as create_tool_calling_agent(...), create_react_agent(...), LLMChain, RetrievalQA, and ConversationBufferMemory are legacy patterns or have moved into langchain-classic.

Core concepts

1. Models

from langchain_openai import ChatOpenAI
from langchain_anthropic import ChatAnthropic
from langchain_google_genai import ChatGoogleGenerativeAI

openai_model = ChatOpenAI(model="gpt-4o")
anthropic_model = ChatAnthropic(model="claude-sonnet-4-5-20250929")
google_model = ChatGoogleGenerativeAI(model="gemini-2.0-flash")

2. Tools

from langchain.tools import tool


@tool
def search_docs(query: str) -> str:
    """Search product documentation."""
    return f"Search results for: {query}"

3. Structured output

from pydantic import BaseModel, Field


class WeatherReport(BaseModel):
    city: str = Field(description="City name")
    temperature: float = Field(description="Temperature in Fahrenheit")
    condition: str = Field(description="Weather condition")


structured_llm = llm.with_structured_output(WeatherReport)
report = structured_llm.invoke("Weather in SF: 65F and sunny")
print(report.city, report.temperature, report.condition)

RAG in LangChain v1

The current LangChain docs show two common approaches:

1. RAG agent: wrap retrieval in a tool and let create_agent(...) decide when to call it.
2. Two-step RAG chain: retrieve documents first, then pass them to the model in a single answer-generation step.

Minimal indexing example

import bs4

from langchain_community.document_loaders import WebBaseLoader
from langchain_core.vectorstores import InMemoryVectorStore
from langchain_openai import OpenAIEmbeddings
from langchain_text_splitters import RecursiveCharacterTextSplitter

loader = WebBaseLoader(
    web_paths=("https://lilianweng.github.io/posts/2023-06-23-agent/",),
    bs_kwargs={
        "parse_only": bs4.SoupStrainer(
            class_=("post-content", "post-title", "post-header")
        )
    },
)
docs = loader.load()

splitter = RecursiveCharacterTextSplitter(
    chunk_size=1000,
    chunk_overlap=200,
    add_start_index=True,
)
splits = splitter.split_documents(docs)

embedding_model = OpenAIEmbeddings()
vector_store = InMemoryVectorStore(embedding=embedding_model)
vector_store.add_documents(splits)

Minimal retrieval tool

from langchain.tools import tool


@tool(response_format="content_and_artifact")
def retrieve_context(query: str):
    """Retrieve information to help answer a query."""
    retrieved_docs = vector_store.similarity_search(query, k=2)
    serialized = "\n\n".join(
        f"Source: {doc.metadata}\nContent: {doc.page_content}"
        for doc in retrieved_docs
    )
    return serialized, retrieved_docs

RAG agent

from langchain.agents import create_agent
from langchain_anthropic import ChatAnthropic

agent = create_agent(
    model=ChatAnthropic(model="claude-sonnet-4-5-20250929"),
    tools=[retrieve_context],
    system_prompt=(
        "Use the retrieval tool whenever you need grounded context. "
        "If the retrieved context is not enough, say you do not know."
    ),
)

Text splitters

Current LangChain docs use the standalone langchain-text-splitters package:

from langchain_text_splitters import RecursiveCharacterTextSplitter

splitter = RecursiveCharacterTextSplitter(
    chunk_size=1000,
    chunk_overlap=200,
)

Persistence and LangGraph relationship

• LangChain is the recommended starting point for high-level agent loops.
• LangGraph is the lower-level orchestration runtime beneath LangChain agents.
• For persistence in LangGraph, the current in-memory checkpointer is InMemorySaver, with separate SQLite and Postgres integrations for durable backends.

Best practices

1. Start with create_agent(...) for new agent work.
2. Prefer provider packages such as langchain-openai or langchain-anthropic over older monolithic integrations.
3. Use langchain-text-splitters for current splitter examples.
4. Treat langchain-classic as the home for legacy helpers you still need to keep around during migrations.
5. Use LangSmith or another trace workflow once an agent has multiple tools, branching behavior, or enough reasoning steps that debugging from final output alone is no longer practical.

References

• Agents Guide - current agent APIs and migration notes
• RAG Guide - indexing, retrieval tools, and RAG agents
• Integration Guide - providers, vector stores, and observability

Resources

• Docs: https://docs.langchain.com
• LangChain overview: https://docs.langchain.com/oss/python/langchain/overview
• LangChain install guide: https://docs.langchain.com/oss/python/langchain/install
• LangChain RAG guide: https://docs.langchain.com/oss/python/langchain/rag
• LangGraph overview: https://docs.langchain.com/oss/python/langgraph/overview
• API Reference: https://reference.langchain.com/python

Source: dhar174/langgraph_system_generator — distributed by TomeVault.