How do you implement a ConversationChain?

A ConversationChain maintains multi-turn dialogue by storing conversation history and injecting it into each new prompt invocation. The legacy approach uses ConversationChain with a memory object; the LCEL approach manages history explicitly in the chain state using MessagesPlaceholder.

LCEL approach (recommended):

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder
from langchain_core.messages import HumanMessage, AIMessage

prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant."),
    MessagesPlaceholder(variable_name="history"),
    ("human", "{input}"),
])

chain = prompt | ChatOpenAI()

# Manually manage history
history = []
def chat(user_input):
    response = chain.invoke({"input": user_input, "history": history})
    history.append(HumanMessage(content=user_input))
    history.append(AIMessage(content=response.content))
    return response.content

print(chat("My name is Alice."))
print(chat("What is my name?"))  # correctly recalls "Alice"

For server-side multi-user conversations, pair this with LangGraph's checkpointing or RunnableWithMessageHistory which wraps the chain and automatically loads/saves history per session ID from a configurable store.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.