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Chat History

There is no server-side "memory" object that magically remembers a user. Memory is just the message list you re-send on every call. Each turn, you append the new human message, invoke the model, append its reply, and send the whole (possibly trimmed) list back next time.

from langchain.chat_models import init_chat_model

model = init_chat_model("anthropic:claude-sonnet-4-6")

history = [{"role": "user", "content": "My name is Ana"}]
response = model.invoke(history)
history.append({"role": "assistant", "content": response.text()})

history.append({"role": "user", "content": "What's my name?"})
response = model.invoke(history)  # sees the whole list, answers "Ana"
Key idea

"Memory" in an LLM app is application state you own, not a feature the model has. If you don't resend a fact, the model doesn't know it.

Keying sessions​

A real app has many concurrent conversations, so history needs a key — a thread_id or session_id — to look up the right list. In a create_agent + checkpointer setup, that key is the thread_id in config["configurable"], and the checkpointer stores the message list on your behalf:

from langchain.agents import create_agent
from langgraph.checkpoint.memory import InMemorySaver

agent = create_agent(
    model="anthropic:claude-sonnet-4-6",
    tools=[],
    checkpointer=InMemorySaver(),
)

config = {"configurable": {"thread_id": "user_42"}}
agent.invoke({"messages": [{"role": "user", "content": "My name is Ana"}]}, config)
agent.invoke({"messages": [{"role": "user", "content": "What's my name?"}]}, config)

Reusing thread_id "user_42" resumes that thread's history; a new thread_id starts empty. See Checkpointing for how the checkpointer itself persists that state.

Wiring history into a prompt​

For a plain LCEL chain (no agent, no checkpointer), thread the history in yourself with MessagesPlaceholder — see Prompt Templates:

from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder

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

Most tutorials online still show ConversationBufferMemory and ConversationChain — both deprecated in LangChain 1.x (see Versions and Migration). The current approach is either a checkpointer-backed agent (above) or explicit MessagesPlaceholder wiring — not a Memory class.

See also​