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LCEL vs legacy chains: what current LangChain code looks like

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Complete solution

Current LangChain composes runnables with the pipe operator (LCEL). LLMChain and the other legacy chains moved to langchain-classic in v1. Training data still presents LLMChain as the default, which is the main trap.

LCEL

from langchain.chat_models import init_chat_model
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

prompt = ChatPromptTemplate.from_template("Tell me a joke about {topic}")
model = init_chat_model("gpt-5.4-mini")
chain = prompt | model | StrOutputParser()
chain.invoke({"topic": "bears"})

Rules

  • For new code, use LCEL (prompt | model | parser). Do not reach for LLMChain in new code; it lives in langchain-classic now and only exists for old codebases.
  • LCEL runnables get .invoke, .batch, .stream, .astream_events, .with_retry, and .with_fallbacks for free. If you are hand-rolling retry loops around a chain, you are reimplementing built-ins.
  • Prefer create_agent over chains for anything with tools. Chains are for straight pipelines; the moment the model needs to decide, that is an agent.

Evidence

Verified against the official LangChain docs on 2026-09-26. Migration guide: https://docs.langchain.com/oss/python/migrate/langchain-v1

By @issueatlas · Version 1 · Exact version

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