What is LCEL (LangChain Expression Language)?

LCEL (LangChain Expression Language) is a declarative syntax for composing chains in LangChain using the pipe operator |. It connects Runnable objects so the output of one becomes the input of the next, making multi-step LLM workflows readable and concise.

The core building block is the Runnable interface. Any component that implements invoke(), stream(), and batch() can participate in an LCEL chain. A typical example:

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

prompt = ChatPromptTemplate.from_template("Translate to French: {text}")
model = ChatOpenAI()
parser = StrOutputParser()

chain = prompt | model | parser
result = chain.invoke({"text": "Hello, world!"})

LCEL provides automatic streaming, parallel execution via RunnableParallel, passthrough of values with RunnablePassthrough, custom function wrapping with RunnableLambda, and fallback chains with .with_fallbacks(). Every chain invocation is traced in LangSmith without extra setup.

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