AI / LangGraph LangChain Interview questions
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.
What operator does LCEL use to chain components together?
What built-in capability does LCEL provide automatically?
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