LangChain Core Workflow A: Chains & Prompts

Overview

Build production chains using LCEL (LangChain Expression Language). Covers prompt templates, output parsers, RunnableSequence, RunnableParallel, RunnableBranch, RunnablePassthrough, and chain composition patterns.

Prerequisites

• langchain-install-auth completed
• @langchain/core and at least one provider installed

Prompt Templates

ChatPromptTemplate

import { ChatPromptTemplate, MessagesPlaceholder } from "@langchain/core/prompts";

// Simple template
const simple = ChatPromptTemplate.fromTemplate(