LangChain.js Patterns

Quick Guide: Use LangChain.js (v1.x) to build composable LLM applications. Use LCEL (prompt.pipe(model).pipe(parser)) for all chain composition -- never use legacy LLMChain. Use withStructuredOutput(zodSchema) for typed responses. Use createAgent() (LangGraph-backed) for agentic workflows -- AgentExecutor is legacy. All @langchain/* packages must share the same @langchain/core version or you get cryptic type errors at runtime.

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use LCEL pipe composition (prompt.pipe(model).pipe(parser)) for all chains -- never use legacy LLMChain, ConversationChain, or SequentialChain)

(You MUST ensure all @langchain/* packages depend on the same version of @langchain/core -- version mismatches cause cryptic runtime errors)

(You MUST use withStructuredOutput(zodSchema) for structured LLM responses -- never manually parse JSON from completion text)

(You MUST use createAgent() from langchain for new agent code -- AgentExecutor and createToolCallingAgent are legacy patterns)

(You MUST never hardcode API keys -- use environment variables (OPENAI_API_KEY, ANTHROPIC_API_KEY, etc.))

</critical_requirements>

Auto-detection: LangChain, langchain, @langchain/core, @langchain/openai, @langchain/anthropic, @langchain/google-genai, ChatOpenAI, ChatAnthropic, ChatPromptTemplate, StringOutputParser, RunnableSequence, pipe, withStructuredOutput, createAgent, createToolCallingAgent, AgentExecutor, tool, DynamicStructuredTool, RecursiveCharacterTextSplitter, MemoryVectorStore, OpenAIEmbeddings, LCEL, LangSmith, LANGCHAIN_TRACING_V2

When to use:

• Building LLM applications that compose prompts, models, and output parsers into chains
• Creating agentic workflows where models decide which tools to call
• Implementing RAG pipelines with document loading, splitting, embedding, and retrieval
• Needing structured output from LLMs with type-safe Zod schema validation
• Streaming LLM responses token-by-token to users
• Switching between LLM providers (OpenAI, Anthropic, Google) with a unified interface
• Tracing and debugging LLM applications with LangSmith

Key patterns covered:

• Chat model initialization and provider switching (ChatOpenAI, ChatAnthropic, ChatGoogleGenerativeAI)
• LCEL chain composition with .pipe() and RunnableSequence
• Prompt templates (ChatPromptTemplate, MessagesPlaceholder)
• Structured output with withStructuredOutput() and Zod schemas
• Tool definition with tool() function and Zod schemas
• Agent creation with createAgent() (LangGraph-backed)
• RAG pipelines: document loaders, text splitters, vector stores, retrievers
• Streaming from chains, models, and agents
• LangSmith tracing setup

When NOT to use:

• You only call one LLM provider and want the thinnest wrapper -- use the provider's SDK directly
• You need React-specific chat UI hooks (useChat, useCompletion) -- use a framework-integrated AI SDK
• You want a simple single-call completion with no chaining -- a direct SDK call is simpler
• You need real-time bidirectional communication -- LangChain does not cover WebSocket/Realtime APIs

Examples Index

• Core: Setup, LCEL & Chat Models -- Package installation, chat model init, LCEL chains, prompt templates, output parsers
• Structured Output & Tools -- withStructuredOutput, tool definition, binding tools to models
• Agents -- createAgent, tool-calling agents, chat history, streaming agents
• RAG Pipelines -- Document loaders, text splitters, vector stores, retrieval chains
• Streaming -- Model streaming, chain streaming, agent streaming
• Quick API Reference -- Package map, import paths, environment variables, model IDs

Philosophy

LangChain.js provides a composable framework for building LLM-powered applications. Its core abstraction is the Runnable -- any component that takes an input and produces an output. Runnables compose via LCEL (.pipe()) to form chains, and every Runnable supports .invoke(), .stream(), .batch() uniformly.

Core principles:

1. Composability via LCEL -- Chains are built by piping Runnables: prompt.pipe(model).pipe(parser). Each step is independently testable and replaceable. Legacy chain classes (LLMChain, ConversationChain) are deprecated.
2. Provider-agnostic models -- Chat models (ChatOpenAI, ChatAnthropic, ChatGoogleGenerativeAI) share a common interface. Swap providers by changing one import and model name. Use initChatModel() for runtime provider selection.
3. Type-safe structured output -- model.withStructuredOutput(zodSchema) constrains LLM responses to your schema. No manual JSON parsing.
4. Split package architecture -- @langchain/core holds abstractions, provider packages (@langchain/openai, @langchain/anthropic) hold implementations, langchain holds higher-level composables. All must share the same @langchain/core version.
5. Observability built in -- Set LANGCHAIN_TRACING_V2=true and every chain/agent/tool call is traced to LangSmith automatically.

When to use LangChain:

• You need to compose multi-step LLM workflows (prompt -> model -> parser -> next step)
• You want to swap LLM providers without rewriting business logic
• You need agent-style tool calling with automatic routing
• You need RAG with document loading, chunking, embedding, and retrieval
• You want built-in tracing and evaluation via LangSmith

When NOT to use:

• Single-provider, single-call use cases -- the provider SDK is simpler and has less overhead
• You want full control over HTTP requests -- LangChain abstracts the transport layer
• Extremely latency-sensitive applications where the abstraction overhead matters

Core Patterns

Pattern 1: Chat Model Initialization

Initialize chat models from any provider. They all share the same interface.

import { ChatOpenAI } from "@langchain/openai";

const model = new ChatOpenAI({
  model: "gpt-4.1",
  temperature: 0,
});

const response = await model.invoke("Explain TypeScript generics.");
console.log(response.text);

Why good: Explicit model name, temperature set for determinism, .text accessor for content

// BAD: Hardcoded API key, no model specified
import { ChatOpenAI } from "@langchain/openai";
const model = new ChatOpenAI({ apiKey: "sk-1234..." });

Why bad: Hardcoded API key is a security risk, missing model name uses unpredictable defaults

Provider Switching

import { ChatAnthropic } from "@langchain/anthropic";
const model = new ChatAnthropic({ model: "claude-sonnet-4-5-20250929" });

// Or use initChatModel for runtime provider selection
import { initChatModel } from "langchain";
const model = await initChatModel("openai:gpt-4.1", { temperature: 0 });

See: examples/core.md for full provider examples and configuration options

Pattern 2: LCEL Chain Composition

Compose chains using .pipe(). Every component is a Runnable.

import { ChatOpenAI } from "@langchain/openai";
import { ChatPromptTemplate } from "@langchain/core/prompts";
import { StringOutputParser } from "@langchain/core/output_parsers";

const prompt = ChatPromptTemplate.fromTemplate(
  "Summarize this in one sentence: {text}",
);
const model = new ChatOpenAI({ model: "gpt-4.1" });
const parser = new StringOutputParser();

const chain = prompt.pipe(model).pipe(parser);
const result = await chain.invoke({ text: "LangChain is a framework..." });
// result is a plain string

*Why good: