Langchain

LangChain Expert Skill

LangChain is an MIT-licensed Python framework for building LLM-powered applications. Since v1.0 (October 2025), it provides a layered architecture: high-level chain composition via LCEL (LangChain Expression Language), agent creation via create_agent (running on the LangGraph runtime underneath), and production observability via LangSmith. With 1000+ integrations and 100K+ GitHub stars, it is the most widely adopted LLM orchestration framework.

Key v1.0 change: All new LangChain agents run on the LangGraph runtime. AgentExecutor is in maintenance mode until December 2026. Use create_agent for new agents. Drop to LangGraph directly when you need full state-machine control.

⚠️ CRITICAL: Do NOT use AgentExecutor for new code. It is in maintenance mode until December 2026. Use create_agent(model, tools, prompt) instead — it generates a LangGraph state machine with streaming, persistence, and observability out of the box.

Core Principles

These principles govern every decision when building with LangChain. Read them before proceeding to the reference guides.

1. LCEL is the composition primitive. The pipe operator (|) chains Runnables. Every component — prompt, model, parser, retriever — implements the Runnable interface. Build everything in LCEL.
2. Agents run on LangGraph. Since v1.0, create_agent generates a LangGraph state machine underneath. You get streaming, persistence, and observability without writing graph code. Drop to LangGraph when you need branching, cycles, or human-in-the-loop.
3. RAG is a chain, not a framework. retriever | prompt | model | parser is the canonical RAG pattern. Document loaders, splitters, and vector stores are all interchangeable components.
4. LangSmith is production observability. Enable tracing at startup. 89% of production teams use observability — without it, debugging agent behavior is guesswork.
5. The ecosystem is the moat. 1000+ integrations mean model providers, vector stores, and tools are swappable with one line. Build against the interface, not the implementation.

Where to Start

You already have...	Start here
Nothing — blank project	Install LangChain, build a basic LCEL chain
Documents to query	Build a RAG chain (load, split, embed, retrieve, generate)
A need for agentic behavior	Use create_agent with tools
Existing AgentExecutor code	Migrate to create_agent — see references/agent-patterns.md
A production deployment	Add LangSmith tracing + LangServe deployment
Comparing frameworks	See the Framework Routing Guide

Pipeline Mode

Mode	When	Phases to run	Skip
Quick	Single chain, exploration	prompt → model → parser	Retrieval, agents, production hardening
RAG	Document Q&A	load → split → embed → retrieve → generate	Agent orchestration, deployment
Agent	Tool-using agents	create_agent + tools + LangGraph runtime	If simple chain suffices
Production	Shipping to users	RAG/Agent + LangSmith + LangServe	Nothing

Quick Reference

Task	Approach	Reference
Basic chain	prompt | model | parser	references/lcel-reference.md
RAG pipeline	retriever | prompt | model | parser	references/rag-strategies.md
Create agent	create_agent(model, tools, prompt)	references/agent-patterns.md
Tool definition	@tool decorator	references/agent-patterns.md
Multi-agent	LangGraph supervisor pattern	references/agent-patterns.md
Observability	Set LANGCHAIN_TRACING_V2=true	references/production-deployment.md
Deployment	LangServe or LangSmith Deployment	references/production-deployment.md
Vector store	One-line swap (Chroma, Pinecone, pgvector)	references/integration-ecosystem.md

When to Use This Skill

Load this skill any time you are:

• Building LCEL chains for LLM-powered applications
• Implementing RAG pipelines over enterprise or personal data
• Creating agents with tool-calling and multi-step reasoning
• Deploying LLM applications to production with observability
• Comparing LangChain with LlamaIndex, Haystack, or raw API calls

Framework Routing Guide

This skill is part of a portfolio of framework skills. When deciding which fits:

Scenario	Reach for	Why
I have chains to compose	LangChain	LCEL is the cleanest pipe-based composition model
I have documents to query	LlamaIndex	Data ingestion and retrieval are first-class primitives
I have agents to orchestrate	LangGraph	State-machine semantics, subgraphs, human-in-the-loop
I have a tool to wrap as an agent	PydanticAI	Type-safe agent definitions with dependency injection
I have search pipelines	Haystack	Pipeline model is more mature for search workloads
Fast prototype of any kind	LangChain	Fastest path from zero to working chain

Reference Files

Reference	Load when	File
LCEL Reference	Building chains with the pipe operator	references/lcel-reference.md
Architecture	Understanding package structure, Runnable, v1.0	references/architecture.md
RAG Strategies	Building RAG pipelines	references/rag-strategies.md
Agent Patterns	Creating agents with tools and multi-agent	references/agent-patterns.md
Production & Deployment	LangServe, LangSmith, deployment	references/production-deployment.md
Integration Ecosystem	Model providers, vector stores, tools	references/integration-ecosystem.md
FAQ & Troubleshooting	Common errors and fixes	references/faq-and-troubleshooting.md
Callbacks System	Custom logging, monitoring, agent auditing	references/callbacks.md
Validation Audit	Research validation of all API claims	references/validation-audit.md

Template Files

Template	When to use	File
Basic Chain	Single prompt→model→parser chain	templates/basic-chain.py
RAG Pipeline	Document Q&A with retrieval	templates/rag-pipeline.py
Agent with Tools	Tool-using agent with LangGraph runtime	templates/agent-with-tools.py
Production Deploy	LangServe deployment with LangSmith	templates/production-deploy.py

Scripts

Script	Purpose	File
check-setup	Verify LangChain installation	scripts/check-setup.py

Troubleshooting Guide

Symptom	Likely cause	Fix	Reference
Chain returns nothing	Output parser not connected	Add .pipe(StrOutputParser()) or equivalent	references/lcel-reference.md
Agent not calling tools	Tool schema mismatch	Check tool has docstring and type hints	references/agent-patterns.md
LangSmith traces missing	LANGCHAIN_TRACING_V2 not set	Set env var before any chain execution	references/production-deployment.md
Deprecation warning	Using AgentExecutor	Migrate to create_agent (LangGraph runtime)	references/agent-patterns.md
Model not found	Integration package missing	Install langchain-openai, langchain-anthropic, etc.	references/integration-ecosystem.md
Streaming not working	LCEL chain not streaming-native	Ensure all components implement stream()	references/lcel-reference.md
Vector store connection fails	Wrong credentials or missing package	Install langchain-community + provider package	references/integration-ecosystem.md

When NOT to Use LangChain

• Single-model, single-prompt application — raw API calls are simpler and more debuggable
• Maximum transparency needed — LangGraph (which LangChain uses underneath) provides more visibility
• Pure multi-agent state machines — LangGraph directly is the correct tool, not the high-level API
• Stateless microservice with no LLM orchestration — LangChain adds overhead without benefit