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

(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.))