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.

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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)