[Callbacks](https://js.langchain.com/docs/concepts/callbacks/): LLM should read this page when learning about LangChain's callback system, implementing event tracking in LLM applications, or customizing monitoring for chains and tools. (Comprehensive guide to LangChain's callback system explaining how to hook into various stages of LLM applications. Covers callback events, callback handlers, and how to pass callbacks at both constructor and request time. Includes details on event types like model starts/ends, token generation, chain events, tool events, and retriever events.)

