LangChain Webhooks & Events

Overview

Event-driven patterns for LangChain: custom callback handlers for lifecycle hooks, webhook dispatching, Server-Sent Events (SSE) for streaming, WebSocket integration, and event aggregation for tracing.

Callback Handler Architecture

LangChain emits events at every stage of chain/agent execution. Custom handlers can observe, log, stream, or dispatch these events.

chain.invoke()
  ├── handleChainStart()
  │   ├── handleLLMStart()
  │   │   ├── handleLLMNewToken()  // streaming
  │   │   └── handleLLMEnd()
  │   ├── handleToolStart()
  │   │   └── handleToolEnd()
  │   └── handleRetrieverStart()
  │       └── handleRetrieverEnd()