langchain-webhooks-events

# 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() └── handleChainEnd() ``` ## Custom Callback Handler ```typescript import { BaseCallbackHandler } from "@langchain/core/callbacks/base"; class WebhookHandler extends BaseCallbackHandler { name = "WebhookHandler"; constructor(private webhookUrl: string) { super(); } async handleLLMStart(llm: any, prompts: string[]) { await this.send("llm_start", { model: llm?.id?.[2], promptCount: prompts.length, }); } async handleLLMEnd(output: any) { await this.send("llm_end", { tokenUsage: output.llmOutput?.tokenUsage, }); } async handleLLMError(error: Error) { await this.send("llm_error", { error: error.message, }); } async handleToolStart(_tool: any, input: string) { await this.send("tool_start", { input: input.slice(0, 200) }); ...