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Documentation Index

Fetch the complete documentation index at: https://docs.vane.build/llms.txt

Use this file to discover all available pages before exploring further.

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Installation

npm install @vane.build/langchain
Peer dependency: @langchain/core >= 0.2.0.

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Three-line setup

import { VaneCallbackHandler } from '@vane.build/langchain';
import { ChatOpenAI } from '@langchain/openai';

const handler = new VaneCallbackHandler({
  baseUrl: process.env.COUNSEL_URL!,
  apiKey: process.env.COUNSEL_API_KEY!,
  agentId: 'researcher-1',
});

const llm = new ChatOpenAI({ callbacks: [handler] });
await llm.invoke('Summarise this contract.');
That’s it. Every LLM call and tool invocation is now attested to Vane.

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VaneCallbackHandler

Extends LangChain’s BaseCallbackHandler. Attach it to any LLM, chain, or agent executor.

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Constructor options

OptionTypeRequiredDescription
baseUrlstringYesBase URL of your Vane instance.
apiKeystringYesCompany-scoped API key.
agentIdstringYesAgent identifier that appears on every record.
companyIdstringNoCompany ID embedded in attestation payloads. If omitted, the API key’s company is used by the server.

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What gets attested

EventactionTypePayload fields
LLM call completed"llm-call"model, input (prompts or messages), output (first generation text), tokenUsage, durationMs
Tool call completed"tool-call"tool, input, output, durationMs
Agent action (tool decision)"agent-action"tool, toolInput
Chat models go through handleChatModelStart (not handleLLMStart). Both paths are covered — you will always get an llm-call record regardless of whether you use a chat model or a completion model.

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Error handling

Attestation calls are fire-and-forget. If the Vane server is unavailable, the error is logged to console.error and the LangChain execution continues uninterrupted. Attestation failures never block or crash your agent.

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Attaching to different LangChain components

LLM or chat model: 
const llm = new ChatOpenAI({ callbacks: [handler] });
Agent executor: 
const executor = AgentExecutor.fromAgentAndTools({
  agent,
  tools,
  callbacks: [handler],
});
Chain: 
const chain = prompt.pipe(llm).withConfig({ callbacks: [handler] });
Global (all callbacks): 
import { setGlobalCallbackManager } from '@langchain/core/callbacks/manager';
// Not recommended for production — attaches to every LangChain operation in the process.

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Example — full agent

import { VaneCallbackHandler } from '@vane.build/langchain';
import { createReactAgent } from '@langchain/langgraph/prebuilt';
import { ChatOpenAI } from '@langchain/openai';
import { tool } from '@langchain/core/tools';
import { z } from 'zod';

const handler = new VaneCallbackHandler({
  baseUrl: process.env.COUNSEL_URL!,
  apiKey: process.env.COUNSEL_API_KEY!,
  agentId: 'contract-reviewer',
  companyId: 'acme',
});

const searchTool = tool(
  async ({ query }) => `Results for: ${query}`,
  { name: 'search', description: 'Web search', schema: z.object({ query: z.string() }) },
);

const agent = createReactAgent({
  llm: new ChatOpenAI({ model: 'gpt-4o', callbacks: [handler] }),
  tools: [searchTool],
});

const result = await agent.invoke({
  messages: [{ role: 'user', content: 'Find the penalty clauses in EU AI Act.' }],
});
Each tool call and LLM completion in this run will produce a signed record in Vane.