@agent-arc-status/adapter-langchain

A LangChain callback handler that emits Agent Arc Status Protocol events at chain/agent boundaries. @langchain/core is a peer dependency.

import { ArcStatusCallbackHandler } from "@agent-arc-status/adapter-langchain";
import { randomUUID } from "node:crypto";

const arc = new ArcStatusCallbackHandler({
  emit: (event) => bus.publish(event), // any EmitFn: HTTP, queue, stdout…
  arcId: randomUUID(),
  arcKind: "agent",
});

await agentExecutor.invoke({ input }, { callbacks: [arc] });

| LangChain callback | arc phase | |---|---| | root handleChainStart (no parent run) | started | | handleAgentAction | milestone | | handleLLMStart / handleToolStart | (dropped — per-call milestones are an anti-pattern, §5.3) | | root handleChainEnd | done | | handleChainError | terminal blocked |

The handler tracks the root run id, so nested chains don't produce extra started/done events — every stream it emits is validateSequence-clean.