Interrupt & resume

LangGraph runs can pause on an interrupt — a human-in-the-loop checkpoint. koel_langgraph surfaces the pause as a canonical AG-UI CUSTOM event (name: "on_interrupt") on the run stream; you read its value and reopen the run on the same thread.

import 'package:koel_core/koel_core.dart';
import 'package:koel_langgraph/koel_langgraph.dart' show LangGraphAgent;

final agent = LangGraphAgent(
  deploymentUrl: Uri.parse('http://localhost:8003/agent'),
);

// Watch the raw event stream for the interrupt.
await for (final event in agent.run(input)) {
  if (event is CustomEvent && event.name == 'on_interrupt') {
    final question = event.value; // whatever the graph paused to ask
    // ... surface `question` to the user, collect an answer ...
    break;
  }
}

// Reopen the run on the same thread with the human's decision.
final resumed = agent.resume(input.threadId, {'approved': true});
await for (final event in resumed) {
  // the run continues from the checkpoint
}

How resume works

resume(threadId, resumeValue) POSTs the value to the same deployment and reopens the SSE stream. LangGraph rebuilds run state server-side from its checkpoint — koel performs no client-side state reconstruction, so the resumeValue is the only thing you send.

Scope

koel ships surface-level interrupt/resume in 1.x: the pause surfaces, you answer, the run resumes. Deep (stateful sub-tree) interrupt/resume is a v2 concern, tracked as OQ-LangGraph-Graduation.

See Connect LangGraph for the agent setup and Events for the CUSTOM family.