How to Fix 'async event loop error in production' in LangGraph (TypeScript)

If you’re seeing async event loop error in production in a LangGraph TypeScript app, it usually means you’re trying to run async graph execution in a runtime that doesn’t match how the graph is being invoked. In practice, this shows up when a request handler, serverless function, or worker is mixing sync and async execution paths, or when the graph is being reused across concurrent requests incorrectly.

The key thing to understand: LangGraph itself is usually not broken. The error is almost always coming from how your code, runtime, or deployment environment is managing async work.

The Most Common Cause

The #1 cause is calling a graph with the wrong execution pattern inside an already-running event loop, or wrapping async graph calls in sync code that hides the real failure.

A common mistake is using invoke() where you should be using ainvoke(), or calling async graph code from a place that expects sync execution.

Broken vs fixed pattern

// ❌ Broken: sync wrapper around async LangGraph execution
import { StateGraph } from "@langchain/langgraph";

const graph = new StateGraph(/* ... */).compile();

export function handler(req: Request) {
  // This can blow up when the runtime already has an active event loop
  const result = graph.invoke({
    messages: [{ role: "user", content: "Hello" }],
  });

  return Response.json(result);
}

// ✅ Fixed: keep everything async and use ainvoke()
import { StateGraph } from "@langchain/langgraph";

const graph = new StateGraph(/* ... */).compile();

export async function handler(req: Request) {
  const result = await graph.ainvoke({
    messages: [{ role: "user", content: "Hello" }],
  });

  return Response.json(result);
}

If you’re using an API route, Lambda handler, Next.js route handler, or Fastify plugin, make the whole path async. Don’t “convert back” to sync just because the framework allows it.

Other Possible Causes

1) Reusing a compiled graph across concurrent requests with shared mutable state

If your node functions mutate shared objects, one request can corrupt another. That often surfaces as runtime instability rather than a clean stack trace.

// ❌ Broken
const sharedMessages: any[] = [];

const graph = builder.compile();

builder.addNode("chat", async () => {
  sharedMessages.push({ role: "assistant", content: "hi" });
  return { messages: sharedMessages };
});

// ✅ Fixed
builder.addNode("chat", async (state) => {
  return {
    messages: [...state.messages, { role: "assistant", content: "hi" }],
  };
});

2) Using Node APIs in an edge/runtime that doesn’t support them

LangGraph TypeScript can run in multiple environments, but your nodes may depend on Node-only modules like fs, net, or some database clients.

// ❌ Broken in edge runtimes
import fs from "node:fs";

builder.addNode("loadConfig", async () => {
  return { config: fs.readFileSync("./config.json", "utf-8") };
});

// ✅ Fixed for edge-compatible runtimes
builder.addNode("loadConfig", async () => {
  const res = await fetch("https://example.com/config.json");
  return { config: await res.text() };
});

If you deploy to Vercel Edge, Cloudflare Workers, or similar runtimes, check compatibility before blaming LangGraph.

3) Mixing callback-based code with unresolved promises inside nodes

A node that never resolves cleanly can look like an event loop problem under production load.

// ❌ Broken
builder.addNode("fetchData", () => {
  return new Promise((resolve) => {
    setTimeout(() => resolve({ ok: true }), 1000);
    // no reject path, no timeout handling
  });
});

// ✅ Fixed
builder.addNode("fetchData", async () => {
  const controller = new AbortController();
  const timeout = setTimeout(() => controller.abort(), 5000);

  try {
    const res = await fetch("https://api.example.com/data", {
      signal: controller.signal,
    });
    return { data: await res.json() };
  } finally {
    clearTimeout(timeout);
  }
});

4) Version mismatch between LangGraph packages and runtime dependencies

This happens when @langchain/langgraph, langchain, and your Node version drift apart. You’ll see weird behavior before you see obvious compile errors.

{
  "dependencies": {
    "@langchain/langgraph": "^0.2.0",
    "langchain": "^0.3.0"
  },
  "engines": {
    "node": "18"
  }
}

Check that:

• •your Node version matches what the package expects
• •related LangChain packages are on compatible versions
• •your lockfile is not pulling mismatched transitive dependencies

How to Debug It

1. •

   Check whether you are calling invoke() instead of ainvoke()

   • •Search for .invoke( across the codebase.
   • •If the surrounding function is already async, switch to await graph.ainvoke(...).
2. •

   Print the exact runtime and deployment target

   • •Log:
     • •process.version
     • •framework name/version
     • •whether you’re on Node, Edge, Lambda, or containerized workers
   • •A lot of “event loop” issues are really environment mismatches.
3. •

   Isolate the failing node

   • •Temporarily replace nodes with simple pass-throughs.
   • •Add logs before and after each node:

     builder.addNode("step1", async (state) => {
       console.log("step1 start");
       console.log("step1 end");
       return state;
     });
     

4. •

   Run one request at a time

   • •Disable concurrency in your test harness.
   • •If the error disappears under single-flight traffic, you likely have shared state or an unclosed resource like a DB client or timer.

Prevention

• •Keep LangGraph execution fully async from entrypoint to node implementation.
• •Treat state as immutable inside nodes; return new objects instead of mutating shared arrays.
• •Pin compatible versions of @langchain/langgraph, related LangChain packages, and Node in production.

If you want one rule to remember: don’t hide async boundaries. In LangGraph TypeScript apps, that’s where most production event-loop failures come from.

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