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

If you’re seeing an async event loop error in LangGraph TypeScript, it usually means you’re trying to run async graph execution in a place where the runtime can’t keep the loop alive or can’t safely nest another execution. In practice, this shows up when calling invoke, stream, or checkpoint-backed graph methods from the wrong context, especially inside already-running async handlers, tests, or serverless functions.

The exact message varies by environment, but the failure often looks like one of these:

• •Error: This event loop is already running
• •Error: async event loop error
• •TypeError: Cannot read properties of undefined after a failed async step
• •LangGraph runtime errors around CompiledStateGraph, Runnable, or checkpointing

The Most Common Cause

The #1 cause is double-running async work inside a sync wrapper, or calling graph execution from a context that already has an active loop and then trying to block on it.

In TypeScript, this usually happens when someone mixes await correctly in most places, but still wraps graph execution in a helper that assumes sync behavior.

Broken pattern vs fixed pattern

// BROKEN
import { StateGraph } from "@langchain/langgraph";

const graph = new StateGraph<any>({ /* ... */ }).compile();

function runWorkflow(input: any) {
  // Bad: pretending async graph work is sync
  let result: any;
  graph.invoke(input).then((r) => {
    result = r;
  });

  return result; // undefined because promise hasn't resolved yet
}

const output = runWorkflow({ query: "hello" });
console.log(output);

// FIXED
import { StateGraph } from "@langchain/langgraph";

const graph = new StateGraph<any>({ /* ... */ }).compile();

async function runWorkflow(input: any) {
  const result = await graph.invoke(input);
  return result;
}

const output = await runWorkflow({ query: "hello" });
console.log(output);

If you’re in an Express route, Next.js handler, or worker callback, the rule is the same: keep the handler async and await the LangGraph call directly.

Other Possible Causes

1. Running LangGraph inside a test that doesn’t support async properly

Jest and Vitest can mask this if you forget to return/await the promise.

// BROKEN
test("runs graph", () => {
  graph.invoke({ query: "hi" });
});

// FIXED
test("runs graph", async () => {
  const result = await graph.invoke({ query: "hi" });
  expect(result).toBeDefined();
});

If your test exits early, you’ll see flaky failures that look like loop issues rather than plain missing awaits.

2. Nesting invoke() inside another stream/invoke callback

This happens when one node triggers another full graph execution instead of returning state for the current run.

// BROKEN
const nodeA = async (state: any) => {
  const nested = await graph.invoke({ query: state.query });
  return { ...state, nested };
};

Better pattern:

// FIXED
const nodeA = async (state: any) => {
  const nestedResult = await someStandaloneRunnable.invoke({ query: state.query });
  return { ...state, nestedResult };
};

If you need composition, use subgraphs intentionally. Don’t recursively call the same compiled graph unless you’ve designed for it.

3. Mixing browser-only and Node-only runtimes

LangGraph TypeScript expects a runtime with proper async primitives. If you compile code for edge/browser but use Node-specific checkpointing or file APIs, event loop behavior gets messy fast.

// Problematic in edge/browser builds if checkpointing uses Node APIs
import { MemorySaver } from "@langchain/langgraph";

Check your runtime target:

• •Next.js route handlers on Edge runtime
• •Cloudflare Workers
• •Bun vs Node.js differences
• •Jest environment mismatches

If you’re using persistence, make sure the checkpointer matches your deployment environment.

4. Misconfigured checkpointer or thread context

A bad checkpointer setup can surface as an event loop problem because the runtime keeps retrying state operations and fails deep inside execution.

const app = workflow.compile({
  checkpointer: memorySaver,
});

If memorySaver is reused across incompatible concurrent requests, isolate it per thread/session where needed. Also confirm your thread_id is stable when using durable state.

How to Debug It

1. •

   Find the first real stack frame

   • •Don’t stop at the top-level error.
   • •Look for whether it points to your route handler, test file, node function, or checkpoint layer.
   • •If the stack includes CompiledStateGraph, Runnable, or a node callback, that’s your entry point.
2. •

   Search for missing await

   • •Grep for .invoke(, .stream(, .batch(.
   • •Every call should either be awaited or explicitly returned.
   • •Bad sign: assigning promise results to variables without awaiting them.
3. •

   Remove nesting

   • •Temporarily replace inner graph calls with plain logging.
   • •If the error disappears, you were recursively invoking or re-entering execution.
   • •Keep only one top-level graph execution per request path.
4. •

   Disable checkpointing temporarily

   • •Remove checkpointer and rerun.
   • •If the error goes away, inspect thread IDs and storage backend.
   • •Reintroduce persistence after confirming basic execution works.

Prevention

• •Keep LangGraph execution paths fully async from entrypoint to exitpoint.
• •Never call a compiled StateGraph recursively unless that pattern is intentional and tested.
• •Match your runtime to your persistence layer:
  • •Node.js + file/memory checkpointing
  • •Edge/browser + compatible storage only
• •In tests, always use async test functions and await every LangGraph call.
• •Treat every .invoke() as an actual network/database-like async boundary.

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By Cyprian Aarons, AI Consultant at Topiax.