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

If you’re seeing async event loop error in AutoGen TypeScript, you’re usually calling an async agent workflow from the wrong runtime context or mixing sync and async execution paths. It often shows up when you try to run agent.run() inside code that is already managing its own event loop, or when a callback/tool handler returns something unexpected.

In practice, this error is less about AutoGen itself and more about how your app is invoking it. The fix is usually to make the whole call chain consistently async and avoid blocking patterns.

The Most Common Cause

The #1 cause is mixing sync code with an async AutoGen workflow.

A common mistake is calling an async AutoGen method without await, then trying to use the result like it was already resolved. In TypeScript, that can trigger runtime failures that look like event loop issues, especially inside frameworks, tests, or server handlers.

// BROKEN
import { AssistantAgent, UserProxyAgent } from "@autogen/agentchat";

const assistant = new AssistantAgent({
  name: "assistant",
  modelClient,
});

const user = new UserProxyAgent({
  name: "user",
});

const result = assistant.run("Summarize this claim note"); // Promise, not final result
console.log(result.messages); // runtime failure / undefined behavior

// FIXED
import { AssistantAgent, UserProxyAgent } from "@autogen/agentchat";

const assistant = new AssistantAgent({
  name: "assistant",
  modelClient,
});

const user = new UserProxyAgent({
  name: "user",
});

async function main() {
  const result = await assistant.run("Summarize this claim note");
  console.log(result.messages);
}

main().catch(console.error);

If you’re inside an HTTP route, make the route handler async too:

app.post("/summarize", async (req, res) => {
  const result = await assistant.run(req.body.text);
  res.json(result);
});

Other Possible Causes

1. Running AutoGen inside a non-async callback

This happens when you call await-style agent code from a callback that doesn’t support it.

// BROKEN
button.onClick(() => {
  const result = assistant.run("Draft email");
});

// FIXED
button.onClick(async () => {
  const result = await assistant.run("Draft email");
});

2. Mixing different AutoGen versions or packages

AutoGen TypeScript has had package split changes. If you mix imports from older and newer packages, you can get runtime mismatches that surface as loop or execution errors.

{
  "dependencies": {
    "@autogen/agentchat": "^0.x",
    "@microsoft/autogen": "^0.y"
  }
}

Fix by using one package family consistently and aligning versions across all AutoGen imports.

3. Tool/function handlers returning the wrong shape

If your tool returns a raw object where AutoGen expects a string or serializable payload, the agent loop can fail during message processing.

// BROKEN
const tools = [{
  name: "lookupPolicy",
  description: "Fetch policy details",
  execute: async () => {
    return new Map([["policyId", "123"]]); // bad for serialization
  }
}];

// FIXED
const tools = [{
  name: "lookupPolicy",
  description: "Fetch policy details",
  execute: async () => {
    return JSON.stringify({ policyId: "123" });
  }
}];

4. Creating multiple competing loops in tests or workers

If your test runner, worker thread, or framework already owns the event loop, spawning another uncontrolled async flow can produce errors like:

• •Error: This event loop is already running
• •UnhandledPromiseRejectionWarning
• •TypeError: Cannot read properties of undefined

Use one orchestrator per test and always await completion.

test("agent run", async () => {
  const result = await assistant.run("Explain denial reason");
  expect(result).toBeDefined();
});

How to Debug It

1. •

   Check whether the failing call is awaited

   • •Search for assistant.run(, groupChatManager.run(, or any agent method returning a Promise.
   • •If there’s no await, fix that first.
2. •

   Inspect the exact stack trace

   • •If you see UnhandledPromiseRejection, it’s usually missing awaits.
   • •If you see serialization errors near tool execution, inspect your function outputs.
   • •If you see framework-specific errors inside Next.js/NestJS/Jest, the issue may be lifecycle-related.
3. •

   Reduce to one agent and one tool

   • •Remove group chat orchestration.
   • •Remove all tools except one trivial function.
   • •Confirm whether AssistantAgent alone runs cleanly before adding complexity back.
4. •

   Log every boundary

   • •Log before and after each async call.
   • •Log tool inputs and outputs.
   • •Log message payloads before they enter AutoGen.

Example:

console.log("before run");
const result = await assistant.run("Draft policy summary");
console.log("after run", result);

If "before run" prints but "after run" never does, the failure is inside execution. If neither prints, your call path never reaches AutoGen.

Prevention

• •

  Make every AutoGen entrypoint explicitly async:

  • •route handlers
  • •CLI commands
  • •test cases
  • •job workers
• •

  Keep tool outputs boring:

  • •return strings or plain JSON objects
  • •avoid class instances, Maps, Dates without serialization
• •

  Pin compatible package versions:

  • •don’t mix old and new AutoGen packages in the same repo
  • •update imports together when upgrading

If you want a stable rule for TypeScript projects: treat every AutoGen call as part of an async pipeline end-to-end. The moment you mix sync wrappers around it, this class of event loop errors starts showing up.

Keep learning

• •The complete AI Agents Roadmap — my full 8-step breakdown
• •Free: The AI Agent Starter Kit — PDF checklist + starter code
• •Work with me — I build AI for banks and insurance companies

By Cyprian Aarons, AI Consultant at Topiax.