streamEvents with for await loop within try catch does not work

[moyara-allgood]

Hi team, I have wasted a tons of time to figure out error messages were swallowed somewhere and finally figured for loop with the streamEvents swallows errors.

This is code to reproduce the issue. I simply copy and pasted from the example in https://langchain-ai.github.io/langgraphjs/concepts/streaming/#streaming-llm-tokens-and-events-streamevents

import { ChatOpenAI } from "@langchain/openai";
import { MessagesAnnotation, StateGraph } from "@langchain/langgraph";

const model = new ChatOpenAI({ model: "gpt-4-turbo-preview" });

function callModel(state: typeof MessagesAnnotation.State) {
  const response = model.invoke(state.messages);
  return { messages: response };
}

const workflow = new StateGraph(MessagesAnnotation)
  .addNode("callModel", callModel)
  .addEdge("__start__", "callModel")
  .addEdge("callModel", "__end__");
const app = workflow.compile();

const inputs = [{ role: "user", content: "hi!" }];

async function main() {
  try {
    for await (const event of app.streamEvents(
      { messages: inputs },
      { version: "v2" },
    )) {
      throw new Error("This should be caught");
    }
  } catch (error) {
    // Error not caught here
    console.error(error);
  }
}
main();

as you can see the code, the error "This should be caught" should be caught in the catch block, but when I run the code, it won't.

by the way this also happens with langchain as well.

import { ChatOpenAI } from "@langchain/openai";

const model = new ChatOpenAI({
  model: "gpt-4o-mini",
  temperature: 0,
});

async function main() {
  try {
    for await (const chunk of model.streamEvents(
      "Hello! Tell me about yourself.",
      {
        version: "v2",
      },
    )) {
      throw new Error("should catch this error");
    }
  } catch (e) {
    // Error not caught here
    console.error(e);
  }
}

main();

Test env:
"@langchain/core": "^0.3.27",
"@langchain/langgraph": "^0.2.39",
"@langchain/openai": "^0.3.16",

For fix, I just catch inside the for await loop, handle the error. But ideally it should catch the error from outside try catch.

[jacoblee93]

Hmm weird - will dig in, thanks for reporting

[jacoblee93]

When I run it it just hangs - that's what you're seeing too right?

[jacoblee93]

Think I have a fix! Will ship tomorrow in @langchain/core.

[jacoblee93]

Can you bump to @langchain/[email protected] and see if that fixes this for you?

[moyara-allgood]

Thanks @jacoblee93 ! just verified it is fixed. Thanks for the quick fix and release!