Malformed JSON Response from Assistant API: Extra Bracket at the Start of Response

[mostafa-gado]

Confirm this is a Node library issue and not an underlying OpenAI API issue

• This is an issue with the Node library

Describe the bug

I created the Assistant with response_format: { type: "json_object" }. Also In the Assistant instructions I informed it to respond with valid JSON only.

The issue is that the response is always returned with an extra characters in the beginning before the actual JSON object. Sometimes it's a {" or [.

After some debugging I found that this extra character is returned in the "textCreated" stream event when running the thread.

To Reproduce

1. create a function that runs a thread this.openai.beta.threads.runs.stream()
2. Print the output in all the events.

Code snippets

const stream = await new Promise<string>((resolve, reject) => {
        let finalResult = ""; // Variable to store the response
        const stream = this.openai.beta.threads.runs
          .stream(thread, {
            assistant_id: assistantId,
          })
          .on("textCreated", (text) => {
            Logger.log("textCreated: " + JSON.stringify(text.value));
            // finalResult += text.value; //>>>>>>>>>The issue happens here. I fixed it by commenting this line
          })
          .on("textDelta", (textDelta, snapshot) => {
            if (textDelta.value) {
              Logger.log("textDelta: " + JSON.stringify(textDelta.value));
              finalResult += textDelta.value;
            }
          })
          .on("end", () => {
            Logger.log("Thread run completed.");
            resolve(finalResult.trim()); // Resolve the promise with the final output
          })
          .on("error", (error) => {
            Logger.error(
              "Error during thread run: " + (error as Error).message
            );
            reject(error); // Reject the promise on error
          });
      });
      Logger.log("Chat with assistant response:\n" + stream);

      return stream;

OS

Windows 10

Node version

Node v22.13.0

Library version

openai v4.78.1