Replacement Character (�) Display Issue in Japanese Chat After Vercel Deployment

Hello, I am currently developing an AI customer support using the code from openai-assistants-quickstart. The chat works fine in my local environment (npm run dev and npm run build & npm run start), but after deploying it to Vercel, Japanese characters appear as replacement characters (�), as shown in the attached image.

Here are the specific details:

  1. Japanese text displays correctly in the local environment.
  2. After deploying to Vercel, Japanese text is replaced with replacement characters (�).
  3. I followed the deployment steps as instructed in the official repository.

Could anyone provide advice on the cause and solution for this issue? I suspect it might be related to environment settings or encoding issues, but I am not sure of the exact solution. Any help would be greatly appreciated. Thank you!

I checked response from{threadId} and I found the message was generated correctly.

Since you have confirmed that the response contains the correct symbols you can try to fix it by manually setting the encoding to UTF-8.

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Thank you for your reply! You saved me.
I manually set the encoding in the response header and it solved the problem!


return new Response(stream.toReadableStream());


return new Response(stream.toReadableStream(), {
    headers: {
      "Content-Type": "text/event-stream; charset=utf-8",
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