Hi, @millerlai!

It looks like the issue you're facing with the Content-Type header showing up as b'text/html; charset=utf-8' is because of the response being interpreted as a byte string.

The b prefix indicates that the string is in bytes format rather than a regular string. This usually happens when there is a mismatch in the encoding or the way the headers are set in your FastAPI response.

To fix this issue, you need to ensure that the Content-Type header is explicitly set and correctly encoded.

Could you try changing:

1response.headers['Content-Type'] = 'text/html; charset=utf-8

Also cross-posting this message from another thread that may be helpful.

https://community.vercel.com/t/vercel-functions-content-type-header-in-vercel-json-not-working/334/2

Hi Pawlean! I'm encountering the same issue here and was not able to fix it. I'm a bit puzzled because it works fine on my local machine and only has this issue on Vercel. I added some middleware, but no dice.

Example header... content-type: b'application/json; charset=utf-8'

Would this imply it's an encoding or decoding issue? My site is https://howdy.new if you'd like to see this in action.

# Middleware to ensure all headers are strings @app.middleware("http") async def ensure_string_headers_middleware(request: Request, call_next): response = await call_next(request)

1if isinstance(response, StreamingResponse):2    # For streaming responses, we need to modify the headers in a different way3    original_headers = response.headers4    response.raw_headers = [(k.encode('utf-8'), v.encode('utf-8')) 5                            for k, v in ensure_string_headers(original_headers).items()]6elif isinstance(response, Response):7    # For regular responses, we can modify the headers directly8    response.headers = ensure_string_headers(response.headers)910return response