Hey @cassidoo,

Thank you for catching this error!

The reason for the demo site breaking is actually really exciting! It's a combination of a few things:

1. The demo site used next-on-netlify v1.2 until this morning. I was planning to update it to v2 soon, but I wanted to demo a few of the new features (getStaticProps and getServerSideProps). So when the site crashed, it was with next-on-netlify v1.2. There are two differences between next-on-netlify v1.2 and v2 that are relevant for this issue:

   1. next-on-netlify v1.2 returned response headers from Netlify Functions as multiValueHeaders. They're like normal headers, but let you set multiple values for the same header.
   2. next-on-netlify v1.2 set the content-length header as an integer (rather than a string).

2. During development of v2, I found out that Netlify Functions actually do not support multiValueHeaders. They just get quietly ignored. I wrote about that in next-on-netlify issue #9 and filed issue #923 on netlify-cli. At the same time, I implemented a workaround in next-on-netlify to convert the multiValueHeaders into plain headers, so that next-on-netlify v2 would have support for response headers.

3. As I converted those multiValueHeaders to plain headers, I also found out that Netlify Functions do not support having an integer in a header. See issue #451 on netlify-cli. Because the content-length header defaults to an integer, I also implemented a workaround to coerce all headers into String for next-on-netlify v2.

4. Now the exciting news: From what I can tell, Netlify actually implemented support for multiValueHeaders in Netlify Functions on Thursday or Friday 🤯 🙌 Because the demo site was running next-on-netlify v1.2 which uses multiValueHeaders and returns content-length as an integer and because Netlify Functions no longer ignore multiValueHeaders now, but still crash when there's an integer in one of the headers, it caused the demo site to crash.

I just updated the demo to use next-on-netlify to v2 which uses plain headers and coerces all headers to string. Everything works again now 🙂

The support for multiValueHeaders is super exciting! Do you have access to the issue I filed with netlify-cli? It was moved to an internal repo and I can no longer access it. But I would love to know if it's true that Netlify Functions now support multiValueHeaders and if this feature is stable!

If multiValueHeaders really are supported now, I can remove the workaround from next-on-netlify and, more importantly, I can add support for preview mode!

Thanks again for flagging this error - much appreciated! 🏴
- Finn

Heck yeah, looks like your feature request was the merged one! :)
This is awesome. You're a really valuable member of the Netlify community, I'm going to share this internally.

Hey @cassidoo, thank you for looking into! That's really great to hear. Netlify rocks 😁

PS: Closing this for now 🙂