nhunzaker / sveltekit-reproduction

Demonstrates unexpected header forwarding.

Header Forwarding Problem

This project demonstrates a weird behavior where initial request headers are forwarded through to external fetch calls when using the svelte-kit node adapter.

You can see this by logging request headers in the externalFetch hook.

Steps to reproduce this issue:

First, run this server locally with yarn run dev.

Then cURL the request with the Accept header:

curl --header "Accept: text/html" http://localhost:3000

In the development console, you'll see that the Request object for the API URL also includes this header (and any other header that you provide):

External Fetch:
https://www.gov.uk/bank-holidays.json
Accept Header: text/htm

External Fetch:
https://www.gov.uk/bank-holidays.json
Accept Header: image/png

This header matches whatever is sent to the request for HTML when connecting to http://localhost:3000, even though this is a request for JSON data.

Why does this matter?

This is unexpected and causes problems for our API interactions. We've hit this in two places:

• A client passes through Accept: image/png to our API endpoint, which results in our API responding with a 406 Not Acceptable error. This is fine 🤷, but pretty weird.
• More frustrating: a client passes through a If-Modified-Since header, our API returns a 304 Not Modified response, which raises an error when generating a server response.

We'd expect these headers to not get automatically passed through. Those headers are for the SvelteKit request, not our API.