This repo provides a minimal reproduction of three unexpected behaviors.

Stripping of ETag header set by middleware when the middleware fully handles the request.

Perform curl --location 'https://<<host>>/strippedEtag'

The middleware for this route will fully handle the request and include the following two headers in the response: { sonos: "rocks", etag: "666" }.

When run locally, the client receives both headers.

When run in Vercel, the client receives sonos: "rocks", but not the etag.

Modifying of ETag header set by middleware when the middleware augments the request.

This one may be uninteresting since it's a bizarre scenario

Perform curl --location 'https://<<host>>/brokenEtag'

The middleware for this route will augment the response with the same two headers.

When run locally, the client receives both headers.

When run in Vercel, the ETag received by the client has been modified

Improperly returning 412 — Precondition Failed

Perform this GET to obtain the proper ETag curl --location --request GET '<<deplyment baseUrl>>/ifMatch?upstream-url=https%3A%2F%2Fapi.test.ws.sonos.com%2Fcontent%2Fapi%2Fv1%2Fgroups%2FRINCON_347E5CE0055E01400%253A365384700%2Fservices%2F16751367%2Faccounts%2F998892611%2Fqueues%2Fc16ad138-a40f-42e2-9bcd-dfb102bf28d1%2Fresources%3Fposition%3DNaN' \ --header 'Content-Type: application/json' \ --header 'Authorization: Bearer <<access token tbs>>' \ --data '{"id":{"accountId":"1","objectId":"spotify:playlist:37i9dQZF1E4ptIIqM81te8","serviceId":"3079"},"type":"PLAYLIST"}'

This will fail when run in Vercel because of the first issue. Running from localhost should work. Currently, the ETag is '18'.

Perform curl --location '<<deplyment baseUrl>>/ifMatch?upstream-url=https%3A%2F%2Fapi.test.ws.sonos.com%2Fcontent%2Fapi%2Fv1%2Fgroups%2FRINCON_347E5CE0055E01400%253A365384700%2Fservices%2F16751367%2Faccounts%2F998892611%2Fqueues%2Fc16ad138-a40f-42e2-9bcd-dfb102bf28d1%2Fresources%3Fposition%3DNaN' \ --header 'If-Match: <<ETag from prior GET step>>' \ --header 'Content-Type: application/json' \ --header 'Authorization: Bearer <<access token tbs>>' \ --data '{"id":{"accountId":"1","objectId":"spotify:playlist:37i9dQZF1E4ptIIqM81te8","serviceId":"3079"},"type":"PLAYLIST"}'

When the POST is sent (implied in the 2nd curl because of --data), it will properly update the resource. However, when running in Vercel, it will 412. When running locally, it 200's.