Available inall subroutines.
When a client connection is using a version of HTTP older than HTTP/3, invoking this function triggers Fastly to advertise HTTP/3 support, on the eventual client response, allowing the client to switch to HTTP/3 for future requests.
When the connection is over HTTP/3 already, this function has no effect.
The addition of the response header happens after the header block has been finalised in
vcl_deliver. As a result, using
error within your VCL code (which will destroy the current response and create a new one), will not affect the addition of the
Alt-Svc header. It is also not possible to read or unset the value of the header in VCL, since it has not yet been added to the response.
resp.http.alt-svc is already set, using this function will cause it to be overwritten.