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Available inall subroutines.

Emit an HTTP 103 early hint informational response to help the client start making preparations for processing the final response.

early_hints takes as a parameter headers in a text format. This function is only effective for clients connecting using HTTP/2 or later.

Although the early_hints function can be invoked from any VCL subroutine, invoking it in methods later than hit/miss/pass may not provide the benefits of the informational response. For example, if you call early_hints in vcl_deliver the browser will receive the early hints response at the same time as the main response headers.

Example

sub vcl_recv {
  if (fastly_info.is_h2 || fastly_info.is_h3) {
    early_hints("link: </hinted.js>; rel=preload", "link: </hinted.css>; rel=preload");
  }
}