Ask for more debug information.

Fastly reads this header from requests. It is proprietary to Fastly.

If an inbound request has the Fastly-Debug header set (conventionally to 1 but actually any value is acceptable), this will prompt Fastly cache servers to output additional response headers:

The presence of the header will also prevent the following headers from being removed, if they are already present on a response:

If you wish to prevent users from triggering debug information in the response on your Fastly service, you can choose to strip the header from the request. Since it is often useful to retain the capability for your own use, you could require it to be set to a more obscure value:

vcl_recv { ... }
Fastly VCL
if (req.http.Fastly-Debug != "o37valg966qgfit63-5iw877gvfgq1") {
unset req.http.Fastly-Debug;