Set when a cookie header is stripped because it is too large.

Fastly writes this header into requests. It is proprietary to Fastly.

When an inbound Cookie header attached to a request exceeds the maximum allowed length, it will be removed and req.http.Cookie will be empty. The header Fastly-Cookie-Overflow will be set to "1" to allow this to be distinguished from a request that had no Cookie header in the first place.

The Fastly-Cookie-Overflow header is automatically removed from requests when they arrive on a Fastly server, before being set if the Cookie header is too long. Therefore, in services that are configured to use clustering, the header is present only in vcl_recv and vcl_deliver, and will not be present in requests made to backends.