X-AC

The HTTP X-AC response header is an unofficial header from Automattic's infrastructure, exposing cache status and server routing information for WordPress.com and related properties.

Table of Contents

Usage

The X-AC header is proprietary to Automattic, the company behind WordPress.com. The header appears on WordPress.com-hosted sites and other Automattic infrastructure. The value contains three space-separated tokens: a server number joined by a dot to a data center code, a second underscore-prefixed data center qualifier, and a cache status token.

The data center codes are airport-style codes tied to Automattic server locations. mxp refers to Milan Malpensa and bur to Burbank, California. The cache status token (HIT, MISS, STALE, or BYPASS) follows the same conventions used in other cache status headers like X-Cache. A MISS means the response was fetched from the origin. A STALE means a cached copy was served because a fresh copy was not yet available.

Note

The "X-" naming convention for HTTP headers, "X" referring to "experimental", has been deprecated and needs to be transitioned to the formal naming convention for HTTP headers.

Values

The header value is a space-separated string with three components:

Server number and data center

The first token joins a numeric server identifier to a data center code with a dot, such as 12.mxp or 31.mxp. The number identifies the specific server or cache tier handling the request, and the code identifies the Automattic data center.

Data center qualifier

The second token is an underscore-prefixed location code, such as _dca, referencing an additional point in the routing path. Longer qualifiers like _atomic_bur or _shield_cache_dfw appear on some responses, indicating specialized cache layers within the infrastructure.

Cache status

One of HIT, MISS, STALE, or BYPASS:

  • HIT: the response was served from cache
  • MISS: no cached copy was available and origin was contacted
  • STALE: a cached copy was served past its freshness window while a fresh copy is being fetched
  • BYPASS: the response skipped the cache entirely, common for logged-in or otherwise personalized requests

Example

A request served by server 12 at the Milan data center with caching bypassed. The response went straight to the origin without touching the cache.

X-AC: 12.mxp _dca BYPASS

A request served from a stale cached copy by server 31 at the same data center. The cache had a copy but its freshness period had expired.

X-AC: 31.mxp _dca STALE

See also

Last updated: August 11, 2026