X-AC

The HTTP X-AC response header is an unofficial header sent by Automattic's infrastructure to expose 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 encodes a server number, a data center location code, and a cache status token separated by spaces.

The data center portion uses airport-style codes tied to Automattic server locations. bur refers to Burbank, California. The cache status suffix (MISS, STALE, or HIT) 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

A numeric identifier for the specific server or cache tier handling the request, such as 20 or 29.

Data center identifier

A location code identifying the Automattic data center. bur corresponds to Burbank. Some values include tier qualifiers like _atomic_bur or _shield_cache_dfw, indicating specialized cache layers within the infrastructure.

Cache status

One of HIT, MISS, or STALE:

  • 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

Example

A request served by server 20 at the Burbank data center with no cached copy available. The origin was contacted to generate the response.

X-AC: 20.bur_bur MISS

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

X-AC: 20.bur_bur STALE

Takeaway

The X-AC header is an Automattic-specific cache diagnostic header encoding a server number, data center location code, and cache status. The header appears on WordPress.com and other Automattic-hosted properties.

See also

Last updated: March 6, 2026