X-Seen-By
The HTTP X-Seen-By response header is an unofficial HTTP header sent by WordPress.com (hosted by Automattic) to expose which internal infrastructure nodes handled the request.
Note
The X- prefix for non-standard headers is
deprecated per RFC 6648.
Usage
WordPress.com runs on Automattic's internal infrastructure, which includes multiple layers: load balancers, application servers, and Caching systems. The X-Seen-By header records which nodes processed a given request as the request traveled through those layers.
Each node appends its identifier to the header before passing the request or response along. The result is a comma-separated list of Base64-encoded hashes, one per node. The hashes are opaque: internal identifiers meaningful only to Automattic engineers diagnosing infrastructure issues.
The header appears on responses from WordPress.com-hosted sites, not on self-hosted WordPress installations. Self-hosted WordPress runs on third-party infrastructure and does not include this header. The header often appears alongside GLB-X-Seen-By, which records the same information from the global load balancer layer.
Values
Node identifier list
The header value is a comma-separated list of Base64-encoded strings. Each string identifies one infrastructure node in the request path. The order reflects the sequence in which the nodes handled the request.
Example
A simple response routed through two internal nodes shows two comma-separated Base64 hashes. The values have no meaning to external clients and are used for internal log correlation.
X-Seen-By: yvSunuo/8ld62ehjr5B7kA==,T7xPrjRFKDMHVv938PYVfx9slopJdhD+WySraMrpIY8=
Requests with more complex routing paths include additional entries. Each added hash corresponds to another layer in the Automattic infrastructure stack.
X-Seen-By: yvSunuo/8ld62ehjr5B7kA==,T7xPrjRFKDMHVv938PYVfx9slopJdhD+WySraMrpIY8=,4kqI2T4hfJQ2y3Ke0kY7Vw==
The header typically appears alongside X-Request-Id for end-to-end request tracing across Automattic's systems.
Takeaway
The X-Seen-By header records the internal Automattic infrastructure nodes processing a WordPress.com request. The values are opaque Base64 hashes intended for internal diagnostics, and they carry no meaning for external clients.