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-" 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.
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.