X-Fastly-Request-Id
The HTTP X-Fastly-Request-Id response header is an unofficial HTTP header carrying a unique identifier assigned by Fastly to each request passing through its network.
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
Fastly assigns a unique request identifier to every request processed by its edge network and returns the value in the X-Fastly-Request-Id header. The identifier follows a hexadecimal hash format. When shielding or clustering is active and the request passes through multiple Fastly PoPs, the ID changes at each hop, identifying the specific edge interaction, not the entire request lifecycle. The value is still useful for correlating log entries and working with Fastly support to diagnose delivery issues.
The header serves a similar purpose to X-Request-Id but is specific to Fastly's platform. Unlike X-Varnish, which exposes Varnish transaction IDs tied to internal cache processing, X-Fastly-Request-Id tracks the identifier for each edge interaction in the request flow. The header often appears alongside other Fastly diagnostics like X-Served-By, X-Timer, and X-Cache-Hits.
Values
Hexadecimal hash
The value is a 40-character lowercase hexadecimal string. Each request receives a distinct hash, making the identifier suitable for log correlation and support ticket references.
Example
A response from a Fastly-powered site includes the request identifier as a 40-character hex string. This value maps to a single request in Fastly's logging pipeline.
X-Fastly-Request-Id: b7fef41e92da921244cc76f95e6bb8e9f9e7e3ac
A different request to the same site produces a completely different identifier. Every request gets its own hash regardless of the resource being fetched.
X-Fastly-Request-Id: dfad9b5d458c0205856d4a71d810e790c45131a4
The header paired with other Fastly diagnostics gives a full picture of how the request was processed. The request ID ties the cache node, timing, and hit count data together into a single traceable event.
X-Fastly-Request-Id: 6c1ecd700880ec267783e0b0ea2495cab8058c17
X-Served-By: cache-sjc1000133-SJC
X-Timer: S1700000000.000000,VS0,VE50
X-Cache-Hits: 1
Takeaway
The X-Fastly-Request-Id header provides a unique hexadecimal identifier for each request processed by Fastly's edge network. The value enables log correlation, request tracing, and support diagnostics across the entire CDN delivery chain.