| Internet-Draft | The Preliminary Request Denied HTTP Stat | February 2026 |
| Nottingham | Expires 27 August 2026 | [Page] |
This specification defines a HTTP status code to indicate that the server is denying a prefetch or preload request.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-nottingham-httpbis-pre-denied/.¶
information can be found at https://mnot.github.io/I-D/.¶
Source for this draft and an issue tracker can be found at https://github.com/mnot/I-D/labels/pre-denied.¶
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This Internet-Draft will expire on 27 August 2026.¶
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[FETCH] introduces a mechanism whereby HTTP [HTTP] user agents can speculatively request a representation of a resource, in order to improve perceived performance.¶
In some circumstances, a server might have information that leads it to believe that sending a full response will not improve performance, and could have negative impacts.¶
When this happens, it is common practice to use a 503 (Service Unavailable) status code. However, this has been shown to cause confusion: a server operator who sees a spike in that status code being sent tends to draw the conclusion that there is a server-side operational issue.¶
While other status codes (e.g., 403 (Forbidden)) could be used, they can also suffer (to varying degrees) from the same problem: being confused with an error, operational problem, or other condition.¶
This specification defines a new status code to specifically address this situation.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The 4xx (Preliminary Request Denied) status code indicates that the server is refusing a preliminary request.¶
A preliminary request is one that contains a Sec-Purpose header field [FETCH] with the value "prefetch".¶
This indication is only applicable to the associated request; future preliminary requests might or might not succeed.¶
The following entry should be registered in the "HTTP Status Codes" registry:¶
The security considerations of [HTTP] and [FETCH] apply. Conceivably, the use of this status code could leak information about the internal state of the server; caution should be exercised to assure that it does not.¶