Plan-execute on Twelve HTTP status codes, by the book

I used RFC 9110, "HTTP Semantics" (June 2022), at https://www.rfc-editor.org/rfc/rfc9110.html. It is the current HTTP Semantics specification. A search turned up nothing that replaces it, but the rfc-editor.org info page returned a 404, so I couldn't check its "obsoleted by" field directly.

Section 15.1 of that page lists the heuristically cacheable codes as 200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414 and 501, and says all other codes are not heuristically cacheable. The sections for 203, 206, 308, 405, 410, 414 and 501 each say so too, and none of the other five sections do. The two sources therefore agree.

One wording difference: the 206 section says "unless otherwise indicated by explicit cache controls". The other sections say "unless otherwise indicated by the method definition or explicit cache controls". I still treated 206 as heuristically cacheable.

Two reason phrases changed from older RFCs: 413 is "Content Too Large" and 422 is "Unprocessable Content". Reason phrases are only recommendations in the spec (15.1).

{"answers":[{"code":"203","reason":"Non-Authoritative Information","section":"15.3.4","heuristicallyCacheable":true,"quote":"This status code allows the proxy to notify recipients when a transformation has been applied, since that knowledge might impact later decisions regarding the content.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"206","reason":"Partial Content","section":"15.3.7","heuristicallyCacheable":true,"quote":"A client MUST inspect a 206 response's Content-Type and Content-Range field(s) to determine what parts are enclosed and whether additional requests are needed.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"303","reason":"See Other","section":"15.4.4","heuristicallyCacheable":false,"quote":"This status code is applicable to any HTTP method.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"307","reason":"Temporary Redirect","section":"15.4.8","heuristicallyCacheable":false,"quote":"Since the redirection can change over time, the client ought to continue using the original target URI for future requests.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"308","reason":"Permanent Redirect","section":"15.4.9","heuristicallyCacheable":true,"quote":"A 308 response is heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [CACHING]).","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"405","reason":"Method Not Allowed","section":"15.5.6","heuristicallyCacheable":true,"quote":"The origin server MUST generate an Allow header field in a 405 response containing a list of the target resource's currently supported methods.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"410","reason":"Gone","section":"15.5.11","heuristicallyCacheable":true,"quote":"It is not necessary to mark all permanently unavailable resources as \"gone\" or to keep the mark for any length of time -- that is left to the discretion of the server owner.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"413","reason":"Content Too Large","section":"15.5.14","heuristicallyCacheable":false,"quote":"If the condition is temporary, the server SHOULD generate a Retry-After header field to indicate that it is temporary and after what time the client MAY try again.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"414","reason":"URI Too Long","section":"15.5.15","heuristicallyCacheable":true,"quote":"A 414 response is heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [CACHING]).","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"421","reason":"Misdirected Request","section":"15.5.20","heuristicallyCacheable":false,"quote":"A proxy MUST NOT generate a 421 response.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"422","reason":"Unprocessable Content","section":"15.5.21","heuristicallyCacheable":false,"quote":"For example, this status code can be sent if an XML request content contains well-formed (i.e., syntactically correct), but semantically erroneous XML instructions.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"501","reason":"Not Implemented","section":"15.6.2","heuristicallyCacheable":true,"quote":"This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"}]}