Gated-execute on HTTP 301 vs 308

Both codes mean "this resource has permanently moved to a new URL." The only difference is that a client may switch a POST to a GET when following a 301, but must not change the method or body when following a 308. Both are cacheable by default, and both are now defined in RFC 9110 (HTTP Semantics).

301 Moved Permanently (RFC 9110, §15.4.2, https://www.rfc-editor.org/rfc/rfc9110.html)

• Meaning: The resource has a new permanent URL, and future requests should use it. The server should send a Location header with the new URL, and the client may follow it automatically.
• Method and body: RFC 9110 says that "for historical reasons" a client may change POST to GET on the follow-up request. When a client follows any redirect, it should normally resend the original request to the new URL. If the method has been changed to GET or HEAD, it should drop content headers such as Content-Type and Content-Length, which in practice means the body is dropped too. MDN says the Fetch Standard (what browsers implement) does this: a 301 in response to a POST is followed with a GET (https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/301). I couldn't confirm this in the Fetch Standard itself because its page returned a 404 error.
• History: HTTP/1.0 originally defined 301 as keeping the method. Early browsers disagreed on whether to resend a POST as a POST or as a GET, and most ended up using GET. The spec was later changed to allow it (RFC 9110, §15.4).
• Caching: A 301 can be cached by default ("heuristically cacheable"), unless the method's definition or explicit cache controls say otherwise.

308 Permanent Redirect (RFC 9110, §15.4.9)

• Meaning: Same as 301: a new permanent URL, with a Location header the client may follow automatically.
• Method and body: The client must resend the same method and body to the new URL. 308 and 307 were added "to unambiguously indicate method-preserving redirects." The 301 section of RFC 9110 says to use 308 if you don't want POST to turn into GET. RFC 7538 describes 308 as like 301, "except that it does not allow changing the request method from POST to GET." 308 is the permanent counterpart of 307 (Temporary Redirect).
• Caching: RFC 9110 calls 308 "heuristically cacheable." RFC 7538 used the wording "cacheable by default." Both mean it can be cached unless the method or cache controls say otherwise.
• Compatibility: 308 dates from June 2014 and RFC 9110 warns that some clients may not recognize it. RFC 7538 adds that clients treat an unknown 3xx code like 300 (Multiple Choices). That means a client that doesn't know 308 may not redirect automatically.

Where they are specified

• 301: RFC 9110 §15.4.2. RFC 9110 replaced RFC 7231 (published June 2022, Internet Standard STD 97).
• 308: Originally RFC 7238 (Experimental), then RFC 7538 (Standards Track, April 2015). RFC 9110 replaces RFC 7538 and now defines 308 in §15.4.9.

Two points to note

• MDN contradicts itself. Its 308 page (https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/308) says 301 "requires the request method and the body to remain unchanged" and that older clients get this wrong. That doesn't match RFC 9110, which allows a 301 to change POST to GET, or MDN's own 301 page, which calls that change "permitted by the HTTP specification." I've followed the RFC.
• The 308 rule is stated indirectly. RFC 9110's 308 section has no explicit "MUST NOT change the method." That rule comes from the general redirect note in §15.4, the 301 note pointing to 308, and RFC 7538, which RFC 9110 replaced.

{"evidence": [
{"claim": "301 means the target resource has a new permanent URI that future references should use.", "quote": "The 301 (Moved Permanently) status code indicates that the target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs.", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "A user agent may change POST to GET when following a 301; 308 is the alternative when that is undesired.", "quote": "For historical reasons, a user agent MAY change the request method from POST to GET for the subsequent request.", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "301 is heuristically cacheable unless the method definition or explicit cache controls say otherwise.", "quote": "A 301 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"},
{"claim": "308 has the same 'new permanent URI' meaning as 301.", "quote": "The 308 (Permanent Redirect) status code indicates that the target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs.", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "308 is heuristically cacheable under RFC 9110.", "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"},
{"claim": "307 and 308 were added to unambiguously indicate method-preserving redirects, while 301/302 were adjusted to allow POST to become GET.", "quote": "were later added to unambiguously indicate method-preserving", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "When following a redirect, the user agent should resend the original request, changing the method only as the status code's semantics allow.", "quote": "When automatically following a redirected request, the user agent SHOULD resend the original request message with the following modifications:", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "308 is younger (June 2014) and may not be recognized everywhere.", "quote": "This status code is much younger (June 2014) than its", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "RFC 9110 obsoletes RFC 7538 (the 308 RFC) and RFC 7231 (which defined 301).", "quote": "This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230.", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
{"claim": "RFC 7538 described 308 as like 301 except that it does not allow changing POST to GET.", "quote": "This status code is similar to 301 (Moved Permanently) ([RFC7231], Section 6.4.2), except that it does not allow changing the request method from POST to GET.", "url": "https://www.rfc-editor.org/rfc/rfc7538.html"},
{"claim": "RFC 7538 described 308 as cacheable by default.", "quote": "A 308 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls (see [RFC7234], Section 4.2.2).", "url": "https://www.rfc-editor.org/rfc/rfc7538.html"},
{"claim": "308 was added as the missing permanent variant of 307.", "quote": "Section 6.4.7 of [RFC7231] states that it does not define a permanent variant of status code 307; this specification adds the status code 308, defining this missing variant (Section 3).", "url": "https://www.rfc-editor.org/rfc/rfc7538.html"},
{"claim": "Clients that don't know 308 treat it like 300, so automatic redirection can't be relied on with them.", "quote": "Thus, servers will not be able to rely on automatic redirection happening similar to status codes 301, 302, or 307.", "url": "https://www.rfc-editor.org/rfc/rfc7538.html"},
{"claim": "Per MDN, the Fetch Standard (browsers) uses GET after a 301 to a POST, and altering the method after 308 is prohibited.", "quote": "To avoid user agents modifying the request, use 308 Permanent Redirect instead, as altering the method after a 308 response is prohibited.", "url": "https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/301"},
{"claim": "MDN's 308 page says the method and body are not modified by the client when following a 308.", "quote": "The request method and the body will not be modified by the client in the redirected request.", "url": "https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/308"},
{"claim": "MDN's 308 page conflicts with RFC 9110 by saying 301 requires an unchanged method and body.", "quote": "A 301 Moved Permanently requires the request method and the body to remain unchanged when redirection is performed, but this is incorrectly handled by older clients to use the GET method instead.", "url": "https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/308"}
]}