Plan-execute-verify on HTTP 301 vs 308

Now I have all the information I need. Let me compile a comprehensive answer about the differences between HTTP status codes 301 and 308.

Answer

HTTP status codes 301 and 308 both indicate that a requested resource has been permanently moved to a new location, but they differ significantly in how they handle the request method and body when a client follows the redirect.

Meaning and Purpose:

Both 301 Moved Permanently and 308 Permanent Redirect indicate that a target resource has been assigned a new permanent URI. According to RFC 9110, "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" (https://www.rfc-editor.org/rfc/rfc9110.html). The 308 code serves the same purpose: "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" (https://www.rfc-editor.org/rfc/rfc9110.html).

Request Method and Body Handling:

The key difference lies in how clients handle the request method when following the redirect. With 301, RFC 9110 explicitly states that "For historical reasons, a user agent MAY change the request method from POST to GET for the subsequent request" (https://www.rfc-editor.org/rfc/rfc9110.html). This means that if a client receives a 301 response to a POST request, it is permitted to send the subsequent request as GET instead of POST. When this method change occurs, content-specific headers and the request body are removed, as RFC 9110 specifies: "If the request method has been changed to GET or HEAD, remove content-specific header fields, including (but not limited to) Content-Encoding, Content-Language, Content-Location, Content-Type, Content-Length, Digest, Last-Modified" (https://www.rfc-editor.org/rfc/rfc9110.html).

In contrast, 308 does not permit this method change. RFC 7538 explicitly notes: "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" (https://www.rfc-editor.org/rfc/rfc7538.html). This means that with 308, the original request method and body must be preserved when following the redirect.

Cacheability:

Both status codes are cacheable. RFC 7231 states that "A 301 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls" (https://www.rfc-editor.org/rfc/rfc7231.html). Similarly, RFC 7538 specifies that "A 308 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls" (https://www.rfc-editor.org/rfc/rfc7538.html). RFC 9110 uses updated terminology, describing both as "heuristically cacheable" (https://www.rfc-editor.org/rfc/rfc9110.html), which has the same practical meaning.

Current Specifications:

The 301 status code is currently specified in RFC 9110 (Section 15.4.2) for HTTP Semantics and Content (https://www.rfc-editor.org/rfc/rfc9110.html), which supersedes RFC 7231 (Section 6.4.2). The 308 status code was originally defined in RFC 7538 and is now also included in RFC 9110 (Section 15.4.9) (https://www.rfc-editor.org/rfc/rfc9110.html).

{"evidence": [
  {"claim": "301 Moved Permanently indicates the target resource has been assigned a new permanent URI", "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": "308 Permanent Redirect indicates the target resource has been assigned a new permanent URI", "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": "With 301, a user agent may change the request method from POST to GET", "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": "With 308, the request method must not be changed from 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": "When method is changed to GET or HEAD in a redirect, content-specific headers are removed", "quote": "If the request method has been changed to GET or HEAD, remove content-specific header fields, including (but not limited to) Content-Encoding, Content-Language, Content-Location, Content-Type, Content-Length, Digest, Last-Modified.", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
  {"claim": "301 response is cacheable by default", "quote": "A 301 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls", "url": "https://www.rfc-editor.org/rfc/rfc7231.html"},
  {"claim": "308 response is cacheable by default", "quote": "A 308 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls", "url": "https://www.rfc-editor.org/rfc/rfc7538.html"},
  {"claim": "301 is specified in RFC 9110 Section 15.4.2", "quote": "15.4.2.  301 Moved Permanently", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"},
  {"claim": "308 is specified in RFC 7538 Section 3", "quote": "3.  308 Permanent Redirect", "url": "https://www.rfc-editor.org/rfc/rfc7538.html"},
  {"claim": "308 is also specified in RFC 9110 Section 15.4.9", "quote": "15.4.9.  308 Permanent Redirect", "url": "https://www.rfc-editor.org/rfc/rfc9110.html"}
]}