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HTTP_STATUS_NOT_SUPPORTED
Unlike 405, which is specific to a known method on one resource, 501 applies when the server does not implement required functionality at all. It can also be emitted by an intermediary that cannot process an extension.
Verify the method, protocol extension, and server version. Switching endpoint paths will not help when the deployed server simply does not support the requested feature.
Unlike 405, which rejects a method for one resource, 501 means the server does not implement the functionality needed for the request. Determine which method or protocol feature is unsupported by that server version. Retrying against the same implementation will not add the missing capability.
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