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HTTP_STATUS_NOT_EXTENDED
It was defined for the experimental HTTP Extension Framework. The response should explain which extension policy or mandatory extension was not satisfied.
Because support is uncommon, verify that the status came from the intended server and inspect its response body. Do not add arbitrary headers unless the application documents the required extension.
RFC 2774 is historical and the mechanism is not a normal requirement for modern HTTP APIs. If 510 appears, first identify the server or gateway that emitted it and read the accompanying explanation. Implement only the extension it actually requires; repeatedly adding unrelated request headers can make diagnosis harder.
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