425 Too Early
Too soon: the server will not risk processing a request that might be a replay.
TLS 1.3 allows sending data on the very first trip — the 0-RTT — but those bytes can be captured and resent by an attacker. RFC 8470 created 425 so the server can refuse to process them when the operation cannot tolerate repetition.
A client receiving it resends the request after the full handshake, when a replay no longer sticks. Few servers emit it — most prefer to simply hold the request — which keeps this specimen on the back shelf.
HTTP/1.1 425 Too Early
Content-Length: 0
Connection: close