HTTP status codes / 4xx — Client errors
The server refuses to process a possibly-replayed early request.
Tied to TLS 1.3 0-RTT ("early data"): the server will not risk processing a request that could be a replay. The client should retry after the handshake completes.
Retry the request normally; if you control the client, disable 0-RTT for non-idempotent requests.
Server-side note: Retry after the handshake completes; disable 0-RTT for writes if you control the client.
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400 Bad Request · 401 Unauthorized · 402 Payment Required · 403 Forbidden · 404 Not Found · 405 Method Not Allowed — or the full reference.
Build the request, send it, read status + headers + timing — on your iPhone. Free to start.