Early-Data: requests sent before the handshake finished

TLS 1.3 lets a client that has connected before send data in its first flight, saving a round trip. The saving is real on a high latency connection, and so is the cost: early data has no protection against replay, so an attacker who captures those bytes can send them again later and the server cannot tell the difference.

That makes early data acceptable for safe, idempotent requests and unacceptable for anything that changes state. A replayed GET wastes a little work; a replayed payment or a replayed delete does exactly what it says twice. The header exists so an application behind a terminating proxy can tell the two situations apart, since it never sees the handshake itself.

The intended response for an unsafe request is 425 Too Early, which tells the client to retry once the handshake completes. Browsers handle that automatically, so the cost of refusing is one extra round trip for the small set of requests that genuinely need it, rather than a visible failure.

The header is added by whatever terminates TLS, so it can be forged by a client if you trust it from anywhere. Treat it exactly like the forwarded headers: honour it only from your own edge, and ignore it entirely when the request arrived from somewhere you do not control.