What 425 Too Early means
425 Too Early is a TLS 1.3 / 0-RTT defense. When TLS 1.3 sends data in 0-RTT (early data) for low-latency reconnects, that data is replayable by an attacker. Servers protecting against replay attacks return 425 for any non-idempotent request that arrived in 0-RTT, telling the client to retry over a fully-established TLS session.
When servers should return it: Return 425 when a non-idempotent request arrives via TLS 1.3 0-RTT and you cannot guarantee it has not been replayed.
How to fix 425 Too Early
- Retry the request without TLS 0-RTT
- Most clients handle 425 automatically and retry
Example response
HTTP/2 425
content-type: text/plain
Replay protection: retry with full handshakeMore references
For a one-page reference of all HTTP status codes, see the HTTP cheat sheet. For testing API responses, try the API Tester tool. For inspecting responses on the command line, the curl cheat sheet covers the most common flags.