demo · v133
Early hints vs final response — on the same timeline
When a server sends 103 Early Hints, the response actually arrives in two stages. Chrome 133 reverts the responseStart tweak (always the FIRST response, including early hints) and adds finalResponseHeadersStart for the 2xx/4xx/5xx. RUM dashboards can now report both without ambiguity.
| field | pre-tweak | brief tweak | 133 (final) |
|---|---|---|---|
| responseStart | final headers time | final headers time (broken interop) | FIRST response time (early hints or final) |
| finalResponseHeadersStart | — | — | final headers time |
why both
An HTTP/103 Early Hints response lets the server tell the browser "here are resources to preload" before the final 200 has even been generated — useful for SSR apps where the final HTML takes a database round-trip. After early hints arrived in 103, performance APIs lacked a way to talk about both timestamps. A brief experiment changed responseStart to mean "final headers" — which broke RUM that compared Chrome with FF/Safari, where it still meant "first response". 133 reverts that and introduces the explicit finalResponseHeadersStart for the new case.