103 Early Hints

HTTP 103 Early Hints allows a server to send some response headers, typically Link headers for preload, preconnect, or dns-prefetch, before it has finished computing the actual response. This is valuable when the final response depends on slow backend work such as a database query or an origin fetch, but the resources the page will need, its stylesheets, fonts, and key scripts, are already known in advance. The browser can start fetching those resources immediately while the server is still assembling the real HTML.

A typical flow has a CDN or edge server hold the connection open, immediately return 103 with Link headers pointing at critical assets, then forward the request to a slower origin, and once the origin replies, send the final 200 (or other) response with its own headers. Because a client might receive zero, one, or multiple 103 responses before the final one, and the final response can repeat or override any header sent early, clients must merge hint headers rather than assume the early ones are authoritative.

Support has grown steadily among CDNs, edge platforms, and modern browsers, which use the Link preload hints to warm up connections and fetch render-blocking assets in parallel with origin processing. It has the most impact on pages with a slow time-to-first-byte, since the browser can start downloading CSS and fonts during that dead time rather than after. Servers that do not support 103 simply skip it, so adding it is a safe, incremental performance optimization.