Early Hints

Server perspective

Use 103 when you can safely send helpful headers, especially Link hints, before the final response is ready.

When to use

• •Return 103 to advertise critical preload or preconnect work early
• •Use it only for hints that remain sensible even before the final response arrives
• •Use it as a performance optimization, not as part of the application's success or error contract
• •Sending Link preload or preconnect hints before the final HTML response is ready
• •Helping browsers start fetching critical assets earlier
• •Reducing page load time when the server already knows which subresources will be needed

How to respond

• •Send the early hint headers first, then follow with the normal final response
• •Hint only headers the client can safely evaluate speculatively, and keep them broadly consistent with the eventual representation
• •You may send multiple 103 responses as new hints become available before the final response, but most browsers only process the first one; additional 103 responses are ignored by browser clients

Headers to consider

• •Link is the common choice for preload or preconnect hints; only send other headers when clients can safely treat them as speculative hints rather than final-response metadata.

Response body

• •Do not include a response body in a 103 response.

Server-side pitfalls

• •Do not expose sensitive or misleading hints before the final response is known
• •Do not overuse preloads that waste bandwidth or contention on the client
• •Do not rely on 103 to carry semantics that must affect how the client interprets the final response
• •Do not send 103 responses over HTTP/1.1 unless every client and intermediary in the chain is verified to handle informational responses correctly; RFC 8297 recommends using HTTP/2 or later for Early Hints
• •Do not send hints when the request may result in a cross-origin redirect; clients must discard 103 hints received before a cross-origin redirect, wasting any preload work already started
• •As a replacement for the final response
• •When the hinted resources are speculative enough to waste bandwidth
• •For application semantics that depend on the final status code or full response body

Examples