HTTP Header

Alt-Svc

Response

Advertises alternative services that can serve the same resource, typically over a different protocol or port.

HTTP header reference, syntax, examples, and developer usage.

What is the Alt-Svc header?

The Alt-Svc HTTP header is used to transmit metadata between a client and server as part of HTTP requests or responses.

HTTP headers define how content should be interpreted, cached, authenticated, secured, or processed by browsers and APIs.

Direction

This is a response header sent by the server to the client.

Syntax

Alt-Svc: <protocol>=<authority>; ma=<seconds> | clear

Example

Alt-Svc: h3=":443"; ma=86400

Common use cases

Advertising HTTP/3 (QUIC) support to clients
Migrating traffic to a new protocol without breaking existing connections
Advertising alternative hostnames or ports

Common mistakes

Using the header in the wrong request or response context
Sending invalid header values
Incorrect header syntax
Assuming the header automatically changes server behaviour

Practical developer insight

Alt-Svc is the primary mechanism browsers use to discover HTTP/3 support. When a server responds with Alt-Svc: h3=":443", the browser will attempt to use QUIC on subsequent requests. The ma (max-age) parameter controls how long the alternative is cached. Use clear to revoke all previously advertised alternatives.

Related headers

upgrade connection