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HTTP Alt-Svc Header Builder / Parser

Network

Build an Alt-Svc response header advertising an alternative protocol/host/port (e.g. HTTP/3 over QUIC) with a max-age and optional persist flag, or parse an existing Alt-Svc header back into its individual alternatives.

Build an Alt-Svc response header telling clients they can switch to a different protocol, host, or port for future requests to this origin, most commonly used to advertise HTTP/3 over QUIC alongside a fallback. Each alternative gets a protocol-id like h3 or h2, an authority (host and port), a max-age in seconds, and an optional persist flag, plus a plain-English note on what that protocol-id means and whether it is an outdated draft version worth dropping. Paste an existing Alt-Svc header, including the special clear value, to get the same breakdown for a header you found on a real response.

alt-svchttpheadershttp3quicnetwork

How to use HTTP Alt-Svc Header Builder / Parser

  • 1.Add one or more alternatives with a protocol-id (e.g. h3), an optional host, a port, a max-age, and whether it should persist across a network change.
  • 2.Copy the generated Alt-Svc header value, or read the per-alternative note to understand what each protocol-id means and whether it is deprecated.
  • 3.Paste an existing Alt-Svc header into the bottom field, including the special "clear" value, to get the same breakdown for a header you found on a real response.

Frequently asked questions

What does the Alt-Svc header actually do?
It tells a client that an origin also serves equivalent content over a different protocol, host, or port, most commonly advertising HTTP/3 over QUIC as an upgrade from HTTP/2 or HTTP/1.1, so the client can try connecting that way on a future request.
Why do h3-29 and similar protocol-ids get flagged?
Those are older IETF HTTP/3 draft version identifiers from before the spec was finalized. Modern clients only match the final "h3" token, so continuing to advertise a draft id is dead weight.
What does the "clear" value mean?
It tells the client to forget every alternative service it has cached for this origin, instead of advertising a new one. It's a distinct header value, not an entry alongside other alternatives.
Does this check whether my server actually serves the advertised protocol?
No, it only builds or parses the header value locally. It can't confirm your server actually accepts connections over the protocol, host, or port you're advertising.

Use via API, SDK, or MCP

cURL# Free: 1,000 req/day · Pro: 10,000 req/day
curl -X POST https://api.utilix.tech/v1/tools/alt-svc-header-builder \
  -H "Authorization: Bearer utx_live_..." \
  -H "Content-Type: application/json" \
  -d '{"alternatives":[{"protocolId":"h3","port":443,"maxAge":86400}],"altSvcHeader":"h3=\":443\"; ma=86400"}'

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