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Security and networkingChecked through WebDiag API

HTTP/2 and HTTP/3 Checker

Enter a URL to check negotiated HTTP/2 signal and HTTP/3 Alt-Svc advertisement.

  • ALPN
  • Alt-Svc
  • bounded
Online tool

Try the tool

Enter a URL: WebDiag will run a safe network check and show the result.

URL

Network check through backend

For HTTP/SEO tools, the URL is sent to the WebDiag API, where SSRF protection, DNS/IP policy, and redirect limits are applied.

Reviewed: 2026-07-17
01

How to use it

  1. 1

    Enter a public HTTPS URL.

  2. 2

    Run the protocol check.

  3. 3

    Review ALPN h2 and Alt-Svc h3 signals.

02

What the tool can do

  • Checks HTTP/2 through TLS ALPN negotiation.
  • Checks HTTP/3 advertisement through the Alt-Svc header.
  • Shows TLS version context.
03

Common use cases

  • Understand whether the site serves HTTP/2 to a modern client.
  • Check whether HTTP/3 is advertised through a CDN.
  • Prepare protocol layer for a performance audit.
04

How it works inside

HTTP/2 is detected from the TLS handshake ALPN result.

HTTP/3 is detected only as an advertised signal in Alt-Svc.

05

Questions and answers

Why is HTTP/3 only advertised?

Full HTTP/3 validation requires QUIC-capable transport. This tool records the verifiable Alt-Svc signal.

Can I test HTTP URLs?

You can, but ALPN works through TLS, so protocol checks need HTTPS.