Check HTTP3 Support for Any URL
Paste a URL and find out whether the server speaks HTTP/3 over QUIC. Inspect Alt-Svc headers, ALPN identifiers, and the negotiated protocol.
We don't log the URLs you submit beyond what's needed to deliver the response.
Everything you need to verify HTTP/3
Simple on the surface, thorough under the hood. Built for everyone who deploys or audits modern web infrastructure.
Real HTTP/3 probe
We don't just guess from headers, we attempt an actual HTTP/3 negotiation and report what the server advertises and which protocol was used.
ALPN identifiers
See the exact ALPN tokens the server advertises (h3, h3-29, and friends), so you know exactly what's wired up.
Full response headers
Browse every response header (Alt-Svc, Server, Content-Type, Status Code and so on) in a clean, readable, and scrollable view.
Elapsed time
See how long the negotiation actually took. A useful sanity check when comparing HTTP/3 performance against your HTTP/2 baseline.
Fast results
The tool takes a few seconds to show the HTTP/3 report, generally ms and up to 5 secs for URLs that don't support HTTP/3 protocol.
No usage limits
You can use this tool for free to test as many URLs as you need, just use it ethically and don't abuse it to create issues.
What exactly is HTTP/3?
HTTP/3 is the third major version of the HTTP protocol. Where HTTP/1.1 and HTTP/2 run on top of TCP, HTTP/3 runs on top of QUIC, a transport protocol built on UDP by Google and standardised by the IETF.
The headline wins: no more head-of-line blocking at the transport layer (a slow stream no longer stalls the others), TLS 1.3 is baked in (so every HTTP/3 connection is encrypted), and the handshake is faster, often just 1-RTT, or 0-RTT for resumed sessions. On lossy mobile networks the difference is night and day.
HTTP/3 is opt-in: servers advertise support via the Alt-Svc response header, and clients upgrade for subsequent requests. This tester reproduces that negotiation flow so you can confirm if your origin or CDN is serving HTTP/3.
HTTPS vs HTTP/3, key advantages
"HTTPS" is about encryption; "HTTP/3" is about which version of the protocol and which transport layer is being used. Here’s how HTTP/3 stacks up against older HTTP/1.1 and HTTP/2 over TLS (aka "HTTPS").
| Aspect | HTTPS (HTTP/1.1 & HTTP/2 over TLS) | HTTP/3 (over QUIC) |
|---|---|---|
| Transport protocol | TCP + TLS 1.2/1.3 |
UDP + QUIC (with TLS 1.3 built in) |
| Connection handshake | 2–3 round trips (TCP + TLS) | 1-RTT typical, 0-RTT for resumed sessions |
| Head-of-line blocking | Yes, one lost packet stalls every stream | No, each stream is independent at the transport layer |
| Encryption | Optional in spec, ubiquitous in practice (TLS) | Mandatory, encryption is part of QUIC itself |
| Connection migration | No, switching networks drops the connection | Yes, connection IDs survive IP changes (Wi-Fi to cellular) |
| Performance on lossy networks | Degrades sharply | Significantly better, independent streams recover in isolation |
| Server signalling | Negotiated via ALPN during TLS handshake | Advertised via Alt-Svc header, upgraded on next request |
| Middlebox / firewall friendliness | Excellent, TCP 443 is universally allowed | Good but uneven, some firewalls may block UDP 443 |
Built for anyone who ships fast websites
From SREs verifying a CDN migration to performance enthusiasts auditing their favourite sites, this tool answers one specific question fast.
DevOps & SREs
Confirm HTTP/3 is actually enabled at your edge after a CDN config change or origin migration, without spinning up curl on five machines.
Frontend & performance engineers
Sanity-check whether the third-party assets you load (fonts, scripts, images) come over HTTP/3, and identify the slow ones still on HTTP/1.1.
Security & compliance
Audit which of your subdomains, partner endpoints and APIs support HTTP/3 with TLS 1.3, and which need attention.
CDN & hosting customers
Your provider says HTTP/3 is on. Verify it from outside your network before you tell stakeholders, the answer takes one click.
Researchers & students
Quickly survey HTTP/3 adoption across a list of well-known sites without setting up a measurement testbed.
The merely curious
Wondering whether your favourite forum, blog or gaming site is on the new stack? You're three clicks away from the answer.
Frequently asked questions
How do I check if a website supports HTTP/3?
The fastest way is the tester at the top of this page: paste a URL, solve the captcha, click Check HTTP/3. We negotiate a real connection and tell you whether the server speaks HTTP/3, plus the ALPN identifiers it advertised.
If you'd rather use the command line, see the curl FAQ below. You can also open Chrome DevTools → Network → enable the "Protocol" column; refresh the page and look for h3 next to your requests.
How can I test HTTP/3 with curl?
If your curl binary was built with HTTP/3 support, you can use the --http3 flag:
# Force HTTP/3, fails if the server doesn't support it
curl --http3 -I https://www.google.com/
HTTP/3 200
content-type: text/html; charset=UTF-8
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000
What is HTTP/3 and how is it different from HTTP/2?
HTTP/3 is the third major version of HTTP. The big shift from HTTP/2 is the underlying transport layer:
- HTTP/2 multiplexes streams over a single TCP connection. If one packet is lost, every stream on that connection stalls until TCP retransmits, which is transport-level head-of-line blocking.
- HTTP/3 multiplexes streams over
QUIC, which runs on UDP and treats each stream independently. A lost packet only stalls the stream it belongs to. Combined with 0-RTT resumption and connection migration, the result is a much more robust experience on lossy or changing networks.
What's the difference between HTTPS and HTTP/3?
HTTPS refers to HTTP running over TLS, it describes that the connection is encrypted, but says nothing about which HTTP version. A site can be HTTPS over HTTP/1.1, HTTP/2, or HTTP/3.
HTTP/3 refers to the protocol version and its transport. Every HTTP/3 connection is encrypted (TLS 1.3 is built into QUIC), so HTTP/3 implies HTTPS, but HTTPS does not imply HTTP/3.
What is the Alt-Svc header?
Alt-Svc (Alternative Services) is an HTTP response header that lets a server advertise that the same resource is also reachable via a different protocol or endpoint. For HTTP/3, you'll see something like:
alt-svc: h3=":443"; ma=2592000, h3-29=":443"; ma=2592000
This tells the client: "I also speak HTTP/3 on port 443, and for the next ma seconds, feel free to upgrade." The first request typically lands on HTTP/2, and the browser switches to HTTP/3 for subsequent requests.
Does enabling HTTP/3 break anything?
Generally no. HTTP/3 is advertised via Alt-Svc response headers, and clients that don't support it simply ignore the hint and fall back to HTTP/2. There's no flag-day, no breaking change.
Two caveats: some corporate firewalls still block UDP on port 443, in which case clients silently fall back to HTTP/2 (you'll see those connections in your server logs). And if you're running a load balancer or middlebox that hasn't been upgraded for QUIC, you'll need to verify it passes UDP correctly before turning it on for production traffic.
Why does the checker sometimes say "supported" but my browser uses HTTP/2?
Browsers use HTTP/3 opportunistically. The first request to a host typically goes over HTTP/2, the server returns an Alt-Svc: h3=… header, and only subsequent requests upgrade to HTTP/3. So a single fresh visit may show HTTP/2 in DevTools even though the server fully supports HTTP/3, refresh the page and you'll usually see h3.
Other reasons: the browser disabled QUIC for the network you're on, your firewall is dropping UDP/443, or the server is rate-limiting QUIC handshakes from your IP address (rare).
Is this tool free? Do you store the URLs I check?
Yes, it's completely free and there's no account to create. We don't keep a long-term log of the URLs you submit; requests are passed to the upstream HTTP/3 probe service to deliver your result and aren't retained beyond what's needed for that round-trip. hCaptcha protects the form from abuse and bots.