1. Source
Live QUIC handshake + Alt-Svc. The result identifies where its evidence came from.
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Free HTTP/3 Test: Live QUIC Handshake and Alt-Svc Check
The HTTP/3 Test attempts a real QUIC connection to the submitted hostname and reads its Alt-Svc discovery signal. The result separates HTTP/3 that is confirmed from HTTP/3 that is only advertised.
Also known as: HTTP/3 test, QUIC
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Definition
What is an HTTP/3 test?
An HTTP/3 test is a check of whether a server answers HTTP/3, the HTTP version carried over QUIC, a UDP-based transport. The Novaverb HTTP/3 Test reads the live HTTPS discovery signal, then attempts a real QUIC handshake and HTTP/3 request, and reports an advertised capability separately from a confirmed one.
Scope
What does the HTTP/3 test check?
The HTTP/3 test checks two signals for the submitted hostname: whether the Alt-Svc header advertises h3, and whether a real QUIC connection offering ALPN h3 completes. A confirmed result also records the QUIC version, whether 0-RTT was accepted, the HTTP status and the timings for first packet, QUIC plus TLS handshake and HTTP/3 response.
Who it is for
Who is the HTTP/3 test for?
The HTTP/3 test is for people who run a site's public edge. An engineer who enabled QUIC at a CDN confirms that the handshake completes. An operator who sees h3 advertised checks whether a firewall blocks UDP 443. A site owner with audiences on unreliable mobile networks checks whether HTTP/3 is reachable at all.
Why it matters
Why use an HTTP/3 test?
An HTTP/3 test catches the gap between announcing HTTP/3 and delivering it. An Alt-Svc header can point at an endpoint that never answers because UDP 443 is blocked or the QUIC listener is misconfigured. Clients then quietly fall back to HTTP/2, so the failure stays unseen until a real handshake is attempted.
Method
How does the HTTP/3 test work?
The HTTP/3 test works in a set order. The Alt-Svc header is read first, because that is how an h3 endpoint is discovered. A real HTTP/3 request is then attempted over QUIC. Support is reported as confirmed only when an actual h3 response is received, so advertisement and confirmation stay separate findings throughout.
- We read the Alt-Svc header that advertises an h3 endpoint for discovery.
- We attempt a real HTTP/3 request over QUIC to confirm support, not just the advertisement.
- We report supported only when an actual h3 response is received.
Input
What do you enter into the HTTP/3 test?
Enter a domain or any URL on the site into the HTTP/3 test. Only the origin is used, so http or https, www or a full path are all normalized. A private host is refused. HTTP/3 travels over UDP, so a network that blocks QUIC can make a supporting site look unsupported from that vantage point.
Use it like this
yourdomain.comAny address form works. http or https, with or without www, a bare domain or a full path - we normalize it for you.https://www.yourdomain.comHTTP/3 rides on QUIC over UDP 443; we resolve the origin and try it directly.Avoid this
Reading 'unsupported' as a tool errorIf UDP 443 is filtered on the path, a real h3 request cannot complete - 'unsupported' is then the correct finding, not a bug.Result
What does the HTTP/3 test report contain?
The HTTP/3 test report contains a verdict, status chips, discovery facts and an evidence boundary. The verdict reads HTTP/3 is confirmed, HTTP/3 is advertised or HTTP/3 was not confirmed. A confirmed result adds protocol evidence and a QUIC timeline. Server and raw header fingerprints are intentionally omitted from the public result.
Reading it
How do you read an HTTP/3 test result?
Read an HTTP/3 test result by its verdict. Confirmed means a live HTTP/3 handshake over QUIC completed in this test. Advertised means the server announced h3 through Alt-Svc but this probe did not complete a QUIC handshake. Not confirmed means neither was observed. A live handshake is stronger evidence than an advertisement alone.
What does an HTTP/3 test measure?
HTTP/3 carries HTTP over QUIC, a UDP-based transport. This checker first reads the live HTTPS discovery signal, then attempts a real QUIC handshake and HTTP/3 request. The result separates an advertised capability from a confirmed one.
How to enable HTTP/3
- Enable QUIC on the public edge: Use a CDN or server that supports HTTP/3 and configure the public HTTPS hostname.
- Allow UDP 443: Make sure network policy and firewalls allow QUIC traffic alongside normal HTTPS.
- Advertise h3: Return an Alt-Svc h3 signal so compatible clients can discover the HTTP/3 endpoint, then retest.
If HTTP/3 is only advertised
Check UDP reachability, CDN edge policy, certificate/SNI configuration and whether the probe is being blocked. An Alt-Svc header alone does not prove that QUIC can complete.
What this test proves
A real QUIC + HTTP/3 handshake when confirmed, or a clearly labeled discovery signal when only h3 is advertised.
What it does not prove
It does not measure page speed, Core Web Vitals, every geographic edge, or HTTP/3 usage by all visitors.
"Confirmed" means a live HTTP/3 (QUIC) handshake completed; "Advertised" means h3 was announced via Alt-Svc but not confirmed by this check.
Source
Where does the HTTP/3 test get its data?
The HTTP/3 test gets its data from a live QUIC handshake and the Alt-Svc header of a live HTTPS response. Both come from one fresh probe sent by the measurement service to the submitted hostname at the time of the test, so the result describes this HTTPS edge as that probe reached it.
Limits
What does the HTTP/3 test not tell you?
The HTTP/3 test does not tell you that HTTP/3 works on every CDN edge, browser, network or visitor path. One fresh QUIC probe is attempted from the measurement service. The result does not measure page speed or Core Web Vitals, does not show HTTP/3 usage by all visitors and cannot rule out filtering on other networks.
Evidence
What does an HTTP/3 test result prove?
An HTTP/3 test result proves whether a real HTTP/3 request over QUIC succeeded from the probe, not whether HTTP/3 is advertised. A confirmed result proves a completed handshake and an h3 response. An unconfirmed result proves only that this probe did not complete, which can be the network path and not the server.
2. Boundary
This attempts a real QUIC connection offering ALPN h3 and also reads the Alt-Svc discovery signal. Confirmed means a live HTTP/3 handshake completed; advertised means h3 was announced but not confirmed here.
3. Next action
Use the finding to verify a problem, then connect a workspace when you need history, monitoring, or site-wide analysis.
Standards
Which standards does the HTTP/3 test apply?
The HTTP/3 test applies three IETF specifications. RFC 9114 defines HTTP/3. RFC 9000 defines the QUIC transport that HTTP/3 runs on. RFC 7838 defines the Alt-Svc header used for discovery. Keeping discovery apart from confirmation follows directly from RFC 7838, which describes an advertisement and not a guarantee.
IETFRFC 9114
HTTP/3
Completes a real HTTP/3 request to confirm support, not just an advertisement.
Read the specificationIETFRFC 9000
QUIC Transport
Opens a QUIC connection over UDP as the transport for HTTP/3.
Read the specificationIETFRFC 7838
HTTP Alternative Services (Alt-Svc)
Reads the Alt-Svc header that advertises an h3 endpoint for discovery.
Read the specificationWe list a standard only where this tool genuinely reads or measures against it. Where a signal is outside a live check, the result says so instead of implying coverage.
Verification
How can you verify an HTTP/3 test result yourself?
Verify an HTTP/3 test result in a browser. Open the developer tools, show the protocol column in the network panel and reload the page, since browsers typically connect over HTTP/2 first, read Alt-Svc and then upgrade on later requests. A request listed as h3 confirms it. Also confirm that firewalls allow inbound UDP on port 443.
Example
What does a typical HTTP/3 test finding look like?
A typical HTTP/3 test finding on a site that enabled HTTP/3 at its CDN edge reads HTTP/3 is confirmed, with QUIC v1, an HTTP status and a QUIC timeline. The finding that needs work reads HTTP/3 is advertised: h3 appears in Alt-Svc, the QUIC handshake did not complete, so check UDP reachability first.
Cost
Is the HTTP/3 test free?
The HTTP/3 test is free and needs no account for a live probe. An account is needed when one probe is not the whole question: a workspace tracks HTTP/3 over QUIC, HTTP/2 and TLS support across the whole site over time and saves the evidence, so a later change at the edge is recorded.
In a workspace
What does a Novaverb workspace add to the HTTP/3 test?
A Novaverb workspace adds monitoring and a site-wide crawl to the HTTP/3 test. Site Health Audit continues the job as a source-aware technical checklist that prioritizes supported findings and records fresh rechecks. Results are saved as evidence, and an advertised but unconfirmed finding can be turned into a task that is rechecked.
When to go further
When is a free HTTP/3 test not enough?
A free HTTP/3 test is not enough when audiences reach the site through many edges and networks. A single probe from the measurement service cannot show every geographic edge. Each additional hostname needs its own test, an edge policy that changes needs recurring checks, and a speed question needs repeated field or browser measurements.
Next step
What should you do after running the HTTP/3 test?
After running the HTTP/3 test, respond to the verdict. For an advertised result, check UDP reachability, CDN edge policy and certificate and SNI configuration. For a not confirmed result, enable QUIC on the public edge, allow UDP 443 and return an Alt-Svc h3 signal. Then retest. A confirmed result needs no further action.
Related tools
Which free tools pair with the HTTP/3 test?
The HTTP/3 test pairs with free tools that cover the rest of the connection. The HTTP/2 Test reads the ALPN protocol on a TLS connection, the fallback when QUIC fails. The Server Response Time Checker measures Time to First Byte with DNS, TCP and TLS latency. The Website Performance Test compares HTTP response timing from available probe locations.
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Questions
What do people ask about the HTTP/3 test?
The HTTP/3 test FAQ answers what HTTP/3 and QUIC are, how confirmed differs from advertised and how clients discover HTTP/3 through Alt-Svc. Other questions cover enabling HTTP/3 on a server, why a site advertises h3 without confirming it, whether HTTP/3 is needed alongside HTTP/2 and whether HTTP/3 requires TLS.
What does the Novaverb HTTP/3 Test confirm?
It attempts a real QUIC connection offering ALPN h3 to confirm your server actually speaks HTTP/3, and separately reads the Alt-Svc header to report whether an h3 endpoint is merely advertised.
What is HTTP/3?
HTTP/3 is the latest HTTP version, defined in RFC 9114, running over QUIC instead of TCP. QUIC uses UDP, integrates TLS 1.3, and eliminates TCP head-of-line blocking, improving performance on lossy or high-latency networks.
What is the difference between HTTP/3 confirmed and advertised?
Confirmed means a live HTTP/3 handshake over QUIC actually completed in this test. Advertised means your server announced an h3 option in the Alt-Svc header, but this test did not verify that endpoint responds.
What is QUIC and how does it relate to HTTP/3?
QUIC is a transport protocol over UDP that combines connection setup with TLS 1.3 encryption and supports stream multiplexing without head-of-line blocking. HTTP/3 is HTTP mapped onto QUIC, so confirming HTTP/3 means completing a QUIC handshake.
How do clients discover HTTP/3 via Alt-Svc?
A server sends an Alt-Svc response header advertising an h3 endpoint, telling clients they may retry over QUIC. Browsers typically connect first over HTTP/2, read Alt-Svc, then upgrade to HTTP/3 on subsequent requests.
How do I enable HTTP/3 on my server?
Enable QUIC and HTTP/3 in your web server or CDN, allow inbound UDP on port 443 through firewalls, and emit an Alt-Svc header advertising h3. Many sites get HTTP/3 simply by enabling it at their CDN edge.
Why does my site advertise h3 but not confirm HTTP/3 here?
Often UDP 443 is blocked by a firewall, the QUIC listener is misconfigured, or only the edge speaks h3 intermittently. Advertising via Alt-Svc is a promise; a failed QUIC handshake means clients quietly fall back to HTTP/2.
Do I need HTTP/3 if I already have HTTP/2?
Not strictly. HTTP/3 mainly helps users on unreliable or high-latency mobile networks by surviving packet loss and network changes better than TCP. It is a worthwhile enhancement rather than a replacement for a working HTTP/2 setup.
Does HTTP/3 require TLS?
Yes. QUIC integrates TLS 1.3 directly into the transport, so HTTP/3 is always encrypted by design. There is no cleartext HTTP/3, and a valid certificate is required for the handshake this test performs.
Does HTTP/3 make a page faster?
It can reduce connection setup cost or recover better on some networks, but a successful handshake does not prove a faster page. Confirm the effect with repeated field or browser measurements for the audiences you serve.