HTTP/3 is the latest version of the HTTP protocol, built on top of QUIC (Quick UDP Internet Connections) instead of TCP. QUIC provides faster connection establishment, improved multiplexing without head-of-line blocking, and better performance on unreliable networks. Our HTTP/3 Checker tests whether a server supports HTTP/3 by analyzing the Alt-Svc response header and QUIC protocol negotiation.
How to Use
Enter the domain name of the website you want to test for HTTP/3 support.
Click Check HTTP/3 to test the server's protocol capabilities.
Review the result to see if HTTP/3 is supported and examine the Alt-Svc header.
Use the information to plan HTTP/3 deployment or verify existing support.
Use Cases
Performance Optimization: Verify HTTP/3 deployment for faster page loads and reduced latency.
CDN Verification: Confirm that your CDN provider is delivering content via HTTP/3 as advertised.
Competitive Analysis: Check if competitor websites have adopted HTTP/3 for performance benchmarking.
Migration Planning: Assess HTTP/3 readiness before enabling QUIC on your own infrastructure.
Frequently Asked Questions
HTTP/3 uses QUIC over UDP instead of TCP, eliminating head-of-line blocking at the transport layer. QUIC also combines TLS 1.3 into its handshake, reducing connection setup from two round trips (TCP + TLS) to just one, significantly improving latency on slow connections.
Enabling HTTP/3 depends on your server software. Nginx supports it via the quiche or boringssl fork, Apache via mod_http3, and cloud services like Cloudflare and Fastly enable it automatically. Most major CDNs and hosting platforms now support HTTP/3 out of the box.
All major browsers including Chrome, Firefox, Edge, and Safari support HTTP/3. Safari has supported it since iOS 14 and macOS Big Sur. Browser support is now widespread, making HTTP/3 deployment practical for general web use.
Because it runs over QUIC on UDP, not TCP with TLS on top. There is no way to complete an HTTP/3 request over an ordinary TCP connection, so the only externally visible signal without a QUIC client is the Alt-Svc header advertising it. HTTP/2 is discoverable through ALPN because it shares TLS with HTTP/1.1, which is why it can be tested properly and HTTP/3 cannot.
The advertisement is missing somewhere in the path. Alt-Svc has to be set on the HTTPS response, and a reverse proxy or CDN in front may strip or rewrite it. Another possibility is that HTTP/3 is enabled only for some hostnames or is failing health checks. Check the header directly with curl on the exact hostname, bypassing any proxy, and compare against what the origin sends.
It is a strong signal and not proof. The header is a promise about an alternative endpoint, and whether that endpoint works is a separate question — a misconfigured advertisement can send clients to a port that refuses connections. Since this page reads the advertisement rather than making a QUIC connection, read a present header as intent declared rather than capability confirmed.
Less than it used to. Earlier implementations varied in maturity and some had implementation bugs, and UDP is more likely to be blocked or throttled by middleboxes than TCP is. Current stacks are mature, and a server advertising Alt-Svc still serves HTTP/2 and HTTP/1.1 to clients that fall back, so enabling it is low risk — provided the fallback genuinely works.
Two things that come from QUIC rather than from the HTTP version. Connection setup is faster because it is built on TLS 1.3 without the TCP handshake beneath it, and loss of one packet no longer stalls every other stream, which removes head-of-line blocking between streams. The second is the larger practical win on lossy mobile networks, where it turns a whole-connection stall into a single-stream one.
How HTTP/3 Support Is Detected
HTTP/3 runs over QUIC, which uses UDP rather than TCP. That makes it impossible to discover through an ordinary TLS request, so the signal available is the header a server sends to advertise an alternative.
1Fetch the site over HTTPS and read the Alt-Svc response header, which advertises alternative endpoints a client may use instead.
2Look for an h3 token in that header, which is how a server announces HTTP/3 availability. The header value is reported so you can see exactly what was advertised, including entries for other protocols.
3Distinguish no header from no support. A missing Alt-Svc means no advertisement was found, which is stated as such rather than as a definite absence of HTTP/3 — a server may support it without advertising, particularly behind a proxy that rewrites headers.
This is advertisement, not proof of capability. Confirming HTTP/3 actually works means making a QUIC connection, which this page does not attempt — so a present header is a strong signal and an absent one is a weak negative.
Tips & Best Practices
•Enable HTTP/3 for improved performance on supported clients
•Verify HTTP/3 QUIC connection security and encryption
•Test HTTP/3 connectivity from different networks and clients
•Monitor HTTP/3 adoption and usage metrics
Technical Details
Protocol: HTTP/3 over QUIC support detection Transport: QUIC transport protocol analysis Security: QUIC encryption and security features Output: HTTP/3 support status with performance analysis
