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HTTP/2 and HTTP/3 offer meaningful performance improvements over HTTP/1.1 (multiplexing, header compression, and for HTTP/3, transport over QUIC instead of TCP). This tool forces a live connection at each protocol version and reports which ones your server actually accepted.
How it works
For each protocol version, a request is made forcing that exact version. Because cURL can silently fall back to a lower version instead of erroring when a forced version isn't available, a version only counts as "supported" when the connection succeeds AND the reported negotiated version matches what was actually requested.
- Enter website URL.
- Click Calculate to see your results.
Examples
A modern, fully upgraded server
A site running a current web server and CDN commonly supports all three — HTTP/1.1 for compatibility, and HTTP/2 or HTTP/3 negotiated automatically by modern browsers when available.
Who should use it
- Verifying a CDN or hosting migration didn't regress HTTP/2 support.
- Auditing a website's modern protocol support as part of a performance review.
Industry applications
- Web performance engineering
- Hosting/DevOps and CDN configuration audits
Advantages
- Actually forces and verifies each protocol version rather than just reporting what was negotiated by default.
- Transparently reports when a version couldn't be tested due to a local limitation, rather than guessing.
Limitations
- HTTP/3 testing is only possible when the checking server's own cURL build includes QUIC support.
Common mistakes to avoid
- Assuming a "not tested" HTTP/3 result means the target website doesn't support it — it may just mean the checking environment's own cURL build lacks QUIC support.
- Forgetting that redirects aren't followed, so a URL that redirects elsewhere is tested at its original address, not its final destination.
Best practices
- If HTTP/3 shows as "not tested," verify separately with a tool or environment known to have QUIC support before concluding the target site lacks it.
- Re-check after any CDN or web server configuration change, since protocol support commonly changes with those.
Tips
- Pair with the TLS Version Support Checker — HTTP/2 and HTTP/3 both effectively require modern TLS to be negotiated at all in real browsers.