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The strictest command-line test is curl --http3-only -I https://example.com/, run with a curl build that includes HTTP/3 support. It forces a QUIC/HTTP/3 connection and fails rather than silently falling back. In a browser, open DevTools, enable the Network panel’s Protocol column, reload, and look for h3. An Alt-Svc header is useful evidence that a server advertises HTTP/3, but only a negotiated request proves that your client and network actually used it.
What an HTTP/3 test must prove
HTTP/3 is HTTP carried over QUIC, which uses UDP rather than TCP. During TLS negotiation, HTTP/3 identifies itself with the ALPN token h3. A useful test therefore answers two separate questions:
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- Does the origin advertise an HTTP/3 endpoint?
- Can this particular client, network and URL complete a real HTTP/3 request?
The first question is answered by response headers such as Alt-Svc: h3=":443". The second requires a negotiated transfer, shown by strict curl or a browser’s protocol information. Test the exact HTTPS hostname you care about: redirects, CDNs, subdomains and third-party assets can negotiate different protocols.
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Method 1: force HTTP/3 with curl
Strict probe (recommended)
Run:
curl --http3-only -I https://example.com/Replace example.com with the target URL. The -I option requests headers, while --http3-only permits no HTTP/2 or HTTP/1.1 fallback. A successful response demonstrates that this curl client reached the URL over HTTP/3. The response status can be 200, 301, 403 or another valid HTTP status; the important fact is that the HTTP/3 exchange completed.
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Check that your curl supports HTTP/3
First inspect the feature list:
curl -VLook for an HTTP/3-capable build. If curl says option --http3-only is unknown, the installed binary or its linked libcurl was built without HTTP/3 support. Install a distribution package that provides HTTP/3, use an up-to-date official build, or compile curl with an HTTP/3-capable QUIC library. Do not interpret that option error as evidence about the website.
Tolerant probe (not proof by itself)
You can allow fallback with:
curl --http3 -I https://example.com/This mode can use HTTP/3 and fall back to HTTP/2 or HTTP/1.1. It is useful for checking general connectivity, but a successful transfer alone does not prove HTTP/3. Use the strict command or a protocol/version indicator when the result matters.
Follow redirects deliberately
Without redirect following, curl tests the URL you supplied and reports its response. To test the final destination as well, add -L:
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Method 2: verify HTTP/3 in browser DevTools
Chrome or Chromium
- Open the target
https://URL. - Open DevTools (for example,
F12orCtrl+Shift+I). - Select Network.
- Open the column chooser, enable Protocol, and reload the page.
- Inspect the document request and important assets. A value of
h3means that request negotiated HTTP/3.
Keep the protocol column visible while testing individual requests. A page can contain resources from several hostnames, and each origin negotiates independently.
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Why the first reload may show h2
Browser discovery is opportunistic. On a first visit, the browser may use HTTP/2, receive an Alt-Svc mapping, cache that mapping, and try HTTP/3 on a later request. Reload after the first response and check the protocol again. A browser can still remain on HTTP/2 when QUIC is disabled, UDP is blocked, a proxy intervenes, or policy prevents the upgrade.
Method 3: inspect the Alt-Svc advertisement
Fetch headers with an ordinary HTTPS request:
curl -I https://example.com/Look for a line like:
Alt-Svc: h3=":443"; ma=86400The h3 token and port advertise an equivalent HTTP/3 service. RFC 9114 describes this as an origin advertising the availability of an equivalent HTTP/3 endpoint through the Alt-Svc response header field. The header is a capability hint, not a success report: the advertised QUIC service could be unreachable, misconfigured, certificate-invalid, rate-limited or blocked in transit. Confirm it with --http3-only or DevTools.
Method 4: test from an independent network
An online HTTP/3 checker such as HTTP3Verify (an external HTTP/3 checker) can perform negotiation from outside your network and show selected ALPN identifiers and response headers. Use an external result as a second vantage point, not as a replacement for a local test: your office firewall, VPN, ISP or proxy may have a different UDP policy than the checker.
For an important diagnosis, compare three observations: strict curl on your network, the browser’s protocol column, and an external probe. Agreement indicates a broad result; disagreement usually identifies a client or network-path issue rather than a simple yes/no property of the site.
How to interpret common outcomes
| Observation | What it establishes | Next action |
|---|---|---|
curl --http3-only returns headers |
This client completed an HTTP/3 request to that URL. | Record the hostname, URL and time; test redirects or other hosts separately. |
Strict curl fails, but Alt-Svc is present |
The origin advertises HTTP/3, but your attempted QUIC connection did not complete. | Check UDP/443 reachability, TLS/certificate errors, proxies and server configuration. |
Browser shows h2 on the first load |
It may not yet have learned the Alt-Svc mapping. | Reload after discovery, then check QUIC and network policy. |
Browser remains on h2, strict curl succeeds |
HTTP/3 works for curl, but the browser path or settings differ. | Check browser QUIC settings, extensions, proxy/VPN and UDP blocking. |
Only one asset shows h3 |
Negotiation differs by hostname, CDN, redirect hop or third-party origin. | Test the document and each relevant hostname directly. |
Plain http:// URL cannot use the normal HTTP/3 procedure |
The standard direct procedure applies to HTTPS origins. | Test the HTTPS endpoint and its redirect behavior. |
Automating the strict check
Shell and cURL
For a script, make curl’s exit status visible and capture headers:
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set -o pipefail
curl --http3-only -sS -D headers.txt -o /dev/null https://example.com/
status=$?
printf 'curl exit status: %sn' "$status"
cat headers.txtA zero exit status means the strict request completed; a nonzero status requires investigating the error text and network conditions. The response status itself is not a guarantee that every page asset used HTTP/3.
Python wrapper
Python’s standard library does not provide a portable HTTP/3 client, so call an HTTP/3-enabled curl binary and preserve its result:
import subprocess
url = "https://example.com/"
result = subprocess.run(
["curl", "--http3-only", "-I", "--silent", "--show-error", url],
text=True,
capture_output=True,
)
print(result.stdout)
if result.returncode:
raise SystemExit(f"HTTP/3 request failed: {result.stderr.strip()}")This tests the curl installed on the machine running the script; verify that machine’s build with curl -V.
Node.js wrapper
Node.js can likewise invoke the system curl binary:
import { execFile } from "node:child_process";
execFile("curl", ["--http3-only", "-I", "--silent", "--show-error", "https://example.com/"], (error, stdout, stderr) => {
if (error) {
console.error(stderr.trim());
process.exit(error.code || 1);
}
console.log(stdout);
});This is intentionally a wrapper rather than a claim that the built-in Node HTTP client negotiates HTTP/3. The curl binary remains the component performing QUIC.
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Troubleshooting HTTP/3 tests
“Option –http3-only is unknown”
Your curl lacks HTTP/3 support. Install or build an HTTP/3-capable version, then rerun curl -V. A newer-looking version number alone is not enough; the build must include the relevant feature.
Strict curl fails while a normal curl succeeds
Normal curl may have used HTTP/2 or HTTP/1.1. Retry with --http3-only and read the specific QUIC, TLS or connection error. Check whether UDP port 443 is blocked by a firewall, VPN, proxy or provider. Also consider an unreachable or incorrectly configured QUIC endpoint.
Alt-Svc is advertised but negotiation fails
The header proves only that the origin announced an alternative service. Verify that the advertised port is reachable, the certificate covers the hostname, and intermediaries are not filtering UDP. Compare with an external probe to separate a local path problem from an origin problem.
The browser never shows h3
Reload after the first response so the browser can learn Alt-Svc. Then inspect browser QUIC settings, enterprise policy, proxies, VPNs and UDP/443 filtering. A successful strict curl result proves the site can work over HTTP/3 for that client path, not that every browser path will choose it.
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Protocol negotiation is per connection and origin. A redirect host, CDN hostname or third-party domain may support a different protocol. Filter the Network panel by hostname and test each relevant URL directly.
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Performance, reliability and testing limits
HTTP/3 support is path-dependent. A server can advertise it while a particular network blocks UDP, and a browser can intentionally use HTTP/2 even when HTTP/3 is available. Record the URL, hostname, client build, network (including VPN or proxy), and whether the result came from an advertisement or a negotiated request. Repeat from another network when the result affects deployment or incident diagnosis.
Do not treat a single protocol result as a universal performance benchmark. These procedures establish negotiation and reachability; they do not measure page speed, loss recovery or application latency. Test the exact routes and assets your users depend on.
Or skip the browser setup
If your goal is a clean visual capture after checking a site, ScreenshotNeo provides a one-call screenshot API. It accepts the page URL and can return PNG, JPEG, WebP or PDF; it is separate from protocol negotiation, so use curl or DevTools above to establish HTTP/3 support.
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See the ScreenshotNeo API documentation for parameters and options. A basic call is:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo accepts cookie or consent banners before capture and removes more than 60 known consent platforms, newsletter popups and chat widgets; each cleanup step can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and the response identifies the page verdict and billing result in X-Page-Verdict and X-Billed headers. Its MCP server includes take_screenshot, get_page_info and capture_pdf for Claude, Cursor and other MCP clients. The Free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 screenshots. Create a free ScreenshotNeo account to try it.
Frequently Asked Questions
Does an Alt-Svc header mean HTTP/3 is enabled for every visitor?
No. It advertises an alternative service; UDP reachability, client support, TLS validation and network policy still determine whether a request negotiates HTTP/3.
Should I test the website’s IP address instead of its hostname?
No. HTTP/3 testing should use the HTTPS hostname because TLS certificates, Alt-Svc and origin routing are hostname-dependent.
Can a site support HTTP/3 for the document but not its assets?
Yes. Assets may come from different hostnames, CDNs or redirect targets, each with its own negotiation result.
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