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SiteCheck Web Intel – HTTP/3 & Alt-Svc Advertiser Detector
x402
SiteCheck Web Intel – HTTP/3 & Alt-Svc Advertiser Detector is a paid API for AI agents from api.webintel-api.workers.dev, paid per call via x402, $0.002/call, status unknown (last checked 2026-10-24).
Checks a domain's Alt-Svc response header to determine which newer HTTP protocols (HTTP/3, draft versions) it advertises and reports the max-age of that advertisement
Price / call$0.002
Success—
StatusUnknown
How this service works
Reports which newer HTTP protocols a site advertises: HTTP/3 (h3) and older draft or HTTP/2 alternatives in its Alt-Svc response header, and the Alt-Svc max-age. This is what the server announces, not a TLS ALPN negotiation: the fetch this tool makes cannot choose the protocol, so a site that negotiates but does not send Alt-Svc will show as not advertised.
Output
Returns a JSON object with the domain checked, the raw Alt-Svc header string, a list of advertised protocols (e.g. ['h3']), a boolean http3Advertised flag, the Alt-Svc max-age in seconds, the HTTP status code of the fetch, the final URL after any redirects, and the timestamp of the check.
Example use cases
Verify HTTP/3 rollout on own domain
“Has our site nginx-demo.example.com actually started advertising HTTP/3 in its Alt-Svc header yet? I want to confirm the rollout is live.”
Competitive protocol benchmark
“Check whether shopify.com advertises HTTP/3 via Alt-Svc — I'm comparing CDN and protocol configurations across major e-commerce platforms.”
Monitor Alt-Svc max-age for caching policy
“What's the Alt-Svc max-age that stripe.com is broadcasting? I want to know how long clients will cache that protocol advertisement.”
Technical details
Endpoint
GEThttps://api.webintel-api.workers.dev/api/security/tls-alpn-detector?utm_source=zero.xyzWhen to prefer
Use this endpoint when you need to quickly determine whether a specific domain is broadcasting HTTP/3 or other protocol upgrades via its Alt-Svc header, without needing to perform a full TLS ALPN handshake. It is ideal for auditing CDN configurations, validating protocol rollouts, benchmarking competitor infrastructure, or monitoring Alt-Svc advertisement policies at scale. Note that it cannot detect HTTP/3 support that is only advertised via TLS ALPN and not via Alt-Svc headers.
Side effects
- Domain does not send an Alt-Svc header — advertised array will be empty and http3Advertised will be false
- Domain is unreachable or returns a non-2xx status — httpStatus reflects the error code
- Invalid or malformed domain input — API may return an error or empty result
- Site negotiates HTTP/3 via TLS ALPN but does not send Alt-Svc — will show as not advertised (by design limitation)
- Redirects to a different final URL — finalUrl field reflects the redirected destination
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| inputrequired | object | |
| output | object |
Output
Schema · Response
{
"type": "json",
"example": {
"altSvc": "h3=\":443\"; ma=86400",
"domain": "cloudflare.com",
"source": "https://cloudflare.com/",
"finalUrl": "https://www.cloudflare.com/",
"checkedAt": "2026-10-24T02:12:26.033Z",
"advertised": [
"h3"
],
"httpStatus": 200,
"http3Advertised": true,
"altSvcMaxAgeSeconds": 86400
}
}Tags
x402
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