# SiteCheck Web Intel – HTTP/3 & Alt-Svc Advertiser Detector

> 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

## Facts

- Endpoint: GET https://api.webintel-api.workers.dev/api/security/tls-alpn-detector?utm_source=zero.xyz
- Price: $0.002/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-24
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/sitecheck-web-intel-http-3-alt-svc-advertiser-detector-f4c5b3a8
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_x55BnCYTYld2fYac2U2du

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability sitecheck-web-intel-http-3-alt-svc-advertiser-detector-f4c5b3a8
```

Example prompt: Can you check whether cloudflare.com advertises HTTP/3 or any newer HTTP protocols in its Alt-Svc response header?

## When to prefer this

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.

## Known failure modes

- 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

## 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.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "domain"
     ],
     "properties": {
      "domain": {
       "type": "string",
       "description": "Domain name such as example.com (required)"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object"
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "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
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/sitecheck-web-intel-http-3-alt-svc-advertiser-detector-f4c5b3a8/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.webintel-api.workers.dev](https://www.zero.xyz/host/api.webintel-api.workers.dev/llms.txt)