# Sentinel 6-Hour URL Watcher

> Sentinel 6-Hour URL Watcher is a paid API for AI agents from sentinel.rootstuff.io, paid per call via x402, $0.2/call, status unknown (last checked 2026-09-15).

Monitors a public URL for 6 hours and delivers signed webhook callbacks whenever its up/down verdict changes

## Facts

- Endpoint: GET https://sentinel.rootstuff.io/x402/watch-6h
- Price: $0.2/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/sentinel-6-hour-url-watcher-f73700f4
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_MF1fAIFOVSp1pKwu_TBcC

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 sentinel-6-hour-url-watcher-f73700f4
```

Example prompt: Watch https://api.myservice.com/health for the next 6 hours and POST a signed callback to https://hooks.myapp.com/sentinel whenever its status changes — include {"service":"myservice","env":"prod"} in the payload so I can identify it.

## When to prefer this

Choose this endpoint when you need short-term URL availability monitoring (up to 6 hours) without running your own polling infrastructure, and you want push-based signed webhook notifications rather than polling for status. It is ideal for deployment windows, maintenance periods, or short-lived incidents where the 24-hour watch would be excessive. Prefer this over the 24-hour watcher when cost or watch duration needs to be minimized. Use when cryptographic signing of callbacks is important for verifying authenticity at your receiver.

## Known failure modes

- Invalid or non-public target URL — private/reserved IP ranges refused
- callback_url is not a public HTTPS endpoint — private or reserved addresses rejected
- callback_url uses redirects — redirects are not followed, delivery will fail
- Payload exceeds 4 KB limit — request rejected
- URL becomes permanently unreachable and no verdict changes occur — no callbacks sent
- Callback endpoint is down when verdict changes — delivery may be lost

## How this service works

Watch a URL for 6 hours: signed callbacks when its verdict changes

## Output

No direct response body is returned to the caller; instead, signed HTTP POST callbacks are delivered to the specified callback_url whenever the monitored URL's up/down verdict changes during the 6-hour watch window. Each callback includes the verdict change details and the echoed custom payload provided at registration. Callbacks are cryptographically signed so the receiver can verify authenticity.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "url": {
   "type": "string",
   "description": "The http(s) URL to watch. Public hosts only."
  },
  "payload": {
   "type": "object",
   "description": "Up to 4 KB of JSON echoed back in every callback."
  },
  "callback_url": {
   "type": "string",
   "description": "Public https URL that receives the POSTs. Private and reserved addresses are refused; redirects are not followed."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/sentinel-6-hour-url-watcher-f73700f4/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from sentinel.rootstuff.io](https://www.zero.xyz/host/sentinel.rootstuff.io/llms.txt)