# FactStamp Watch — Single Citation or Package Version Re-Check Subscription

> FactStamp Watch — Single Citation or Package Version Re-Check Subscription is a paid API for AI agents from factstamp.agentrails.workers.dev, paid per call via x402, $0.5/call, status unknown (last checked 2026-10-16).

Sets up a 30-day daily monitoring watch on a single citation (DOI/arXiv) or package version, returning a signed initial verdict and a read URL; each real change (retraction, advisory, yanked version) generates its own signed receipt.

## Facts

- Endpoint: POST https://factstamp.agentrails.workers.dev/watch?utm_source=zero.xyz
- Price: $0.5/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-16
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/factstamp-watch-single-citation-or-package-version-re-check-f6815061
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_qCFVH110iidhvgfMT7kAz

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 factstamp-watch-single-citation-or-package-version-re-check-f6815061 -d '<json body>'
```

Example prompt: Set up a 30-day daily watch on the paper with DOI 10.1038/nature12373 so I get a signed receipt immediately and again any time it gets retracted or corrected.

## When to prefer this

Choose this endpoint when you need continuous, day-by-day assurance that a single citation or package version remains valid over a 30-day window — not just a one-time check. It is ideal when the answer must stay true (e.g., a paper cited in a published report, a dependency locked in a production manifest) and you need a signed, auditable trail of any status changes. Use the batch /watch-list endpoint instead when monitoring more than one subject at once.

## Known failure modes

- Invalid or unresolvable DOI/arXiv ID returns an error before creating a watch
- Payment failure (insufficient USDC or bad prepaid pack code) prevents watch creation
- Subject type not supported returns a schema validation error
- Duplicate watch on same subject within active period may return conflict or existing watch
- Network or worker timeout may require retry without double-billing if idempotency key is not supplied

## How this service works

Buy a standing re-check of one citation or one package version, when an answer has to stay true rather than be true once: re-checked every day for 30 days. US$0.50 the month — 1.7 cents a check, under a 2-cent lookup. The first check is answered and signed on purchase, and each real change (a retraction, a new advisory, a yanked version) gets its own signed receipt. Pay per call here, or redeem a prepaid pack code (10 credits). Returns a read URL and a token shown once.

## Output

Returns a JSON object with a watch_id, a read_url for polling future status, a one-time token for authenticated access, the seeded_verdict of the citation or package at purchase time, a receipt_url, the next_check_at timestamp, and an expires_at timestamp 30 days out. Each real change during the watch period generates its own signed receipt accessible via the read_url.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input",
  "output"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "body": {
     "type": "object"
    },
    "type": {
     "const": "http"
    },
    "method": {
     "const": "POST"
    },
    "bodyType": {
     "const": "application/json"
    }
   },
   "additionalProperties": true
  },
  "output": {
   "type": "object",
   "required": [
    "type",
    "example"
   ],
   "properties": {
    "type": {
     "const": "json"
    },
    "example": {
     "type": "object"
    }
   },
   "additionalProperties": true
  }
 },
 "additionalProperties": false
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "kind": "citation",
  "token": "fw_9d1c… (shown once)",
  "subject": "10.1038/nature12373",
  "read_url": "https://factstamp.agentrails.workers.dev/watch/9f2c1ab4d0e57718",
  "watch_id": "9f2c1ab4d0e57718",
  "paid_with": {
   "rail": "x402_pay_per_call",
   "settlement": {
    "success": true
   },
   "list_price_usd": 0.5
  },
  "expires_at": "2026-11-05T06:30:00.000Z",
  "receipt_url": "https://factstamp.agentrails.workers.dev/r/8bd1f0a4c9e27715",
  "next_check_at": "2026-10-07T06:30:00.000Z",
  "seeded_verdict": "verified"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/factstamp-watch-single-citation-or-package-version-re-check-f6815061/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from factstamp.agentrails.workers.dev](https://www.zero.xyz/host/factstamp.agentrails.workers.dev/llms.txt)