# Factstamp Watch — Single Citation or Package Version Monitor

> Factstamp Watch — Single Citation or Package Version Monitor is a paid API for AI agents from agents.factstamp.dev, paid per call via x402, $0.5/call, status unknown (last checked 2026-10-21).

Registers a single citation (DOI/arXiv) or package version for daily re-checks over 30 days, returning a signed receipt on purchase and a new signed receipt whenever a real change (retraction, advisory, yanked version) is detected.

## Facts

- Endpoint: POST https://agents.factstamp.dev/watch?utm_source=zero.xyz
- Price: $0.5/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-21
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/factstamp-watch-single-citation-or-package-version-monitor-8a4debc7
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_wXaceTQez-JZ_D8S-dIK5

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-monitor-8a4debc7 -d '<json body>'
```

Example prompt: Watch the paper with DOI 10.1016/j.cell.2023.01.001 every day for the next 30 days and send me a signed alert if it gets retracted or corrected.

## When to prefer this

Use this endpoint when you need continuous, daily integrity monitoring of a single citation or package version over a 30-day period — not just a one-time check. Prefer this over a single-lookup endpoint (like /check or /retraction) when the subject must remain trustworthy for weeks, not just at a point in time. Choose /watch/portfolio instead if you have a list of subjects. Ideal when an agent or workflow needs auditable, signed proof of each detected change throughout the monitoring window.

## Known failure modes

- Subject identifier not recognized (invalid DOI, arXiv ID, or package name) — returns error indicating unresolvable subject
- Subject already being watched under the same account — may return conflict or duplicate warning
- Payment not received or insufficient USDC — 402 payment required response
- Attempting to watch multiple subjects in one call — should use /watch/portfolio instead
- Pack code invalid or already redeemed — authentication/credit error

## How this service works

A standing re-check of ONE citation or package version, for an answer that must stay true for a month rather than once: daily re-checks 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. One subject only: for a list use /watch/portfolio. Pay per call here, or redeem a pack code (10 credits).

## Output

On purchase the endpoint returns a signed receipt confirming the first check result for the subject. For each subsequent day over 30 days, if a real change is detected (retraction, correction, security advisory, yanked package version), the agent receives a new signed receipt documenting that change. No change means no additional receipt for that day. The signed receipts include a timestamp, the subject identifier, and the nature of the detected change.

## 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
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/factstamp-watch-single-citation-or-package-version-monitor-8a4debc7/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agents.factstamp.dev](https://www.zero.xyz/host/agents.factstamp.dev/llms.txt)