# FactStamp Probe — Single Claim Fact-Check

> FactStamp Probe — Single Claim Fact-Check is a paid API for AI agents from agents.factstamp.dev, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-22).

Fact-checks a single dated figure, company name, DOI/arXiv ID, or package@version against primary sources and returns a cited verdict with retrieval metadata, paid per call at $0.02 USDC on Base.

## Facts

- Endpoint: POST https://agents.factstamp.dev/probe?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-22
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/factstamp-probe-single-claim-fact-check-e4b159c3
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_VgssKW2gglOoNyFnyUfdH

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-probe-single-claim-fact-check-e4b159c3 -d '<json body>'
```

Example prompt: Can you fact-check this claim for me — 'US unemployment rate was 3.7% in January 2024' — and give me the source it's verified against?

## When to prefer this

Choose this endpoint when you need a single, fast, receipted fact-check on one discrete claim — a number, a company name, a paper ID, or a package version — and need a cited source rather than just a model's opinion. It is ideal for compliance, publishing, or audit workflows where provenance matters. Prefer the batch or document-scanning siblings when you have multiple claims to verify at once. Use this probe endpoint for spot-checks or when cost per call needs to stay minimal at $0.02.

## Known failure modes

- Claim is too vague or ambiguous to match a specific source — returns 'unknown' verdict
- DOI or arXiv ID does not exist in indexed databases — returns not-found or unknown
- Package@version not found in registry — returns unknown
- Payment not completed or x402 handshake fails — request rejected before processing
- Rate or input format errors if claim type cannot be inferred — returns validation error
- Primary source is temporarily unavailable — may return unknown rather than a stale cached answer

## How this service works

A two-cent probe of the FactStamp engine: one small claim in, one cited answer out, receipted. Send a dated figure, a company name, a DOI/arXiv id or a package@version and get the verdict, the value and the source it rests on with our retrieval time — the same engine and the same sources the full-price doors read. Unknown is a valid answer and nothing is invented. Paid per call in USDC on Base; no pack code and no free tier.

## Output

Returns a structured JSON object containing: a verdict (verified, stale, contradicted, or unknown), the retrieved value or fact as found in the primary source, the specific source URL or identifier the answer rests on, and the retrieval timestamp. 'Unknown' is a valid and honest answer; nothing is fabricated. The response is receipted, meaning it includes provenance metadata suitable for audit trails.

## 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-probe-single-claim-fact-check-e4b159c3/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)