# Verity Suite – Faithful (Faithfulness Checker)

> Verity Suite – Faithful (Faithfulness Checker) is a paid API for AI agents from suite.veritylayer.dev, paid per call via x402, $0.15/call, status unknown (last checked 2026-09-14).

Checks whether a candidate summary or translation faithfully represents its source text, returning a calibrated verdict and claim-level reasons.

## Facts

- Endpoint: POST https://suite.veritylayer.dev/faithful/pro
- Price: $0.15/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/verity-suite-faithful-faithfulness-checker-9c22ec6b
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_TXyVzksl3fQEWbjd7riif

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 verity-suite-faithful-faithfulness-checker-9c22ec6b -d '<json body>'
```

Example prompt: Check if this summary is faithful to the original article — source: 'The board unanimously voted to increase the dividend by 10% following record quarterly profits.' Candidate: 'The board voted to raise the dividend after strong earnings.' Mode: summary.

## When to prefer this

Choose this endpoint when you need a calibrated, auditable faithfulness verdict for AI-generated summaries or translations, especially in pipelines where hallucination or factual drift must be caught before content is published, acted upon, or passed downstream. The signed receipt feature makes it suitable for compliance-sensitive workflows where proof of verification is needed. Prefer it over generic LLM self-evaluation when you need structured verdicts with claim-level reasoning rather than free-form critique.

## Known failure modes

- Source or candidate text exceeds 2000 character limit — request rejected with validation error
- Missing required fields (source, candidate) — returns schema validation error
- Ambiguous mode when both summary and translation interpretations conflict — applies stricter union check which may yield 'uncertain' verdict
- Network or service unavailability — fail-closed behavior means no permissive fallback is returned
- Invalid bodyType or method — request rejected at schema validation

## How this service works

The trust fabric for AI agents — calibrated, fail-closed services agents pay per call.

## Output

Returns a verdict ('faithful', 'minor_drift', 'unfaithful', or 'uncertain'), a faithfulness score, and an array of concrete claim-level reasons citing specific source-vs-candidate discrepancies or confirmations. Optionally includes an Ed25519-signed VerityLayer receipt for independent verification.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "mode": {
   "anyOf": [
    {
     "type": "string",
     "maxLength": 2000
    },
    {
     "type": "null"
    }
   ],
   "title": "Mode",
   "default": null,
   "description": "'summary' or 'translation'. If absent, the checker applies the stricter union of both checks rather than guessing a mode."
  },
  "source": {
   "type": "string",
   "title": "Source",
   "maxLength": 2000,
   "minLength": 1,
   "description": "the original source text; the sole ground truth the candidate is judged against"
  },
  "candidate": {
   "type": "string",
   "title": "Candidate",
   "maxLength": 2000,
   "minLength": 1,
   "description": "the summary or translation to check against the source"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/verity-suite-faithful-faithfulness-checker-9c22ec6b/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from suite.veritylayer.dev](https://www.zero.xyz/host/suite.veritylayer.dev/llms.txt)