# 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, $0.06/call, status unknown (last checked 2026-09-14).

Checks whether a candidate summary or translation is faithful to its source text, returning a verdict, faithfulness score, and claim-level reasons.

## Facts

- Endpoint: POST https://suite.veritylayer.dev/faithful
- Price: $0.06/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-b530c256
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_irr8HGkHGSjtgd1gMeqAP

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-b530c256 -d '<json body>'
```

Example prompt: Check if this summary is faithful to the original article — source: 'The Fed raised interest rates by 0.25% citing persistent inflation, with the next meeting scheduled for June.' Candidate: 'The Fed cut rates due to cooling inflation.' Run it in summary mode.

## When to prefer this

Choose this endpoint when you need a calibrated, claim-level faithfulness verdict with an auditable signed receipt — especially in agent pipelines where hallucination or distortion in summaries or translations must be caught before downstream use. Prefer it over generic LLM self-checks when you need a deterministic, pay-per-call, fail-closed verification layer with structured output and no risk of the model rationalizing its own errors.

## Known failure modes

- Source or candidate text missing — 422 validation error
- Text exceeds 2000-character limit — validation error
- Invalid mode value (not 'summary' or 'translation') — validation error
- Uncertain verdict returned when claims cannot be definitively verified against source
- Payment not received or insufficient — 402 Payment Required
- Service temporarily unavailable — 5xx error

## How this service works

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

## Output

Returns a JSON object with: a verdict enum (faithful, minor_drift, unfaithful, or uncertain), a faithfulness score, an array of concrete claim-level reasons citing specific source-vs-candidate discrepancies or confirmations, and an optional Ed25519-signed VerityLayer receipt for auditability.

## 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-b530c256/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)