# Verity Suite Faithful Pro — Faithfulness Checker

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

Checks whether a summary or translation faithfully preserves the source text, returning a calibrated faithfulness probability, verdict, hallucinations, and omissions.

## Facts

- Endpoint: POST https://verity-suite.onrender.com/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-pro-faithfulness-checker-2fe29027
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_-CW4G2UyFbPPo6ICftd8T

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-pro-faithfulness-checker-2fe29027 -d '<json body>'
```

Example prompt: Check whether this summary is faithful to the original article — the source is: 'The Federal Reserve raised interest rates by 0.25% on Wednesday, citing persistent inflation and a strong labor market.' and the candidate summary is: 'The Fed cut interest rates by 0.25% due to falling inflation.' Tell me the verdict, faithfulness score, and any hallucinations or dropped claims.

## When to prefer this

Use this endpoint when you need a calibrated, fail-closed faithfulness judgment on a summary or translation — not just a heuristic similarity score. It is specifically designed for AI agent pipelines where hallucinations and factual drift must be detected reliably. Prefer it over embedding-similarity checks when you need claim-level explanations, hallucination enumeration, and omission detection. It covers both summarization and translation modes from a single endpoint.

## Known failure modes

- Empty or truncated input text returns verdict='uncertain' with a faithfulness score near 0
- Gibberish or unreadable input in either field returns verdict='uncertain'
- Texts in completely misaligned languages or domains that cannot be mapped return verdict='uncertain'
- Payment failure (x402) causes the call to be rejected before processing
- Missing required fields 'source' or 'candidate' cause a validation 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 calibrated faithfulness probability from 0 to 1, an array of concrete claim-level reasons, a list of hallucinated claims found in the candidate, and a list of material omissions from the source. The checker fails closed — it never returns faithful or minor_drift when uncertain.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "required": [
  "source",
  "candidate"
 ],
 "properties": {
  "mode": {
   "type": "string",
   "description": "'summary' or 'translation'. If absent, the checker applies the stricter union of both checks rather than guessing a mode."
  },
  "source": {
   "type": "string",
   "description": "the original source text; the sole ground truth the candidate is judged against"
  },
  "candidate": {
   "type": "string",
   "description": "the summary or translation to check against the source"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "object",
 "title": "faithful_out",
 "required": [
  "verdict",
  "faithfulness",
  "reasons"
 ],
 "properties": {
  "reasons": {
   "type": "array",
   "items": {
    "type": "string"
   },
   "title": "Reasons",
   "description": "concrete claim-level reasons, each either citing a specific source-vs-candidate discrepancy or confirming specific support; at least one reason required"
  },
  "verdict": {
   "enum": [
    "faithful",
    "minor_drift",
    "unfaithful",
    "uncertain"
   ],
   "type": "string",
   "title": "Verdict",
   "description": "faithful=every candidate claim is supported by the source and no material source claim is dropped or distorted; minor_drift=only small non-material omissions/nuance loss that change no core claim, entity, quantity, date, or stance; unfaithful=at least one hallucination, contradiction, negation flip, wrong/invented entity-number-date, dropped material claim, or distortion; uncertain=cannot reliably compare (empty/unreadable/truncated/gibberish input or texts too misaligned to map). FAIL CLOSED: never faithful or minor_drift when unsure."
  },
  "omissions": {
   "type": "array",
   "items": {
    "type": "string"
   },
   "title": "Omissions",
   "description": "material claims present in the source but dropped or weakened in the candidate"
  },
  "faithfulness": {
   "type": "number",
   "title": "Faithfulness",
   "maximum": 1,
   "minimum": 0,
   "description": "calibrated probability the candidate fully preserves source meaning with no hallucination, contradiction, or material distortion; 1=certainly faithful, 0=certainly unfaithful. A single confirmed defect drives this well below 0.5."
  },
  "hallucinations": {
   "type": "array",
   "items": {
    "type": "string"
   },
   "title": "Hallucinations",
   "description": "claims asserted in the candidate that are not supported by the source (added or invented content)"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/verity-suite-faithful-pro-faithfulness-checker-2fe29027/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from verity-suite.onrender.com](https://www.zero.xyz/host/verity-suite.onrender.com/llms.txt)