Quality score for any x402 service, earned by real paid probes with on-chain receipts: composite 0-100, component breakdown (payment settlement, schema conformance, ground-truth accuracy, LLM-judged quality), 7-day trend, probe count. Scores publish only after 20+ probes across days; daily corpus digests are Bitcoin-anchored via OpenTimestamps. Query: /score?service={url-encoded resource URL}. Free: /tier/{url}, /leaderboard. Agent guide: https://assay.nominal-labs.com/SKILL.md