Assert a price answer

Check the number before you ship it. Most of what agents pay for in the Bazaar is data and finance: a price, a quote, a balance. The answer an agent writes from a feed is where the number goes wrong, rounded, stale, or copied from the wrong field, and nobody downstream can tell. assert checks the written answer against the feed’s own text: the JSON shape, the number in a sane range, the timestamp in ISO form, the quoted figure found verbatim in the source, the URL under the feed’s domain. The feed answer here is a fixed sample (a BTC/USD quote as a price API returns it); the two agent answers are a faithful one and one that rounded the price and wrote “today” for a time.

Run 2026-10-11 05:26 UTC against api.s2ar.dev 0.1.1 · 4 calls · 607 ms · list price $0.01 for a stranger (these ran on the operator's key and cost nothing) · every number on this page re-derived by tools/walkthrough.py before it was written

1. The quote: HTTP 402

The price, the rails and the witnesses of this route. The quote costs nothing.

curl -s -X POST 'https://api.s2ar.dev/v1/assert' \
  -H 'X-Assay-Quote: 1' \
  -H 'content-type: application/json' \
  -d '{"text": "{\"symbol\": \"BTC-USD\", \"price\": 67234.51, \"as_of\": \"2026-10-10T14:00:00Z\", \"quote\": \"\\\"usd\\\":67234.51\", \"source_url\": \"https://feed.example.com/v1/price/BTC-USD\"}", "checks": [{"check": "json_valid"}]}'

→ HTTP 402 · 197 ms

{
  "price_usd": 0.005,
  "rails": [
    "x402",
    "credits"
  ],
  "determinism": "replayable",
  "witnesses": [
    "checks"
  ],
  "free.per_day_keyless": 10
}

2. A faithful answer: five checks, five passes

The shape holds, the price lies in range, the timestamp is ISO 8601, the quoted figure is in the feed’s text word for word, the URL is the feed’s.

curl -s -X POST 'https://api.s2ar.dev/v1/assert' \
  -H "Authorization: Bearer $S2AR_KEY" \
  -H 'content-type: application/json' \
  -d '{"text": "{\"symbol\": \"BTC-USD\", \"price\": 67234.51, \"as_of\": \"2026-10-10T14:00:00Z\", \"quote\": \"\\\"usd\\\":67234.51\", \"source_url\": \"https://feed.example.com/v1/price/BTC-USD\"}", "source": "{\"symbol\":\"BTC-USD\",\"price\":{\"usd\":67234.51,\"currency\":\"USD\"},\"as_of\":\"2026-10-10T14:00:00Z\",\"exchange\":\"composite\",\"source\":\"https://feed.example.com/v1/price/BTC-USD\"}", "checks": [{"check": "json_schema", "schema": {"type": "object", "required": ["symbol", "price", "as_of", "quote", "source_url"], "properties": {"symbol": {"type": "string", "pattern": "^[A-Z]{3,5}-[A-Z]{3}$"}, "price": {"type": "number"}, "as_of": {"type": "string"}, "quote": {"type": "string"}, "source_url": {"type": "string"}}}}, {"check": "number_in_range", "path": "price", "min": 1000, "max": 1000000}, {"check": "regex_match", "pattern": "\"as_of\": \"\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}Z\""}, {"check": "quotes_in_source", "path": "quote", "min_words": 1}, {"check": "urls_allowed", "domains": ["feed.example.com"]}]}'

PASS → HTTP 200 · 140 ms · list price $0.005 · receipt sha256:4baf94e59c6d02c0…

{
  "verdict": "pass",
  "passed": 5,
  "checks": 5,
  "score": 1.0,
  "results.1.check": "number_in_range",
  "results.1.evidence": {
    "value": 67234.51,
    "min": 1000,
    "max": 1000000
  },
  "results.3.check": "quotes_in_source",
  "results.3.ok": true,
  "record.kind": "assertion",
  "record.replayable": true,
  "record_sha256": "420f0b00a3bf2cda0b6f4e3a2e0f9ff974cb5fb4d3bba9b08d7b1854a7216c65",
  "x-assay-record": "420f0b00a3bf2cda0b6f4e3a2e0f9ff974cb5fb4d3bba9b08d7b1854a7216c65"
}

3. A rounded, undated answer: two of five fail

The schema and the range still pass (67000 is a plausible price), the domain passes; the timestamp is not a time and the quoted figure is not in the feed. The record names both.

curl -s -X POST 'https://api.s2ar.dev/v1/assert' \
  -H "Authorization: Bearer $S2AR_KEY" \
  -H 'content-type: application/json' \
  -d '{"text": "{\"symbol\": \"BTC-USD\", \"price\": 67000, \"as_of\": \"today\", \"quote\": \"\\\"usd\\\":67000\", \"source_url\": \"https://feed.example.com/v1/price/BTC-USD\"}", "source": "{\"symbol\":\"BTC-USD\",\"price\":{\"usd\":67234.51,\"currency\":\"USD\"},\"as_of\":\"2026-10-10T14:00:00Z\",\"exchange\":\"composite\",\"source\":\"https://feed.example.com/v1/price/BTC-USD\"}", "checks": [{"check": "json_schema", "schema": {"type": "object", "required": ["symbol", "price", "as_of", "quote", "source_url"], "properties": {"symbol": {"type": "string", "pattern": "^[A-Z]{3,5}-[A-Z]{3}$"}, "price": {"type": "number"}, "as_of": {"type": "string"}, "quote": {"type": "string"}, "source_url": {"type": "string"}}}}, {"check": "number_in_range", "path": "price", "min": 1000, "max": 1000000}, {"check": "regex_match", "pattern": "\"as_of\": \"\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}Z\""}, {"check": "quotes_in_source", "path": "quote", "min_words": 1}, {"check": "urls_allowed", "domains": ["feed.example.com"]}]}'

PARTIAL → HTTP 200 · 159 ms · list price $0.005 · receipt sha256:3f27ebdbadb5f553…

{
  "verdict": "partial",
  "passed": 3,
  "checks": 5,
  "results.2.check": "regex_match",
  "results.2.ok": false,
  "results.2.evidence": {
    "reason": "no match"
  },
  "results.3.check": "quotes_in_source",
  "results.3.ok": false,
  "results.3.evidence": {
    "quotes": 1,
    "missing": [
      "\"usd\":67000"
    ],
    "path": "quote"
  },
  "record.outcome": "partial",
  "record_sha256": "7bcba6b794a9716d80c632b5c4aac731b6e38a8f6fea7530bdd689cbae9946c7"
}

4. Anyone verifies the verdict, free

curl -s -X GET 'https://api.s2ar.dev/v1/verify/7bcba6b794a9716d80c632b5c4aac731b6e38a8f6fea7530bdd689cbae9946c7'

PARTIAL → HTTP 200 · 111 ms

{
  "found": true,
  "seal_ok": true,
  "signature_ok": true,
  "key_pinned": true,
  "certificate.kind": "assertion",
  "certificate.verdict": "partial",
  "certificate.payload.passed": 3
}

No model reads the answer; five checks do, and the record names which one failed and where. An agent that calls assert before it ships a number spends half a cent and gets a signed verdict it can hand on with the figure. The same preset serves any feed: swap the schema’s keys, the range and the domain.

What this cannot buy

From the signed proceedings (/.well-known/proceedings.json), verbatim:

• placement or routing position
• a different verdict or grade
• buyer data
• amendment access
• a revocation

Failed calls are never charged. Nothing about a caller is kept. The rules: the charter.