Deterministic, evidence-backed answers for things AI agents can't verify themselves. Pay per call via x402 (USDC on Base) — no signup, no API key, no account required.
The x402 marketplace has no real vetting of its own. Check live whether a seller exists, whether its own declared schema is internally consistent, and whether its payout address is sanctioned — plus a standalone wallet-safety check for any address, before you send it money. Check a seller or wallet →
Checked against Crossref, OpenAlex, the Wayback Machine, CourtListener, SEC EDGAR, openFDA, and NIH RePORTER. Flags fabricated citations, retractions, dead links, invented case law, and misattributed filings, drug records, or grants.
See all 7 checks → RealDepChecked against npm/PyPI/crates.io plus OSV.dev, and the GitHub REST API. Catches "slopsquatting" and an agent depending on a hallucinated or abandoned package or repo.
See both checks →Every response, from any product, returns confirmed, contradicted, or an honest unknown — never a guess — with the evidence used to decide, so you can check our work:
{
"verdict": "contradicted",
"checks": { "exists": true, "retracted": true, ... },
"canonical": { "doi": "...", "title": "...", ... },
"evidence": [
{ "source": "crossref", "url": "https://api.crossref.org/works/...", "retrieved_at": "2026-08-30T12:00:00Z" }
],
"latency_ms": 312,
"cost": { "amount": "0.005", "currency": "USDC" }
}
Built to be read by code, not people: one verdict enum, boolean checks, no prose to interpret — the exact same shape whether you're checking a citation or a repo. Every field is typed and documented in the OpenAPI spec — nothing to guess at.
Crossref, npm, GitHub, SEC EDGAR, openFDA, NIH RePORTER, and Chainalysis's sanctions oracle are all free and public — why not just call them yourself? You can. What you're paying for is the part after the fetch: normalizing a messy source response into a real yes/no/unknown, handling the pagination and edge cases correctly (an old filing isn't "fake" just because it's not in the first page of results), and getting the exact same verdict/checks/evidence shape back whether you're checking a citation, a package, a filing, a drug recall, a grant, or a wallet address — instead of writing and maintaining ten different source-specific parsers yourself.
Send the request with no auth header. You'll get a 402 back with a payment challenge (USDC on Base); an x402-aware client or agent framework settles it and retries automatically — no account, no key, no commitment. $0.005/call across every endpoint, $0.01 for /verify/case. A failed lookup (our source was down) is always free.
curl -i https://goodsong.dev/verify/citation \
-H "Content-Type: application/json" \
-d '{"doi": "10.1234/example.doi"}'
# → 402 Payment Required, with the x402 payment challenge in the response body
Full per-endpoint body shapes and field references are on each product's page (RealCite, RealDep, RealX402) and in the OpenAPI spec. See x402.org for client libraries.