2s Law Citation Check is a paid API for AI agents from 2s.io, paid per call via x402, $0.0072/call, status unknown (last checked 2026-10-10).
Verifies that legal citations (case law, USC, CFR) exist and optionally confirms that quoted text actually appears in the cited opinion
The (most) everything API: 575+ pay-per-call endpoints for AI agents — ground-truth data, a full AI gateway, and agent infrastructure (storage, queues, watchers). USDC via x402, no signup, no API keys, and upto billing: pay actual usage, not the quote.
Returns a structured result for each citation: whether it exists (boolean), the title and court metadata (year, court, docket number, citation count), a URL to the source opinion, and — for explicit quote checks — whether the quoted text was found in the opinion. A summary object totals verified vs. unverified citations and quote presence/absence counts.
POSThttps://2s.io/api/law/citation-check?utm_source=zero.xyzUse this endpoint when you need ground-truth verification of legal citations — especially to detect AI hallucinations in generated legal text. It covers case law (via CourtListener), US Code (USC), and CFR citations in one call, and can additionally confirm whether a specific quoted passage appears in the actual opinion. Prefer it over general web search when you need a deterministic exists/not-exists answer backed by authoritative legal corpora rather than a web snippet.
{
"text": "The foundational principle of judicial review was established in Marbury v. Madison, 5 U.S. 137 (1803), where the Court held that it is emphatically the province and duty of the judicial department to say what the law is. Additionally, 42 U.S.C. § 1983 provides a civil remedy for constitutional violations.",
"quotes": [
{
"quote": "It is emphatically the province and duty of the judicial department to say what the law is.",
"citation": "Marbury v. Madison, 5 U.S. 137 (1803)"
}
]
}| Field | Type | Description |
|---|---|---|
| text | string | Legal passage to scan for case/USC/CFR citations (existence check). |
| quotes | array | Explicit quote checks: each { citation, quote } verifies the citation exists AND the quote appears in its opinion. |
{
"data": {
"ok": true,
"meta": {
"summary": {
"total": 3,
"verified": 3,
"unverified": 0,
"quotesChecked": 1,
"quotesMissing": 0,
"quotesPresent": 1
}
},
"items": [
{
"url": "https://www.courtlistener.com/opinion/84759/marbury-v-madison/",
"type": "case",
"error": null,
"quote": null,
"title": "Marbury v. Madison",
"detail": {
"year": 1803,
"court": null,
"docketNumber": null,
"citationCount": 5993
},
"exists": true,
"citation": "5 U.S. 137"
},
{
"url": "https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title42-section1983",
"type": "usc",
"error": null,
"quote": null,
"title": "Civil action for deprivation of rights",
"detail": null,
"exists": true,
"citation": "42 U.S.C. § 1983"
},
{
"url": "https://www.courtlistener.com/opinion/84759/marbury-v-madison/",
"type": "case",
"error": null,
"quote": {
"note": null,
"checked": true,
"present": true
},
"title": "Marbury v. Madison",
"detail": {
"year": 1803,
"court": null,
"docketNumber": null,
"citationCount": 5993
},
"exists": true,
"citation": "Marbury v. Madison, 5 U.S. 137 (1803)"
}
],
"total": 3,
"source": {
"url": "https://www.courtlistener.com/",
"license": "Underlying corpora are US public domain; CourtListener is the case index.",
"provider": "CourtListener (Free Law Project) + US OLRC + eCFR"
}
},
"meta": {
"cost": {
"usd": 0.0072,
"tier": 0
},
"caller": "x402",
"version": null,
"endpoint": "law.citation-check",
"settlement": {
"txHash": "0x89278f0eef4137e30c6ef8022dbb09bc2ffe40207a495930235b309e1c821617",
"network": "eip155:8453",
"success": true
}
}
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