# 2s Law Citation Check

> 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

## Facts

- Endpoint: POST https://2s.io/api/law/citation-check?utm_source=zero.xyz
- Price: $0.0072/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-10
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/2s-law-citation-check-062660f9
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_iUKHpuWg6qUPTWdETcbPs

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 2s-law-citation-check-062660f9 -d '<json body>'
```

Example prompt: Can you verify these citations from the legal memo I'm reviewing? Check that 'Brown v. Board of Education, 347 U.S. 483 (1954)' exists, that '42 U.S.C. § 1983' is real, and confirm that the quote 'separate educational facilities are inherently unequal' actually appears in the Brown opinion.

## When to prefer this

Use 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.

## Known failure modes

- Citation not found in CourtListener or OLRC index — exists=false returned
- Quote provided but does not appear in the opinion — quotesPresent count unchanged, quoteMissing incremented
- Malformed citation string that cannot be parsed — error field populated on that item
- Network or upstream source unavailability — non-200 or partial results
- Payment failure via x402 — request not processed

## How this service works

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.

## Output

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.

## Example request

```json
{
 "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)"
  }
 ]
}
```

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "text": {
   "type": "string",
   "description": "Legal passage to scan for case/USC/CFR citations (existence check)."
  },
  "quotes": {
   "type": "array",
   "items": {
    "type": "object",
    "required": [
     "citation",
     "quote"
    ],
    "properties": {
     "quote": {
      "type": "string",
      "description": "The text attributed to that case."
     },
     "citation": {
      "type": "string",
      "description": "Case citation, e.g. \"Marbury v. Madison, 5 U.S. 137 (1803)\"."
     }
    }
   },
   "description": "Explicit quote checks: each { citation, quote } verifies the citation exists AND the quote appears in its opinion."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/2s-law-citation-check-062660f9/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from 2s.io](https://www.zero.xyz/host/2s.io/llms.txt)