law.citation-check

Anti-hallucination checker for legal references — verify that cited cases, US Code sections, and CFR regulations in a passage actually EXIST, and (deterministically) that an attributed QUOTE actually appears in the cited opinion. POST { text } to scan a brief/passage: every case citation (via CourtListener), 'N U.S.C. § X', and 'N C.F.R. § X' is checked for existence with canonical metadata + a source URL, or flagged unverified. POST { quotes: [{ citation, quote }] } to verify specific quotations: each citation is resolved and its opinion full text is checked for the quote (ellipsis-aware), returning quote.present true/false. Pass both. Returns per-reference results + a summary (verified/unverified, quotesPresent/quotesMissing). Catches fabricated cases AND fabricated quotations — the LLM legal-output failure mode that gets attorneys sanctioned. Note: this checks existence + quote presence (facts), NOT whether a case legally supports a proposition. Sources: CourtListener (Free Law Project), US OLRC, eCFR (public domain).

price
$0.0072 USDC per call
method
POST/api/law/citation-check
payment
x402 v2 · USDC on Base (EIP-3009) or Solana (SPL transfer)
auth
None. Sign the payment, retry with PAYMENT-SIGNATURE.
tier
Tier 0 — no paid upstream

Parameters

NameTypeDescription
textstring
min 1 chars · max 50000 chars
quotesarray

Code samples

cURLbash
# 1. Probe the endpoint with no auth — receive 402 with PaymentRequirements
curl -sS -X POST 'https://2s.io/api/law/citation-check' \
  -H 'Content-Type: application/json' \
  -d '{"text":"example","quotes":[{"citation":"xx","quote":"xxxx"}]}'

# 2. Sign the EIP-3009 transferWithAuthorization for the advertised price +
#    payTo from the 402 envelope, then retry with PAYMENT-SIGNATURE:
curl -sS -X POST 'https://2s.io/api/law/citation-check' \
  -H 'Content-Type: application/json' \
  -H 'PAYMENT-SIGNATURE: <base64-json-payload>' \
  -d '{"text":"example","quotes":[{"citation":"xx","quote":"xxxx"}]}'

# Or just use the canonical runner — it handles the whole loop:
#   EVM_PRIVATE_KEY=0x... node --env-file=.env.local \
#     --experimental-strip-types scripts/x402-pay.ts \
#     'https://2s.io/api/law/citation-check'
TypeScript / Node — @2sio/sdktypescript
import { TwoS } from '@2sio/sdk'

const client = new TwoS({
  privateKey: process.env.EVM_PRIVATE_KEY as `0x${string}`,
})

const result = await client.law.citationCheck({
  "text": "example",
  "quotes": [
    {
      "citation": "xx",
      "quote": "xxxx"
    }
  ]
})

console.log('endpoint:', result.endpoint)
console.log('cost:', result.costUsd, 'USDC')
console.log('tx:', result.settlement?.txHash)
console.log('data:', result.data)
Python — 2siopython
import os
from twosio import TwoS

client = TwoS(private_key=os.environ["EVM_PRIVATE_KEY"])

result = client.law.citation_check(text="example", quotes=[{"citation":"xx","quote":"xxxx"}])

print("endpoint:", result.endpoint)
print("cost:", result.cost_usd, "USDC")
print("tx:", (result.settlement or {}).get("tx_hash"))
print("data:", result.data)
MCP — Claude Desktop / AgentKit / any MCP hostjson
// 1. Add @2sio/mcp to your MCP host config (Claude Desktop example below).
//    EVM_PRIVATE_KEY funds x402 payments per call.

// claude_desktop_config.json
{
  "mcpServers": {
    "2sio": {
      "command": "npx",
      "args": ["-y", "@2sio/mcp"],
      "env": { "EVM_PRIVATE_KEY": "0x..." }
    }
  }
}

// 2. Once the server is running, agents call this tool via standard MCP:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "law.citation-check",
    "arguments": {
      "text": "example",
      "quotes": [
        {
          "citation": "xx",
          "quote": "xxxx"
        }
      ]
    }
  }
}

Response

FieldTypeDescription
okboolean
one of: true
itemsarray
totalintegerTotal matching rows upstream; null when unknown.
sourceobject
metaobject
Example response datajson
{
  "ok": true,
  "items": [
    {
      "type": "case",
      "citation": "example",
      "exists": false,
      "title": "example",
      "detail": {
        "court": "example",
        "year": 1,
        "docketNumber": "example",
        "citationCount": 1
      },
      "url": "example",
      "error": "example",
      "quote": {
        "checked": false,
        "present": false,
        "note": "example"
      }
    }
  ],
  "total": 1,
  "source": {
    "provider": "example",
    "url": "example",
    "license": "example"
  },
  "meta": {
    "summary": {
      "total": 1,
      "verified": 1,
      "unverified": 1,
      "quotesChecked": 1,
      "quotesPresent": 1,
      "quotesMissing": 1
    }
  }
}

Discovery

2s.io is x402-native. Every call is paid per-request from a USDC-funded EVM wallet on Base — no signup, no API keys, no monthly fees. Source code: github.com/2s-io/sdk.