An adversarial second opinion
for AI agents.

curl -s -X POST https://secondopinionx402.com/v1/try \
  -H 'content-type: application/json' \
  -d '{"claim":"Ed25519 signatures can be verified offline using only the public key"}'

Why agents call this

Agents fail worst when they are most confident, and self-checking shares the same blind spots as the original reasoning. Second Opinion is a different model instance with fresh context, live web access, and one paid incentive: refute the claim if it can be refuted. Call it at the moment of irreversible commitment — before code, money, publication, or a promise:

Every verdict doubles as an audit receipt: independent check, timestamped, payment on-chain — something self-verification can never produce.

Endpoints & pricing

Verdicts are refuted, supported, or inconclusive, each with calibrated confidence, findings, and the URLs actually consulted. Failed sync requests (5xx) are never settled, and failed async jobs retry free — you only pay for delivered verdicts.

Chains: pay on Algorand · Base · Polygon · Solana — same price, same product. Algorand is the preferred rail (listed first in every 402). Solana payers must use the /async routes: Solana transactions expire in ~1 minute, so the async flow settles your payment in seconds and serves the verdict via the free result URL.

Call it

Client libraries for JS/TS and Python handle the payment, polling, and receipt verification for you — keys sign locally and never leave your process:

npm install secondopinion-x402   # Node — pays on Base, Algorand or Solana
pip install secondopinion-x402   # Python — pays on Base; LangChain + CrewAI tools

import { SecondOpinion } from "secondopinion-x402";

const so = new SecondOpinion();   // wallet key from env; freeTry() needs none
const verdict = await so.verify("Library X v3 supports streaming responses natively", {
  context: "About to migrate our retry logic to rely on this",
});
if (verdict.verdict === "refuted") { /* stop and re-plan */ }
await so.verifyReceipt(verdict);  // Ed25519 receipt, verified offline

Or wire any x402 client yourself

import { wrapFetchWithPayment, x402Client } from "@x402-avm/fetch";
import { toClientAvmSigner, ALGORAND_MAINNET_CAIP2, NETWORK_TO_ALGOD } from "@x402-avm/avm";
import { ExactAvmScheme } from "@x402-avm/avm/exact/client";

// signer: your Algorand account (holds a little USDC on algorand mainnet)
const signer = toClientAvmSigner(process.env.PAYER_SK_B64);
// Pin the scheme to the network's algod node — an unconfigured wildcard
// registration silently builds testnet transactions and fails verification.
const client = new x402Client().register(
  ALGORAND_MAINNET_CAIP2,
  new ExactAvmScheme(signer, { algodUrl: NETWORK_TO_ALGOD[ALGORAND_MAINNET_CAIP2] }),
);
const payingFetch = wrapFetchWithPayment(fetch, client);

const res = await payingFetch("https://secondopinionx402.com/v1/verify", {
  method: "POST",
  headers: { "content-type": "application/json" },
  body: JSON.stringify({
    claim: "Library X v3 supports streaming responses natively",
    context: "About to migrate our retry logic to rely on this",
  }),
});
const verdict = await res.json();
if (verdict.verdict === "refuted") { /* stop and re-plan */ }

Or inspect the payment requirements yourself — request without payment and read the 402:

curl -s -X POST https://secondopinionx402.com/v1/verify \
  -H 'content-type: application/json' \
  -d '{"claim":"HTTP 402 was reserved in 1997 and first standardised for real use by x402"}'
# → HTTP 402 + JSON: accepts[], network, asset, payTo, amount — pay, then retry with X-PAYMENT header

Add it to your AI app (hosted MCP — zero install)

Add one URL to Claude, Cursor, or any MCP-capable agent stack and five tools appear — pricing quotes and job results free, verdicts pay-per-call from the caller's own wallet. Keys never touch this server: payments are built by your wallet and passed through.

https://secondopinionx402.com/mcp

Open /mcp in a browser for the human walkthrough. Prefer local signing? npx secondopinion-x402 mcp runs the connector on your machine — the free-trial tool needs no wallet at all, and paid tiers switch on with one env var.

Example verdict

{
  "id": "so_9f2c…",
  "mode": "single",
  "verdict": "refuted",
  "confidence": 0.93,
  "summary": "The claim conflates two products. v3 added streaming to the Pro tier only; the open-source core still buffers responses (changelog, issue #4812).",
  "findings": [
    { "point": "v3.0 changelog scopes streaming to Pro", "direction": "against_claim", "source_url": "https://…/changelog" }
  ],
  "citations": ["https://…/changelog", "https://…/issues/4812"],
  "engine": { "model": "claude-sonnet-5", "searched_web": true },
  "checked_at": "2026-07-21T12:00:00Z"
}

Honesty properties

Privacy by design