NOMOS x402 Buyer Quickstart

Buy one governed NOMOS full-chain verification for 0.001 USDC on Base mainnet and cryptographically verify the signed execution receipt. Everything runs over the open x402 v2 protocol — no account, no API key, no OAuth. This page is a technical quickstart, not a revenue or security guarantee.

Downloads

• package.json — pinned dependencies (@x402/core 2.19.0 · @x402/evm 2.19.0 · @x402/fetch 2.19.0 · viem 2.48.11)
• buyer_quickstart.mjs — the client (preflight gates, one payment, receipt check)
• receipt_verify.mjs — standalone receipt verification
• env.example — placeholder env file (save your copy as .env)
• test/offline_compat_test.mjs — offline self-test suite (21 tests; save into a test/ subfolder)
• README.md — full documentation incl. troubleshooting

What you are buying

Five steps

# 1. download the files above into an empty folder (test file into test/)
# 2. install pinned dependencies
npm install
# 3. configure your signer (never committed, never printed)
cp env.example .env    # then edit BUYER_PRIVATE_KEY
# 4. run the offline self-test (no network, no payment, no key needed)
npm test
# 5. buy exactly once
node --env-file=.env buyer_quickstart.mjs

No source edits are needed. Step 5 sends exactly one unpaid challenge request and at most one signed paid retry, spends at most 0.001 USDC, and prints: HTTP status, settlement success, the truncated on-chain transaction, the execution id, the truncated receipt digest and receipt_verified: true|false.

Built-in safety

• every contract field (x402Version=2, scheme=exact, network=eip155:8453, USDC asset, payTo, amount ceiling 1000 atomic, resource URL) is validated before the signature is created — on any mismatch nothing is signed and nothing is sent;
• exactly one challenge request, at most one paid retry; a hard latch prevents a second signature; redirects are never followed; one global 120 s budget; a timeout never re-sends;
• the private key, the payment payload and the PAYMENT-SIGNATURE value are never printed.

Compatibility note (2.19.0 + CDP facilitator)

The 2.19 SDK client attaches an optional resource object to the payment payload which the Coinbase (CDP) facilitator's /verify schema currently does not accept. This quickstart therefore removes that optional field after payload creation (delete payload.resource) — omitting an optional field is fully x402-v2-conform, and verification then succeeds. If you build your own client on stock @x402/fetch 2.19 and see verify rejections, check this first.

Verifying the receipt

Every paid response carries chain.receipt.verification with the JWKS URL and the recipe. receipt_verify.mjs implements it: recompute the receipt digest over the canonical JSON of chain.receipt.document.receipt (sorted keys, compact separators), check the kid against the public JWKS at https://feedoracle.io/.well-known/nomos-execution-jwks.json (Ed25519), verify the signature over "nomos.execution.receipt.v1" || 0x00 || canonical(receipt), and confirm the delivered result is hash-bound into the signed receipt. Anyone can re-run this at any time.

Troubleshooting — official x402 error codes

On a non-200 the machine-readable reason is in the fresh PAYMENT-REQUIRED header's error field (verify stage) or in PAYMENT-RESPONSE.errorReason (settlement stage). Codes marked x402 are protocol-standard; facilitator codes come from the CDP facilitator.

Discovery: /.well-known/x402 · agent-card.json · OpenAPI