HTTP 402 is a payment request, not an instruction to pay. An x402-capable server can return a quote, and your agent can inspect that quote without signing or spending anything. The difficult part is not constructing a transaction. It is refusing terms that depart from your purchasing policy.

Disclosure: I work on Skavio LLC. Our public x402 endpoint is the example below. The preview flow is tested against production; no funded settlement is claimed here.

A real, unpaid example

Skavio's USDC transfer-verification endpoint returns an x402 v2 quote when called without payment:

curl -i --max-time 15 \
 'https://getskavio.com/x402/v1/usdc/verify?tx_hash=0x98809948204426c574b6f0aabdec03baa996b5c43ce28cc9ca704fa5d07e592e&network=base'

On November 6, 2026, the observed offer was 20,000 base units of USDC, or 0.02 USDC, on Base (eip155:8453). The PAYMENT-REQUIRED header carries base64-encoded JSON payment terms. Prices may change, so the agent must evaluate each fresh quote.

The following five tests are more important than any slick "pay automatically" demo.

Test 1: Wrong chain

A quote asking for a payment on Ethereum mainnet is not equivalent to the same amount on Base.

Allow only the configured network for this specific service:

if (offer.network !== "eip155:8453") {
  throw new Error("Chain is not approved");
}

If you support several chains, configure which ones are allowed per service, not globally.

Test 2: Wrong token

On Base, the native Circle USDC contract address is:

0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913

Don't treat every token with the symbol USDC as equivalent. Compare the chain and the exact on-chain asset contract, case-insensitively for an EVM hex address.

const expectedAsset = "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913";
if (String(offer.asset).toLowerCase() !== expectedAsset) {
  throw new Error("Asset is not approved");
}

Test 3: Different recipient

A payment price can be correct while its destination is wrong. Your buyer should have a known, configured merchant or intermediary address for each tool.

const trustedMerchant = "0x62bb64e9ab0e8104def0d7845223580319d4c139";
if (String(offer.payTo).toLowerCase() !== trustedMerchant) {
  throw new Error("Recipient is not approved");
}

This address belongs to our example only. Never copy it as a universal x402 default.

Test 4: Excessive or ambiguous amount

USDC has six decimals, so 20,000 base units = 0.02 USDC. Do not compare decimal strings using floating point.

const capBaseUnits = 20_000n;
if (!/^[0-9]+$/.test(String(offer.amount))) {
  throw new Error("Invalid integer amount");
}
const amount = BigInt(offer.amount);
if (amount <= 0n || amount > capBaseUnits) {
  throw new Error("Price exceeds authorized cap");
}

Also check protocol version 2, the exact payment scheme and that exactly one acceptable payment option remains after filtering. Ambiguous options should not be resolved by "try the first".

A per-call cap is not a daily budget: a thousand approved 0.02 USDC calls would still spend 20 USDC. Add a separate aggregate limit.

Test 5: A retry after an uncertain payment

This one can be expensive even when all the payment fields are correct.

Imagine a buyer signs a payment, the server completes the request, but the connection drops before the buyer receives the response. If the agent immediately pays again with a new request identity, the user may be charged twice.

Instead, persist the business request ID, offered terms, attempted payment and any receipt. Query or reconcile the previous settlement before another charge. Where supported, reuse a stable idempotency key.

Distinguish these events in logs:

An HTTP 402 proves none of the last two events. An HTTP 200 alone is also not proof of an independently verified settlement.

A free five-minute test

We published runnable Python and Node.js quote inspection clients with negative unit tests:

Skavio x402 safe buyer – code and tests

git clone https://github.com/skavio-eu/skavio-agent-tools.git
cd skavio-agent-tools/examples/x402-safe-buyer

python3 preview_x402.py
node skavio-x402.mjs --preview
node --test

The preview works without an account, a wallet or a purchase. Our three Node.js unit tests passed and both clients fetched a real unpaid 402 offer. The optional wallet-enabled paid branch has not been settlement-tested with real mainnet funds.

For the original, longer walkthrough about payment quotes and spending policy, see our DEV Community tutorial.

Practical rule: Make your agent prove it can reject the wrong offer before teaching it to accept the right one.

Skavio LLC develops the example API and maintains the linked GitHub code. This is an original technical explanation, not an independent endorsement or a paid-transaction success report.