title: x402 (HTTP 402 payment) description: "Native support for the HTTP 402 Payment Required protocol — agents pay paywalled APIs automatically."

x402 is a Coinbase-led standard for HTTP-level micro- payments. When a server returns 402 Payment Required with payment details, the client pays and retries. CardZero implements x402 client-side so your agent can transact without manual payment plumbing.

The protocol in 60 seconds

Agent makes a request: GET https://example.com/data
Server checks: not paid → returns HTTP 402 Payment Required with payment details in headers (USDC amount, recipient address, network).
Agent's CardZero integration constructs a USDC payment, signs it, includes the payment proof in X-PAYMENT header, retries.
Server verifies payment, returns HTTP 200 + the data.

All in milliseconds. No human in the loop. No prior account.

CardZero's x402 implementation

The POST /v1/x402/pay endpoint:

curl -X POST https://api.cardzero.ai/v1/x402/pay \
  -H "Authorization: Bearer $CARDZERO_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://example.com/api/data",
    "maxAmount": "1.0",
    "recipient": "0xMerchantAddress",
    "network": "eip155:8453",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"
  }'

Returns:

{
  "paymentId": "pay_x402_…",
  "txHash": "0x…",
  "paymentHeader": "x402.v1.eip155:8453.…",
  "status": "confirmed",
  "amount": "0.50"
}

The agent then includes X-PAYMENT: <paymentHeader> when retrying the original request.

When you'd use x402

Paywalled APIs: provider wants $0.001 per call from autonomous agents.
Per-request data feeds: news, market data, web scraping APIs.
Compute on demand: pay-per-inference LLM endpoints.
Content unlocks: per-article paywalls, subscription bypasses.

Differences from direct payment

| | x402 | Direct payment | | --- | --- | --- | | Endpoint | POST /v1/x402/pay | POST /v1/payments | | Triggers | HTTP 402 response | Application logic | | Result | Payment header for retry | Plain confirmation | | Use case | Server-mediated paywall | Send USDC anywhere |

Both fully respect wallet spending rules. Both are 2% fee on the platform side.

Spec compliance

CardZero implements x402 v1:

✅ eip155:8453 (Base mainnet)
✅ USDC asset (0x833589fC…2913)
✅ Standard X-PAYMENT header format
✅ ERC-3009 transferWithAuthorization signature
✅ Verifiable on-chain by recipient

What we don't yet support (P2):

❌ Other chains (would require additional contract deployments)
❌ Other ERC-20s besides USDC
❌ Subscription / recurring payment flow (x402 v2 draft)

Best practices

Set maxAmount lower than the wallet's per-tx limit. If a server demands more, the call fails fast at the rule level.
Use a strict whitelist if you only pay known providers.
Idempotency: x402 payments include an idempotencyKey to prevent double-charging on retry.
Audit txHash: always verifiable on Basescan.

Recipe: pay an x402 paywall →