What is the x402 payment protocol?
x402 is an open payment protocol that revives the long-dormant HTTP 402 "Payment Required" status code so software — especially AI agents — can pay for APIs per request, in crypto like USDC, with no accounts, no API keys, and no checkout pages. When a client calls a resource without paying, the server answers 402 with exact machine-readable payment terms; the client signs a payment authorization and retries the same request; the server settles it on-chain and returns the resource with a payment receipt. Launched by Coinbase in 2025 and stewarded by the Linux Foundation.
Live receipt — this is not theory. ScriptMasterLabs runs a production x402 v2 contract you can inspect right now:
curl https://squeezeos-api.onrender.com/api/marketplace
returns the payment terms: protocol x402-v2, network eip155:8453 (Base), asset USDC, one-time listing fee 0.01 USDC, buyer sends the payment signature in the PAYMENT-SIGNATURE header and gets the settlement receipt in PAYMENT-RESPONSE. Receiver: 0xc29185fa176357612f3194735753e520e91adc46. (Verified live September 22, 2026. Cached page text is never the payment authority — the fresh 402 challenge is.)
The 4-step flow, exactly
| # | Step | What actually happens |
|---|
| 1 | Request | Client calls the resource with no payment, like any normal HTTP request. |
| 2 | 402 challenge | Server answers 402 Payment Required with an offer: token, chain, amount, payTo address, and expiry. The offer is in a PAYMENT-REQUIRED header or body. |
| 3 | Sign & retry | Client signs a payment authorization (e.g. USDC on Base) and retries the same request with a PAYMENT-SIGNATURE header. No account, no API key, no person approving. |
| 4 | Settle & deliver | A facilitator verifies the authorization, broadcasts the transaction, and pays the network fee. Server returns the resource plus a PAYMENT-RESPONSE receipt with the on-chain transaction reference. |
What's new in x402 (September 2026)
- Cardano joined the x402 SDK — September 21, 2026. The Cardano Foundation announced that any app or AI agent can now pay for API calls in ADA or Cardano Native Tokens over a web request, with working client, server, facilitator, and end-to-end examples merged into the protocol's repository. The foundation joined x402 as an associate member in July 2026 alongside Visa, Mastercard, Stripe, Google, AWS, and Coinbase.
- Ripple shipped an XRP x402 developer kit — September 17, 2026. The v1.1 XRPL AI Starter Kit added support for Stripe/Tempo’s MPP alongside its x402 integration (x402 wired in since June 2026), giving developers a single surface for XRP payments in AI workflows.
- Most x402 volume isn't from AI agents — September 15, 2026. TRM Labs analyzed about $52.7 million across 198.9 million x402 settlement transactions on Base, Solana, and Polygon (since May 2025) and estimated only 0.6% to 7.5% of the remaining payment value looked agentic after filtering. Ordinary scripts and scheduled processes can run the same flow. The protocol works; the "agent economy" narrative runs ahead of the agents.
- Live per-call billing exists. ProBlocks put a live x402 integration on Base (September 17, 2026): every API call priced and settled automatically, with a public demo endpoint at 0.001 USDC per call and a listing in Coinbase's Bazaar discovery catalogue. A controlled mainnet payment on September 7, 2026 returned a verifiable on-chain receipt.
Real prices from real endpoints
| Endpoint | Price | Network |
|---|
| SML Provider Marketplace listing | 0.01 USDC, one time | Base (eip155:8453) |
| ProBlocks demo endpoint | 0.001 USDC per call | Base |
| Squeeze Wire per-issue x402 | $0.25, proposed — launching soon | TBD |
The protocol sets no price — each seller does. Verify the price in the fresh 402 challenge before you sign anything.
Do it yourself
As a buyer (agent or script):
- Pick an x402 endpoint. Inspect its terms first — always free:
curl https://squeezeos-api.onrender.com/api/marketplace.
- Read the offer: price, asset, network,
payTo, expiry. Confirm they match what you expected before signing.
- Fund a compatible wallet with USDC on Base. Sign the payment authorization with an x402-compatible client and retry with the
PAYMENT-SIGNATURE header.
- Keep the
PAYMENT-RESPONSE receipt — it carries the on-chain transaction reference. Settlement proves value moved; it does not prove the API returned correct, complete data. Verify delivery separately.
As a seller (put a meter on your API):
- Place x402 in front of your API — from zero-code proxying to full control of the payment flow.
- Set your per-call price (cost-based, usage-based, or outcome-based).
- Get discovered: list in Coinbase's Bazaar catalogue or on the SML Provider Marketplace (one-time 0.01 USDC x402 listing; discovery is free).
Before you sign anything: verify price, asset, network, recipient, expiry, and resource binding in the challenge; cap amounts; restrict destinations. Settlement and accepted delivery are separate evidence stages.
Where ScriptMasterLabs fits
Sources
- Cardano Foundation x402 SDK announcement — X, September 21, 2026 (via crypto-economy.com, coin-turk.com, bitcoininsider.org)
- Ripple x402 XRP developer kit — September 17, 2026 (blockchainacademics.com)
- TRM Labs x402 analysis — via PYMNTS, September 15, 2026; Decrypt summary September 13, 2026
- ProBlocks live x402 on Base — press release via EINPresswire, September 17, 2026; controlled mainnet payment September 7, 2026
- SML marketplace contract — fetched live from squeezeos-api.onrender.com, September 22, 2026
- x402 protocol origin: Coinbase, 2025; Linux Foundation stewardship
SCRIPTMASTERLABS · Truth First. Proof Always. Pay Only for Accepted Delivery.