Which blockchains does x402 support?

x402 settles on public blockchains. Documented live usage includes Base, Solana, and Polygon (TRM Labs analysis, September 2026), with ADA and Cardano Native Tokens added via the Cardano Foundation's x402 SDK integration (September 2026) and XRP via Ripple's x402 developer kit (September 2026). USDC is the most common settlement asset.

Does x402 require AI agents?

No. x402 is designed for machine-to-machine payments and works for any software client. TRM Labs found in September 2026 that only 0.6% to 7.5% of remaining x402 payment value, after filtering, appeared to come from autonomous AI agents — ordinary scripts can run the same flow.

How much does an x402 payment cost?

The protocol has no fixed price; each service sets its own per-call price. Real examples: ProBlocks demonstrated a live x402 endpoint on Base at 0.001 USDC per call (September 2026), and ScriptMasterLabs' Provider Marketplace charges a one-time 0.01 USDC listing fee on Base.

What is the x402 payment protocol?

Q: What is the x402 payment protocol?

x402 is an open payment protocol that revives the HTTP 402 'Payment Required' status code so software, especially AI agents, can pay for APIs per request without accounts, API keys, or checkout pages. A client requests a resource, the server answers 402 with machine-readable payment terms, the client signs a payment authorization and retries the same request, and the server settles payment and returns the resource with a receipt.

x402 revives HTTP 402 so AI agents can pay for APIs per request in USDC — no accounts, no API keys. TRM Labs analyzed $52.7 million across 198.9 million x402 settlements (September 2026, per the page's sources) and estimated only 0.6% to 7.5% of the value looked agentic. Coinbase launched it in 2025; the Linux Foundation stewards it.

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: 0x5bbd32e0478e49a7540ef2d760e0b0f9aa6328b2. (Verified live September 22, 2026. Cached page text is never the payment authority — the fresh 402 challenge is.)

What is the 4-step x402 flow?

Four steps: request, 402 challenge, sign and retry, settle and deliver.

#	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)

September 2026 brought the Cardano Foundation's x402 SDK, Ripple's XRP developer kit, TRM Labs' finding that only 0.6% to 7.5% of x402 volume looked agentic, and ProBlocks' live per-call billing on Base.

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.

What do real x402 endpoints charge?

Real prices from the table: the SML Provider Marketplace listing at 0.01 USDC one-time, the ProBlocks demo endpoint at 0.001 USDC per call, and Squeeze Wire per-issue x402 at a proposed $0.25.

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.

How do I do x402 payments myself?

Free payment-safety check: Inspect an x402 quote before signing. Compare the actual payment challenge with Base USDC, an independently verified recipient and your per-call ceiling. A valid quote is not a payment receipt.

As a buyer: pick an endpoint, read the offer, fund a wallet, sign and keep the receipt. As a seller: put x402 in front of your API, set a per-call price, and get discovered.

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 does ScriptMasterLabs fit in x402?

SML fits through the Provider Marketplace live x402 v2 listing contract, the x402 Protocol Guide PDF, Squeeze Wire per-issue pricing, and the machine manifest.

SML Provider Marketplace — the live x402 v2 listing contract quoted above. Discovery free, one-time 0.01 USDC listing fee.
x402 Protocol Guide (PDF, $29) — the full field guide: challenge inspection, narrow authorization, settlement vs. delivery evidence, operational controls.
Squeeze Wire — per-issue squeeze briefs priced natively for machines (x402 per-issue, proposed $0.25) and humans.
Machine manifest: squeezeos-api.onrender.com/.well-known/x402

What are the sources for this x402 guide?

Sources: the Cardano Foundation x402 SDK announcement, Ripple's XRP developer kit, the TRM Labs x402 analysis, ProBlocks' live x402 integration on Base, the SML marketplace contract, and the x402 protocol origin.

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

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One topic, one page per question — the x402 explainer cluster

This page is the canonical answer to “what is the x402 payment protocol?” Every neighbouring page in this cluster has a different job, and links back here for the definition:

HTTP 402: the status code that became money — the status code’s own history, spec story, and search demand — not the protocol built on it.
Which payment networks support the x402 protocol? — the network-support question, answered from live facilitator probes.
Which blockchains support x402? — chain-by-chain support, verified with a three-layer probe on October 15, 2026.
Is an HTTP 402 challenge proof of payment? — why a challenge is a price quote, not a receipt — the single sharpest question in the cluster.
x402 Protocol Guide (PDF) — the paid 4-page field guide (SKU SML-X402-001, $29 one-time) that maps the full loop step by step.
HTTP 402 Payment Required Explained (PDF) — the paid field guide (SKU SML-X402-002, $29 one-time) on 402 semantics and implementation boundaries.
x402 Integration Checklist (PDF) — the paid implementation checklist for putting the flow into production.

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