x402

x402 is an open HTTP-native payment protocol that revives the long-dormant HTTP 402 Payment Required status code as a machine-payable handshake. Servers gate API responses on payment; clients retry with proof of payment. Designed for AI agents, machine-to-machine commerce, and any caller without a human in the loop. Reference spec at x402.org, maintained by Coinbase Developer Platform with broad community participation.

Client requests a paid endpoint. Server responds 402 Payment Required with a JSON body containing x402Version, resource, and an accepts array (scheme, network, amount, asset, payTo, maxTimeoutSeconds). For the Coinbase reference exact scheme on EVM networks like Base, the client signs an EIP-3009 transferWithAuthorization (gasless from the agent’s side), wraps it as a PaymentPayload (signature plus authorization fields from, to, value, validAfter, validBefore, nonce), and base64-encodes the result. The client retries with header X-PAYMENT set to that base64 string. The server’s facilitator broadcasts the authorization, verifies on-chain settlement, and returns 200 with the data plus a PAYMENT-RESPONSE header containing the settlement receipt. No accounts, no API keys for billing, no signup form, no invoice cycle.

As of May 2026, the live adopter set includes Coinbase (reference SDK and the spec itself), Amazon Bedrock AgentCore Payments (Preview, launched May 7 2026, with native x402 execution and a Coinbase x402 Bazaar MCP server exposing 10,000+ pay-per-use endpoints), Stripe (Link for agents using the stripe x402 method variant), TensorFeed (14 paid premium endpoints, AFTA-certified, end-to-end USDC loop verified on Base mainnet April 27 2026), TerminalFeed (federated with TensorFeed via the AFTA cross-Worker rail), the @coinbase/x402 SDK, the tensorfeed Python and JavaScript SDKs, the afta-gateway Cloudflare Worker template, and the tensorfeed-mcp MCP server. Live adopter directory at tensorfeed.ai/x402-adopters.

Traditional payment APIs assume a human at a keyboard filling out a signup form, attaching a credit card, and copying an API key. AI agents have none of those primitives. x402 lets agents act on their own spending decisions in a single round trip, settling on-chain in seconds with no chargebacks, no merchant-account termination risk, and microtransactions that are economically viable (sub-cent gas on L2s like Base). Stripe Link for agents reuses the x402 envelope with a stripe method variant, so the protocol is cooperative not adversarial.

In x402 V2, services that support the protocol publish a discovery manifest at /.well-known/x402 listing every paid endpoint with its accepts block (asset, network, amount, payTo), input/output schemas, and metadata. Facilitators like CDP Bazaar and x402scan crawl this manifest to auto-index services so x402-compatible agents can discover them without manual directory submission. TensorFeed publishes its manifest at /.well-known/x402.json.

Two coexisting formats are in the wild as the protocol matures. V1 is the original Coinbase reference (network and asset declared as separate fields). V2 added the discovery manifest and the eip155-style network identifier (e.g. eip155:8453 for Base mainnet). x402scan currently lists V2 publishers; x402.org documents both. The cleanest path for new publishers is V2 with the well-known manifest plus an x-payment-info extension on each paid OpenAPI operation.

Three options ranked by effort. Drop-in: fork github.com/RipperMercs/afta-gateway, set 3 secrets (Worker secret, USDC payTo wallet, Ed25519 receipt signing key), deploy. Library: install @coinbase/x402 and add its middleware to your Express, Fastify, or Hono app. Hand-rolled: implement the 402 handshake yourself in ~50 lines, verify the X-Payment-Tx receipt against an RPC node. The afta-gateway template is the recommended path for any publisher who also wants AFTA primitives (signed receipts, code-enforced no-charge ledger).