complete archive
x402 tag
Publications
Ordered by date created, newest first. All 10 publications are shown on this page.
- How an x402 payment works, step by step, from a real 402 response 2026-10-21 · article
Decode a live x402 v2 challenge header by header: the 402, PAYMENT-REQUIRED, the signed EIP-3009 authorization, settlement and the receipt. - What an x402 seller should do that the spec doesn't require 2026-10-21 · article
Eight seller behaviors beyond the x402 spec, each running on a live endpoint: refuse before charging, recover purchases, pin payees, one catalog. - Red Cup Engineering: x402 sellers, agent-facing web infrastructure and content-addressed evidence 2026-10-20 · article
Articles on x402 seller engineering, Belnap four-valued HTTP, content-addressed names, ActivityPub and publishing from the 561 Group's engineering company. - Repeat x402 purchases with X-Payment-Recovery: get the same bytes after a dropped connection 2026-10-14 · article
How an x402 seller returns an already-paid response when the buyer repeats the same signed request with a private X-Payment-Recovery token. - Per-byte x402 pricing: deterministic workload bands in atomic USDC 2026-10-14 · article
How a seller prices x402 calls by canonical JSON size: $0.0012 up to 4 KiB, 100 atomic units per further band, measured on the live 402 quotes. - Sign-in-with-X on an x402 402: stateless wallet login for returning agents 2026-10-14 · article
How an x402 seller attaches a CAIP-122 sign-in challenge to its 402, verifies EIP-191 and EIP-1271 proofs with stateless HMAC nonces, and grants wallets access. - Belnap four-valued logic as an API: knowledge join, truth join, negation and conflation per call 2026-10-13 · article
Eight pay-per-call endpoints implement Belnap's FOUR algebra over x402: join, meet, negate, conflate, convert and census, with truth tables and live examples. - Refuse before you charge: how an x402 API stops billing agents for bad requests 2026-10-13 · article
Validate the body and refuse with 400, 415 or 422 before any 402 challenge, and cancel settlement if the handler fails. Real live responses included. - Never trust the payee in a 402 challenge: buyer-side x402 payee verification 2026-10-13 · article
An x402 buyer must take the payee from its own authority, not the seller's 402 challenge. How an expectedPayTo check, a budget and quote-drift refusal work. - One catalog, five machine faces: MCP, A2A, ActivityPub, llms.txt and x402 discovery 2026-10-13 · article
One 130-offer catalog compiles into x402 discovery, an MCP server, an A2A agent card, an ActivityPub outbox and llms.txt so agents find the same prices.