ATXN: Agent-to-Agent Transaction Definition Protocol

This document defines a canonical, defensible, machine-checkable primitive for an Agent-to-Agent (A2A) transaction. It establishes the bundle of cryptographically signed elements that constitute a recorded value exchange between two software agents acting as instruments of identified principals, the conformance tiers that determine which elements are required, the rail-specific Profiles that map the bundle to existing payment infrastructure, and the two- tier validity model that distinguishes externally-adjudicable transactions from operationally-valid uncontested exchanges. ATXN is the foundational legal and technical primitive for escrow, dispute resolution, audit, and liability allocation in agentic commerce. It is designed to operate under existing contract law (UCC 2-204, UETA 14, Restatement (Third) of Agency, CISG) without requiring statutory recognition of agent personhood. It maps directly to AP2, Stripe ACP, Visa TAP, Mastercard Agent Pay, and x402 as Profiles of a single canonical bundle. Companion specifications (all co-submitted as Internet-Drafts, work in progress): * AIVS (draft-stone-aivs-01): cryptographic audit-trail substrate that ATXN bundles inherit from * VCAP (draft-stone-vcap-01): verified-commerce escrow rails that consume ATXN bundles * ATEP (draft-stone-atep-01): trust passports that bind agents to capacity-attested principals * ADRP (draft-stone-adrp-00): dispute resolution protocol invoked when an ATXN bundle enters the disputed state