FADP: A Sovereignty-Native HTTP Payment Protocol for Autonomous Agent Transactions

Abstract

We present FADP (the Fluid Agentic Payment Protocol), an HTTP-native twophase protocol for secure DeFi agent-to-agent transactions and agentic payments that couples on-chain transactions with cryptographic identity attestation in a single round-trip. Every FADP identity proof is unique per agent, unforgeable under standard cryptographic assumptions, and unreplayable by construction via four-dimensional nonce protection — making it the first HTTP payment protocol with formal guarantees on all three properties simultaneously. FADP extends RFC 7231's HTTP 402 status code [1] with three header namespaces — X-FADP-* for payment challenge and proof, XPauli-* for zero-knowledge identity binding [2], and X-FLDP-* for ECDSA request signing — yielding a wire format in which the server can verify who placed an order, that the request is fresh, and that the payment is on-chain final, all from headers alone. The protocol operates in a strict two-phase model: Phase α (initialization) provisions seven keys across three categories (local private, server public, internal proving), and Phase β (runtime) executes the 402 → onchain settlement → 200 cycle. Private keys never cross the network at any phase; the server holds only public material; the chain is the source of truth for payment finality. We prove four theorems — protocol correctness, liveness independence, replay impossibility, and identity-payment binding — and introduce three new metrics for HTTP-native payment protocols: Authentication Round-Trip Count (ART), Payment Atomicity Score (PAS), and Sovereignty Inheritance (SI). The reference implementation is deployed on Base Mainnet as a beta MVP prototype and submitted to the IETF as draft-fluid-fadp-01. Median end-to-end cycle latency is ~160–215 ms (analytical from measured components: 2 RTT plus on-chain confirmation), and per-call cost is approximately $0.001–$0.01 in stablecoin payment. To our knowledge, FADP is the first published HTTP payment protocol that (i) couples payment with cryptographic identity attestation in a single response, (ii) operates entirely within the existing RFC 7231 status-code framework with no L4 or L3 modifications, and (iii) inherits strict self-custody guarantees (Σ = 5) from a companion identity standard.