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

Autonomous AI agents — software systems that independently execute tasks, trade digital assets, and consume services — require machine-native payment infrastructure with formal cryptographic guarantees. Existing protocols (OAuth, JWT, API keys) provide no binding between payment and agent identity, leaving a critical gap in the emerging agentic economy. This paper introduces FADP (Fluid Agentic Payment Protocol), the first HTTP-native two-phase payment protocol designed for autonomous agent transactions with provable self-custody guarantees. FADP extends RFC 7231's HTTP 402 status code with three header namespaces — X-FADP-, X-Pauli-, and X-FLDP-* — and a seven-key architecture partitioned across three trust levels, where private keys satisfy strict locality: ∀k ∈ K_local, k ∉ Network. Every payment proof π is constructed as π = Groth16.Prove(PK, w) bound to a four-dimensional nonce vector N₄ = (n_time, n_chain, n_req, n_agent), guaranteeing uniqueness, unforgeability, and replay impossibility simultaneously. We formally prove four theorems — protocol correctness, liveness independence (L ⊥ S), replay impossibility, and identity-payment binding — and introduce three novel metrics: Authentication Round-Trip Count (ART), Payment Atomicity Score (PAS), and Sovereignty Inheritance (SI), with provable sovereignty score Σ = 5. The reference implementation deploys on Base Mainnet achieving median latency of 160–215 ms and per-call cost of $0.001–$0.01 in stablecoin, submitted to IETF as draft-fluid-fadp-01. To our knowledge, FADP is the first protocol coupling cryptographic identity attestation, payment finality, and self-custody (Σ = 5) in a single HTTP response.