Fluid Agentic DeFi Protocol (FADP/1.0): HTTP-Native Micropayment Authentication for Autonomous AI Agents

This document defines the Fluid Agentic DeFi Protocol (FADP), version 1.0. FADP is an application-layer protocol layered atop HTTP that enables autonomous AI agents to pay for access to web resources using on-chain cryptocurrency transfers, with cryptographic proof of payment embedded directly in HTTP headers. FADP extends HTTP 402 (Payment Required) with two new header fields: X-FADP-Required, which a server uses to communicate payment terms to an agent, and X-FADP-Proof, which an agent uses to supply a verifiable on-chain payment receipt. A nonce-challenge mechanism prevents proof replay. An optional verification endpoint allows third-party on-chain confirmation without requiring the verifying party to run blockchain infrastructure. FADP is designed for agent-to-server and agent-to-agent payment flows operating at sub-dollar granularity (micropayments), where credit- card or OAuth-based billing is impractical. The reference implementation targets USDC on Base (an Ethereum Layer 2), though the protocol is token- and chain-agnostic.