APEX: Agent Payment Execution with Policy for Autonomous Agent API Access
Mohd Safwan Uddin, Mohammed Mouzam, Mohammed Imran, Syed Badar Uddin Faizan

TL;DR
APEX is a system that enables fiat-based payment gating for autonomous agents, ensuring secure, policy-compliant API access without relying on cryptocurrencies.
Contribution
It introduces a complete, adaptable payment infrastructure tailored for fiat systems like UPI, maintaining security and policy enforcement in autonomous agent interactions.
Findings
Policy enforcement reduces total spending by 27.3%.
Achieves 100% block rate for replay attacks and invalid tokens.
Maintains a 52.8% success rate for legitimate requests.
Abstract
Autonomous agents are moving beyond simple retrieval tasks to become economic actors that invoke APIs, sequence workflows, and make real-time decisions. As this shift accelerates, API providers need request-level monetization with programmatic spend governance. The HTTP 402 protocol addresses this by treating payment as a first-class protocol event, but most implementations rely on cryptocurrency rails. In many deployment contexts, especially countries with strong real-time fiat systems like UPI, this assumption is misaligned with regulatory and infrastructure realities. We present APEX, an implementation-complete research system that adapts HTTP 402-style payment gating to UPI-like fiat workflows while preserving policy-governed spend control, tokenized access verification, and replay resistance. We implement a challenge-settle-consume lifecycle with HMAC-signed short-lived tokens,…
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