Smart Contracts as Semantic Signalling for AI Agents on Blockchains

Abstract

Smart contract security mainly aims to discover vulnerabilities in contract code based on the assumption that malicious behavior emerges from exploitable execution paths. This assumption is increasingly misaligned with modern blockchain systems, where off-chain automated agents continuously observe on-chain state and act autonomously. In this paper, we argue that smart contracts now act as public signalling mechanisms, capable of encoding command-and-control (C2) primitives through events and persistent storage variables. Off-chain agents, including LLM-based agents, convert benign-looking state changes into coordinated, high-impact actions across protocols and chains, semantically interpreting these signals. This kind of behaviour does not require code-level exploitation and thus can be built to evade existing analysis methods by construction. We explore how such a kind of coordination enables delayed activation, selective targeting, and deception, and explain why contract-centric security models fall short of such risks. We conclude by examining the main challenges and the directions for securing agent-mediated blockchain ecosystems.