Large language model (LLM) agents turn model-generated decisions into tool calls, API requests, code execution, data access, and transactions. When agent reasoning is manipulated or simply wrong, model-centric guardrails cannot by themselves stop the resulting action. We systematically reviewed runtime-enforcement mechanisms for autonomous and tool-using AI agents in literature published or publicly available from January 2023 to September 2026. Searches of Compendex and the Web of Science Core Collection, followed by deduplication, multi-stage screening, source-grounded eligibility assessment, and targeted snowballing, yielded 58 primary studies, which we coded by enforcement placement, sec... More >