No Red Lines: The Impossibility of Formal Safety Guarantees in Large Language Models V2.0

Large language models (LLMs) are being deployed in control systems that require reliable safety guarantees. This rests on a false assumption: that natural-language safety instructions create boundaries comparable to those in rule-based systems. We show this is categorically false. Rule-based systems enforce constraints through program structure, creating syntactic boundaries that cannot be violated. LLMs implement “safety” as shifts in probability mass, semantic boundaries that dissolve under contextual pressure. Using architectural analysis and a concrete scenario, we show no training technique can convert statistical pattern-matching into formal constraint enforcement. This is an architectural, not an engineering, limitation. Accordingly, formal guarantees are impossible with LLMs alone; responsible use requires external verification, mandatory disclosure, and prohibitions on LLM-only control in safety-critical contexts. Current certification regimes, designed for syntactic guarantees, cannot assess systems whose boundaries are semantic.