CES-CORE: A Deterministic Execution Discipline for LLM-Assisted Engineering

Abstract Large language models have accelerated software development but introduced a persistent failure mode: engineering drift. Over extended sessions, outputs become difficult to verify, reproduce, or trust, even when they appear locally correct. Existing workflows rely on probabilistic validation through logs, tests, and reconstruction, none of which guarantee that a produced result is identical to the original decision. This paper introduces the Constraint Execution System (CES), a deterministic execution discipline that restricts engineering actions to a minimal set of legal moves and enforces a strict proof condition for correctness. CES replaces probabilistic validation with binary legality: an operation either satisfies an exact proof condition or it fails. CES defines a minimal execution grammar (READ, PATCH, PROVE), a strict PASS condition, a current-state authority model, and an ordered failure resolution system. By constraining the execution space rather than optimizing within it, CES eliminates entire classes of failure modes and ensures that outputs are reproducible, attributable, and exact under replay. A worked case demonstrates how CES collapses ambiguous engineering decisions into a single legal action that enforces system invariants. The result is a shift from probabilistic validation to deterministic legality in LLM-assisted engineering workflows.