Beyond Function Equivalence: Method-Equivalence Verification for AI Coding Agents and the Explosion of Enterprise Defect Surface

Abstract

Abstract. AI coding agents deliver on two of the three enterprise KPIs: velocity and cost. GPT-5.3-Codex reaches 56.8% on SWE-Bench Pro and 77.3% on Terminal-Bench 2.0 [3]; Claude Code crossed $1B in annualized revenue within six months [13]. Yet even the velocity claim is fragile: METR’s randomized controlled trial found that experienced developers using AI tools were 19% slower, yet believed they were 20% faster [15]. The third KPI—quality, measured by defects—is moving in the wrong direction, invisibly. These agents are evaluated on function equivalence: whether a patch makes tests pass. They are not evaluated on method equivalence: whether the computational approach, algorithmic contract, and structural properties of the code are preserved. SlopCodeBench reveals that GPT-5.3 Codex achieves 51.6% on core tests but only 23.7% in isolation; Claude Opus 4.6 scores 53.8% on core tests but only 21.5% in isolation [1]. The gap—27.9 and 32.3 percentage points—quantifies code that passes every quality gate the enterprise operates while manufacturing latent defects: god functions exceeding 954 lines, clone densities of 116–174 duplicated lines per thousand LOC, and abstractions built then broken across successive edits. At enterprise scale, these invisible defects compound across interconnected software, data, and business estates into an explosion of defect surface that no dashboard currently detects. If velocity gains are illusory for experienced maintainers and the defect signal is hidden, we may have merely accelerated defect generation. We formalize the method equivalence gap, present an empirically-grounded four-type defect taxonomy, propose MEV (Method-Equivalence Verification)- a three-layer CI/CD gate - and argue, drawing on Heckman’s selection-bias framework [14], that enterprises must maintain airgapped human-authored baselines to prevent MEV’s own detection thresholds from silently recalibrating to the degraded normal.