Constraint-Relative Platform Selection Under a Hardened Boundary Calculus

Post-Horizon Instrumental Lineage — 4 August 2026 This record forms part of the Post-Horizon Instrumental Lineage of the Architecture of Limitation research programme. The designation and conditions governing subsequent review are established in K1 Open Concordance Phase: A Structured Validation and Evidence-Alignment Programme for the Open Research Release of AoLOS, v1.0, 2 August 2026, doi:10.5281/zenodo.21758275. The record is retained unchanged as evidence of the conditions, tools, concepts, and architectural pressures through which the programme developed. Inclusion in this lineage does not imply retrospective K1 validation or completion of article-level concordance; any proposition-specific qualification, reanalysis, companion publication, or unresolved pressure will be recorded separately through the phase’s declared review process. -- This study develops a constraint-relative framework for evaluating large language model platforms under a declared instrumental boundary calculus derived from the Architecture of Limitation (AoL). Rather than ranking models globally, platform behavior is analyzed relative to a fixed kernel (K0G), a high-stress artifact, and a formal experiment contract. Operational Boundary Metrics (OBM) are extracted and used to construct composite sensitivity and stability indices. Full mathematical derivations of the Instrument Sensitivity Index (ISI), Structural Stability Index (SSI_S), and Stability–Sensitivity Ratios (SSR) are provided. Cross-platform divergence is shown to be weight-dependent, with crossover occurring at approximately 18% operational weighting. The results support a stewardship-centered methodology for tool selection under explicit constraint declaration. The findings further demonstrate that platform selection functions as a constraint-sensitive parameter in high-stress reasoning contexts, requiring explicit declaration within the evaluative process. This work belongs to the Architecture of Limitation research program.