What Scaffolding Survives Model Improvement — and How Do You Know When a Line Turns Harmful?

Three-question synthesis of the harness-evolution cluster. (1) Not all scaffolding migrates inward: behavior *requests* dissolve (and past-due ones turn harmful), but five classes survive — boundary enforcement (verification, isolation, security rules as constraints), org-specific record (repo-local truth, decisions no model can infer), deliberate identity (character/brand voice, kept stable across capability jumps by design), inference/deployment structure (no 'inward' to migrate to), and human-facing legibility (which grows as models improve) — plus one class flowing the *opposite* way (communication calibration, added as defaults lengthen). The bitter-lesson exemption rule sorts them: structure encoding a task prior migrates; structure encoding boundaries, records, identity, or serving arithmetic doesn't. (2) Compounding lines have a detectable signature — ablation non-inferiority (removal holds quality, cuts tokens) and inverted dose-response (stronger phrasing → worse outcome, the effort-inversion fingerprint) — with native-behavior baselining as the cheap pre-filter; no tool in the corpus automates it yet. (3) If model improvement stalls, build-for-the-next-model degrades gracefully: it is a cheap call option on the release cadence — a stall costs the premium (prototypes-in-waiting expire), not the firm; latent-capability overhang keeps effective capability rising post-stall; harness re-accretion becomes correct engineering again; and the durable layers become the competitive surface