Underwriting is described as rules or models. Loan-level federal data shows it is a surface — and each lender's surface has its own topology: cliffs where denial jumps discontinuously, plateaus where nothing matters, and in one case, no cliffs at all.
The DTI-45 cliff. Four of six lenders show a discontinuous jump between DTI 44 and 45 — stable in location across 2023–2025 even as overall denial levels moved. CrossCountry's cliff sits one step right, at 46: its own institutional line, three years running.
The CLTV-85 wall. Above 85% CLTV, denial explodes for most lenders — Rocket barely accepts files there at all. Part of this is FHA program mechanics (cash-out LTV cap), but the lender-to-lender difference inside the same band is behavioral: in 2025, Guild denied 18% of the 85–89 band while Rocket denied 83%.
The null signature. Mr. Cooper's surface is flat on both axes — no DTI cliff, no CLTV wall, denial 4–14% everywhere. This is the strongest evidence the cliffs are institutional choices rather than data artifacts: if the cliffs came from FHA rules alone, every lender would show them.
Practical reading: a file at DTI 46 or CLTV 87 is not "unfinanceable" — it is unfinanceable at lenders whose surface has a cliff there. The same file sits on a flat region of another lender's map. Routing is geometry.
Mr. Cooper's flatness may partly reflect portfolio composition (streamline-refi-heavy pipelines underwrite DTI/LTV differently) — flat surface is a verified observation, "overlay-free philosophy" is an interpretation. The CLTV-85 jump includes product-eligibility mechanics, not pure overlay. DTI is only reported as exact integers in the 36–49 window, so the surface is mapped where federal data permits. FHA only; conventional surfaces may differ.