https://jeonghwankim.dev/writing I build and ship software end-to-end — from system design to deployment. Full-stack web, native iOS/watchOS, and real-time systems, across AI, audio, mental health, legal tech, and neuroscience research. en https://jeonghwankim.dev/writing/the-model-wasnt-the-problem https://jeonghwankim.dev/writing/the-model-wasnt-the-problem Thu, 06 Aug 2026 00:00:00 GMT My cross-validation reported 0.61. The pooled out-of-fold number was 0.52. Both were correct, and the gap had nothing to do with the model — it was five folds scoring on five incompatible scales. Part 5 of The Differential. https://jeonghwankim.dev/writing/from-case-reports-to-ct-scans https://jeonghwankim.dev/writing/from-case-reports-to-ct-scans Tue, 04 Aug 2026 00:00:00 GMT The text benchmark left me arguing about what counts as a finding. So I moved to pancreatic CT, where a wrong answer is just wrong. The first run scored 0.52 AUROC — and the way it failed was worth more than the score. Part 4 of The Differential. https://jeonghwankim.dev/writing/the-model-didnt-hallucinate https://jeonghwankim.dev/writing/the-model-didnt-hallucinate Sat, 01 Aug 2026 00:00:00 GMT 50 manual extraction runs across two clinical NLP benchmarks. Source grounding was 100% and the unsupported-claim rate was zero — but the evidence units, entity boundaries, and temporal relations fell apart. Part 3 of The Differential. https://jeonghwankim.dev/writing/when-evidence-changes https://jeonghwankim.dev/writing/when-evidence-changes Wed, 29 Jul 2026 00:00:00 GMT I repeated every perturbation condition three times. The leading diagnosis was highly reproducible; the ordering and scoring below it was not — which killed my original product hypothesis. Part 2 of The Differential. https://jeonghwankim.dev/writing/getting-the-diagnosis-right https://jeonghwankim.dev/writing/getting-the-diagnosis-right Wed, 29 Jul 2026 00:00:00 GMT A frontier model ranked the correct diagnosis first on a masked neuromuscular case. Then I started removing evidence — and its confidence didn't move the way it should. Part 1 of The Differential. https://jeonghwankim.dev/writing/a-self-audit-of-a-production-saas https://jeonghwankim.dev/writing/a-self-audit-of-a-production-saas Sun, 19 Apr 2026 00:00:00 GMT I sat down for a day and attacked my own app. Six categories of finding — and what I'd build into the next project from day one.