Litmus: The Admissible Set (System Architecture)

This companion document specifies the system architecture implied by Litmus: The Admissible Set. The design is one substrate applied to many workflows: a model-pluggable agent runtime, an extensible registry of standardized tools, and a fail-closed verification spine whose criteria are curated per specialty. The machine acts alone where physician consensus is explicit, routes an evidence-bounded case to the certified specialist where it is not, and records every claim, gate decision, and verdict in a provenance layer. Feedback from physician verdicts, payer adjudication, and clinical outcomes widens what can be verified over time. The document details the layers, the runtime flow, an end-to-end medical coding and appeals workflow, the build sequence from existing components, and how the architecture scales across specialties and absorbs advances in model capability.