mock.health serves US Core 6.1.0 profiled resources on FHIR R4 (4.0.1) — 26 resource-type profiles covering all 22 USCDI v3 data classes.
USCDI v3 is a data-element standard, not an API standard. It names the classes and elements a certified system must exchange — patient demographics, problems, medications, allergies and intolerances, laboratory, vital signs, procedures, immunizations, clinical notes, care team members, goals, health insurance information, and the rest. US Core 6.1 is the FHIR implementation guide that binds those elements to concrete R4 profiles, must-support flags, and terminology bindings.
Every USCDI v3 data class maps to at least one profiled resource in the sandbox, populated with real terminology: LOINC for laboratory and vital signs, SNOMED CT for problems and procedures, RxNorm for medications, CVX for immunizations, ICD-10-CM where a billing-shaped code is expected. Must-support elements are populated rather than omitted, which is the difference that makes a certification rehearsal meaningful — a fixture set that leaves must-support fields empty passes your parser and fails the real test.
The same corpus backs the Inferno (g)(10) rehearsal path, and the conformance benchmark publishes how mock.health and other FHIR servers score against the base R4 and US Core suites. Per-resource profile details live in the API reference.