Version 4.0.1 · Round 2026-q2-r000. Official spec: https://hl7.org/fhir/R4/index.html.
The FHIR R4 base specification defines the core RESTful API every conformant server must implement: CapabilityStatement discovery, resource read/search/history interactions, and search parameter semantics. Our test suite buckets every assertion as MUST (base-spec mandatory), SHOULD (recommended), or MAY (optional) so you can see exactly where each server diverges from the spec.
Every assertion in the FHIR R4 base spec suite is bucketed as MUST (mandatory to conform), SHOULD (recommended), or MAY (optional). We deliberately don't roll these into a single composite. Read the full methodology.