I Almost Hand-Wrote a FHIR Schema. Then I Found Out I Didn't Have To.

A hospital-network client wanted our system to output patient data in actual FHIR format - the standard interoperability format healthcare systems use to talk to each other - instead of whatever shape we felt like inventing. Made total sense from their side, their EHR software only accepts FHIR resources, not our custom JSON. From my side, it meant I now had to get an LLM to produce a Patient resource that was FHIR R4 compliant, field for field.

I opened the FHIR R4 spec page for Patient to see what I was dealing with. Closed the tab about four minutes later. It's not one flat object - names have their own nested structure with use/family/given arrays, telecom is a list of typed contact points, addresses have their own multi-field shape, and half the fields have specific allowed value sets straight out of a separate FHIR terminology spec. This was not going to be a quick z.object({...}).

Two days into hand-writing it, and I wasn't even done

I started anyway, because what else was I going to do:

const PatientSchema = z.object({