Blog

Deep-dive notes on FHIR R4, SMART on FHIR, US Core 6.1, HAPI internals, and synthetic patient data — written by an engineer who ships sandbox infrastructure nightly.

• The FHIR Sandbox Problem: Why Open Epic Isn't Enough — You opened a Patient resource and found TEST TEST. The sandbox is built for certification, not demos. Here's what's missing and the fix.
• The Contents of That Dumpster Are Private — ONC mandates it. CMS requires it. Every EHR vendor supports it. But production FHIR data is trash — here's what that actually looks like.
• Three Ways to Build a Multi-Tenant FHIR Server — Separate databases, partitioned tables, or tag-based filtering — every multi-tenant FHIR deployment picks one. Here's how to choose.
• Same FHIR Specification Different Answers — I loaded the same Synthea patient into six open-source FHIR servers and ran the same conformance probes against each.
• 6 Open FHIR Servers Until They Break — Six FHIR servers. 64,000 Synthea patients. Here is what happened under loads far below your average hospital.
• HAPI vs GCP Healthcare API: Same FHIR, Different Results — I loaded 1,000 Synthea patients into HAPI and GCP Healthcare API. Same data, same queries — the two stores disagreed on every single one.
• Testing FHIR Integrations Without a Hospital — You can't get hospital access without a working integration. You can't build a working integration without hospital data. Here's how to break the catch-22.
• Your Clinical AI Agent Needs More Than 5 Patients — Your prior auth agent works in testing. Then it meets a 68-year-old with CKD, hypertension, and a specialist referral — and crashes.
• Build a SMART on FHIR App in 30 Minutes — Build a standalone SMART on FHIR app in a single HTML file with zero build tools. You can do it.
• FHIR Is Not Enough: Real-Time Integrations Still Need HL7v2 — FHIR answers 'what does this patient look like now?' To ask for changes you'll have to remember how HL7v2 works.
• Read from FHIR? np. Write to FHIR? please. — A federal survey of 2,253 hospitals: read-side FHIR is table stakes. The real opportunity is write-side clinical integrations.
• Your FHIR Architecture Determines Your Test Data Strategy — Facade, hybrid, or FHIR-native — each architecture breaks differently. Here's what to test for each model and what breaks when you don't.
• Building a FHIR API Gateway: What HAPI Won't Do for You — HAPI stores FHIR and runs queries. It doesn't auth users, enforce access, or fix URLs behind a load balancer. Here's the gateway layer.
• How to Make Claude Write Valid Synthea Modules — LLMs generate valid Synthea JSON but hallucinate the medical codes. Here's a Skill to grounds every SNOMED and LOINC lookup.
• FHIR, USCDI, and US Core: What They Are, How They Fit — FHIR says how to send data. USCDI says what data. US Core says exactly how to format it. Here's how the three standards fit together.
• How 10,000 Synthetic Patients at mock.health Stack Up. — We ran 10K patients through Census, CDC, and comorbidity benchmarks. 15/15 pairs correct. 18/20 prevalences in range. Honest numbers.