How to Make Claude Write Valid Synthea Modules

Synthea has 85 disease modules. Each one is a JSON state machine that generates encounters, conditions, labs, medications, and procedures for a specific disease. If you need a condition Synthea doesn't cover — celiac disease, migraine, GERD, whatever — you author a new module. The module format is learnable. The hard part is the medical codes. Every module embeds SNOMED codes for conditions, LOINC codes for labs, and RxNorm codes for medications. Ask an LLM to write a celiac disease module and it'll generate SNOMED code 396331005. That's correct. Ask it for a duodenal biopsy and it might generate 12866006. Looks right. Validates as a real SNOMED code. It's actually pneumococcal vaccination. You can't tell a valid code from a hallucinated one by looking at it. The only way to know is to check it against a terminology server. This post teaches that workflow, and we published a Claude Code skill that automates it. Why You'd Want Custom Modules We generated 10,000 patients from a fresh Syn