Topics discussed:

• The first version of the API will be R4-compatible. Arjun Sanyal was careful about the wording — it isn't an R4 spec, it's R4-compatible, because what they're publishing is a logical model rather than a real R4 resource. Nikolai's reasoning was blunt: his personal view is that the world will sit on R4 for many years, so publishing something that works there is good enough, and the remaining work is editorial rather than a matter of open decisions.
• The reason not to hand-build a multi-version spec is that FHIR may be about to grow one properly. FHIR-I is pulling some resources out of core and into IGs for R6, and there's an active discussion about whether those IGs should be multi-version — nobody wants to maintain a separate spec per FHIR version, or per US Core version for that matter. If IG Publisher grows support for that, SQL on FHIR would use it. So: R4 now, and wait rather than invent conditional machinery by hand.
• Arjun made the case that FHIR to OMOP shouldn't be a one-way street. Filling data into OMOP is worthwhile and everyone should go to town on it, but he wants the more controversial direction too — OMOP back out into tabular FHIR, or whatever you call the output of a ViewDefinition. He also thinks the first is the stepping stone to the second: build tabular schemas of FHIR resources shaped for loading into OMOP, and those same schemas become the targets when you reverse the transformation. Underneath it is his general position that there is no one true data model and never will be — you shape data for the use case, which is exactly why multiple representations of the same thing exist.
• Rust FHIRPath engines keep multiplying — the Helios one is out and open source, another is being built in the Octo tooling, and others are in progress. Nikolai's warning came from doing it: a good FHIRPath engine is roughly a year of work, and his own team is months in with prior experience and still not done. The interesting side effect is that building one seriously surfaces real gaps in the FHIRPath specification itself, which is where his recent run of questions on the FHIRPath channel came from.