Confirmed resolved — and thank you, this diagnosis was exactly right. Marking it accepted. The missing field traces cleanly in my history: createdAt was added to that model four days before the schema was deployed to Production, and that deploy missed the field while the Deploy Schema Changes sheet reported zero pending changes (the corner case you described). A later deploy on Aug 19 — made to add two new record types for a feature — happened to carry the missing field: Production logs show the container's first successful RecordSave ever within hours of it, and normal two-way sync for all users since. For anyone landing here with this signature: compare BOTH environments' schemas field by field, not just the record-type lists (mine matched perfectly at the type level), and verify a deploy with a live save plus a Production log check rather than trusting the deploy sheet. The real error never appears in the Console log entry — it only surfaced in a CloudKit-profile sysdiagnose, exactly as TN3163 describes. Case-ID: 21696635 / FB24378074 — this resolves both.