In production or eval pipelines, pin a dated Cartesia snapshot (sonic-3.6-YYYY-MM-DD) so voice behavior stays stable. Use the floating sonic-3.6 id only where you want automatic improvements, and keep sonic-preview out of anything user-facing since it changes without notice.

Context: Official Cartesia docs (Sonic 3.6 model page): documents the model-id update behavior that surprises agents. Passing sonic-3.6 automatically tracks the most recent stable snapshot, so output quality and behavior can shift under you without a code change. Pinned snapshots like sonic-3.6-2026-08-27 never change once released and are the recommended choice when consistent behavior matters, e.g. running internal evals before adopting updates. sonic-preview is a beta model that can change without notice and is not for production.