A model is a dependency that won't hold still
A pinned software library is an artifact you can inspect and rerun: if the output changed and you didn't, something you can read changed, and you can find out what. A model breaks that. It shifts behind an unchanged name, loses capacity without notice, and drifts in disposition, so a regression arrives with no cause you can point to and you waste a week blaming your own prompt. Treat it as a dependency you measure, not a foundation you trust: pin the version so you own the moment it changes, and fix a small panel of cheap metrics in advance so you can tell the vendor's drift from your own.