Consider deprecating old models API

**Problem**
When working with auth, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue names no files, tests, or entry points. Start by locating the old models API and authentication error handling, then clarify the deprecation scope and required error-message behavior; done requires agreed, testable acceptance criteria that the current issue does not provide.

Written by the indexing model from the issue text.