Consider deprecating old auth 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.

Start by locating the old auth API and the error-handling paths it uses. Clarify which API is being deprecated, identify the intended detailed error messages, and confirm completion with focused auth error tests or reproducible debugging cases; the issue names no files or tests.

Written by the indexing model from the issue text.