Consider deprecating old core API

**Problem**
When working with models, 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 core API and the model error-handling paths, then confirm the deprecation scope and expected error-message details with maintainers. Done means the API decision, migration or compatibility requirements, and descriptive error behavior are clearly defined.

Written by the indexing model from the issue text.