Consider deprecating old models API

**Problem**
When working with services, 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 the service error-handling paths, then clarify which API is deprecated and which detailed messages are required; completion should include an agreed deprecation plan and tests for the resulting errors.

Written by the indexing model from the issue text.