Consider deprecating old core API

**Problem**
When working with types, 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 type-handling error paths, then clarify which API is being considered for deprecation and what error details are required. Done should include an agreed compatibility scope and tests covering the affected error messages.

Written by the indexing model from the issue text.