Consider deprecating old controllers API

**Problem**
When working with validation, 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 does not identify any files, tests, or entry points. First locate the controllers API and its validation error handling, then clarify which old API is being deprecated, the migration path, and the tests that should define completion.

Written by the indexing model from the issue text.