Consider deprecating old handlers 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.

Start by locating the validation error-handling code and the old handlers API. Review how handlers are currently used and check existing tests or documentation for validation messages. Done means the deprecation scope is agreed, affected usage is identified, and detailed error behavior is covered.

Written by the indexing model from the issue text.