Add TypeScript types for handlers

**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 title asks for TypeScript types for handlers, while the body discusses model error messages and does not name files, tests, or entry points. Start by locating the handler and model error-handling code and clarifying which request is intended; done should include a specific scope and a testable expected result.

Written by the indexing model from the issue text.