Refactor types for better readability

**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.

No file, test, or entry point is named. Start by locating the validation and error-handling code in the terminal agent, then identify which messages need more detail. Done means validation failures are handled clearly and their messages provide useful debugging context.

Written by the indexing model from the issue text.