Add TypeScript types for models

**Problem**
When working with api, 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 model definitions and API error-handling entry points. Confirm which models need TypeScript types and which error messages are in scope before coding, since the issue does not name files or tests. Done means the requested model types exist and the API produces the agreed detailed error messages.

Written by the indexing model from the issue text.