Consider deprecating old models API

**Problem**
When working with middleware, 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 middleware and the old models API in the repository; the issue names no files, entry points, or tests. First determine which error paths need more descriptive messages, then define completion by covering the intended cases with tests and confirming the updated messages improve debugging.

Written by the indexing model from the issue text.