Consider deprecating old middleware API

**Problem**
When working with core, 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 issue names no files, tests, or entry points. Start by locating the core middleware API and its current error-handling path, then clarify which old API is in scope and how detailed messages should be defined. Done requires an agreed deprecation scope and identified tests for the resulting error-handling changes.

Written by the indexing model from the issue text.