Add TypeScript types for parsers

**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.

The issue names parsers and middleware but gives no file or test; first locate those entry points and clarify whether the request is TypeScript declarations or middleware error handling. Done should be defined by the agreed scope, with coverage for the affected behavior and more descriptive errors.

Written by the indexing model from the issue text.