What determines accessibility of PyPI metadata for external tooling?

PyPI Metadata Accessibility

The goal is to ensure external tools can reliably interpret package metadata for workflow integration.

Constraints

The current system relies on standard metadata fields like classifiers and dependencies, but the exact accessibility for non-standard parsers is uncertain.

What specific metadata fields are guaranteed to be universally parsable across all modern PyPI index interactions? Does the dependency resolution logic strictly enforce semantic versioning across all declared constraints? When should a tooling integration expect metadata availability regardless of the underlying packaging backend?