What is an authentication scheme in ASP.NET Core, and how do authenticate, challenge, and forbid differ?

In production I make authenticate, challenge, forbid, sign-in, and sign-out defaults explicit when several schemes coexist. I avoid forwarding based on unvalidated token content, and I keep each API policy clear about accepted issuers and audiences. A fallback policy protects endpoints that were not annotated, while intentional anonymous routes remain explicit. Custom result handling can normalize API errors, but it must preserve the semantic distinction between 401 and 403 and avoid revealing protected resource existence. Tests exercise cookie and bearer requests against the same endpoint matrix, including missing credentials, invalid tokens, valid users without permission, and correct users. Telemetry records scheme and failure category without credentials, which makes a sudden challenge loop or wrong-handler regression diagnosable after configuration changes.