Most auth migrations do not fail because the new provider is weak. They fail because teams treat authentication like an identity project and ignore that it is also a session-behavior project . That sounds less exciting than debating providers, passkeys, JWTs, or SSO standards, which is probably why teams keep skipping it. But users do not feel your identity architecture. They feel whether they got logged out unexpectedly, whether one tab still works while another does not, whether their trusted device suddenly is not trusted, and whether support can explain what happened. So the practical recommendation comes first: plan the session lifecycle before you plan the migration launch . If you cannot explain how sessions are issued, refreshed, downgraded, revoked, and retired across web, API, mobile, and admin surfaces, your auth migration strategy is incomplete.…