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The gnubok-home-ok cache cookie stored only the host, so any account that signed in within the 15-minute TTL window inherited the previous user's "this is home" verdict in the same browser and skipped the brand-host bounce entirely: a user with no ties to a white-label brand could land inside the branded shell instead of being redirected to the canonical domain (amnas account-switch repro, two logins 9 s apart). The cookie value is now userId~host and the middleware only skips the affinity check when both match the current session. Old host-only cookies never match, so the check re-runs and the format self-migrates; no sign-out hook is needed because a foreign verdict misses by construction. Claude-Session: https://claude.ai/code/session_01EBJAE7b6B64HaKybjaC66B Co-authored-by: Claude Fable 5 <noreply@anthropic.com>