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Users in the plans-2026-phase-1=enabled split test can no longer purchase the AI Assist add-on via crafted HTTP requests. The preview and purchase endpoints return 404/redirect for these users. Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com> GitOrigin-RevId: 2c75eb622cf44dc91019a692290ac646b51fd72c