A paid Claude plan: Pro or Max. Pro is $20 a month, or $17 a month with annual billing ($200 up front). Max starts at $100 a month. Duet itself adds no cost during the research preview.
A Claude account in the US, and 18 or older per Anthropic's consumer terms. Access also needs an invitation from Anthropic's waitlist: invitations go out in waves, so joining sets a place in line, not a date.
Duet runs as its own iPhone and Android app, and on the web at claude.ai once you're admitted. You need nothing beyond a browser to join the waitlist.
One honesty note up front: no invitation had reached our test accounts when we checked on October 21, so the in-app steps below come from Anthropic's get-started guide rather than our hands-on run. The waitlist steps we did run.