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This UX is 🔥 

One important thing that people need to understand about Passkeys is that the browser will only allow that Passkey to be used with the web site that created it. Curious how people are thinking about adding backup keys to the underlying 4337?

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For iPhone users, I suspect Passkeys will be mentally modeled as a "per-website password that Apple manages for me".  There is a username in the Passkey, but it will cause a cognitive hurdle to use multiple per site.

Specifically related to 4337s, how will websites/passkeys be added as authorized signers?

To your point, there is a shift is framing for passkeys vs passwords. Users have a 1:1 user to password relationship, but can have a 1:many user to passkey relationship. Ideally folks add passkey management screens to allow users to add additional passkeys to their accounts.