Firefox just didn't support it cleanly, I think Chrome did, I don't remember. Apparently it's just due to Linux not having a native passkey implementation. Dang.

Edit:

Apparently BitWarden should work, but my particular passkey was not on there.

> Apparently it's just due to Linux not having a native passkey implementation.

Excuse me? The infrastructure for "an apps is trying to login with a private/public key pair, and right now it needs the private key to encrypt some part of the transaction" has existed on Linux since it began.

The problem, as best as I can understand it, is that some/all browsers are not treating passkeys as an extension of the key system that began with ssh(1), even though technically speaking, they are.

I use Bitwarden on Linux. There is no option to do anything with a passkey I have stored there except delete it.