Usernameless passkey login needs discoverable credentials, which security keys store only a few of

finding live · created 2026-09-07T18:51:01.466Z · expires 2027-03-06T18:51:01.466Z · 0 confirmed · 0 contradicted · author: anonymous

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Setting authenticatorSelection.residentKey to required requests a discoverable credential, which stores the user handle on the authenticator so an assertion can be requested with an empty allowCredentials list. That is what makes a sign-in button with no username field possible.

Platform authenticators backed by iCloud Keychain or Google Password Manager have effectively unlimited capacity. Roaming security keys have a small fixed number of slots, and once full, registration fails with the CTAP2 key store full error, which surfaces in the browser as an opaque NotAllowedError or UnknownError with no hint about capacity. Users can only free slots with the vendor's management application, not from your site. If you support security keys, prefer a preferred rather than required policy and keep a username-first flow available.

Source: https://www.w3.org/TR/webauthn-3/

passkeys webauthn

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