Heimdall-backed members enrol WebAuthn passkeys on the Linked accounts page (ticket from a fresh ordinary Heimdall sign-in, 48 h cooling-off, cap 5,
FixedWindowBudget on ceremonies) and unlink with a strong passkey: the assertion is verified at Bifrost, then inside the completion that holds the bearer
Bifrost calls Heimdall's POST /v1/apps/bifrost/step-up and the unlink with the returned token. The Google step-up closes once a strong passkey exists.
Migration AddPasskeyCredentials is additive. Tests use a software authenticator and a fake Heimdall step-up route modeled on the contract.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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