Safari 18.0.1 on macOS 15.01 doesn't support the Passkey PRF extension during cross-device WebAuthn authentication when using QR code scanning, while it works correctly with iCloud passkeys. Steps to Reproduce: Clone and setup: git clone https://github.com/quocle108/passkey-prf-test yarn yarn start Test iCloud Passkey Flow: Open http://localhost:3000 in Safari Open DevTools (Cmd+Option+I) Click "Register" Choose "Passkey on iCloud" Expected console output: PRF supported: true Test Cross-Device Flow: Click "Register" Choose "Phone/Tablet" Scan QR with mobile device Expected: PRF supported: true PRF extension should be supported in cross-device flow, matching iCloud passkey behavior. Actual: PRF supported: false Cross-device flow returns empty extension results. Verify in Chrome Repeat steps 2-3 in Chrome Both flows return proper PRF extension results: PRF supported: true Test Environment: Browser: Safari 18.1.1 , Chrome 131.0.6778.70 OS: macOS 15.01 Mobile: iOS 18.x / Galaxy Note9 Android 10 Test repo: https://github.com/quocle108/passkey-prf-test

Test Scenario: Initial Setup: Register a passkey on Chrome (MacBook) with cross-platform option The passkey syncs to iPhone via iCloud Both devices share same iCloud account Authentication Tests: Chrome on MacBook: Using hybrid transport (QR code with iPhone) → PRF output A Using platform authenticator → PRF output B (different) Safari on MacBook: Only uses platform authenticator → PRF output B Expected Behavior: When using same credential ID and salt, PRF output should be consistent across browsers/devices