# Passkeys

## Objective

Sign in with device biometrics instead of a password or a code.

WebAuthn credentials registered per device, listed and named by the user, with a defined fallback for browsers and devices that cannot use them.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Let a user register several passkeys and name each one. One passkey on one phone is a lockout waiting to happen.
2. Detect support before offering it, and keep the existing sign-in methods visible. Never make a passkey the only credential on an account.
3. Verify the assertion on the server — challenge, origin, and signature counter. A client that says authentication succeeded is not evidence.
4. Show each credential with its name, creation date, and last-used date, and let the user remove any of them, with reauthentication required to add or remove.
5. Do not require the user to know what a passkey is. Label it by outcome — sign in with Face ID, fingerprint, or device PIN — not by protocol.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Registration must refuse a credential already registered to the account rather than creating a silent duplicate.
- Removing the last passkey must be allowed only if another usable sign-in method remains.
- Cross-device sign-in via a QR code and a phone must be an explicit, explained path, not an unlabelled fallback screen.
- A user who wipes or loses the device keeps stale credential records; make them removable from another signed-in session and via account recovery.
- A cancelled or timed-out prompt is not a failure — return the user to a working sign-in screen without an alarming error.
- Synced passkeys appear on new devices without registration; do not assume one credential means one physical device.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] A user can register, name, list, and remove multiple passkeys.
- [ ] Assertions are verified server-side, including challenge and origin.
- [ ] Unsupported browsers fall back to existing sign-in methods without a dead end.
- [ ] Duplicate credential registration is refused.
- [ ] An account can never be left with no usable sign-in method.
- [ ] Cancelled prompts return cleanly to the sign-in screen.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.