# Social Sign-In

## Objective

Let users sign in with an identity they already have.

OAuth used as authentication: first-time account creation from a provider identity, safe matching to existing accounts, and handling providers that return incomplete or unverified data.

## 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. Key the account on the provider's stable subject identifier, not on the email address. Emails change hands; subject IDs do not.
2. Only auto-link to an existing account when the provider asserts the email is verified. An unverified email from a provider is an account-takeover route — send those users through an ownership check instead.
3. Request the minimum scopes needed to identify someone. Sign-in does not need calendar or contact access.
4. Handle missing profile fields as normal: many providers return no name, no avatar, and some return no email at all. Do not block account creation on data you cannot guarantee.
5. Adding or removing a sign-in method on an already-authenticated account is owned by Account Linking, and OAuth for data access is owned by OAuth Integration Connections. Extend those rather than duplicating them here.

## 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

- A user who revoked access at the provider must get a clear reconnect prompt, not a generic failure, and must still be able to sign in another way.
- Two providers returning the same email must not silently become two accounts — or one account, without an explicit decision.
- The provider changing a user's email must not orphan the account or create a duplicate.
- A cancelled or denied consent screen returns an error, not a crash; land the user back on sign-in with an explanation.
- State must be validated on the callback, and the returned state must match a value tied to the originating session.
- The first sign-in for a brand-new user must still complete any required onboarding, not drop them into a half-configured account.

## 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

- [ ] Accounts are keyed on the provider subject identifier.
- [ ] Auto-linking happens only for provider-verified email addresses.
- [ ] Only identity scopes are requested.
- [ ] Missing name, avatar, or email does not block account creation.
- [ ] Denied consent and revoked access produce clear, recoverable states.
- [ ] The callback rejects mismatched or missing state.
- [ ] 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.