# Twilio SMS Delivery

## Objective

Send transactional SMS with delivery tracking and opt-out handling that actually holds.

An SMS delivery path with canonical phone numbers, per-message delivery state, and an enforced opt-out list.

## 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. Identify the small set of messages that genuinely warrant SMS — verification codes, delivery alerts, appointment reminders — and route everything else to the app's existing email or in-app channels. SMS is expensive and intrusive.
2. Normalise every phone number to a canonical international form at the point it is captured, store it that way, and reject anything that cannot be parsed rather than attempting a send on a guess.
3. Send through the app's background-job system, and record a message row before dispatch that carries a stable identifier, the recipient, the purpose, and the current delivery state.
4. Check the opt-out list immediately before dispatch, not only when the message is queued. A message queued an hour ago must not go out to someone who has since replied to stop.
5. Do not treat a successful hand-off to the provider as delivery. Accepted means queued; the delivery outcome arrives later on a callback and the message state must reflect that distinction everywhere it is shown.

## 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

- Numbers entered with local formatting, spaces, or a missing country code will fail or reach the wrong person. Normalise to a canonical international format on capture and store only that form.
- Standard opt-out keywords must unsubscribe the number immediately and permanently, and regional rules on quiet hours, sender identity, and consent differ by country. Encode the destination country's constraints rather than applying one policy everywhere.
- Queued, sent, delivered, undelivered, and failed are distinct states with different consequences. Model all of them, and never collapse undelivered into failed, because a temporarily unreachable handset is not a wrong number.
- Status callbacks arrive over a public endpoint. Verify their authenticity before trusting them, and make them idempotent, because the provider will resend the same event and events can arrive out of order.
- A job retried after a network timeout must not send the message again. Use a stable per-message key so a repeat attempt reconciles with the existing message instead of creating a second one.
- Long messages split into multiple segments and are billed per segment. Show the segment count where a message is composed, and be deliberate about characters that force a more expensive encoding.
- A per-recipient and per-account send ceiling must exist, so a loop or a retry storm cannot run up a bill or flood one person overnight.
- When the provider is degraded, the triggering action must still complete, the message must stay queued rather than being marked failed, and any user-facing copy must say the message is on its way rather than claiming it was delivered.

## 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

- [ ] Every stored phone number is in a single canonical international format.
- [ ] Opt-out is checked at dispatch time and an opted-out number is never sent to again.
- [ ] Each message carries queued, sent, delivered, undelivered, or failed state, updated from verified callbacks.
- [ ] Status callbacks are authenticated and processing them repeatedly changes nothing.
- [ ] A retried job reconciles with the existing message rather than sending a duplicate.
- [ ] Per-recipient and per-account send ceilings are enforced.
- [ ] Provider downtime leaves messages queued and the triggering action successful.
- [ ] 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.