# Idempotent Form Submission

## Objective

Make one submit mean one result, even when the network retries.

Idempotency keys on high-impact writes so a double-click, a retry, or a refresh cannot create the same record twice.

## 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. Find the writes where a duplicate is expensive — payments, orders, invitations, provisioning, anything that sends money or email.
2. Generate an idempotency key when the form is rendered, send it with the submission, and persist it server-side alongside the result of the first successful attempt.
3. When the same key arrives again, return the stored result instead of doing the work a second time. When the same key arrives with a different payload, reject it — that is a client bug, not a retry.
4. Also disable the submit control while a request is in flight and show a pending state, so the common case never reaches the server twice.
5. Do NOT treat the disabled button as the protection. It only covers one browser tab; the endpoint is still reachable directly, from a retry, or from a second device. The server-side key is the actual guarantee.

## 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

- The key must survive a page refresh and a browser back-navigation, or the user gets a fresh key and a second record.
- The client can time out after the server has already committed. The retry must find the stored result, not start over.
- Two requests with the same key arriving concurrently must not both proceed — hold a lock or a unique constraint on the key.
- Keys must expire, but not so quickly that a slow retry slips through as a new write.
- A failed attempt must not burn the key permanently; the user should be able to fix the input and submit again.
- Do not key on the payload hash alone — two genuinely intended identical submissions are legal and must both go through.

## 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 high-impact write accepts and requires an idempotency key.
- [ ] A repeated key returns the original result and performs no second write.
- [ ] A repeated key with a changed payload is rejected with a clear error.
- [ ] Submit controls disable during flight and show a pending state.
- [ ] Concurrent duplicate requests resolve to exactly one record.
- [ ] Key expiry is a documented, deliberate window.
- [ ] 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.