Passkeys
Sign in with device biometrics instead of a password or a code.
What it adds
WebAuthn credentials registered per device, listed and named by the user, with a defined fallback for browsers and devices that cannot use them.
What your agent is told to do
5
What your agent is told to do
5-
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.
Edge cases it handles
6
Edge cases it handles
6- 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.
Definition of done
8
Definition of done
8- 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.
Related features
Password Generator
Password Generator
Offer a strong password in one click wherever someone sets or changes credentials.
What it does
A generate control on password fields that produces a strong value, shows it, and hands it to the user safely.
How it works
- 1 Add the control to every place a password is set: sign-up, password change, password reset completion, and any admin screen that provisions credentials for someone else.
- 2 Generate the value in the browser using the platform's cryptographic random source. The value must never be produced on the server or sent anywhere.
- 3 Reveal the generated password by default so the user can record it, and offer a regenerate control beside it. A hidden generated password that the user cannot see is a password they will immediately reset.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/password-generator
Idempotent Form Submission
Idempotent Form Submission
Make one submit mean one result, even when the network retries.
What it does
Idempotency keys on high-impact writes so a double-click, a retry, or a refresh cannot create the same record twice.
How it works
- 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.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/idempotent-form-submission
Client Version Compatibility
Client Version Compatibility
Handle the tab that has been open since two deploys ago.
What it does
The server tells the client which build it expects, and the client responds — quietly for an optional update, insistently when the old code can no longer talk to the API.
How it works
- 1 Stamp every build with a version identifier and return it on API responses, either in a header or a small metadata endpoint. The client compares it against the version it was built as.
- 2 Separate two cases and treat them differently. A newer build being available is an optional update — offer it and let the user finish what they are doing. An API contract the running client cannot satisfy is a hard incompatibility and must block further writes.
- 3 For an optional update, show an unobtrusive, dismissible prompt and apply the new version at the next natural navigation. Do not interrupt.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/client-version-compatibility
How it works
-
1
Copy the link
Grab the Markdown instruction URL for this feature.
-
2
Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
-
3
It inspects, then implements
Your agent reads your existing app first, then adds the feature to fit it.
Works with your stack
These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.
Need it tighter than that? Customize the feature and tell it exactly what you're running.