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
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Let a user register several passkeys and name each one. One passkey on one phone is a lockout waiting to happen.
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Detect support before offering it, and keep the existing sign-in methods visible. Never make a passkey the only credential on an account.
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Verify the assertion on the server — challenge, origin, and signature counter. A client that says authentication succeeded is not evidence.
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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.
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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
Rate Limiting
Rate Limiting
Stop one client from ruining it for everyone.
What it does
Throttling on the endpoints that get abused: auth, search, exports, and public forms.
How it works
- 1 Identify the endpoints worth protecting: sign-in, sign-up, password reset, search, exports, and anything unauthenticated.
- 2 Limit by a stable identifier — user ID where signed in, IP otherwise. Be aware IP is shared behind NAT and proxies.
- 3 Return 429 with a Retry-After header. Do not silently drop the request.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/rate-limiting
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
Data Integrity Constraints
Data Integrity Constraints
Enforce your important rules in the database, not just in the form.
What it does
Database-level uniqueness, foreign key, check, and not-null constraints backing the business rules the application currently only validates in code.
How it works
- 1 Inventory the rules the app relies on — required fields, uniqueness, valid enum values, positive amounts, referential links — and note which exist only in application code.
- 2 Before adding each constraint, query for rows that already violate it and decide explicitly whether to fix, delete, or exclude them. Every mature database has violating legacy rows.
- 3 Add constraints in a way that does not hold a long exclusive lock: create indexes concurrently, add checks as NOT VALID and validate them in a second step.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/data-integrity-constraints
How it works
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1
Copy the link
Grab the Markdown instruction URL for this feature.
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2
Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
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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.