Inline Validation
Tell people what's wrong before they hit submit.
What it adds
Field-level validation that catches problems early without nagging.
What your agent is told to do
4
What your agent is told to do
4-
1
Validate on blur, not on every keystroke — validating while someone is still typing their email is hostile.
-
2
Once a field has errored, re-validate as they fix it so the error clears immediately.
-
3
Say what is wrong AND how to fix it. 'Invalid' is not an error message.
-
4
Keep server-side validation as the source of truth. Client-side validation is a convenience, never a security boundary.
Edge cases it handles
4
Edge cases it handles
4- Submitting must still surface server-side errors the client didn't catch.
- The first errored field must receive focus on a failed submit.
- Async validation (is this username taken?) must be debounced and must not block submit forever.
- Errors must survive a failed submit — never clear the user's input.
Definition of done
8
Definition of done
8- Fields validate on blur, not per keystroke.
- Errors clear as soon as they're fixed.
- Messages explain how to fix the problem.
- Server-side validation still runs and its errors are shown.
- Errors are tied to inputs with aria-describedby and announced.
- A failed submit never discards what the user typed.
- The feature matches the existing design system.
- No existing functionality is broken.
Related features
Unit Converter
Unit Converter
Let people convert a value between units without leaving the page they are on.
What it does
An inline converter that takes a typed value in one unit and shows the equivalent in another, in place.
How it works
- 1 Start from the units the app actually deals in. A shipping tool needs weight and length; a recipe tool needs volume and temperature. Ship those and leave the rest out.
- 2 Convert as the user types, with the result visible beside the input rather than behind a submit button. Let either side be edited so the conversion runs in both directions.
- 3 Reuse the app's existing number parsing and display formatting rather than writing new ones. If Locale-Aware Numbers and Dates or Number Formatting Component is already present, the converter must render through it so grouping and decimal marks match the rest of the app.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/unit-converter
Character Limits
Character Limits
Show how much room is left before text gets rejected or cut off.
What it does
Live remaining-character feedback on limited fields, counting the same way the server does.
How it works
- 1 Find every field with a length limit — database column, API constraint, or downstream service — and surface that limit in the UI.
- 2 Count the same units the server counts. If the server counts Unicode characters, do not count UTF-16 code units in the browser; the two disagree on emoji and the user gets rejected for text that looked fine.
- 3 Show remaining count rather than used count once the field is near the limit, and change its emphasis at a threshold rather than colouring it from the start.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/character-limits
Field State Matrix
Field State Matrix
Decide once how every field looks and behaves in every state, before screens improvise.
What it does
A defined and documented set of visual and behavioural states covering every input type in the app.
How it works
- 1 Enumerate the app's input types and cross them against every state they can enter: empty, filled, focused, hovered, invalid, valid, disabled, read-only, loading, and browser-autofilled.
- 2 Define disabled and read-only as genuinely different states — a read-only field shows a real value the user may select and copy, a disabled field is inert and excluded from submission.
- 3 Define the loading state so the field is visibly busy and not editable, and say what happens to a keystroke that arrives mid-load rather than letting it be silently swallowed.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/field-state-matrix
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.