Input Masks
Guide people into the expected format without fighting what they type.
What it adds
Formatted input fields that insert separators as the user types, while submitting a clean, normalized value.
What your agent is told to do
6
What your agent is told to do
6-
1
Keep the caret where the user expects it after every insertion and deletion. Reformatting that jumps the caret to the end is the single most common failure of masked inputs.
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2
Accept pasted values in any shape — with separators, without them, with spaces or dashes — and normalize on paste rather than rejecting.
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3
Submit the normalized value to the server and format only for display. The server must never receive a display string it has to strip.
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4
Deleting a separator must delete the adjacent character, not fight the user by re-inserting it.
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5
Do not put the mask pattern in the field's accessible name, and do not let screen readers read every separator character. Announce the field's purpose and expected format once via the label and description.
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6
Never use a mask as your only validation. The value must be validated server-side regardless of what the field allowed.
Edge cases it handles
6
Edge cases it handles
6- Selecting the whole value and typing over it must reformat cleanly from scratch.
- Undo and redo must step through logical edits, not through each reformat.
- Autofill inserts a whole value at once, bypassing keystroke handlers — normalize on change, not only on keypress.
- Mobile keyboards behave differently; test composition and predictive input rather than assuming key events.
- A partially entered value must be preserved on validation failure, not cleared.
- The field must remain usable with JavaScript disabled or failed — a plain, unmasked input that still submits.
Definition of done
8
Definition of done
8- The caret stays put during insertion, deletion, and mid-string editing.
- Pasted values are accepted with or without formatting.
- The server receives normalized values, never display strings.
- Screen readers announce the field's format once, not per separator.
- Autofill and mobile input produce correctly formatted values.
- Server-side validation exists independently of the mask.
- 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
Voice Dictation
Voice Dictation
Put a microphone in text fields so people can speak their input instead of typing it.
What it does
A dictation control on the app's longer text inputs that transcribes speech into the field as the user talks.
How it works
- 1 Add the control to the fields where typing is genuinely tedious: long descriptions, notes, comments, and message bodies. A microphone on a postcode field is clutter.
- 2 Show interim results in the field as they arrive, visually distinct from committed text, so the speaker can see it is working and correct course before they finish.
- 3 Reuse the app's existing Voice Transcription for the server-side path so one recognition configuration and one set of language settings serve both dictation and recorded audio.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/voice-dictation
Code Editor Field
Code Editor Field
Edit code, JSON, and config with syntax highlighting instead of a bare textarea.
What it does
A form field backed by a real code editor: syntax highlighting, a language mode per field, and inline validation that tolerates work in progress.
How it works
- 1 Load the editor library lazily, only on views that use it, and render a usable plain textarea until it arrives. It must never block first paint.
- 2 Set the language mode per field from what the field actually holds, and validate against that mode.
- 3 Report syntax errors as you go, but never block typing or reset the content. Half-written JSON is a normal intermediate state; only submission should be gated.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/code-editor-field
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.