Currency Input
Let people enter money naturally without rounding surprises.
What it adds
A money field that keeps currency selection separate from the amount and stores exact values, not floats.
What your agent is told to do
6
What your agent is told to do
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Store money as integer minor units or a fixed-precision decimal. Never a floating-point number — 0.1 + 0.2 is not 0.3, and money bugs found in production are always expensive.
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Keep currency selection a distinct value from the amount. An amount without a currency is meaningless, and a currency inferred from the user's locale will be wrong for anyone working across borders.
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Handle currencies whose minor unit is not two digits: JPY and KRW have none, BHD and KWD have three. A hardcoded ×100 is a real defect for real users.
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Parse the separators the user's locale actually uses — 1.234,56 and 1,234.56 are the same amount. Accept both rather than rejecting one.
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Decide and document how negative amounts are entered and shown, including the parenthesis convention in accounting contexts.
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Caret behaviour, paste handling, and separator insertion are owned by Input Masks; extend that rather than writing a second masking implementation. Read-only formatting of stored amounts is owned by Locale-Aware Numbers and Dates.
Edge cases it handles
6
Edge cases it handles
6- Changing currency after typing an amount must not silently convert it — clarify whether the number stays or resets.
- More decimal places than the currency allows must be handled by an explicit rounding rule, not truncated at random.
- Very large amounts must not lose precision at the boundaries of the storage type.
- An empty field and a zero amount are different; do not collapse them.
- Amounts displayed after storage must match what was typed, to the minor unit.
- The numeric keyboard on mobile must offer the decimal separator the locale actually uses.
Definition of done
8
Definition of done
8- Amounts are stored as minor units or fixed-precision decimals, never floats.
- Currency is an explicit field, never inferred from locale alone.
- Zero-decimal and three-decimal currencies round-trip correctly.
- Both comma and full-stop decimal conventions parse to the same value.
- Negative and zero amounts have defined, documented behaviour.
- A stored amount redisplays exactly as it was entered.
- 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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Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
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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.