Code Editor Field
Edit code, JSON, and config with syntax highlighting instead of a bare textarea.
What it adds
A form field backed by a real code editor: syntax highlighting, a language mode per field, and inline validation that tolerates work in progress.
What your agent is told to do
5
What your agent is told to do
5-
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.
-
4
Preserve the content exactly: tabs versus spaces, trailing whitespace, line endings, and the final newline. An editor that reformats on save destroys diffs.
-
5
Offer an explicit escape hatch to the plain textarea, and use it automatically when the fancy editor cannot initialise.
Edge cases it handles
6
Edge cases it handles
6- Screen readers and browser autofill do not cope with syntax-highlighted layers — the fallback textarea must be a real, labelled control.
- Pasting a very large file must not freeze the tab; cap the size, disable highlighting above a threshold, and say so.
- Tab must indent inside the editor but must not trap keyboard users — give them a documented way out.
- Content saved with CRLF line endings must come back the same way unless the app deliberately normalises them.
- A failed save must keep the buffer intact; never clear the editor on error.
- Highlighting themes must follow the app's light and dark themes, not ship their own colours.
Definition of done
8
Definition of done
8- The editor library is lazily loaded and never blocks initial render.
- Each field highlights and validates using its declared language.
- Invalid intermediate content can be typed freely; only submit is blocked.
- Whitespace, tabs, and line endings survive a save-and-reload round trip.
- A labelled plain-textarea fallback exists and is used when the editor fails.
- Editor colours match the app's theme in both light and dark.
- 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
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
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
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.