I Rebuilt Wordtune in 10 Minutes — Here's the Prompt

The tool we rebuilt

Wordtune is one of the cleanest examples of AI dressed up well. You highlight a sentence on any webpage, a small toolbar pops up next to it, and you click "Casual" or "Formal" or "Shorter" or "Longer" to get rewrite suggestions. The interaction is so smooth it feels native to the page. The pricing — $9.99/month for Plus — is on the lower end of AI writing tools, which is part of what makes it popular.

It's also a tool whose entire functionality is one OpenAI API call per click. The interesting question, as with every AI writing tool we look at, is whether you can rebuild the experience without paying the subscription.

The answer is yes, and the rebuild took 10 minutes. The hard part was matching Wordtune's specific UI polish — the way the toolbar floats next to selected text, the way suggestions appear as cards you can swap in with one click. The actual AI part is trivial.

Worth flagging up front: Wordtune at $9.99/month is the cheapest AI writing tool in this rebuild series. The cost case is the weakest here. You rebuild Wordtune for ownership and customization, not for savings. If you write only occasionally and Wordtune covers your needs, paying for it makes sense.

What I built

A Chrome extension that adds an inline rewrite toolbar when you select text on any webpage. When you highlight a sentence (or paragraph), a small floating toolbar appears above the selection with four rewrite options: Casual, Formal, Shorter, Longer. Click any option to see 3 rewrite suggestions in a side panel, then click any suggestion to replace the original text in-place.

It took 10 minutes total. Most of those minutes were Chrome's reload latency. The actual code is straightforward — text selection detection, floating UI positioning, OpenAI API call, text replacement.

Output is a Manifest V3 extension, four files, about 220 lines of code. No external dependencies.

The prompt I used

A few notes on the prompt:

The "follow the text selection" requirement matters more than it sounds. Wordtune's toolbar tracks the selection precisely, which makes the interaction feel native. Without that, the toolbar appears in a fixed spot and feels disconnected from what the user just selected. Including this in the prompt produced the right behavior in the first generation.

The "5 words minimum" threshold filters out accidental selections. Users frequently select 1-2 words while clicking around. Without the threshold, the toolbar appears constantly and is annoying.

The mention of custom tone presets adds extensibility from day one. Wordtune's 4 modes are reasonable defaults, but real users have preferences ("how I write to my CEO" vs "how I write on Twitter"). Custom presets let the rebuild scale to those needs.

How it works

Content script listens for selectionchange events on the document. When a selection is detected with at least 5 words, it computes the bounding rectangle of the selection and injects a floating toolbar at the rectangle's top edge (positioned above the selection, centered horizontally).

The toolbar uses absolute positioning with z-index high enough to overlay above page content. CSS transitions handle the fade-in (200ms ease-out).

When the user clicks any button on the toolbar, the content script:

1. Captures the currently selected text
2. Sends a message to the background worker with the text and the requested transformation
3. Opens a side panel placeholder that shows a loading state

Background worker receives the message and constructs the OpenAI request:

const transformPrompts = {
  casual: 'Rewrite this sentence in a more casual, conversational tone. Make it sound natural and friendly. Provide 3 different variations.',
  formal: 'Rewrite this sentence in a more formal, professional tone. Use polished language. Provide 3 different variations.',
  shorter: 'Rewrite this sentence to be more concise while preserving the meaning. Provide 3 different variations.',
  longer: 'Expand this sentence with more detail and nuance. Keep the original meaning. Provide 3 different variations.'
};

const response = await fetch('https://api.openai.com/v1/chat/completions', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${apiKey}`,
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    model: 'gpt-4o-mini',
    messages: [
      { role: 'system', content: transformPrompts[action] + ' Return only the 3 variations, one per line, no preamble.' },
      { role: 'user', content: selectedText }
    ]
  })
});

The response gets parsed into 3 variations and sent back to the content script.

Content script (continued) receives the 3 variations and populates the side panel with three cards, each showing one variation and a "Use this" button. When the user clicks "Use this":

1. The content script restores the original text selection (which may have been lost while interacting with the side panel)
2. Calls document.execCommand('insertText', false, chosenVariation) to replace the selected text with the chosen rewrite
3. Closes the side panel

The execCommand API is technically deprecated in some contexts but still works reliably for text input replacement in 2026 Chrome. The modern replacement (Range manipulation via Selection API) is more verbose but produces identical results. Either works.

Popup page is the settings page with API key field, default tone preferences, and a section for adding custom tone presets. Custom presets are stored as { label: 'Professional', prompt: 'Rewrite in a polished, executive tone...' } objects in chrome.storage.local. The content script reads these and adds extra buttons to the toolbar.

That's the entire extension. The polish difference vs Wordtune lives in small details: the exact timing of the fade-in animation, the precise positioning of the toolbar relative to selected text on multi-line selections, the visual treatment of the side panel cards. Each of those is a 5-10 minute CSS adjustment if you want to invest in matching Wordtune more closely.

Cost breakdown — the honest numbers

The straightforward math: Wordtune is $9.99/month. PlugThis Starter is $29/month + $2-6 in API costs. Total around $31-35/month. That's roughly 3x Wordtune's price.

So why rebuild?

For users who only need Wordtune, you shouldn't. $9.99/month for a tool that works well is reasonable.

For users who want more than Wordtune offers, the rebuild starts to make sense. A few specific cases:

• You want different tone presets. Wordtune's 4 modes are fixed. The rebuild lets you add unlimited custom presets — "how I write technical docs," "how I write to my mom," "how I write product launches."

• You want to use Claude or Gemini instead of OpenAI. Wordtune picks the model for you. The rebuild lets you swap providers in a single line of code.

• You're already paying for PlugThis Starter for another extension. The marginal cost of adding this one is just the API costs ($2-6/month) since PlugThis Starter includes one extension. In that case the rebuild is genuinely cheaper than Wordtune.

• You want the same workflow but for content Wordtune doesn't support well. Custom system prompts let you target specific writing styles (academic, technical, creative) that Wordtune's general-purpose modes don't handle well.

If none of these apply, stay with Wordtune. The economics don't beat their pricing for most use cases. The reason to rebuild is ownership, not cost.

Side-by-side feature comparison

Honest take: Wordtune's UI is meaningfully better than the rebuild. The animations, the responsiveness, the way the toolbar gracefully follows multi-line selections — all of it is more polished. If UI polish matters to you, Wordtune is the better choice.

Where the rebuild wins is customization. Adding "rewrite in the voice of [X]" or "make this sound 30% more confident" or "translate to Portuguese while keeping the tone" — these are 30-second prompt changes in the rebuild, impossible in Wordtune.

Customization ideas

Voice-cloned rewriting. Add 5-10 samples of your own writing to the system prompt as context. Tell GPT-4: "rewrite the user's input to match this voice." Now every rewrite sounds like you.

Industry-specific modes. Add presets like "more technical," "more academic," "more journalistic," "more conversational." Each is a custom prompt in your settings.

Multi-style suggestions. Instead of 3 variations of the same style, generate 1 variation each across 3 different styles (casual + formal + concise simultaneously). Useful for picking the right register.

Translation as rewrite. Add a "Translate to [X]" button. The same UI pattern but the prompt becomes "Translate to Spanish while preserving meaning and tone."

Plagiarism-aware rewriting. Add a mode that rewrites to maximize semantic equivalence while changing wording. Useful for paraphrasing academic content (with proper citation).

Tone matching to surrounding text. Detect the tone of the text around the selection (the previous few sentences). Rewrite to match that tone rather than a fixed preset.

Each is a small change. Custom prompts in chrome.storage.local make these user-configurable rather than developer-only.

Try it yourself

Drop the prompt above into PlugThis, provide your OpenAI API key in the settings page, load into Chrome. About 10 minutes to a working rewriter.

Modify before generating if you want different default modes, different model, or different UI behavior. The prompt is the source of truth.

Related reading

• Wordtune alternatives — structured comparison — competitor analysis with side-by-side
• I rebuilt WriteSeed in 8 minutes — AI writing assistant rebuild
• I rebuilt QuillBot's paraphraser — paraphrasing-focused rebuild
• The complete Chrome extension building guide — how Chrome extensions work technically

The Wordtune rebuild is the clearest case where cost isn't the reason to rebuild. The reason is everything else: custom presets, model choice, voice matching, language flexibility. If Wordtune's defaults cover your needs, pay for it. If you want more control over how AI helps you write, the rebuild gives you that for roughly the same total cost with full ownership.