Claude Code
Good for building a browser extension or Mac app that calls an LLM API.
AI writing · updated 2026-11-03
Can you vibe code a Grammarly alternative?
Medium: Yes, with care An AI rewrite tool for your own writing is easy with an LLM API; matching Grammarly's inline, everywhere experience is the hard part.
2–6 h
MVP build time
$1–$13/mo
Your running cost
$12/mo/seat
Grammarly Pro
up to $132
Saved per year
The honest verdict
Large language models made grammar fixes and rewrites a single API call, so a web app or editor plugin that improves your text is quick to build. Grammarly's moat is the experience: underlines as you type in every text field of every app, low latency and privacy controls. A browser extension or Mac menu bar tool for yourself is a medium project, with model costs that stay small for one person.
Grammarly is aI writing assistant for grammar, tone and rewrites across apps and the browser. This page is about building your own version for personal or internal use, or to learn how this kind of product works, not about copying Grammarly to compete with it. How we grade.
What's easy and what's hard
Easy with AI
- Paste text, get corrections and rewrites from an LLM
- Tone presets such as formal, friendly or concise
- Showing a diff of changes
- Custom style rules in the prompt
Hard (where AI won't save you)
- Inline suggestions inside every text field across sites and apps
- Low latency while typing without huge API bills
- Mapping model output back to exact character positions
- Handling private text safely
Grammarly feature checklist
Difficulty is for a solo builder using an AI coding tool. Skip means not worth building yourself.
| Feature | Difficulty | Why |
|---|---|---|
| Rewrite and fix grammar | Easy | One LLM call with a clear system prompt. |
| Tone presets | Easy | Prompt variants stored as presets. |
| Diff view | Easy | Use a text diff library to show changes. |
| Browser extension | Medium | Content scripts for textareas; rich editors like Google Docs are much harder. |
| Inline underlines while typing | Hard | Needs debouncing, position mapping and a fast model. |
| Personal style guide | Easy | Store rules and add them to the prompt. |
| Plagiarism checking | Skip | Requires a web index; leave it out. |
Build time and running costs
Your own version vs Grammarly, at personal or small-team scale.
Build time
- Working MVP for yourself
- 2–6 h
- Polished, relied on daily
- 4–10 days
- MVP time at $50/h equals
- 18 months of Grammarly
Grammarly price (not independently verified)
Pro: $12/mo/seat billed annually
Pro is listed at 12 per member per month on annual billing (shown in EUR on the official page; third parties report $12 USD annual and $30 monthly).
Source: grammarly.com/plans. Prices change; check before deciding.
| Running cost | Per month | Notes |
|---|---|---|
| LLM API usage | $1–$8 | Small models cost cents per day for one person's writing. |
| Hosting | $0–$5 | Only needed for the web version or a proxy for your API key. |
| Total | $1–$13/mo | Saves up to $132 a year vs Grammarly Pro per seat; at the top of the range it costs $12 a year more. |
Recommended vibe coding tools
Starter prompt
Paste it into your tool, then ask for a plan before any code. Edit the specifics to match what you need.
prompt.txt
Build a personal writing assistant for my own use. Stack: a Chrome extension (Manifest V3) plus a small Next.js API route that calls an LLM API with my key. Features: 1. Select text in any textarea or contenteditable field, press a shortcut, and get a panel with: fixed grammar, a clearer version and a shorter version. 2. Show a word-level diff between the original and each suggestion, and a button to replace the selection. 3. Tone presets (neutral, friendly, formal, concise) and a personal style guide stored in extension settings. 4. A web page version where I can paste text. 5. Keep a local history of the last 50 rewrites in extension storage only. Security and privacy: never put the API key in the extension; call my own API route, which checks a per-user secret and rate-limits requests; do not log text on the server; skip password fields and fields marked autocomplete=cc-number; request the narrowest extension permissions possible (activeTab instead of all sites where it works).
Security pitfalls to check
AI tools often get these wrong by default. Check each one before anyone else uses the app.
- 01Shipping your LLM API key inside an extension lets anyone extract it and run up your bill.
- 02Broad all-sites extension permissions plus remote code is a large security risk; request the minimum.
- 03Sending text from password or payment fields to an API leaks secrets.
- 04Server logs that store submitted text keep private writing longer than you intend.
Indie makers who built a real ai writing business
They went far past the prototype: support, payments, reliability and years of iteration. Their profiles show what that took.
Mathias Michel built RewriteBar
Mac app for rewriting text with AI in any app.
$150K/yrAyush Chaturvedi built Elephas
AI writing assistant for Mac, $150K/yr.
Or just use these indie-made tools instead
If you need Grammarly's job done rather than a project, these are built and supported by independent makers.
RewriteBar
rewritebar.com
Native macOS menu bar app that rewrites, corrects and translates selected text in any app using AI.
by Mathias Michel
Elephas
elephas.app
Private AI assistant for Mac and Windows that answers and writes from your own documents, with sources.
$150K/yrby Ayush Chaturvedi
What building it teaches you
You learn how browser extensions interact with page text, and how to use LLM APIs cheaply and safely.
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Vibe coding Grammarly: FAQ
Can you vibe code a Grammarly alternative?
Medium. An AI rewrite tool for your own writing is easy with an LLM API; matching Grammarly's inline, everywhere experience is the hard part.
How long does it take to vibe code a Grammarly alternative?
A working personal or internal MVP takes about 2–6 h with a tool like Claude Code. A polished version you would rely on every day takes about 4–10 days, mostly spent on inline suggestions inside every text field across sites and apps.
How much does it cost to run your own Grammarly alternative?
Expect $1–$13/mo at personal or small-team scale (llm api usage, hosting). Grammarly Pro costs $12/mo/seat (not independently verified), so the most you would save is $132 a year, before counting your time.
Can I build my own Grammarly with ChatGPT or Claude?
Yes, a tool that fixes and rewrites selected text takes a few hours with an LLM API. Matching Grammarly's real-time underlines in every app is much harder.
How much would a homemade Grammarly cost to run?
For one person, LLM API costs are usually a few dollars a month or less with a small, fast model.
Is it safe to send my writing to an AI API?
Check the provider's data retention policy, keep your API key on a server, and avoid sending passwords, payment data or confidential text.
Is it legal to build your own Grammarly clone?
Building your own tool that solves the same problem is common and generally fine. Copying Grammarly's code, design assets, brand name or content is not. Use your own name and design, respect their terms of service, and get proper advice if you plan to sell it. This page is not legal advice.