# How to Vibe Code Your Own LanguageTool (and Stop Paying for It) > AI-powered multilingual writing assistant and grammar checker - Site: https://languagetool.org - Category: Productivity - Platforms: Web app, Browser extension, macOS app, Windows app - Verdict: **Serious undertaking** (42/100 vibecodeable) - Estimated effort: 6-8 weeks of part-time development ## Verdict Build a web-based clone with an AI grammar and paraphrasing engine yourself, but keep paying for LanguageTool if you rely on its native office plugins and browser extensions. Replicating the core editing web interface and connecting it to an LLM for grammar correction and paraphrasing is manageable. However, building the ecosystem of native browser extensions that mutate arbitrary DOMs without breaking web apps, combined with deterministic n-gram matching engines across 30 languages, crosses into serious engineering territory. For personal use, a Next.js web editor backed by an LLM is achievable, but you will miss the instant out-of-the-box system-wide integration. ### What you can't replicate - Two decades of curated open-source rule engines and massive n-gram datasets for 30+ languages - Deep native integrations into Microsoft Word, Google Docs, Apple Pages, and desktop apps - The massive installed base of 2 million active users and enterprise trust ## What it does An advanced writing assistant that checks text across more than 30 languages for spelling, grammar, punctuation, style, and tonality issues, complete with an AI paraphrasing tool. ### Core features - Real-time text error detection (spelling, grammar, punctuation) - Multi-language support (30+ languages and dialects) - Context-aware style suggestions and Picky Mode - AI-powered sentence paraphrasing (formal, fluent, concise) - Cross-platform browser extensions and desktop client integration - Personal dictionary and custom style rule management - Writing statistics and productivity tracking ## The business ### Pricing - Free: Free - Individual Premium (Annual): $5.83/mo - Team: €56.90/user/yr Founded 2003. Team size: 30-80+. ## The hard parts - Sub-millisecond parsing and diff-matching across arbitrary text nodes in active browser DOMs - Balancing custom deterministic NLP grammar rules with statistical models across 30+ languages - Low-latency real-time debouncing and incremental checking without lagging the host input field - Building seamless native integrations across disparate desktop environments and office suites ## How to vibe code LanguageTool ### Prerequisites - Node.js (free): Runtime environment for the Next.js full-stack application - GitHub (free): Source code control and deployment pipeline integration ### Recommended AI tools - Claude Code: Agentic terminal coding tool to scaffold the Next.js app, API routes, and UI components - Cursor: AI-native code editor for fine-tuning frontend components and diff-matching UI highlights ### Stack - Frontend: Next.js with React and Tailwind CSS - Backend: Next.js API routes with Vercel AI SDK - Database: Turso (SQLite at the edge) - Auth: better-auth - Payments: none - Other: Anthropic API, PostHog ### Hosting - Vercel (Hosting the Next.js web application and serverless API endpoints): $0/mo - Turso (Storing user texts, personal dictionary items, and usage statistics): $0/mo ### Build guide 1. **Project Scaffolding and Database Setup** — Initialize the Next.js application with TypeScript, Tailwind CSS, and configure Turso with better-auth for user accounts. ``` Create a new Next.js project using App Router, Tailwind CSS, and TypeScript. Set up better-auth with email/password authentication backed by a Turso SQLite database using the libSQL client. Create database migration files for users, saved texts, personal dictionary words, and usage statistics. Ensure the project structure follows best practices with clear separation between app routes, components, and server actions. ``` 2. **Distraction-Free Editor Layout** — Build the core web editor interface with a rich text area, sidebar for corrections and rephrasing, and dark/light mode toggle. ``` Build a distraction-free web editor interface in Next.js. The main view should feature a large, responsive editable text area with real-time word and character counting. Implement a collapsible right-hand sidebar with tab switching between 'Corrections' and 'Rephrasing'. Add a top navigation bar showing user statistics, account settings, and a dark/light mode toggle. Style everything cleanly using Tailwind CSS matching modern productivity tool aesthetics. ``` 3. **Grammar and Spell Checking API Integration** — Integrate the Vercel AI SDK and Anthropic API to analyze input text and return structured JSON containing error positions, types, and suggestions. ``` Implement a server action in Next.js using the Vercel AI SDK and Anthropic API (Claude 3.5 Sonnet) that accepts raw text input and returns a structured JSON array of writing errors. Each error object must specify the start index, end index, error category ('spelling', 'grammar', 'punctuation', 'style'), original substring, and an array of suggested replacements. Set up debouncing on the frontend text area so checks trigger automatically 600ms after the user stops typing. ``` 4. **Interactive Text Highlighting and Correction Overlay** — Overlay grammar underlines on the editor text and build popover suggestion cards for applying corrections with a single click. ``` Create a custom text overlay component in React that renders underlines beneath errors detected in the text area (red for spelling, blue for style, yellow for grammar). When a user clicks an underlined segment, display a floating popover card showing the error explanation and suggestion buttons. Clicking a suggestion must instantly replace the erroneous text in the underlying state and trigger a clean re-render. ``` 5. **AI Paraphrasing Tool Implementation** — Add sentence rewriting capabilities for formal, fluent, and concise styles via a dedicated rephrasing endpoint. ``` Implement sentence-level paraphrasing in the sidebar and via selection popovers. Create a backend API route that takes a selected sentence and requested style tone ('formal', 'fluent', 'shorter') and queries Claude via the Vercel AI SDK to generate three alternative rephrasings. Display these alternatives in the 'Rephrasing' tab of the sidebar with a one-click 'Apply' action to swap the sentence in the main text editor. ``` 6. **Statistics Dashboard and Polish** — Build a personal writing statistics dashboard tracking daily text checks, error correction rates, and productivity metrics. ``` Build a statistics dashboard page that aggregates user writing metrics stored in Turso. Display charts or summary cards for total texts checked, errors corrected, most frequent error types, and language distribution. Add a personal dictionary settings page where users can add custom words to ignore during checks. Polish all UI states, loading skeletons, and error handling for production readiness. ``` ### Cost vs paying **Starting costs (one-time):** - Domain name (optional): $12 one-time - Claude Pro (for building with agent): $20 one-time - Total: ~$32 one-time **Ongoing costs (monthly):** - Vercel Hobby Hosting: $0/mo - Turso Free Tier: $0/mo - Anthropic API usage (personal text checking & rephrasing): ~$3-5/mo - Total: ~$4/mo - Paying for the SaaS instead: $5.83/mo - Build time: 25-35 hours - AI tool credits: $20 one-time (Claude Pro) - Break-even: Personal project (built for learning and custom control) ## Sources - [LanguageTool Official Website](https://languagetool.org) - [LanguageTool Pricing & Subscription Tiers](https://languagetool.55.org/legal/pricing) - [Wikipedia Overview on LanguageTool Architecture](https://en.wikipedia.org/wiki/LanguageTool)