{"slug": "i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser", "title": "I Built a Free AI Grammar Checker That Runs Entirely in the Browser", "summary": "A developer built a free AI grammar checker that runs entirely in the browser, using DeepSeek's API for grammar checking and local computation for readability stats. The tool, called AI Grammar Checker, costs under $2 per month to run and catches 91% of errors compared to GPT-4o's 93%, making it a cost-effective alternative. It requires no signup and stores no user text on servers.", "body_md": "A few months ago I got tired of copy-pasting my writing between five different tools — Grammarly for grammar, Hemingway for readability, some word counter for stats. So I built [AI Grammar Checker](https://grammaraicheck.com/) — a single tool that does all of that in one shot, for free, with no signup.\n\nYou paste English text, hit check, and get back:\n\nAll client-side except the AI check call. No server stores your text.\n\n```\nHTML + vanilla JS + CSS\n↓\nDeepSeek API (OpenAI-compatible, $0.27/million tokens)\n↓\nResult rendered directly in the DOM\n```\n\nNo React. No Next.js. No build step. Just `app.js`\n\n, `style.css`\n\n, and `index.html`\n\n. The \"backend\" is a tiny Cloudflare Worker that proxies the DeepSeek API call so the API key stays server-side.\n\nHere's roughly how the grammar check works:\n\n``` js\nasync function runGrammarCheck(text, intensity) {\n  const systemPrompt = `You are a professional English editor.\nAnalyze the text sentence by sentence.\nFor each issue, return: original text, corrected text, explanation.\nAlso return: readability score, CEFR level, tone.`;\n\n  const response = await fetch('/api/deepseek', {\n    method: 'POST',\n    headers: { 'Content-Type': 'application/json' },\n    body: JSON.stringify({\n      model: 'deepseek-chat',\n      messages: [\n        { role: 'system', content: systemPrompt },\n        { role: 'user', content: text }\n      ],\n      temperature: 0.3\n    })\n  });\n\n  return response.json();\n}\n```\n\nThe intensity slider is the interesting bit — it adjusts the system prompt to be more or less aggressive about suggesting changes. At low intensity it only flags actual errors. At high intensity it rewrites for style too.\n\nThree reasons:\n\n**Cost.** DeepSeek is roughly 1/10th the price of GPT-4o for comparable quality on grammar tasks. At the free tier usage levels, my API bill is literally under $2/month.\n\n**Quality on structured output.** I tested both on the same 50-sample test set (emails, essays, blog posts). DeepSeek caught 91% of errors vs GPT-4o's 93%. The 2% gap isn't worth 10x the cost.\n\n**No content moderation false positives.** This is the one nobody talks about. OpenAI's moderation API sometimes flags academic writing about medical or legal topics. DeepSeek just checks the grammar.\n\n**Offline mode.** The Hemingway stats (passive voice, reading time, word count) are computed locally. The grammar check needs the API. I'd like to bundle a small on-device model via WebLLM eventually.\n\n**Better mobile UX.** The tool works on mobile but the text area + results layout isn't great on narrow screens. Bootstrap or Tailwind would've helped but I wanted zero dependencies.\n\n**API key flow.** Right now users bring their own DeepSeek key. New DeepSeek users get 5M free tokens, which covers a LOT of grammar checks. But the UX of pasting an API key is friction. Considering a free tier with a shared key + rate limiting.\n\nCompletely free, no signup, no email. If you write in English regularly — especially if you're a non-native speaker — I'd love feedback on what's missing.\n\nAlso have a comparison page if you're curious how it stacks up against Grammarly, ProWritingAid, and others: [Best AI Grammar Checkers 2026](https://grammaraicheck.com/best-ai-grammar-checkers/).", "url": "https://wpnews.pro/news/i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser", "canonical_source": "https://dev.to/dayu2333jinyul/i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser-cec", "published_at": "2026-06-26 06:15:18+00:00", "updated_at": "2026-06-26 06:33:41.650073+00:00", "lang": "en", "topics": ["artificial-intelligence", "large-language-models", "developer-tools", "ai-products", "natural-language-processing"], "entities": ["DeepSeek", "Grammarly", "Hemingway", "OpenAI", "Cloudflare", "AI Grammar Checker", "GPT-4o", "WebLLM"], "alternates": {"html": "https://wpnews.pro/news/i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser", "markdown": "https://wpnews.pro/news/i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser.md", "text": "https://wpnews.pro/news/i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser.txt", "jsonld": "https://wpnews.pro/news/i-built-a-free-ai-grammar-checker-that-runs-entirely-in-the-browser.jsonld"}}